Heads up: There are no amended sections in this chapter.
Chapter A1 Seismic Strengthening Provisions for Unreinforced Masonry Bearing Wall Buildings
About this appendix: Appendix A provides guidelines for upgrading the seismic-resistance capacity of different types of existing buildings. It is organized into separate chapters that deal with buildings of different types, including unreinforced masonry buildings, reinforced concrete and reinforced masonry wall buildings, and light-frame wood buildings.
The purpose of this chapter is to promote public safety and welfare by reducing the risk of death or injury from the effects of earthquakes on existing unreinforced masonry bearing wall buildings.
The provisions of this chapter are intended as minimum standards for structural seismic resistance, and are established primarily to reduce the risk of life loss or injury. Compliance with these provisions will not necessarily prevent loss of life or injury, or prevent earthquake damage to retrofitted buildings.
The provisions of this chapter shall apply to all existing buildings not more than six stories in height above the base of the structure and having not fewer than one unreinforced masonry bearing wall. The elements regulated by this chapter shall be determined in accordance with Table A102.1. Except as provided herein, other structural provisions of the building code shall apply. This chapter does not apply to the alteration of existing electrical, plumbing, mechanical or fire safety systems.
[BS] TABLE A102.1
ELEMENTS REGULATED BY THIS CHAPTER
|≥ 0.067g < 0.133g||≥ 0.133g < 0.20g||≥ 0.20g < 0.30g||≥ 0.30g|
|Walls, h/t ratios||X||X||X|
|Walls, in-plane shear||X||X||X|
|Diaphragms, shear transferb||X||X||X|
|Diaphragms, demand-capacity ratiosb||X||X|
The provisions of this chapter shall not apply to the strengthening of buildings in Risk Category III or IV. Such buildings shall be strengthened to meet the requirements of the International Building Code for new buildings of the same risk category or other such criteria approved by the code official.
For the purpose of this chapter, the applicable definitions in the building code shall also apply.
[BS] BED JOINT. The horizontal layer of mortar on which a masonry unit is laid.
[BS] COLLAR JOINT. The vertical space between adjacent wythes. A collar joint may contain mortar or grout.
[BS] CROSSWALL. A new or existing wall that meets the requirements of Section A111.3. A crosswall is not a shear wall.
[BS] CROSSWALL SHEAR CAPACITY. The unit shear value times the length of the crosswall, νcLc.
[BS] DETAILED BUILDING SYSTEM ELEMENTS. The localized elements and the interconnections of these elements that define the design of the building.
[BS] DIAPHRAGM EDGE. The intersection of the horizontal diaphragm and a shear wall.
[BS] DIAPHRAGM SHEAR CAPACITY. The unit shear value times the depth of the diaphragm, νuD.
[BS] FLEXIBLE DIAPHRAGM. A diaphragm of wood or untopped metal deck construction in which the horizontal deformation along its length is at least two times the average story drift.
HEAD JOINT. The vertical mortar joint placed between masonry units within the wythe.
[BS] NORMAL WALL. A wall perpendicular to the direction of seismic forces.
[BS] OPEN FRONT. An exterior building wall line on one side only without vertical elements of the seismic force-resisting system in one or more stories.
[BS] POINTING. The process of removal of deteriorated mortar from between masonry units and placement of new mortar. Also known as repointing or tuckpointing for purposes of this chapter.
[BS] REPOINTING. See "Pointing."
[BS] RIGID DIAPHRAGM. A diaphragm of concrete construction or concrete-filled metal deck construction.
[BS] TUCKPOINTING. See "Pointing."
[BS] UNREINFORCED MASONRY (URM). Includes burned clay, concrete or sand-lime brick; hollow clay or concrete block; plain concrete; and hollow clay tile. These materials shall comply with the requirements of Section A106 as applicable.
[BS] UNREINFORCED MASONRY BEARING WALL. A URM wall that provides the vertical support for the reaction of floor or roof-framing members for which the total superimposed vertical load exceeds 100 pounds per linear foot (1459 N/m) of wall length.
[BS] UNREINFORCED MASONRY WALL. A masonry wall that relies on the tensile strength of masonry units, mortar and grout in resisting design loads, and in which the area of reinforcement is less than the minimum amounts as defined for reinforced masonry walls.
[BS] YIELD STORY DRIFT. The lateral displacement of one level relative to the level above or below at which yield stress is first developed in a frame member.
For the purpose of this chapter, the following notations supplement the applicable symbols and notations in the building code.
an = Diameter of core multiplied by its length or the area of the side of a square prism.
A = Cross-sectional area of unreinforced masonry pier or wall, square inches (10-6 m2).
Ab = Total area of the bed joints above and below the test specimen for each in-place shear test, square inches (10-6 m2).
An = Area of net mortared or grouted section of a wall or wall pier.
D = In-plane width dimension of pier, inches (10-3 m), or depth of diaphragm, feet (m).
DCR = Demand-capacity ratio specified in Section A111.4.2.
f'm = Lower bound masonry compressive strength.
fsp = Tensile-splitting strength of masonry.
Fwx = Force applied to a wall at level x, pounds (N).
H = Least clear height of opening on either side of a pier, inches (10-3m).
h/t = Height-to-thickness ratio of URM wall. Height, h, is measured between wall anchorage levels and/or slab-on-grade.
L = Span of diaphragm between shear walls, or span between shear wall and open front, feet (m).
Lc = Length of crosswall, feet (m).
Li = Effective diaphragm span for an open-front building specified in Section A111.8, feet (m).
P = Applied force as determined by standard test method of ASTM C496 or ASTM E519, pounds (N).
PD = Superimposed dead load at the location under consideration, pounds (N). For determination of the rocking shear capacity, dead load at the top of the pier under consideration shall be used.
PD+L = Stress resulting from the dead plus actual live load in place at the time of testing, pounds per square inch (kPa).
Ptest = Splitting tensile test load determined by standard test method ASTM C496, pounds (N).
Pw = Weight of wall, pounds (N).
R = Response modification factor for Ordinary plain masonry shear walls in Bearing Wall System from Table 12.2-1 of ASCE 7, where R = 1.5.
SDS = Design spectral acceleration at short period, in g units.
SD1 = Design spectral acceleration at 1-second period, in g units.
va = The shear strength of any URM pier, vmA/1.5 pounds (N).
vc = Unit shear strength for a crosswall sheathed with any of the materials given in Table A108.1(1) or Table A108.1(2), pounds per foot (N/m).
vmL = Shear strength of unreinforced masonry, pounds per square inch (kPa).
Vaa = The shear strength of any URM pier or wall, pounds (N).
Vca = Total shear capacity of crosswalls in the direction of analysis immediately above the diaphragm level being investigated, vcLc, pounds (N).
Vcb = Total shear capacity of crosswalls in the direction of analysis immediately below the diaphragm level being investigated, vcLc, pounds (N).
Vp = Shear force assigned to a pier on the basis of its relative shear rigidity, pounds (N).
Vr = Pier rocking shear capacity of any URM wall or wall pier, pounds (N).
vtest = Load at incipient cracking for each in-place shear test performed in accordance with Section A22.214.171.124, pounds (N).
vtl = Lower bound mortar shear strength, pounds per square inch (kPa).
vto = Mortar shear test values as specified in Section A126.96.36.199, pounds per square inch (kPa).
vu = Unit shear capacity value for a diaphragm sheathed with any of the materials given in Table A108.1(1) or A108.1(2), pounds per foot (N/m).
Vwx = Total shear force resisted by a shear wall at the level under consideration, pounds (N).
W = Total seismic dead load as defined in the building code, pounds (N).
Wd = Total dead load tributary to a diaphragm level, pounds (N).
Ww = Total dead load of a URM wall above the level under consideration or above an open-front building, pounds (N).
Wwx = Dead load of a URM wall assigned to level x halfway above and below the level under consideration, pounds (N).
ΣvuD = Sum of diaphragm shear capacities of both ends of the diaphragm, pounds (N).
ΣΣvuD = For diaphragms coupled with crosswalls, vuD includes the sum of shear capacities of both ends of diaphragms coupled at and above the level under consideration, pounds (N).
ΣWd = Total dead load of all the diaphragms at and above the level under consideration, pounds (N).
The seismic force-resisting system specified in this chapter shall comply with the International Building Code and referenced standards, except as modified herein.
The following construction information shall be included in the plans required by this chapter:
- Dimensioned floor and roof plans showing existing walls and the size and spacing of floor and roof-framing members and sheathing materials. The plans shall indicate all existing URM walls, and new crosswalls and shear walls, and their materials of construction. The location of these walls and their openings shall be fully dimensioned and drawn to scale on the plans.
- Dimensioned URM wall elevations showing openings, piers, wall classes as defined in Section A188.8.131.52, thickness, heights, wall shear test locations, cracks or damaged portions requiring repairs, the general condition of the mortar joints, and if and where pointing is required. Where the exterior face is veneer, the type of veneer, its thickness and its bonding and/or ties to the structural wall masonry shall be noted.
- The type of interior wall and ceiling materials, and framing.
- The extent and type of existing wall anchorage to floors and roof where used in the design.
- The extent and type of parapet corrections that were previously performed, if any.
- Repair details, if any, of cracked or damaged unreinforced masonry walls required to resist forces specified in this chapter.
- All other plans, sections and details necessary to delineate required retrofit construction.
- The design procedure used shall be stated on both the plans and the permit application.
- Details of the anchor prequalification program required by Section A107.5.3, if used, including location and results of all tests.
- Quality assurance requirements of special inspection for all new construction materials and for retrofit construction including: anchor tests, pointing or repointing of mortar joints, installation of adhesive or mechanical anchors, and other elements as deemed necessary to ensure compliance with this chapter.
Structural observation, in accordance with Section 1704.6 of the International Building Code, shall be required for all structures in which seismic retrofit is being performed in accordance with this chapter. Structural observation shall include visual observation of work for compliance with the approved construction documents and confirmation of existing conditions assumed during design.
Structural testing and inspection for new and existing construction materials shall be in accordance with the building code, except as modified by this chapter.
Special inspection as described in Section A105.3, Item 10, shall be provided equivalent to Level 3 as prescribed in TMS 402, Table 3.1(2).
Existing materials used as part of the required vertical load-carrying or seismic force-resisting system shall be evaluated by on-site investigation and: determined to be in good condition (free of degraded mortar, degraded masonry units or significant cracking); or shall be repaired, enhanced, retrofitted or removed and replaced with new materials. Mortar joint deterioration shall be patched by pointing or repointing of the eroded joint in accordance with Section A184.108.40.206. Existing significant cracks in solid unit unreinforced and solid grouted hollow unit masonry shall be repaired.
Unreinforced masonry walls used to support vertical loads or seismic forces parallel and perpendicular to the wall plane shall be tested as specified in this section. Masonry that does not meet the minimum requirements established by this chapter shall be repaired, enhanced, removed and replaced with new materials, or alternatively, shall have its structural functions replaced with new materials and shall be anchored to supporting elements.
Unreinforced masonry walls shall be laid in a running bond pattern.
The facing and backing wythes of multiple-wythe walls shall be bonded so that not less than 10 percent of the exposed face area is composed of solid headers extending not less than 4 inches (102 mm) into the backing wythes. The clear distance between adjacent header courses shall not exceed 24 inches (610 mm) vertically or horizontally. Where backing consists of two or more wythes, the headers shall extend not less than 4 inches (102 mm) into the most distant wythe, or the backing wythes shall be bonded together with separate headers for which the area and spacing conform to the foregoing. Wythes of walls not meeting these requirements shall be considered to be veneer, and shall not be included in the effective thickness used in calculating the height-to-thickness ratio and the shear capacity strength of the wall.
Exception: Where SD1 is 0.3 g or less, veneer wythes anchored and made composite with backup masonry are permitted to be used for calculation of the effective thickness.
Lay-up patterns other than those specified in Section A220.127.116.11 are allowed if their performance can be justified.
Grouted or ungrouted hollow concrete masonry units shall be tested in accordance with ASTM C140. Grouted or ungrouted structural clay load-bearing tile shall be tested in accordance with ASTM C67.
Mortar joint shear test values, vto, shall be obtained by one of the following:
- ASTM C1531.
- For masonry walls that have high shear strength mortar, or where in-place testing is not practical because of crushing or other failure mode of the masonry, alternative procedures for testing shall be used in accordance with Section A18.104.22.168.
The splitting tensile strength of existing masonry, fsp, or the prism strength of existing masonry, f'm, is permitted to be determined in accordance with ASTM C496 and calculated by the following equation:
The shear tests shall be taken at locations representative of the mortar conditions throughout the building. Test locations shall be determined at the building site by the registered design professional in charge. Results of all tests and their locations shall be recorded.
The minimum number of tests per masonry class shall be determined as follows:
- At each of both the first and top stories, not less than two tests per wall or line of wall elements providing a common line of resistance to seismic forces.
- At each of all other stories, not less than one test per wall or line of wall elements providing a common line of resistance to seismic forces.
- In any case, not less than one test per 1,500 square feet (139.4 m2) of wall surface and not less than a total of eight tests.
- Mortar shear test values, vto, in pounds per square inch (kPa), shall be obtained for each in-place shear test in accordance with the following equation:(Equation A1-2)where:Vtest = Load at first observed movement.Ab = Total area of the bed joints above and below the test specimen.PD+L = Stress resulting from actual dead plus live loads in place at the time of testing.
- Individual unreinforced masonry walls with more than 50 percent of mortar test values, vto, less than 30 pounds per square inch (207 kPa) shall be pointed prior to and retested.
- The lower bound mortar shear strength, vtL, is defined as the mean minus one standard deviation of the mortar shear test values, vto.
- Unreinforced masonry with mortar shear strength, vtL, less than 30 pounds per square inch (207 kPa) shall be pointed and retested or shall have its structural function replaced, and shall be anchored to supporting elements in accordance with Sections A106.2.1 and A113.8. When existing mortar in any wythe is pointed to increase its shear strength and is retested, the condition of the mortar in the adjacent bed joints of the inner wythe or wythes and the opposite outer wythe shall be examined for extent of deterioration. The shear strength of any wall class shall be not greater than that of the weakest wythe of that class.
Where the alternative procedures of Section A22.214.171.124 are used to determine masonry quality, the following minimums apply:
- The minimum average value of splitting tensile strength, fsp, as calculated by Equation A1-1 shall be 50 pounds per square inch (344.7 kPa).
- Individual unreinforced masonry walls with average splitting tensile strength of less than 50 pounds per square inch (344.7 kPa) shall be pointed and retested.
- The lower-bound mortar strength fspL is defined as the mean minus one standard deviation PD+L of the splitting tensile test values fsp.
The collar joints shall be inspected at the test locations during each in-place shear test, and estimates of the percentage of surfaces of the adjacent wythe that are covered with mortar shall be reported along with the results of the in-place shear tests.
Existing unreinforced masonry shall be categorized into one or more classes based on shear strength, quality of construction, state of repair, deterioration and weathering. A class shall be characterized by the masonry shear strength determined in accordance with Section A108.2. Classes are defined for whole walls, not for small areas of masonry within a wall. Discretion in the definition of classes of masonry is permitted to avoid unnecessary testing.
Deteriorated mortar joints in unreinforced masonry walls shall be pointed in accordance with the following requirements:
- Joint preparation. Deteriorated mortar shall be cut out by means of a toothing chisel or nonimpact power tool until sound mortar is reached, to a depth not less than 3/4 inch (19.1 mm) or twice the thickness of the joint, whichever is less, but not greater than 2 inches (50 mm). Care shall be taken not to damage the masonry edges. After cutting is complete, all loose material shall be removed with a brush, or air or water stream.
- Mortar preparation. The mortar mix shall be proportioned as required by the construction specifications and manufacturer's approved instructions.
- Packing. The joint into which the mortar is to be packed shall be dampened but without free-standing water. The mortar shall be tightly packed into the joint in layers not exceeding 1/4 inch (6.4 mm) deep until it is filled; then it shall be tooled to a smooth surface to match the original profile.
Preparation and mortar pointing shall be performed with special inspection.
Exception: At the discretion of the code official, incidental pointing may be performed without special inspection.
Existing wall anchors used as all or part of the required tension anchors shall be tested in pullout according to Section A107.5.1. Not fewer than four anchors tested per floor shall be tested in pullout, with not fewer than two tests at walls with joists framing into the wall and two tests at walls with joists parallel to the wall, but not less than 10 percent of the total number of existing tension anchors at each level.
New wall anchors embedded in URM walls shall be subject to special inspection prior to placement of the anchor and grout or adhesive in the drilled hole. Five percent of all anchors that do not extend through the wall shall be subject to a direct-tension test, and an additional 20 percent shall be tested using a calibrated torque wrench. Testing shall be performed in accordance with Section A107.5.
New wall anchors embedded in URM walls resisting tension forces or a combination of tension and shear forces shall be subject to special inspection, prior to placement of the anchor and grout or adhesive in the drilled hole. Five percent of all anchors resisting tension forces shall be subject to a direct-tension test, and an additional 20 percent shall be tested using a calibrated torque wrench. Testing shall be performed in accordance with Section A107.5.
Exception: New bolts that extend through the wall with steel plates on the far side of the wall need not be tested.
Tests of anchors in unreinforced masonry walls shall be in accordance with Sections A107.5.1 through A107.5.3. Results of all tests shall be reported to the authority having jurisdiction. The report shall include the test results of maximum load for each test; pass-fail results; corresponding anchor size and type; orientation of loading; details of the anchor installation, testing apparatus and embedment; wall thickness; and joist orientation and proximity to the tested anchor.
The test apparatus shall be supported by the masonry wall. The test procedure for prequalification of tension and shear anchors shall comply with ASTM E488. Existing wall anchors shall be given a preload of 300 pounds (1335 N) before establishing a datum for recording elongation. The tension test load reported shall be recorded at 1/8 inch (3.2 mm) relative movement between the existing anchor and the adjacent masonry surface. New embedded tension anchors shall be subject to a direct tension load of not less than 2.5 times the design load but not less than 1,500 pounds (6672 N) for 5 minutes.
Exception: Where obstructions occur, the distance between the anchor and the test apparatus support shall be not less than one-half the wall thickness for existing anchors and 75 percent of the embedment length for new embedded anchors.
Anchors embedded in unreinforced masonry walls shall be tested using a torque-calibrated wrench to the following minimum torques:
- 1/2-inch-diameter (12.7 mm) bolts: 40 foot pounds (54.2 N-m).
- 5/8-inch-diameter (15.9 mm) bolts: 50 foot pounds (67.8 N-m).
- 3/4-inch-diameter (19.1 mm) bolts: 60 foot pounds (81.3 N-m).
ASTM E488 or the test procedure in Section A107.5.1 is permitted to be used to determine tension or shear strength values for anchors greater than those permitted by Table A108.1(2). Anchors shall be installed in the same manner and using the same materials as will be used in the actual construction. Not fewer than five tests for each bolt size and type shall be performed for each class of masonry in which they are proposed to be used. The tension and shear strength values for such anchors shall be the lesser of the average ultimate load divided by 5.0 or the average load at which 1/8 inch (3.2 mm) elongation occurs for each size and type of anchor and class of masonry.
- Strength values for existing materials are given in Table A108.1(1) and for new materials in Table A108.1(2).
- The strength reduction factor, ϕ, shall be taken equal to 1.0.
- The use of materials not specified herein shall be based on substantiating research data or engineering judgment, as approved by the code official.
[BS] TABLE A108.1(1)
STRENGTH VALUES FOR EXISTING MATERIALS
|EXISTING MATERIALS OR CONFIGURATION OF MATERIALSa||STRENGTH VALUES|
|x 14.594 for N/m|
|Horizontal diaphragms||Roofs with straight sheathing and roofing applied directly to the sheathing.||300 lbs. per ft. for seismic shear|
|Roofs with diagonal sheathing and roofing applied directly to the sheathing.||750 lbs. per ft. for seismic shear|
|Floors with straight tongue-and-groove sheathing.||300 lbs. per ft. for seismic shear|
|Floors with straight sheathing and finished wood flooring with board edges offset or perpendicular.||1,500 lbs. per ft. for seismic shear|
|Floors with diagonal sheathing and finished wood flooring.||1,800 lbs. per ft. for seismic shear|
|Metal deck welded with minimal welding.c||1,800 lbs. per ft. for seismic shear|
|Metal deck welded for seismic resistance.d||3,000 lbs. per ft. for seismic shear|
|Crosswallsb||Plaster on wood or metal lath.||600 lbs. per ft. for seismic shear|
|Plaster on gypsum lath.||550 lbs. per ft. for seismic shear|
|Gypsum wallboard, unblocked edges.||200 lbs. per ft. for seismic shear|
|Gypsum wallboard, blocked edges.||400 lbs. per ft. for seismic shear|
|Existing footing, wood framing, structural steel, reinforcing steel||Plain concrete footings.||f'c = 1,500 psi unless otherwise shown by tests|
|Douglas fir wood.||Same as D.F. No. 1|
|Reinforcing steel.||Fy = 40,000 psi maximum|
|Structural steel.||Fy = 33,000 psi maximum|
For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2, 1 pound = 4.4 N, 1 pound per square inch = 6894.75 N/m2, 1 pound per foot = 14.43 N/m.
- Material must be sound and in good condition.
- Shear values of these materials may be combined, except the total combined value should not exceed 900 pounds per foot.
- Minimum 22-gage steel deck with welds to supports satisfying the standards of the Steel Deck Institute.
- Minimum 22-gage steel deck with 3/4-inch diameter plug welds at an average spacing not exceeding 8 inches and with sidelap welds appropriate for the deck span.
[BS] TABLE A108.1(2)
STRENGTH VALUES OF NEW MATERIALS USED IN CONJUNCTION WITH EXISTING CONSTRUCTION
|NEW MATERIALS OR CONFIGURATION OF MATERIALS||STRENGTH VALUES|
|Horizontal diaphragms||Plywood sheathing applied directly over existing straight sheathing with ends of plywood sheets bearing on joists or rafters and edges of plywood located on center of individual sheathing boards.||675 lbs. per ft.|
|Crosswalls||Plywood sheathing applied directly over wood studs; no value should be given to plywood applied over existing plaster or wood sheathing.||1.2 times the value specified in the current building code.|
|Drywall or plaster applied directly over wood studs.||The value specified in the current building code.|
|Drywall or plaster applied to sheathing over existing wood studs.||50 percent of the value specified in the current building code.|
|Tension anchorsf||Anchors extending entirely through unreinforced masonry wall secured with bearing plates on far side of a wall 30 square inches of area.b, c|| |
5,400 lbs. per anchor for three-wythe minimum walls.
2,700 lbs. for two-wythe walls.
|Shear boltse, f||Anchors embedded not less than 8 inches into unreinforced masonry walls; anchors should be centered in 21/2-inch-diameter holes with dry-pack or nonshrink grout around the circumference of the anchor.||The value for plain masonry specified for solid masonry TMS 402; and no value larger than those given for 3/4-inch bolts should be used.|
|Combined tension and shear anchorsf||Through-anchors—anchors meeting the requirements for shear and for tension anchors.b, c||Tension—same as for tension anchors. |
Shear—same as for shear anchors.
|Embedded anchors—anchors extending to the exterior face of the wall with a 21/2-inch round plate under the head and drilled at an angle of 221/2 degrees to the horizontal; installed as specified for shear anchors.a, b, c||Tension—3,600 lbs. per anchor. |
Shear—same as for shear anchors.
|Infilled walls||Reinforced masonry infilled openings in existing unreinforced masonry walls; provide keys or dowels to match reinforcing.||Same as values specified for unreinforced masonry walls.|
|Reinforced masonryd||Masonry piers and walls reinforced per the current building code.||The value specified in the current building code for strength design.|
|Reinforced concreted||Concrete footings, walls and piers reinforced as specified in the current building code.||The value specified in the current building code for strength design.|
For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2, 1 pound = 4.4 N, 1 degree = 0.017 rad, 1 pound per foot = 14.43 N/m, 1 foot = 304.8 mm.
- Embedded anchors to be tested as specified in Section A107.4.
- Anchors shall be 1/2 inch minimum in diameter.
- Drilling for anchors shall be done with an electric rotary drill; impact tools should not be used for drilling holes or tightening anchors and shear bolt nuts.
- Load factors or capacity reduction factors shall not be used.
- Other bolt sizes, values and installation methods may be used, provided that a testing program is conducted in accordance with Section A107.5.3. The strength value shall be determined by multiplying the calculated allowable value, determined in accordance with Section A107.5.3, by 3.0, and the usable value shall be limited to not greater than 1.5 times the value given in the table. Bolt spacing shall not exceed 6 feet on center and shall be not less than 12 inches on center.
- An alternative adhesive anchor bolt system is permitted to be used providing: its properties and installation conform to an ICC Evaluation Service Report; and the report states that the system's use is in unreinforced masonry as an acceptable alternative to Sections A107.4 and A113.1 or TMS 402, Section 2.1.4. The report's allowable values shall be multiplied by a factor of three to obtain strength values and the strength reduction factor, ϕ, shall be taken equal to 1.0.
The unreinforced masonry shear strength, vmL, shall be determined for each masonry class from one of the following equations:
- When testing is performed in accordance with Section A126.96.36.199, the unreinforced masonry shear strength, vm, shall be determined by Equation A1-3.(Equation A1-3)The mortar shear strength values, vtL, shall be determined in accordance with Section A188.8.131.52.
- When alternate testing is performed in accordance with Section A184.108.40.206, unreinforced masonry shear, vmL, shall be determined by Equation A1-4.(Equation A1-4)
Where any increase in wall dead plus live load compression stress occurs, the maximum compression stress in unreinforced masonry, QG/An, shall not exceed 300 pounds per square inch (2070 kPa).
Unreinforced masonry shall be assumed to have no tensile capacity.
The tension strength of wall anchors shall be the average of the tension test values for anchors having the same wall thickness and framing orientation.
For existing foundations, new total dead loads are permitted to be increased over the existing dead load by 25 percent. New total dead load plus live load plus seismic forces may be increased over the existing dead load plus live load by 50 percent. Higher values may be justified only in conjunction with a geotechnical investigation.
The elements of buildings hereby required to be analyzed are specified in Table A102.1.
Buildings shall be analyzed to resist minimum lateral forces assumed to act nonconcurrently in the direction of each of the main axes of the structure in accordance with the following:
Parts and portions of a structure not covered in Section A110.3 shall be analyzed and designed per the current building code, using force levels defined in Section A110.1.
- Unreinforced masonry walls for which height-tothickness ratios do not exceed ratios set forth in Table A110.2 need not be analyzed for out-of-plane loading. Unreinforced masonry walls that exceed the allowable h/t ratios of Table A110.2 shall be braced according to Section A113.5.
- Parapets complying with Section A113.6 need not be analyzed for out-of-plane loading.
- Where walls are to be anchored to flexible floor and roof diaphragms, the anchorage shall be in accordance with Section A113.1.
[BS] TABLE A110.2
ALLOWABLE VALUE OF HEIGHT-TO-THICKNESS RATIO OF UNREINFORCED MASONRY WALLS
|WALL TYPES||0.13g ≤ SD1 < 0.25g||0.25g ≤ SD1 < 0.4g||SD1 ≥ 0.4g BUILDINGS WITH CROSSWALLSa||SD1 ≥ 0.4g ALL OTHER BUILDINGS|
|Walls of one-story buildings||20||16||16b, c||13|
|First-story wall of multiple-story building||20||18||16||15|
|Walls in top story of multiple-story building||14||14||14b, c||9|
|All other walls||20||16||16||13|
For SI:1 pound per square inch = 6894.75 N/m2.
- Applies to the special procedures of Section A111 only. See Section A111.7 for other restrictions.
- This value of height-to-thickness ratio shall be used where mortar shear tests establish a tested mortar shear strength, vt, of not less than 100 pounds per square inch. This value shall also be used where the tested mortar shear strength is not less than 60 pounds per square inch, and where a visual examination of the collar joint indicates not less than 50-percent mortar coverage.
- Where a visual examination of the collar joint indicates not less than 50-percent mortar coverage, and the tested mortar shear strength, vt, is greater than 30 pounds per square inch but less than 60 pounds per square inch, the allowable height-to-thickness ratio may be determined by linear interpolation between the larger and smaller ratios in direct proportion to the tested mortar shear strength.
Vertical seismic force-resisting elements shall be analyzed in accordance with Section A112.
Any redundancy or overstrength factors contained in the building code may be taken as unity. The vertical component of seismic force (Ev) may be taken as zero.
The special procedures of this section shall be applied only to buildings having the following characteristics:
- Flexible diaphragms at all levels above the base of the structure.
- Vertical elements of the seismic force-resisting system consisting predominantly of masonry or a combination of masonry and concrete shear walls.
- Except for single-story buildings with an open front on one side only, not fewer than two lines of vertical elements of the seismic force-resisting system parallel to each axis of the building (see Section A111.8 for open front buildings).
Crosswalls shall meet the requirements of this section.
A crosswall is a wood-framed wall sheathed with any of the materials described in Table A108.1(1) or Table A108.1(2) or other system as defined in Section A111.3.5. Crosswalls shall be spaced not more than 40 feet (12 192 mm) on center measured perpendicular to the direction of consideration, and shall be placed in each story of the building. Crosswalls shall extend the full story height between diaphragms.
- Crosswalls need not be provided at all levels where used in accordance with Section A111.4.2, Item 4.
- Existing crosswalls need not be continuous below a wood diaphragm at or within 4 feet (1219 mm) of grade, provided that:2.1. Shear connections and anchorage requirements of Section A111.5 are satisfied at all edges of the diaphragm.2.2. Crosswalls with total shear capacity of 0.5SD1ΣWd interconnect the diaphragm to the foundation.2.3. The demand-capacity ratio of the diaphragm between the crosswalls that are continuous to their foundations does not exceed 2.5, calculated as follows:(Equation A1-6)
Within any 40 feet (12 192 mm) measured along the span of the diaphragm, the sum of the crosswall shear capacities shall be not less than 30 percent of the diaphragm shear capacity of the strongest diaphragm at or above the level under consideration.
Existing crosswalls shall have a maximum height-to-length ratio between openings of 1.5 to 1. Existing crosswall connections to diaphragms need not be investigated as long as the crosswall extends to the framing of the diaphragms above and below.
New crosswall connections to the diaphragm shall develop the crosswall shear capacity. New crosswalls shall have the capacity to resist an overturning moment equal to the crosswall shear capacity times the story height. Crosswall overturning moments need not be cumulative over more than two stories.
Other systems, such as moment-resisting frames, may be used as crosswalls provided that the yield story drift does not exceed 1 inch (25 mm) in any story.
A diaphragm is acceptable if the point (L,DCR) on Figure A111.4.1 falls within Region 1, 2 or 3.
Demand-capacity ratios shall be calculated for the diaphragm at any level according to the following formulas:
- For a diaphragm without qualifying crosswalls at levels immediately above or below:(Equation A1-7)
- For a diaphragm in a single-story building with qualifying crosswalls, or for a roof diaphragm coupled by crosswalls to the diaphragm directly below:(Equation A1-8)
- For diaphragms in a multiple-story building with qualifying crosswalls in all levels:(Equation A1-9)DCR shall be calculated at each level for the set of diaphragms at and above the level under consideration. In addition, the roof diaphragm shall meet the requirements of Equation A1-10.
- For a roof diaphragm and the diaphragm directly below, if coupled by crosswalls:(Equation A1-10)
An analysis for diaphragm flexure need not be made, and chords need not be provided.
An analysis of diaphragm collector forces shall be made for the transfer of diaphragm edge shears into vertical elements of the lateral force-resisting system. Collector forces may be resisted by new or existing elements.
- Diaphragm forces at corners of openings shall be investigated and shall be developed into the diaphragm by new or existing materials.
- In addition to the demand-capacity ratios of Section A111.4.2, the demand-capacity ratio of the portion of the diaphragm adjacent to an opening shall be calculated using the opening dimension as the span.
- Where an opening occurs in the end quarter of the diaphragm span, the calculation of vuD for the demand-capacity ratio shall be based on the net depth of the diaphragm.
Diaphragms shall be connected to shear walls and new vertical seismic force-resisting elements with connections capable of developing the diaphragm-loading tributary to the shear wall or new seismic force-resisting elements given by the lesser of the following formulas:
using the Cp values in Table A111.5, or
[BS] TABLE A111.5
HORIZONTAL FORCE FACTOR, Cp
|CONFIGURATION OF MATERIALS||Cp|
|Roofs with straight or diagonal sheathing and roofing applied directly to the sheathing, or floors with straight tongue-and-groove sheathing.||0.50|
|Diaphragms with double or multiple layers of boards with edges offset, and blocked plywood systems.||0.75|
|Diaphragms of metal deck without topping:|
|Minimal welding or mechanical attachment.||0.6|
|Welded or mechanically attached for seismic resistance.||0.68|
The wall story force distributed to a shear wall at any diaphragm level shall be the lesser value calculated as:
but need not exceed
The wall story shear shall be the sum of the wall story forces at and above the level of consideration.
New seismic force-resisting elements such as moment frames, braced frames or shear walls shall be designed as required by the building code, except that the seismic forces shall be as specified in Section A111.6.1, and the story drift ratio shall be limited to 0.015, except as further limited by Section A112.4.2 for moment frames.
The provisions of Section A110.2 are applicable, except the allowable height-to-thickness ratios given in Table A110.2 shall be determined from Figure A111.4.1 as follows:
- In Region 1, height-to-thickness ratios for buildings with crosswalls may be used if qualifying crosswalls are present in all stories.
- In Region 2, height-to-thickness ratios for buildings with crosswalls may be used whether or not qualifying crosswalls are present.
- In Region 3, height-to-thickness ratios for "all other buildings" shall be used whether or not qualifying crosswalls are present.
Where diaphragms above and below the wall under consideration have demand-capacity ratios in different regions of Figure A111.4.1, the lesser height-to-thickness ratio shall be used.
A single-story building with an open front on one side and crosswalls parallel to the open front may be designed by the following procedure:
- Effective diaphragm span, Li, for use in Figure A111.4.1 shall be determined in accordance with the following formula:(Equation A1-16)
- Diaphragm demand-capacity ratio shall be calculated as:(Equation A1-17)
The following requirements are applicable to both the general procedure and the special procedure for analyzing vertical elements of the lateral force-resisting system.
Flexural components of deflection need not be considered in determining the rigidity of an unreinforced masonry wall.
Wall piers shall be analyzed according to the following procedure, which is diagrammed in Figure A112.2.2.
For any pier,1.1. The pier shear capacity shall be calculated as:(Equation A1-18)where:An = area of net mortared or grouted section of a wall or wall pier.1.2. The pier rocking shear capacity shall be calculated as:(Equation A1-19)
The wall piers at any level are acceptable if they comply with one of the following modes of behavior:2.1. Rocking controlled mode. Where the pier rocking shear capacity is less than the pier shear capacity, in other words, Vr < va, for each pier in a level, forces in the wall at that level, Vwx, shall be distributed to each pier in proportion to PDD/H.
For the wall at that level:(Equation A1-20)2.2. Shear controlled mode. Where the pier shear capacity is less than the pier rocking capacity, in other words, va < Vr in one or more pier(s) in a level, forces in the wall at the level, Vwx, shall be distributed to each pier in proportion to D/H.For each pier at that level:(Equation A1-21)(Equation A1-22)
- Masonry pier tension stress. Unreinforced masonry wall piers need not be analyzed for tension stress.
Shear walls without openings shall be analyzed the same as for walls with openings, except that Vr shall be calculated as follows:
Plywood-sheathed shear walls may be used to resist lateral forces for URM buildings with flexible diaphragms analyzed according to provisions of Section A111. Plywood-sheathed shear walls shall not be used to share lateral forces with other materials along the same line of resistance.
Moment resisting frames shall not be used with an unreinforced masonry wall in a single line of resistance unless the wall has piers that have adequate shear capacity to sustain rocking in accordance with Section A112.2.2. The frames shall be designed in accordance with the building code to resist 100 percent of the seismic forces tributary to that line of resistance, as determined from Section A111.2. The story drift ratio shall be limited to 0.0075.
Unreinforced masonry walls shall be anchored at the roof and floor levels as required in Section A110.2. Ceilings of plaster or similar materials, where not attached directly to roof or floor framing and where abutting masonry walls, shall either be anchored to the walls at a maximum spacing of 6 feet (1829 mm) or be removed.
Anchorage of masonry walls to each floor or roof shall resist a minimum force determined as 0.9SDS times the tributary weight or 200 pounds per linear foot (2920 N/m), whichever is greater, acting normal to the wall at the level of the floor or roof. Existing wall anchors, if used, must be tested and meet the requirements of Section A107.5.1 or be upgraded.
At the roof and floor levels, both shear and tension anchors shall be provided within 2 feet (610 mm) horizontally from the inside of the corners of the walls.
Anchors transmitting shear forces shall have a maximum spacing of 6 feet (1829 mm) and shall have nuts installed over malleable iron or plate washers where bearing on wood, and heavy-cut washers where bearing on steel.
Collector elements shall be provided that are capable of transferring the seismic forces originating in other portions of the building to the element providing the resistance to those forces.
Ties and continuity shall conform to the requirements of the building code.
Vertical bracing members shall be attached to floor and roof construction for their design loads independently of required wall anchors. Horizontal spacing of vertical bracing members shall not exceed one-half of the unsupported height of the wall or 10 feet (3048 mm). Deflection of such bracing members at design loads shall not exceed one-tenth of the wall thickness.
The wall height may be reduced by bracing elements connected to the floor or roof. Horizontal spacing of the bracing elements and wall anchors shall be as required by design, but shall not exceed 6 feet (1829 mm) on center. Bracing elements shall be detailed to minimize the horizontal displacement of the wall by the vertical displacement of the floor or roof.
Parapets and exterior wall appendages not conforming to this chapter shall be removed, or stabilized or braced to ensure that the parapets and appendages remain in their original positions.
The maximum height of an unbraced unreinforced masonry parapet above the lower of either the level of tension anchors or the roof sheathing shall not exceed the height-to-thickness ratio shown in Table A113.6. If the required parapet height exceeds this maximum height, a bracing system designed for the forces determined in accordance with the building code shall support the top of the parapet. Parapet corrective work must be performed in conjunction with the installation of tension roof anchors.
The height of a URM parapet above any wall anchor shall be not less than 12 inches (305 mm).
Exception: If a reinforced concrete beam is provided at the top of the wall, the height above the wall anchor is permitted to be not less than 6 inches (152 mm).
[BS] TABLE A113.6
MAXIMUM ALLOWABLE HEIGHT-TO-THICKNESS RATIO FOR PARAPETS
|0.13g ≤ SD1 ≤ 0.25g||0.25g ≤ SD1 < 0.4g||SD1 ≥ 0.4g|
|Maximum allowable height-to-thickness ratios||2.5||2.5||1.5|
- Veneer shall be anchored with approved anchor ties conforming to the required design capacity specified in the building code and shall be placed at a maximum spacing of 24 inches (610 mm) with a maximum supported area of 4 square feet (0.372 m2).Exception: Existing anchor ties for attaching brick veneer to brick backing shall be acceptable, provided that the ties are in good condition and conform to the following minimum size and material requirements.Existing veneer anchor ties shall be considered adequate if they are of corrugated galvanized iron strips not less than 1 inch (25 mm) in width, 8 inches (203 mm) in length and 1/16 inch (1.6 mm) in thickness, or the equivalent.
- The location and condition of existing veneer anchor ties shall be verified as follows:2.1. An approved testing laboratory shall verify the location and spacing of the ties and shall submit a report to the code official for approval as part of the structural analysis.2.2. The veneer in a selected area shall be removed to expose a representative sample of ties (not less than four) for inspection by the code official.
Unreinforced masonry walls that do not carry design vertical or lateral loads and that are not required by the design to be part of the lateral force-resisting system shall be adequately anchored to new or existing supporting elements. The anchors and elements shall be designed for the out-of-plane forces specified in the building code. The height- or length-to-thickness ratio between such supporting elements for such walls shall not exceed nine.
Where trusses and beams other than rafters or joists are supported on masonry, independent secondary columns shall be installed to support vertical loads of the roof or floor members.
Exception: Secondary supports are not required where SD1 is less than 0.3 g.
Where elements of adjacent buildings do not have a separation of 5 inches (127 mm) or greater, the allowable height-to-thickness ratios for "all other buildings" per Table A110.2 shall be used in the direction of consideration.
Walls of unburned clay, adobe or stone masonry construction shall conform to the following:
- Walls of unburned clay, adobe or stone masonry shall not exceed a height- or length-to-thickness ratio specified in Table A114.1.
- Adobe shall be allowed a maximum value of 9 pounds per square inch (62.1 kPa) for shear unless higher values are justified by test.
- Mortar for repointing may be of the same soil composition and stabilization as the brick, in lieu of cement mortar.
[BS] TABLE A114.1
MAXIMUM HEIGHT-TO-THICKNESS RATIO FOR ADOBE OR STONE WALLS
|0.13g ≤ SD1 < 0.25g||0.25g ≤ SD1 < 0.4g||SD1 ≥ 0.4g|
Chapter A2 Earthquake Hazard Reduction in Existing Reinforced Concrete and Reinforced Masonry Wall Buildings With Flexible Diaphragms
The purpose of this chapter is to promote public safety and welfare by reducing the risk of death or injury as a result of the effects of earthquakes on reinforced concrete and reinforced masonry wall buildings with flexible diaphragms. Based on past earthquakes, these buildings have been categorized as being potentially hazardous and prone to significant damage, including possible collapse in a moderate to major earthquake. The provisions of this chapter are minimum standards for structural seismic resistance established primarily to reduce the risk of life loss or injury on both subject and adjacent properties. These provisions will not necessarily prevent loss of life or injury, or prevent earthquake damage to an existing building that complies with these standards.
The provisions of this chapter shall apply to wall anchorage systems that resist out-of-plane forces and to collectors in existing reinforced concrete or reinforced masonry buildings with flexible diaphragms. Wall anchorage systems that were designed and constructed in accordance with the 1997 Uniform Building Code or the 2000 or subsequent editions of the International Building Code shall be deemed to comply with these provisions.
For the purpose of this chapter, the applicable definitions in the International Building Code and the following shall apply:
- [BS] CONTINUITY CONNECTOR. A component, typically a plate, rod, strap or hold-down, that ensures load path continuity along the full length of a crosstie or strut.
- [BS] CROSSTIE. A member or group of members continuous across the main diaphragm that connects opposite wall lines and transfers out-of-plane wall anchorage forces into the diaphragm.
- [BS] FLEXIBLE DIAPHRAGM. A roof or floor sheathed with plywood, wood decking (1-by or 2-by) or metal deck without a concrete topping slab.
- [BS] STRUT. A member or group of members continuous across a subdiaphragm that transfers out-of-plane wall anchorage forces into the subdiaphragm.
- [BS] WALL ANCHORAGE SYSTEM. The components comprising a complete load path for out-of-plane wall forces from the wall to the main diaphragm, typically including anchors embedded in or fastened to the wall; rods, straps, plates, hold-downs or other hardware; subdiaphragms and their chords; crossties; struts; and continuity connectors.
- [BS] WALL SEGMENT. Any length of concrete wall with continuous horizontal reinforcing and not interrupted or intersected by a pilaster or vertical construction joint, or any length of reinforced masonry wall with continuous horizontal reinforcing and not interrupted or intersected by a pilaster or vertical control joint.
For the purpose of this chapter, the applicable symbols and notations in the International Building Code shall apply.
The seismic-resisting elements specified in this chapter shall comply with applicable provisions of Section 1613 of the International Building Code and Chapter 12 of ASCE 7, except as modified herein.
The plans shall accurately reflect the results of the engineering investigation and design and shall show all pertinent dimensions and sizes for plan review and construction. The following shall be provided:
- Floor plans and roof plans shall show existing framing construction, diaphragm construction, proposed wall anchors, crossties and collectors. Existing nailing, anchors, crossties and collectors shall be shown on the plans if they are considered part of the lateral force-resisting systems.
- Typical wall panel details and sections with panel thickness, height, pilasters and location of anchors shall be provided.
- Details shall include existing and new anchors and the method of developing anchor forces into the diaphragm framing, existing and new crossties, and existing and new or improved support of roof and floor girders at pilasters or walls.
- The basis for design and the building code used for the design shall be stated on the plans.
Structural observation, in accordance with Section 1704.6 of the International Building Code is required, regardless of seismic design category, height or other conditions. Structural observation shall include visual observation of work for conformance to the approved construction documents and confirmation of existing conditions assumed during design.
In addition to the requirements of Section 1705.13 of the International Building Code, special inspection shall be required for:
- Installation of anchors into existing concrete or masonry walls to form part of a wall anchorage system.
- Fastening of new or existing steel deck forming part of a wall anchorage system.
- Installation of continuity connectors along the length of crossties, to ensure compliance with Section A206.2. This inspection may be periodic special inspection.
Testing shall show that the existing anchors can sustain a test load of 1.5 times the design tension load without noticeable deformation or damage to the anchor, to the masonry or concrete element, or to any part of the existing load path between the anchor and new retrofit components. Three anchors of each existing detail type shall be tested, and all three shall satisfy the requirement. Prior to testing, the design professional shall submit a test plan for code official approval identifying the expected locations of the existing anchors in question, the locations of the proposed tests, and the test procedure and criteria. After testing, the design professional shall submit a report of the satisfactory testing showing the test results, the design strengths derived from them, and the size and spacing as confirmed by investigation.
Structural testing and inspection for new construction materials, submittals, reports and certificates of compliance shall be in accordance with Sections 1704 and 1705 of the International Building Code. Work done to comply with this chapter shall not be eligible for Exception 1 to Section 1704.2 of the International Building Code or Exception 2 to Section 1705.13 of the International Building Code.
Concrete and masonry walls shall be anchored to all floors and roofs that provide lateral support for the wall in accordance with Section 12.11.2 of ASCE 7. The anchorage shall provide a direct connection capable of resisting 75 percent of the forces specified in Section 220.127.116.11 of ASCE 7.
- Existing walls need not be evaluated or retrofitted for bending between anchors.
- Work required by this chapter need not consider shrinkage, thermal changes or differential settlement.
For any site designated as Site Class E, the value of Fa shall be taken as 1.2. Site-specific procedures are not required for compliance with this chapter. Mitigation of existing geologic site hazards such as liquefiable soil, fault rupture or landslide is not required for compliance with this chapter.
The wall anchorage system shall comply with the requirements of this section and Section 18.104.22.168 of ASCE 7.
The maximum spacing between wall anchors shall be 8 feet (2438 mm), and each wall segment shall have at least two wall anchors.
The wall anchorage system, excluding subdiaphragms and existing roof or floor framing members, shall be designed and installed to limit the relative movement between the wall and the diaphragm to no more than 1/8 inch before engagement of the anchors. Wall anchors shall be provided to resist out-of-plane forces, independent of existing shear anchors.
Where new members are added as crossties, they shall be spaced no more than 24 feet (7315 mm) apart. Where existing girders are used as crossties, their actual spacing shall be deemed adequate even where the spacing exceeds 24 feet (7315 mm), as long as the girders are provided with continuity connectors as required.
Wall anchorage shall not be provided by fastening the edge of plywood sheathing to steel ledgers. Wall anchorage shall not be provided solely by fastening the edge of steel decking to steel ledgers unless analysis demonstrates acceptable capacity. The existing connections shall be subject to field verification and the new connections shall be subject to special inspection.
New wall anchors shall be provided to resist the full wall anchorage design force independent of existing shear or tension anchors.
Exception: Existing cast-in-place anchors shall be permitted as part of the wall anchorage system if the tie element can be readily attached to the anchors, and if the anchors are capable of resisting the total vertical and lateral shear load (including dead load) while being acted on by the maximum wall anchorage tension force caused by an earthquake. Acceptable tension values for the existing anchors shall be established by testing in accordance with Section A205.4.
Development of the required anchorage forces into roof and floor diaphragms shall comply with the requirements of this section and Section 22.214.171.124 of ASCE 7.
Lengths of development of anchor loads in wood diaphragms shall be based on existing field nailing of the sheathing unless existing edge nailing is positively identified on the original construction plans or at the site.
Where pilasters are present, the wall anchorage system shall comply with the requirements of this section and Section 126.96.36.199.7 of ASCE 7. The pilasters or the walls immediately adjacent to the pilasters shall be anchored directly to the roof framing such that the existing vertical anchor bolts at the top of the pilasters are bypassed without permitting tension or shear failure at the top of the pilasters.
Exception: If existing vertical anchor bolts at the top of the pilasters are used for the anchorage, additional exterior confinement shall be provided as required to resist the total anchorage force.
New anchors used in combination on a single framing member shall be of compatible behavior and stiffness.
Existing interior reinforced concrete or reinforced masonry walls that extend to the floor above or to the roof diaphragm shall be anchored for out-of-plane forces per Sections A206.1 and A206.3. Walls extending through the roof diaphragm shall be anchored for out-of-plane forces on both sides, and continuity ties shall be spliced across or continuous through the interior wall to provide diaphragm continuity.
Collectors designed in accordance with this section shall be provided at reentrant corners and at interior shear walls. Existing or new collectors shall have the capacity to develop into the diaphragm a force equal to the lesser of the rocking or shear capacity of the reentrant wall or the tributary shear based on 75 percent of the diaphragm design forces specified in Section 12.10 of ASCE 7. The capacity of the collector need not exceed the capacity of the diaphragm to deliver loads to the collector. A connection shall be provided from the collector to the reentrant wall to transfer the full collector internal force . If a truss or beam other than a rafter or purlin is supported by the reentrant wall or by a column integral with the reentrant wall, then an independent secondary column is required to support the roof or floor members whenever rocking or shear capacity of the reentrant wall is less than the tributary shear.
Existing mezzanines relying on reinforced concrete or reinforced masonry walls for vertical or lateral support shall be anchored to the walls for the tributary mezzanine load. Walls depending on the mezzanine for lateral support shall be anchored per Sections A206.1, A206.2 and A206.3.
Exception: Existing mezzanines that have independent lateral and vertical support need not be anchored to the walls.
Materials permitted by the building code, including their appropriate strength or allowable stresses, shall be used to meet the requirements of this chapter.
Chapter A3 Prescriptive Provisions for Seismic Strengthening of Cripple Walls and Sill Plate Anchorage of Light, Wood-Frame Residential Buildings
The provisions of this chapter are intended to promote public safety and welfare by reducing the risk of earthquake-induced damage to existing wood-frame residential buildings. The requirements contained in this chapter are prescriptive minimum standards intended to improve the seismic performance of residential buildings; however, they will not necessarily prevent earthquake damage.
This chapter sets standards for strengthening that may be approved by the code official without requiring plans or calculations prepared by a registered design professional. The provisions of this chapter are not intended to prevent the use of any material or method of construction not prescribed herein. The code official may require that construction documents for strengthening using alternative materials or methods be prepared by a registered design professional.
The provisions of this chapter apply to residential buildings of light-frame wood construction containing one or more of the structural weaknesses specified in Section A303.
Exception: The provisions of this chapter do not apply to the buildings, or elements thereof, listed as follows. These buildings or elements require analysis by a registered design professional in accordance with Section A301.3 to determine appropriate strengthening:
- Group R-1.
- Group R with more than four dwelling units.
- Buildings with a lateral force-resisting system using poles or columns embedded in the ground.
- Cripple walls that exceed 4 feet (1219 mm) in height.
- Buildings exceeding three stories in height and any three-story building with cripple wall studs exceeding 14 inches (356 mm) in height.
- Buildings where the code official determines that conditions exist that are beyond the scope of the prescriptive requirements of this chapter.
- Buildings or portions thereof constructed on concrete slabs on grade.
The details and prescriptive provisions herein are not intended to be the only acceptable strengthening methods permitted. Alternative details and methods shall be permitted to be used where approved by the code official. Approval of alternatives shall be based on a demonstration that the method or material used is at least equivalent in terms of strength, deflection and capacity to that provided by the prescriptive methods and materials.
For the purpose of this chapter, in addition to the applicable definitions in the building code, certain additional terms are defined as follows:
- [BS] ADHESIVE ANCHOR. An assembly consisting of a threaded rod, washer, nut, and chemical adhesive approved by the code official for installation in existing concrete or masonry.
- [BS] CRIPPLE WALL. A wood-frame stud wall extending from the top of the foundation to the underside of the lowest floor framing.
- [BS] EXPANSION ANCHOR. An approved post-installed anchor, inserted into a predrilled hole in existing concrete or masonry, that transfers loads to or from the concrete or masonry by direct bearing or friction or both.
- [BS] PERIMETER FOUNDATION. A foundation system that is located under the exterior walls of a building.
- [BS] SNUGTIGHT. As tight as an individual can torque a nut on a bolt by hand, using a wrench with a 10-inch-long (254 mm) handle, and the point at which the full surface of the plate washer is contacting the wood member and slightly indenting the wood surface.
- [BS] WOOD STRUCTURAL PANEL. A panel manufactured from veneers, wood strands or wafers or a combination of veneer and wood strands or wafers bonded together with waterproof synthetic resins or other suitable bonding systems. Examples of wood structural panels are:
- Composite panels. A wood structural panel that is comprised of wood veneer and reconstituted wood-based material and bonded together with waterproof adhesive.
- Oriented strand board (OSB). A mat-formed wood structural panel comprised of thin rectangular wood strands arranged in cross-aligned layers with surface layers normally arranged in the long panel direction and bonded with waterproof adhesive.
- Plywood. A wood structural panel comprised of plies of wood veneer arranged in cross-aligned layers. The plies are bonded with waterproof adhesive that cures on application of heat and pressure.
For the purposes of this chapter, any of the following conditions shall be deemed a structural weakness:
- Sill plates or floor framing that are supported directly on the ground without a foundation system that conforms to the building code.
- A perimeter foundation system that is constructed only of wood posts supported on isolated pad footings.
- Perimeter foundation systems that are not continuous.Exceptions:
- Existing single-story exterior walls not exceeding 10 feet (3048 mm) in length, forming an extension of floor area beyond the line of an existing continuous perimeter foundation.
- Porches, storage rooms and similar spaces not containing fuel-burning appliances.
- A perimeter foundation system that is constructed of unreinforced masonry or stone.
- Sill plates that are not connected to the foundation or that are connected with less than what is required by the building code.
- Cripple walls that are not braced in accordance with the requirements of Section A304.4 and Table A304.3.1, or cripple walls not braced with diagonal sheathing or wood structural panels in accordance with the building code.
The structural weaknesses noted in Section A303 shall be strengthened in accordance with the requirements of this section. Strengthening work may include both new construction and alteration of existing construction. Except as provided herein, all strengthening work and materials shall comply with the applicable provisions of the International Building Code.
Existing wood materials that will be a part of the strengthening work (such as sills, studs and sheathing) shall be in a sound condition and free from defects that substantially reduce the capacity of the member. Any wood material found to contain fungus infection shall be removed and replaced with new material. Any wood material found to be infested with insects or to have been infested with insects shall be strengthened or replaced with new materials to provide a net dimension of sound wood equal to or greater than its undamaged original dimension.
Floor joists framed perpendicular or at an angle to perimeter foundations shall be restrained either by an existing nominal 2-inch-wide (51 mm) continuous rim joist or by a nominal 2-inch-wide (51 mm) full-depth block between alternate joists in one- and two-story buildings, and between each joist in three-story buildings. Existing blocking for multiple-story buildings must occur at each joist space above a braced cripple wall panel.
Existing connections at the top and bottom edges of an existing rim joist or blocking need not be verified in one story buildings. In multiple-story buildings, the existing top edge connection need not be verified; however, the bottom edge connection to either the foundation sill plate or the top plate of a cripple wall shall be verified. The minimum existing bottom edge connection shall consist of 8d toenails spaced 6 inches (152 mm) apart for a continuous rim joist, or three 8d toenails per block. Where this minimum bottom edge-connection is not present or cannot be verified, a supplemental connection installed as shown in Figure A304.1.3 or A304.1.4(2) shall be provided.
Where an existing continuous rim joist or the minimum existing blocking does not occur, new 3/4-inch (19.1 mm) or 23/32-inch (18 mm) wood structural panel blocking installed tightly between floor joists and nailed as shown in Figure A304.1.4(3) shall be provided at the inside face of the cripple wall. In lieu of wood structural panel blocking, tight fitting, full-depth 2-inch (51 mm) blocking may be used. New blocking may be omitted where it will interfere with vents or plumbing that penetrates the wall.
Where existing floor joists are parallel to the perimeter foundations, the end joist shall be located over the foundation and, except for required ventilation openings, shall be continuous and in continuous contact with the foundation sill plate or the top plate of the cripple wall. Existing connections at the top and bottom edges of the end joist need not be verified in one-story buildings. In multiple-story buildings, the existing top edge connection of the end joist need not be verified; however, the bottom edge connection to either the foundation sill plate or the top plate of a cripple wall shall be verified. The minimum bottom edge connection shall be 8d toenails spaced 6 inches (152 mm) apart. If this minimum bottom edge connection is not present or cannot be verified, a supplemental connection installed as shown in Figure A304.1.4(1), A304.1.4(2) or A304.1.4(3) shall be provided.
New perimeter foundations shall be provided for structures with the structural weaknesses noted in Items 1 and 2 of Section A303. Soil investigations or geotechnical studies are not required for this work unless the building is located in a special study zone as designated by the code official or other authority having jurisdiction.
Partial perimeter foundations or unreinforced masonry foundations shall be evaluated by a registered design professional for the force levels specified in Section A301.3. Test reports or other substantiating data to determine existing foundation material strengths shall be submitted to the code official. Where approved by the code official, these existing foundation systems shall be strengthened in accordance with the recommendations included with the evaluation in lieu of being replaced.
Exception: In lieu of testing existing foundations to determine material strengths, and where approved by the code official, a new nonperimeter foundation system designed for the forces specified in Section A301.3 shall be used to resist lateral forces from perimeter walls. A registered design professional shall confirm the ability of the existing diaphragm to transfer seismic forces to the new nonperimeter foundations.
All new perimeter foundations shall be continuous and constructed according to Figure A304.2.3(1) and Table A304.2.3(1) or Figure A304.2.3(2) and Table A304.2.3(2). New construction materials shall comply with the requirements of building code. Where approved by the code official, the existing clearance between existing floor joists or girders and existing grade below the floor need not comply with the building code.
[BS] TABLE A304.2.3(1)
NEW REINFORCED CONCRETE FOUNDATION SYSTEM
|MINIMUM FOUNDATION DIMENSIONS||MINIMUM FOUNDATION REINFORCING|
|NUMBER OF STORIES||W||F||Da, b, c||T||H||VERTICAL REINFORCING|
|Single-pour wall and footing||Footing placed separate from wall|
|1||12 inches||6 inches||12 inches||6 inches||≤ 24 inches||#4 @ 48 inches on center||#4 @ 32 inches on center|
|2||15 inches||7 inches||18 inches||8 inches||≥ 36 inches||#4 @ 48 inches on center||#4 @ 32 inches on center|
|3||18 inches||8 inches||24 inches||10 inches||≥ 36 inches||#4 @ 48 inches on center||#4 @ 18 inches on center|
For SI: 1 inch = 25.4 mm.
- Where frost conditions occur, the minimum depth shall extend below the frost line.
- The ground surface along the interior side of the foundation may be excavated to the elevation of the top of the footing.
- Where the soil is designated as expansive, the foundation depth and reinforcement shall be approved by the code official.
[BS] TABLE A304.2.3(2)
NEW MASONRY CONCRETE FOUNDATION
|MINIMUM FOUNDATION DIMENSIONS||MINIMUM FOUNDATION REINFORCING|
|NUMBER OF STORIES||W||F||Da, b, c||T||H||VERTICAL REINFORCING||HORIZONTAL REINFORCING|
|1||12 inches||6 inches||12 inches||6 inches||≤ 24 inches||#4 @ 24 inches on center||#4 continuous at top of stem wall|
|2||15 inches||7 inches||18 inches||8 inches||≥ 24 inches||#4 @ 24 inches on center||#4 @ 16 inches on center|
|3||18 inches||8 inches||24 inches||10 inches||≥ 36 inches||#4 @ 24 inches on center||#4 @ 16 inches on center|
For SI: 1 inch = 25.4 mm.
- Where frost conditions occur, the minimum depth shall extend below the frost line.
- The ground surface along the interior side of the foundation may be excavated to the elevation of the top of the footing.
- Where the soil is designated as expansive, the foundation depth and reinforcement shall be approved by the code official.
New concrete foundations shall have a minimum compressive strength of 2,500 pounds per square inch (17.24 MPa) at 28 days.
New hollow-unit masonry foundations shall be solidly grouted. The grout shall have minimum compressive strength of 2,000 pounds per square inch (13.79 MPa). Mortar shall be Type M or S.
Where new sill plates are used in conjunction with new foundations, they shall be minimum two times nominal thickness and shall be preservative-treated wood or naturally durable wood permitted by the building code for similar applications, and shall be marked or branded by an approved agency. Fasteners in contact with preservative-treated wood shall be hot-dip galvanized or other material permitted by the building code for similar applications. Anchors, that attach a preservative-treated sill plate to the foundation, shall be permitted to be of mechanically deposited zinc-coated steel with coating weights in accordance with ASTM B695, Class 55 minimum. Metal framing anchors in contact with preservative-treated wood shall be galvanized in accordance with ASTM A653 with a G185 coating.
Where the building has an existing continuous perimeter foundation, all perimeter wall sill plates shall be anchored to the foundation with adhesive anchors or expansion anchors in accordance with Table A304.3.1.
Anchors shall be installed in accordance with Figure A304.3.1(1), with the plate washer installed between the nut and the sill plate. The nut shall be tightened to a snugtight condition after curing is complete for adhesive anchors and after expansion wedge engagement for expansion anchors. Anchors shall be installed in accordance with manufacturer's recommendations. Expansion anchors shall not be used where the installation causes surface cracking of the foundation wall at the locations of the anchor.
Where existing conditions prevent anchor installations through the top of the sill plate, this connection shall be made in accordance with Figure A304.3.1(2), A304.3.1(3) or A304.3.1(4). Alternative anchorage methods having a minimum shear capacity of 900 pounds (4003 N) per connection parallel to the wall shall be permitted. The spacing of these alternative connections shall comply with the maximum spacing requirements of Table A304.3.1 for 1/2-inch (12.7 mm) bolts.
[BS] TABLE A304.3.1
SILL PLATE ANCHORAGE AND CRIPPLE WALL BRACING
|NUMBER OF STORIES ABOVE CRIPPLE WALLS||MINIMUM SILL PLATE CONNECTION AND MAXIMUM SPACINGa, b, c||AMOUNT OF BRACING FOR EACH WALL LINEd, e, f|
|A Combination of Exterior Walls Finished with Portland Cement Plaster and Roofing Using Clay Tile or Concrete Tile Weighing More than 6 psf (287 N/m2)||All Other Conditions|
|One story||1/2 inch spaced 6 feet, 0 inch center-to-center with washer plate||Each end and not less than 50 percent of the wall length||Each end and not less than 40 percent of the wall length|
|Two stories||1/2 inch spaced 4 feet, 0 inch center-to-center with washer plate; or 5/8 inch spaced 6 feet, 0 inch center-to-center with washer plate||Each end and not less than 70 percent of the wall length||Each end and not less than 50 percent of the wall length|
|Three stories||5/8 inch spaced 4 feet, 0 inch center-to-center with washer plate||100 percent of the wall lengthg||Each end and not less than 80 percent of the wall lengthg|
For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square foot = 47.88 N/m2.
- Sill plate anchors shall be adhesive anchors or expansion anchors in accordance with Section A304.3.1.
- All washer plates shall be 3 inches by 3 inches by 0.229 inch minimum. The hole in the plate washer is permitted to be diagonally slotted with a width of up to 3/16 inch larger than the bolt diameter and a slot length not to exceed 13/4 inches, provided that a standard cut washer is placed between the plate washer and the nut.
- This table shall also be permitted for the spacing of the alternative connections specified in Section A304.3.1.
- See Figure A304.4.2 for braced panel layout.
- Braced panels at ends of walls shall be located as near to the end as possible.
- All panels along a wall shall be nearly equal in length and shall be nearly equal in spacing along the length of the wall.
- The minimum required underfloor ventilation openings are permitted in accordance with Section A304.4.4.
Anchors shall be placed within 12 inches (305 mm), but not less than 9 inches (229 mm), from the ends of sill plates and shall be placed in the center of the stud space closest to the required spacing. New sill plates may be installed in pieces where necessary because of existing conditions. For lengths of sill plates 12 feet (3658 mm) or greater, anchors shall be spaced along the sill plate as specified in Table A304.3.1. For other lengths of sill plate, anchor placement shall be in accordance with Table A304.3.2.
Exception: Where physical obstructions such as fireplaces, plumbing or heating ducts interfere with the placement of an anchor, the anchor shall be placed as close to the obstruction as possible, but not less than 9 inches (229 mm) from the end of the plate. Center-to-center spacing of the anchors shall be reduced as necessary to provide the minimum total number of anchors required based on the full length of the wall. Center-to-center spacing shall be not less than 12 inches (305 mm).
[BS] TABLE A304.3.2
SILL PLATE ANCHORAGE FOR VARIOUS LENGTHS OF SILL PLATEa, b
|NUMBER OF STORIES||LENGTHS OF SILL PLATE|
|Less than 12 feet to 6 feet||Less than 6 feet to 30 inches||Less than 30 inchesc|
|One story||Three connections||Two connections||One connection|
|Two stories||Four connections for 1/2-inch anchors or bolts or three connections for 5/8-inch anchors or bolts||Two connections||One connection|
|Three stories||Four connections||Two connections||One connection|
For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm.
- Connections shall be either adhesive anchors or expansion anchors.
- See Section A304.3.2 for minimum end distances.
- Connections shall be placed as near to the center of the length of plate as possible.
Wood structural panel sheathing shall be not less than 15/32-inch (12 mm) thick and shall be installed in accordance with Figure A304.4.1(1) or A304.4.1(2). Individual pieces of wood structural panels shall be nailed with 8d common nails spaced 4 inches (102 mm) on center at all edges and 12 inches (305 mm) on center at each intermediate support with not less than two nails for each stud. Nails shall be driven so that their heads are flush with the surface of the sheathing and shall penetrate the supporting member not less than 11/2 inches (38 mm). When a nail fractures the surface, it shall be left in place and not counted as part of the required nailing. A new 8d nail shall be located within 2 inches (51 mm) of the discounted nail and be hand-driven flush with the sheathing surface. Where the installation involves horizontal joints, those joints shall occur over nominal 2-inch by 4-inch (51 mm by 102 mm) blocking installed with the nominal 4-inch (102 mm) dimension against the face of the plywood.
Vertical joints at adjoining pieces of wood structural panels shall be centered on studs such that there is a minimum 1/8 inch (3.2 mm) between the panels. Where required edge distances cannot be maintained because of the width of the existing stud, a new stud shall be added adjacent to the existing studs and connected in accordance with Figure A304.4.1(3).
See Table A304.3.1 and Figure A304.4.2 for the distribution and amount of bracing required for each wall line. Each braced panel length must be not less than two times the height of the cripple stud. Where the minimum amount of bracing prescribed in Table A304.3.1 cannot be installed along any walls, the bracing must be designed in accordance with Section A301.3.
Exception: Where physical obstructions such as fireplaces, plumbing or heating ducts interfere with the placement of cripple wall bracing, the bracing shall then be placed as close to the obstruction as possible. The total amount of bracing required shall not be reduced because of obstructions.
Where bracing materials are installed on the interior face of studs forming an enclosed space between the new bracing and the existing exterior finish, each braced stud space must be ventilated. Adequate ventilation and access for future inspection shall be provided by drilling one 2-inch to 3-inch-diameter (51 mm to 76 mm) round hole through the sheathing, nearly centered between each stud at the top and bottom of the cripple wall. Such holes should be spaced not less than 1 inch (25 mm) clear from the sill or top plates. In stud spaces containing sill bolts, the hole shall be located on the centerline of the sill bolt but not closer than 1 inch (25 mm) clear from the nailing edge of the sheathing. Where existing blocking occurs within the stud space, additional ventilation holes shall be placed above and below the blocking, or the existing block shall be removed and a new nominal 2-inch by 4-inch (51 mm by 102 mm) block shall be installed with the nominal 4-inch (102 mm) dimension against the face of the plywood. For stud heights less than 18 inches (457 mm), only one ventilation hole need be provided.
Existing underfloor ventilation shall not be reduced without providing equivalent new ventilation as close to the existing ventilation as possible. Braced panels may include underfloor ventilation openings where the height of the opening, measured from the top of the foundation wall to the top of the opening, does not exceed 25 percent of the height of the cripple stud wall; however, the length of the panel shall be increased a distance equal to the length of the opening or one stud space minimum. Where an opening exceeds 25 percent of the cripple wall height, braced panels shall not be located where the opening occurs. See Figure A304.4.1(3).
Exception: For homes with a post and pier foundation system where a new continuous perimeter foundation system is being installed, new ventilation shall be provided in accordance with the building code.
All work shall be subject to inspection by the code official including, but not limited to:
- Placement and installation of new adhesive or expansion anchors installed in existing foundations. Special inspection is not required for adhesive anchors installed in existing foundations regulated by the prescriptive provisions of this chapter.
- Installation and nailing of new cripple wall bracing.
- Any work shall be subject to special inspection where required by the code official in accordance with the building code.
All nails specified in this chapter shall be common wire nails of the following diameters and lengths:
- 8d nails = 0.131 inch (3.3 mm) by 21/2 inches (64 mm).
- 10d nails = 0.148 inch (3.8 mm) by 3 inches (76 mm).
- 12d nails = 0.148 inch (3.8 mm) by 31/4 inches (83 mm).
- 16d nails = 0.162 inch (4.1 mm) by 31/2 inches (89 mm).
Nails used to attach metal framing connectors directly to wood members shall be as specified by the connector manufacturer in an approved report.
Chapter A4 Earthquake Risk Reduction in Wood-Frame Residential Buildings With Soft, Weak or Open Front Walls
The purpose of this chapter is to promote public welfare and safety by reducing the risk of death or injury as a result of the effects of earthquakes on existing wood-frame, multiple-unit residential buildings. The ground motions of past earthquakes have caused the loss of human life, personal injury and property damage in these types of buildings. This chapter creates minimum standards to strengthen the more vulnerable portions of these structures. Where fully followed, these minimum standards will improve the performance of these buildings but will not necessarily prevent all earthquake-related damage.
Notwithstanding the applicable definitions, symbols and notations in the building code, the following definitions shall apply for the purposes of this chapter:
- [BS] ASPECT RATIO. The span-width ratio for horizontal diaphragms and the height-length ratio for shear walls.
- [BS] NONCONFORMING STRUCTURAL MATERIALS. Wall bracing materials other than wood structural panels or diagonal sheathing.
- [BS] OPEN-FRONT WALL LINE. An exterior wall line, without vertical elements of the lateral force-resisting system, that requires tributary seismic forces to be resisted by diaphragm rotation or excessive cantilever beyond parallel lines of shear walls. Diaphragms that cantilever more than 25 percent of the distance between lines of lateral force-resisting elements from which the diaphragm cantilevers shall be considered to be excessive. Exterior exit balconies of 6 feet (1829 mm) or less in width shall not be considered excessive cantilevers.
- [BS] RETROFIT. An improvement of the lateral force-resisting system by alteration of existing structural elements or addition of new structural elements.
- [BS] SOFT WALL LINE. A wall line whose lateral stiffness is less than that required by story drift limitations or deformation compatibility requirements of this chapter. In lieu of analysis, a soft wall line may be defined as a wall line in a story where the story stiffness is less than 70 percent of the story above for the direction under consideration.
- [BS] STORY. A story as defined by the building code, including any basement or underfloor space of a building with cripple walls exceeding 4 feet (1219 mm) in height.
- [BS] WALL LINE. Any length of wall along a principal axis of the building used to provide resistance to lateral loads. Parallel wall lines separated by less than 4 feet (1219 mm) shall be considered to be one wall line for the distribution of loads.
- [BS] WEAK WALL LINE. A wall line in a story where the story strength is less than 80 percent of the story above in the direction under consideration.
Modifications required by the provisions in this chapter shall be designed in accordance with the International Building Code provisions for new construction, except as modified by this chapter.
Exception: Buildings for which the prescriptive measures provided in Section A404 apply and are used.
This chapter requires the alteration, repair, replacement or addition of structural elements and their connections to meet the strength and stiffness requirements herein. The lateral load-path analysis shall include the resisting elements and connections from the wood diaphragm immediately above any soft, weak or open-front wall lines to the foundation soil interface or to the uppermost story of a podium structure comprised of steel, masonry, or concrete structural systems that supports the upper wood-framed structure. Stories above the uppermost story with a soft, weak or open-front wall line shall be considered in the analysis but need not be modified. The lateral load-path analysis for added structural elements shall include evaluation of the allowable soil-bearing and lateral pressures in accordance with the building code. Where any portion of a building within the scope of this chapter is constructed on or into a slope steeper than one unit vertical in three units horizontal (33-percent slope), the lateral force-resisting system at and below the base level diaphragm shall be analyzed for the effects of concentrated lateral forces at the base caused by this hillside condition.
The design base shear in a given direction shall be permitted to be 75 percent of the value required for similar new construction in accordance with the building code. The value of R used in the design of the strengthening of any story shall not exceed the lowest value of R used in the same direction at any story above. The system overstrength factor, Ω0, and the deflection amplification factor, Cd, shall be not less than the largest respective value corresponding to the R factor being used in the direction under consideration.
- For structures assigned to Seismic Design Category B, values of R, Ω0 and Cd shall be permitted to be based on the seismic force-resisting system being used to achieve the required strengthening.
- For structures assigned to Seismic Design Category C or D, values of R, Ω0 and Cd shall be permitted to be based on the seismic force-resisting system being used to achieve the required strengthening, provided that when the strengthening is complete, the strengthened structure will not have an extreme weak story irregularity defined as Type 5b in ASCE 7 Table 12.3-2.
- For structures assigned to Seismic Design Category E, values of R, Ω0 and Cd shall be permitted to be based on the seismic force-resisting system being used to achieve the required strengthening, provided that when the strengthening is complete, the strengthened structure will not have an extreme soft story, a weak story, or an extreme weak story irregularity defined, respectively, as Types 1b, 5a and 5b in ASCE 7 Table 12.3-2.
- For retrofit systems involving different seismic force-resisting systems in the same direction within the same story, resisting elements are permitted to be designed using the least value of R for the different structural systems found in each independent line of resistance if all of the following conditions are met:4.1. The building is assigned to Risk Category I or II.4.2. The building height is no more than four stories above grade plane.4.3. The seismic force-resisting systems of the retrofitted building comprise only wood structural panel shear walls, steel moment-resisting frames, steel cantilever columns and steel-braced frames. Values for C and Ω0 shall be consistent with the R value used.
- With reference to ASCE 7 Table 12.2-1, ordinary, intermediate and special steel systems, and all light-frame systems shall be permitted without limitation where those systems are used only for retrofit to comply with the requirements of this chapter.
Despite any other requirement of Section A403.3 or A403.4, the total expected strength of retrofit elements added to any story need not exceed 1.7 times the expected strength of the story immediately above in a two-story building, or 1.3 times the expected strength of the story immediately above in a three-story or taller building, as long as the retrofit elements are located symmetrically about the center of mass of the story above, or so as to minimize torsion in the retrofitted story. Calculation of expected story strength and identification of irregularities in Section A403.3 shall be based on the expected strength of all wall lines, even if sheathed with nonconforming materials. The strength of a wall line above the retrofitted story shall be permitted to be reduced to account for inadequate load path or overturning resistance.
For any site designated as Site Class E, the value of F shall be taken as 1.2. Site-specific procedures are not required for compliance with this chapter. Mitigation of existing geologic site hazards such as liquefiable soil, fault rupture or landslide is not required for compliance with this chapter.
The calculated story drift for each retrofitted story shall not exceed the allowable deformation compatible with all vertical load-resisting elements and 0.025 times the story height. The calculated story drift shall not be reduced by the effects of horizontal diaphragm stiffness but shall be increased where these effects produce rotation. Drift calculations shall be in accordance with the building code.
The effects of rotation and soil stiffness shall be included in the calculated story drift where lateral loads are resisted by vertical elements whose required depth of embedment is determined by pole formulas. The coefficient of subgrade reaction used in deflection calculations shall be based on a geotechnical investigation conducted in accordance with the building code.
The requirements of the building code shall apply, except as modified herein. Structural framing elements and their connections not required by design to be part of the lateral force-resisting system shall be designed and detailed to be adequate to maintain support of expected gravity loads when subjected to the expected deformations caused by seismic forces. Increased demand caused by P Δ effects and story sidesway stability shall be considered in retrofit stories that rely on the strength and stiffness of cantilever columns for lateral resistance.
Floor diaphragms within the scope of Section A403.2 shall be shown to have adequate strength at the following locations:
- For straight lumber sheathed diaphragms without integral hardwood flooring throughout the diaphragm: The code official is authorized to waive the requirement where it is shown that the condition occurs in areas small enough not to affect overall building performance.
- For all other diaphragms adequate strength shall be shown to be provided at locations where forces are transferred between the diaphragm and each new or strengthened vertical element of the seismic force-resisting system. Collector elements shall be provided where needed to distribute the transferred force over a greater length of diaphragm.
Exception: Where the existing vertical elements of the seismic force-resisting system are shown to comply with this chapter, diaphragms need not be evaluated.
Wood-framed shear walls shall have strength and stiffness sufficient to resist the seismic loads and shall conform to the requirements of this section. Where new sheathing is applied to existing studs to create new wood-framed shear walls, the new wall elements shall be considered bearing wall systems for purposes of determining seismic design parameters.
Wood structural panel shear walls shall meet the story drift limitation of Section A403.4. Conformance to the story drift limitation shall be determined by approved testing or calculation. Individual shear panels shall be permitted to exceed the maximum aspect ratio, provided that the allowable story drift and allowable shear capacities are not exceeded.
Shear walls are permitted to be designed for continuity around openings in accordance with the building code. Blocking and steel strapping shall be provided at corners of the openings to transfer forces from discontinuous boundary elements into adjoining panel elements. Alternatively, perforated shear wall provisions of the building code are permitted to be used.
Expansion anchors that provide tension strength by friction resistance shall not be used to connect hold-down devices to existing concrete or masonry elements.
The required depth of embedment or edge distance for the anchor used in the hold-down connector shall be provided in the concrete or masonry below any plain concrete slab unless satisfactory evidence is submitted to the code official that shows that the concrete slab and footings are of monolithic construction.
Steel retrofit systems shall have strength and stiffness sufficient to resist the seismic loads and shall conform to the requirements of this section.
Steel special moment frames shall comply with all applicable provisions of AISC 341, except that Section E3.4a addressing strong-column/weak-beams of AISC 341, is not required for columns that carry no gravity load.
Cantilevered column systems shall be permitted to be designed as inverted special, intermediate or ordinary moment frames, with corresponding moment frame seismic design coefficients, where the system satisfies the following conditions:
- The columns carry no gravity load.
- The columns are configured in pairs or larger groups connected by a continuous reinforced concrete foundation or grade beam.
- The foundation or grade beam shall be designed to resist the expected plastic moment at the base of each column, computed as RyFyZ in accordance with AISC 341.
- The flexibility of the foundation or grade beam, considering cracked section properties of the reinforced concrete, shall be included in computing the deformation of the steel frame system.
- The column height shall be taken as twice the actual height when checking lateral torsional buckling.
These prescriptive measures shall apply only to two-story buildings and only where deemed appropriate by the code official. These prescriptive measures rely on rotation of the second floor diaphragm to distribute the seismic load between the side and rear walls around a ground floor open area. In the absence of an existing floor diaphragm of wood structural panel or diagonal sheathing at the top of the first story, a new wood structural panel diaphragm of minimum thickness of 3/4 inch (19.1 mm) and with 10d common nails at 6 inches (152 mm) on center shall be applied.
To qualify for these prescriptive measures, the following additional conditions need to be satisfied by the retrofitted structure:
- Diaphragm aspect ratio L/W is less than 0.67, where W is the diaphragm dimension parallel to the soft, weak or open-front wall line and L is the distance in the orthogonal direction between that wall line and the rear wall of the ground floor open area.
- Minimum length of side shear walls = 20 feet (6096 mm).
- Minimum length of rear shear wall = three-fourths of the total rear wall length.
- Plan or vertical irregularities shall not be other than a soft, weak or open-front wall line.
- Roofing weight less than or equal to 5 pounds per square foot (240 N/m2).
- Aspect ratio of the full second floor diaphragm meets the requirements of the building code for new construction.
The anchor size and spacing shall be not less than 3/4 inch (19.1 mm) in diameter at 32 inches (813 mm) on center. Where existing anchors are inadequate, supplemental or alternative approved connectors (such as new steel plates bolted to the side of the foundation and nailed to the sill) shall be used.
Shear wall top plates shall be connected to blocking or rim joist at upper floor with not less than 18-gage galvanized steel angle clips 41/2 inches (114 mm) long with 12-8d nails spaced not farther than 16 inches (406 mm) on center, or by equivalent shear transfer methods.
The shear wall sheathing shall be not less than 15/32-inch (11.9 mm), 5-ply Structural I with 10d nails at 4 inches (102 mm) on center at edges and 12 inches (305 mm) on center at field; blocked all edges with 3 by 4 board or larger. Where existing sill plates are less than 3-by thick, place flat 2-by on top of sill between studs, with flat 18-gage galvanized steel clips 41/2 inches (114 mm) long with 12-8d nails or 3/8-inch-diameter (9.5 mm) lags through blocking for shear transfer to sill plate. Stagger nailing from wall sheathing between existing sill and new blocking. Anchor new blocking to foundation as specified in this section.
Shear walls shall be provided with hold-down anchors at each end. Two hold-down anchors are required at intersecting corners. Hold-downs shall be approved connectors with a minimum 5/8-inch-diameter (15.9 mm) threaded rod or other approved anchor with a minimum allowable load of 4,000 pounds (17.8 kN). Anchor embedment in concrete shall be not less than 5 inches (127 mm). Tie-rod systems shall be not less than 5/8 inch (15.9 mm) in diameter unless using high-strength cable. High-strength cable elongation shall not exceed 5/8 inch (15.9 mm) under a 4,000 pound (17.8 kN) axial load.
New materials shall meet the requirements of the International Building Code, except where allowed by this chapter.
The physical condition, strengths and stiffnesses of existing building materials shall be taken into account in any analysis required by this chapter. The verification of existing materials conditions and their conformance to these requirements shall be made by physical observation, material testing or record drawings as determined by the registered design professional subject to the approval of the code official.
Where the required calculations rely on design values for common nails or surfaced dry lumber, their use in construction shall be verified by exposure.
Where verification of the existing plywood is by use of record drawings alone, plywood shall be assumed to be of three plies.
All existing structural steel shall be permitted to be assumed to comply with ASTM A36. Existing pipe or tube columns shall be assumed to be of minimum wall thickness unless verified by testing or exposure.
All existing concrete footings shall be permitted to be assumed to be plain concrete with a compressive strength of 2,000 pounds per square inch (13.8 MPa). Existing concrete compressive strength taken greater than 2,000 pounds per square inch (13.8 MPa) shall be verified by testing, record drawings or department records.
The analysis of existing cast-in-place anchors shall be permitted to assume proper anchor embedment for purposes of evaluating shear resistance to lateral loads.
The plans shall show all information necessary for plan review and for construction and shall accurately reflect the design. The plans shall contain a note that states that this retrofit was designed in compliance with the criteria of this chapter.
The plans shall show existing diaphragm and shear wall sheathing and framing materials; fastener type and spacing; diaphragm and shear wall connections; continuity ties; collector elements; and the portion of the existing materials that needs verification during construction. If the cap allowed by Section A403.3.1 is used to limit the scope of retrofit, the foregoing information shall be shown for each retrofitted story and at least one story above the uppermost retrofitted story. If the cap allowed by Section A403.3.1 is not used, the foregoing information need only be shown for each retrofitted story and for the floor at the top of that story.
The foundation plan shall include the size, type, location and spacing of all anchor bolts with the required depth of embedment, edge and end distance; the location and size of all shear walls and all columns for braced frames or moment frames; referenced details for the connection of shear walls, braced frames or moment-resisting frames to their footing; and referenced sections for any grade beams and footings.
The framing plan shall include the length, location and material of shear walls; the location and material of frames; references or details for the column-to-beam connectors, beam-to-wall connections and shear transfers at floor and roof diaphragms; and the required nailing and length for wall top plate splices.
Shear walls shall have a referenced schedule on the plans that includes the correct shear wall capacity in pounds per foot (N/m); the required fastener type, length, gage and head size; and a complete specification for the sheathing material and its thickness. The schedule shall also show the required location of 3-inch (76 mm) nominal or two 2-inch (51 mm) nominal edge members; the spacing of shear transfer elements such as framing anchors or added sill plate nails; the required hold-down with its bolt, screw or nail sizes; and the dimensions, lumber grade and species of the attached framing member.
Notes shall show required edge distance for fasteners of structural wood panels and framing members; required flush nailing at the plywood surface; limits of mechanical penetrations; and the sill plate material assumed in the design. The limits of mechanical penetrations shall be detailed showing the maximum notching and drilled hole sizes.
General notes shall show the requirements for material testing, special inspection and structural observation.
Structural observation, in accordance with Section 1704.6 of the International Building Code is required, regardless of seismic design category, height or other conditions. Structural observation shall include visual observation of work for conformance to the approved construction documents and confirmation of existing conditions assumed during design.
Structural testing and inspection for new construction materials, submittals, reports and certificates of compliance shall be in accordance with Sections 1704 and 1705 of the International Building Code. Work done to comply with this chapter shall not be eligible for Exceptions 1, 2, or 3 of Section 1704.2 of the International Building Code or for the exception to Section 1705.13.2 of the International Building Code.
Chapter A5 Referenced Standards
See Table A501.1 for standards that are referenced in various sections of this appendix. Standards are listed by the standard identification with the effective date, standard title, and the section or sections of this appendix that references the standard.
|STANDARD ACRONYM||STANDARD NAME||SECTIONS HEREIN REFERENCED|
|AISC 341-16||Seismic Provisions for Structural Steel Buildings||A403.10.1, A403.10.2|
|ASCE/SEI 7 —16||Minimum Design Loads for Buildings and Other Structures with Supplement No. 1||A104.1, A205.1, A206.1, A206.2, A206.3, A206.4, A206.7, A403.3|
|ASTM A36/A36M— 14||Specification for Carbon Structural Steel||A405.3.3|
|ASTM A653/A653M —15||Standard Specification for Steel Sheet, Zinc Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by Hot-Dip Process||A304.2.6|
|ASTM B695 —04(2009)||Standard Specification for Coating of Zinc Mechanically Deposited on Iron and Steel||A304.2.6|
|ASTM C67- 14||Test Methods of Sampling and Testing Brick and Structural Clay Tile||A188.8.131.52|
|ASTM C140/C140M —15||Standard Test Methods for Sampling and Testing Concrete Masonry Units and Related Units||A184.108.40.206|
|ASTM C496 —96/C496M —11||Standard Test Method for Splitting Tensile Strength of Cylindrical Concrete Specimens||A104.1, A220.127.116.11|
|ASTM C1531—15||Standard Test Methods for In Situ Measurement of Masonry Mortar Joint Shear Strength Index||A18.104.22.168|
|ASTM E488/E488M —15||Standard Test Methods for Strength of Anchors in Concrete and Masonry Elements||A107.5.1, A107.5.3|
|ASTM E519/E519M —2010||Standard Test Method for Diagonal Tension (Shear) in Masonry Assemblages||A104.1|
|IBC—00||International Building Code||A202.1|
|IBC—03||International Building Code||A202.1|
|IBC—06||International Building Code||A202.1|
|IBC—09||International Building Code||A202.1|
|IBC—12||International Building Code||A202.1|
|IBC—15||International Building Code||A202.1|
|IBC—18||International Building Code||A202.1|
|IBC—21||International Building Code||A102.2, A105.1, A105.4, A202.1, A203.1, A204.1, A205.1, A205.3, A205.3.1, A205.4, A301.3, A304.1.1, A403.1, A405.1, A407.1, A407.2, A407.3|
|UBC—97||Uniform Building Code||A202.1|