Chapter A3 Prescriptive Provisions for Seismic Strengthening of Cripple Walls and Sill Plate Anchorage of Light, Woodframe 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 building official without requiring plans or calculations prepared by an architect or engineer. The provisions of this chapter are not intended to prevent the use of any material or method of construction not prescribed herein. The building official may require that construction documents for strengthening using alternative materials or methods be prepared by an architect or engineer.
The provisions of this chapter apply to residential buildings of light-frame wood construction assigned to Seismic Design Category C, D or E of the International Building Code 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 below. These buildings or elements require analysis by an engineer or architect in accordance with Section A301.3 to determine appropriate strengthening:
- Group R-1, R-2 or R-4 occupancies 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 building 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 may be used when approved by the building official. Approval of alternatives shall be based on test data showing 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.
The provisions of this chapter may be used to strengthen historic structures, provided they are not in conflict with other related provisions and requirements that may apply.
For the purpose of this chapter, in addition to the applicable definitions in the building code, certain additional terms are defined as follows:
CHEMICAL ANCHOR. An assembly consisting of a threaded rod, washer, nut and chemical adhesive approved by the building official for installation in existing concrete or masonry.
COMPOSITE PANEL. A wood structural panel product composed of a combination of wood veneer and wood-based material, and bonded with waterproof adhesive.
CRIPPLE WALL. A wood-frame stud wall extending from the top of the foundation to the underside of the lowest floor framing.
EXPANSION BOLT. A single assembly approved by the building official for installation in existing concrete or masonry. For the purpose of this chapter, expansion bolts shall contain a base designed to expand when properly set, wedging the bolt in the pre-drilled hole. Assembly shall also include appropriate washer and nut.
ORIENTED STRAND BOARD (OSB). A mat-formed wood structural panel product composed of thin rectangular wood strands or wafers arranged in oriented layers and bonded with waterproof adhesive.
PERIMETER FOUNDATION. A foundation system that is located under the exterior walls of a building.
PLYWOOD. A wood structural panel product composed of sheets of wood veneer bonded together with the grain of adjacent layers oriented at right angles to one another.
SNUG-TIGHT. 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.
WAFERBOARD. A mat-formed wood structural panel product composed of thin rectangular wood wafers arranged in random layers and bonded with waterproof adhesive.
WOOD STRUCTURAL PANEL. A structural panel product composed primarily of wood and meeting the requirements of United States Voluntary Product Standard PS 1 and United States Voluntary Product Standard PS 2. Wood structural panels include all-veneer plywood, composite panels containing a combination of veneer and wood-based material, and mat-formed panels such as oriented strand board and waferboard.
- Sill plates or floor framing that are supported directly on the ground without an approved foundation system.
- A perimeter foundation system that is constructed only of wood posts supported on isolated pad footings.
- Perimeter foundation systems that are not continuous.
- 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.
Exception: When approved by the building official, connections of a sill plate to the foundation made with other than sill bolts may be accepted if the capacity of the connection is equivalent to that required by the building code.
- Cripple walls that are not braced in accordance with the requirements of Section A304.4 and Table A3-A, or cripple walls not braced with diagonal sheathing or wood structural panels in accordance with the building code.
|NUMBER OF STORIES ABOVE CRIPPLE WALLS||MINIMUM SILL PLATE CONNECTION
AND MAXIMUM SPACINGa, b
|AMOUNT OF BRACING FOR EACH WALL LINEc,d,e|
|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 (12.7 mm) spaced 6 feet, 0 inch (1829 mm) 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 (12.7 mm) spaced 4 feet, 0 inch (1219 mm) center-to-center with washer plate; or 5/8 inch (15.9 mm) spaced 6 feet, 0 inch (1829 mm) 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 (15.9 mm) spaced 4 feet, 0 inch (1219 mm) center-to-center with washer plate||100 percent of the wall lengthf||Each end and not less than 80 percent of the wall lengthf|
- Sill plate anchors shall be chemical anchors or expansion bolts in accordance with Section A304.3.1.
- All washer plates shall be 2 inches by 2 inches by 3/16 inch (51 mm by 51 mm by 4.8 mm) minimum.
- See Figure A3—10 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.
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 blocking between alternate joists in one-and two-story buildings, and between each joist in three-story buildings. Existing blocking for multistory 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 multistory 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. When this minimum bottom edge-connection is not present or cannot be verified, a supplemental connection installed as shown in Figure A3-8 shall beprovided.
Where an existing continuous rim joist or the minimum existing blocking does not occur, new 3/4-inch (19 mm) wood structural panel blocking installed tightly between floor joists and nailed as shown in Figure A3-8 shall be provided at the inside face of the cripple wall. In lieu of 3/4-inch (19 mm) wood structural panel blocking, tightfitting, 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.
Partial perimeter foundations or unreinforced masonry foundations shall be evaluated by an engineer or architect for the force levels noted in Section A301.3. Test reports or other substantiating data to determine existing foundation material strengths shall be submitted for review. When approved by the building official, these foundation systems may be strengthened in accordance with the recommendations included with the evaluation in lieu of being replaced.
All new perimeter foundations shall be continuous and constructed according to one of the details shown in Figure A3-1 or A3-2.
- When approved by the building official, the existing clearance between existing floor joists or girders and existing grade below the floor need not comply with the building code.
- When approved by the building official, and when designed by an engineer or architect, partial perimeter foundations may be used in lieu of a continuous perimeter foundation.
|MINIMUM FOUNDATION DIMENSIONS||MINIMUM FOUNDATION REINFORCING|
|W||F||D a, b, c||T||H||VERTICAL
|1||12 inches (305 mm)||6 inches
|6 inches (152 mm)||≤ 24 inches (610 mm)||#4 @ 24 inches
(610 mm) on center
|#4 continuous at top of stem wall|
|7 inches (178 mm)||18 inches (457 mm)||8 inches (203 mm)||≥ 24 inches
|#4 @ 24 inches
(610 mm) on center
|#4 @ 16 inches
(406 mm) on center
|8 inches (203 mm)||24 inches (610 mm)||10 inches (254 mm)||≥ 36 inches (914 mm)||#4 @ 24 inches
(610 mm) on center
|#4 @ 16 inches
(406 mm) on center
When the building has an existing continuous perimeter foundation, all perimeter wall sill plates shall be bolted to the foundation with chemical anchors or expansion bolts in accordance with Table A3-A.
Anchors or bolts shall be installed in accordance with Figure A3-3, with the plate washer installed between the nut and the sill plate. The nut shall be tightened to a snug-tight condition after curing is complete for chemical anchors and after expansion wedge engagement for expansion bolts. The installation of nuts on all bolts shall be subject to verification by the building official. Where existing conditions prevent anchor or bolt installation through the sill plate, this connection may be made in accordance with Figure A3-4A, A3-4B or A3-4C. The spacing of these alternate connections shall comply with the maximum spacing requirements of Table A3-A. Expansion bolts shall not be used when the installation causes surface cracking of the foundation wall at the location of the bolt.
Chemical anchors or expansion bolts 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 when necessary because of existing conditions. For lengths of sill plate greater than 12 feet (3658 mm), anchors or bolts shall be spaced along the sill plate as noted in Table A3-A. For other lengths of sill plate, see Table A3-B. For lengths of sill plate less than 30 inches (762 mm), a minimum of one anchor or bolt shall be installed.
|NUMBER OF STORIES||LENGTHS OF SILL PLATE|
|Less than 12 feet (3658 mm) to
6 feet (1829 mm)
|Less than 6 feet (1829 mm) to 30 inches (762 mm)||Less than 30 inches
footnote (762 mm)c
|One story||Three connections||Two connections||One connection|
|Two stories||Four connections for 1/2-inch (12.7 mm) anchors or bolts or
Three connections for 5/8-inch (15.9 mm) anchors or bolts
|Two connections||One connection|
|Three stories||Four connections||Two connections||One connection|
- Connections shall be either chemical anchors or expansion bolts.
- 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 not be less than 15/32-inch (12 mm) thick and shall be installed in accordance with Figure A3-5 or A3-6. All 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 a minimum of 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. All horizontal joints must 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 existing studs such that there is a minimum 1/8 inch (3.2 mm) between the panels, and such that the nails are placed a minimum of 1/2 inch (12.7 mm) from the edges of the existing stud. Where such 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 A3-7.
See Table A3-A and Figure A3-10 for the distribution and amount of bracing required for each wall line. Each braced panel length must be at least two times the height of the cripple stud. Where the minimum amount of bracing prescribed in Table A3-A cannot be installed along any walls, the bracing must be designed in accordance with Section A301.3.
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 when 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 A3-7.
All work shall be subject to inspection by the building official including, but not limited to:
- Placement and installation of new chemical anchors or expansion bolts installed in existing foundations. Special inspection is not required for chemical anchors installed in existing foundations regulated by the prescriptive provisions of this chapter.
- Installation and nailing of new cripple wall bracing.
- Any work may be subject to special inspection when required by the building official in accordance with the building code.
When approved by the building official, the strengthening work contained in this chapter may be completed in phases. The strengthening work in any phase shall be performed on two parallel sides of the structure at the same time.
|W||F||D a, b, c||T||H||VERTICAL REINFORCING|
|1||12 inches (305 mm)||6 inches
|6 inches (152 mm)||≤ 24 inches (610 mm)||Single-pour wall and footing||Footing poured separate from wall|
|7 inches (178 mm)||18 inches (457 mm)||8 inches (203 mm)||≥ 36 inches
|#4 @ 48 inches
(1219 mm) on center
|#4 @ 32 inches
(813 mm) on center
|8 inches (203 mm)||24 inches (610 mm)||10 inches (254 mm)||≥ 36 inches (914 mm)||#4 @ 48 inches
(1219 mm) on center
|#4 @ 18 inches
(457 mm) on center
- 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.
- When expansive soil is encountered, the foundation depth and reinforcement shall be as directed by the building official.