The purpose of this chapter is to promote public safety and welfare by reducing the risk of death or injury that may result 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 rehabilitated buildings.
For the purpose of this chapter, the applicable definitions in the building code shall also apply.
[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.
[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.
[BS] TUCKPOINTING. See "Pointing."
[BS] UNREINFORCED MASONRY. 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.
[BS] UNREINFORCED MASONRY (URM) 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 25 percent of the minimum ratio required by the building code for reinforced masonry.
[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).|
|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||=||Compressive strength of masonry.|
|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 (10-3 m).|
|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 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 (kN). For determination of the rocking shear capacity, dead load at the top of the pier under consideration shall be used.|
|PD+L||=||Press resulting from the dead plus actual live load in place at the time of testing, pounds per square inch (kPa).|
|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 capacity value for a crosswall sheathed with any of the materials given in Table A1-D or A1-E, pounds per foot (N/m).|
|vm||=||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).|
|vt||=||Mortar shear strength as specified in Section A22.214.171.124, pounds per square inch (kPa).|
|Vtest||=||Load at incipient cracking for each in-place shear test performed in accordance with Section A126.96.36.199, pounds (kN).|
|vto||=||Mortar shear test values as specified in Section A188.8.131.52, pounds per square inch (kPa).|
|vu||=||Unit shear capacity value for a diaphragm sheathed with any of the materials given in Table A1-D or A1-E, 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 following construction information shall be included in the plans required by this chapter:
Structural observation, in accordance with Section 1708 of the Florida Building Code, Building, 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 conformance with the approved construction documents and confirmation of existing conditions assumed during design.
Structural testing and inspection for new construction materials shall be in accordance with the building code, except as modified by this chapter.
Existing materials used as part of the required vertical load-carrying or lateral forceresisting system shall be in sound condition, or shall be repaired or removed and replaced with new materials. All other unreinforced masonry materials shall comply with the following requirements:
The use of materials not specified herein or in Section A108.1 shall be based on substantiating research data or engineering judgment, with the approval of the building official.
The facing and backing 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. The clear distance between adjacent full-length headers shall not exceed 24 inches (610 mm) vertically or horizontally. Where the 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 with their area and spacing conforming to the foregoing. Wythes of walls not bonded as described above shall be considered veneer. Veneer wythes shall not be included in the effective thickness used in calculating the height-to-thickness ratio and the shear capacity of the wall.
Exception: Where SD1 is not more than 0.3, veneer wythes anchored as specified in the building code and made composite with backup masonry may be used for calculation of the effective thickness.
The quality of mortar in all masonry walls shall be determined by performing in-place shear tests in accordance with the following:
The tensile-splitting strength of existing masonry, fsp, or the prism strength of existing masonry, f'm, may be determined in accordance with one of the following procedures:
- (Equation A1-1)
- (Equation A1-2)
The minimum number of tests per class shall be as follows:
- (Equation A1-3)
All new embedded bolts shall be subject to periodic special inspection in accordance with the building code, prior to placement of the bolt and grout or adhesive in the drilled hole. Five percent of all bolts 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 bolts that extend through the wall with steel plates on the far side of the wall need not be tested.
Exception: Special inspection in accordance with the building code may be provided during installation of new anchors in lieu of testing.
All new embedded bolts resisting tension forces or a combination of tension and shear forces shall be subject to periodic special inspection in accordance with the building code, prior to placement of the bolt and grout or adhesive in the drilled hole. Five percent of all bolts 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. New through-bolts need not be tested.
Bolts 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).
This section is applicable when it is desired to use tension or shear values for anchors greater than those permitted by Table A1-E. The direct-tension test procedure set forth in Section A107.5.1 for existing anchors shall be used to determine the allowable tension values for new embedded through bolts, except that no preload is required. Bolts shall be installed in the same manner and using the same materials as will be used in the actual construction. A minimum of 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 allowable tension values for such anchors shall be the lesser of the average ultimate load divided by a safety factor of 5.0 or the average load at which 1/8 inch (3.2 mm) elongation occurs for each size and type of bolt and class of masonry.
The test procedure for prequalification of shear bolts shall comply with ASTM E488 or another approved procedure.
The allowable values determined in this manner shall be permitted to exceed those set forth in Table A1-E.
The unreinforced masonry shear strength, vm, shall be determined for each masonry class from one of the following equations:
- (Equation A1-4)
- (Equation A1-5)
- (Equation A1-6)
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:
The special procedures of this section may be applied only to buildings having the following characteristics:
A crosswall is a wood-framed wall sheathed with any of the materials described in Table A1-D or A1-E or other system as defined in Section A111.3.5. Crosswalls shall be spaced no 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.
- (Equation A1-8)
Demand-capacity ratios shall be calculated for the diaphragm at any level according to the following formulas:
- (Equation A1-9)
- (Equation A1-10)
- (Equation A1-11)
- (Equation A1-12)
Diaphragms shall be connected to shear walls with connections capable of developing the diaphragm-loading tributary to the shear wall given by the lesser of the following formulas:
using the Cp values in Table A1-C, or
The wall story force distributed to a shear wall at any diaphragm level shall be the lesser value calculated as:
but need not exceed
Wall piers shall be analyzed according to the following procedure, which is diagrammed in Figure A1-2.
- (Equation A1-20)
- (Equation A1-21)
- (Equation A1-22)
- (Equation A1-23)(Equation A1-24)
Shear walls without openings shall be analyzed the same as for walls with openings, except that Vr shall be calculated as follows:
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 A1-F. 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 minimum height of a parapet above any wall anchor shall be 12 inches (305 mm).
Exception: If a reinforced concrete beam is provided at the top of the wall, the minimum height above the wall anchor may be 6 inches (152 mm).
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 0.3g.
Walls of unburned clay, adobe or stone masonry construction shall conform to the following:
[BS] TABLE A1-A
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|
[BS] TABLE A1-B
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 multistory building||20||18||16||15|
|Walls in top story of multistory building||14||14||14b,c||9|
|All other walls||20||16||16||13|
[BS] TABLE A1-C
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 |
|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|
[BS] TABLE A1-D
STRENGTH VALUES FOR EXISTING MATERIALS
|EXISTING MATERIALS OR CONFIGURATION OF MATERIALSa||STRENGTH VALUES|
|× 14.594 for N/m|
|Roofs with straight sheathing and roofing applied directly to the |
|300 lbs. per ft. for seismic shear|
|Roofs with diagonal sheathing and roofing applied directly to the |
|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 (10.34 MPa) unless otherwise |
shown by tests
|Douglas fir wood.||Same as D.F. No. 1|
|Reinforcing steel.||Fy= 40,000 psi (124.1 N/mm2) maximum|
|Structural steel.||Fy= 33,000 psi (137.9 N/mm2) maximum|
For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2, 1 pound = 4.4 N.
[BS] TABLE A1-E
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 ofplywood 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 woodsheathing.
|1.2 times the value specified in the current|
|Drywall or plaster applied directly over wood studs.||The value specified in the current building |
|Drywall or plaster applied to sheathing over existing wood studs.||50 percent of the value specified in the |
current building code.
|Tension boltse||Bolts extending entirely through unreinforced masonry wall secured |
with bearing plates on far side of a three-wythe-minimum wall withat least 30 square inches of area.b,c
|5,400 lbs. per bolt. |
2,700 lbs. for two-wythe walls.
|Shear boltse||Bolts embedded a minimum of 8 inches into unreinforced masonry walls; bolts should be centered in 21/2-inch-diameter holes with dry-pack |
or nonshrink grout around the circumference of the bolt.
|The value for plain masonry specified for |
solid masonry in the current building code;no value larger than those given for 3/4-inch
bolts should be used.
|Combined tension and |
|Through-bolts—bolts meeting the requirements for shear and for tension bolts.b,c||Tension—same as for tension bolts. |
Shear—same as for shear bolts.
|Embedded bolts—bolts extending to the exterior face of the wall |
with a 21/2-inch round plate under the head and drilled at an angle of221/2 degrees to the horizontal; installed as specified for shear
|Tension—3,600 lbs. per bolt. |
Shear—same as for shear bolts.
|Infilled walls||Reinforced masonry infilled openings in existing unreinforced |
masonry walls; provide keys or dowels to match reinforcing.
|Same as values specified for unreinforced |
|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.
[BS] TABLE A1-F
MAXIMUM ALLOWABLE HEIGHT-TO-THICKNESS RATIOS 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|