Division 92 Voluntary — Earthquake Hazard Reduction in Existing Wood Frame Residential Buildings With Weak Cripple Walls and Unbolted Sill Plates
Prior to 1960, most wood frame residential buildings were built with raised wood floors supported by short wood stud walls known as cripple walls. These cripple walls are typically braced with weak seismic materials such as portland cement plaster or horizontal wood siding. In addition, wood frame buildings built under building codes in effect prior to July 1938 were not required to be bolted to their foundations. Recent earthquakes have shown that if a building has weak cripple walls or is unbolted, it may fall off its foundation even in moderate earthquakes. Fallen buildings have collapsed, caught fire or needed extensive repairs to restore their occupancy.
This division sets prescriptive standards for strengthening of underfloor enclosures that shall be permitted by the Superintendent of Building without requiring plans or calculations prepared by an architect or an engineer. This division also provides a design standard for the use of alternate materials or an alternate method of construction in lieu of the prescriptive standards. Construction documents for strengthening using alternate materials or methods shall be prepared by an architect or engineer.
The provisions of this division do not apply to the buildings or building elements, listed below. These buildings or elements require analysis by an engineer or architect in accordance with Division 16, Article 1, Chapter IX of the LAMC or other approved standards to determine appropriate strengthening.
- Buildings with a lateral force resisting system using poles or columns embedded in the ground.
- Cripple walls that exceed four feet (1234 mm) in height.
- Buildings exceeding three stories in height and any three-story building with cripple wall studs exceeding 14 inches (360 mm) in height.
- Buildings, or portions of buildings, constructed on a concrete slab on grade or constructed on or into a slope steeper than three horizontal to one vertical.
- Buildings where the Superintendent of Building determines that conditions exist that are beyond the scope of the requirements of this division.
V = The total design lateral force or shear at the base.
W = The total seismic dead load defined in Section 12.7.2 of ASCE 7.
ANCHOR SIDE PLATE is a metal plate or plates used to connect a sill plate to the side of a concrete or masonry stem wall.
CRIPPLE WALL is a wood-framed stud wall extending from the top of the foundation to the underside of the lowest floor framing.
EXPANSION ANCHOR is a mechanical fastener placed in hardened concrete or assembled masonry, designed to expand in a self-drilled or pre-drilled hole of a specified size and engage the sides of the hole in one or more locations to develop shear and/or tension resistance to applied loads without grout, adhesive or drypack.
PERIMETER FOUNDATION is a foundation system which is located under the exterior walls of a building.
SNUG-TIGHT is as tight as an individual can torque a nut on a bolt by hand using a wrench with a 10-inch (254 mm) long handle and the point at which the full surface of the plate washer is contacting the wood member and slightly indents the wood surface.
UNREINFORCED MASONRY includes adobe, burned clay, concrete or sand-lime brick, hollow clay or concrete block, hollow clay tile, rubble, cut stone and unburned clay masonry walls in which the area of reinforcement is less than 50 percent of the minimum steel ratios required for reinforced masonry.
- Sill plates or floor framing which are supported directly on the ground without an approved foundation system.
- A perimeter foundation system which is constructed of wood posts supported on isolated pad footings.
- Perimeter foundation systems that are not continuous.
- A perimeter foundation system which is constructed of unreinforced masonry.
- Sill plates which are not connected to the foundation or are connected with less than what is required by LAMC Subdivision 91.9204.3.1.
- Cripple walls that are not braced in accordance with the requirements of LAMC Subsection 91.9204.4 and LAMC Table 92-A.
Existing connections at the top edge of an existing rim joist or blocking need not be verified. The bottom edge connection to either the foundation sill plate or top plate of a cripple wall shall be verified unless a supplemental connection is provided. The minimum existing bottom edge connection shall consist of 8d toe nails spaced six inches (152 mm) apart for a continuous rim joist or three 8d toe nails per block. When this minimum bottom edge connection is not present, or is not verified, a supplemental connection shall be provided.
When an existing continuous rim joist or the minimum existing blocking does not occur, new 1-1/8 inch (29 mm) wood structural panel blocking installed tightly between floor joists and nailed with 10d common nails at four inches on center to the sill or wall top plate shall be provided at the inside face of the cripple wall. In lieu of 1-1/8 inch (29 mm) wood structural panel blocking, tight fitting, full or near full depth two inches nominal width (51 mm) lumber blocking shall be allowed provided it does not split during installation. New blocking is not required where it will interfere with vents or plumbing which penetrates the wall.
- 3-inch by 6-inch (76 mm by 152 mm) by 0.036-inch-thick (0.9 mm) galvanized steel and nailed with six 8d nails at each end.
- 1-1/2 inches (38 mm) by 12-inch (305 mm) by 0.058 inches (1.47 mm) galvanized steel nailed with six 16d nails at each end.
- 2-inch by 4-inch by 12-inch wood blocking nailed with six 16d nails at each end.
- When approved by the Superintendent of Building, the existing clearance between existing floor joists or girders and existing grade below the floor need not comply with CBC Section 2304.11.2.1. This exception shall not be permitted when buildings are relocated on new foundations.
- When approved by the Superintendent of Building, and when designed by an engineer or architect, partial perimeter foundations may be used in lieu of a continuous perimeter foundation.
Anchors shall be installed 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 adhesive anchors and after expansion wedge engagement for expansion anchors. The installation of nuts on all anchors shall be subject to verification by the Superintendent of Building. Torque testing shall be performed for 25 percent of all adhesive or expansion anchors. Minimum test values shall be 30 foot pounds (41 N-m) for 1/2-inch (12.7 mm) and 40 foot pounds (55 N-m) for 5/8-inch (15.9 mm) diameter anchors.
Anchor side plates shall be permitted when conditions prevent anchor installation vertically through the sill plate. Anchor side plates shall be spaced as required for adhesive or expansion anchors but only one anchor side plate is required on individual pieces of sill plate less than 32 inches (813 mm) in length. Wood structural panel shims shall be used on sill plates for single plate anchor side plates when the foundation stem wall is from 3/16 inch (4.8 mm) to 3/4 inch (19 mm) wider than the sill plate. The shim length shall extend a minimum of two inches (50.8 mm) past each end of the anchor side plate. Two plate anchor side plates shall be used when the total thickness of the required shim exceeds 3/4 inch (19 mm).
All anchor side plates, which use lag or wood screws shall pre-drill the sill plate to prevent splitting as required by CBC Section 2304.9. Lag or wood screws shall be installed in the center of the thickness of the existing sill plate.
Expansion anchors shall not be used in unreinforced masonry or concrete or masonry grout of poor quality. Adhesive anchors shall be required when expansion anchors will not tighten to the required torque or their installation causes surface cracking of the foundation wall.
All horizontal joints must occur over nominal two-inch by four-inch (51 mm by 102 mm) blocking installed with the nominal four-inch (102 mm) dimension against the face of the plywood. All vertical joints must occur over studs. 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. Nails shall be placed a minimum of 1/2 inch (12.7 mm) from the edges of the existing stud. When such edge distance cannot be maintained because of the width of the existing stud, a new stud shall be added adjacent to the existing and connected with 16d common nails at eight inches (206 mm) on center. A minimum of three such nails shall be provided.
The braced panel must be at least two times the height of the cripple stud wall but not less than 48 inches (1219 mm) in width. All panels along a wall shall be nearly equal in length and shall be nearly equally spaced along the length of the wall. Braced panels at ends of walls shall be located as near the end as possible.
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 but the required length of bracing need not exceed the length of the wall.
Underfloor ventilation openings shall be maintained in accordance CBC Section 1203. Braced panels may include underfloor ventilation openings when the height of the solid portion of the panel meets or 75% of the height of the cripple stud wall. When the minimum amount of bracing prescribed in LAMC Table 92-A cannot be installed due to obstructions along any wall, the bracing must be designed by an architect or engineer in accordance with CBC Section 1203.3.
When 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 two-inch (51 mm) by four-inch (102 mm) block installed with the nominal four-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.
- Placement and installation of new adhesive or expansion anchors or anchor side plates installed in existing foundations.
- Placement of required blocking and framing anchors.
- Installation and nailing of new cripple wall bracing.
- "I am responsible for this building's seismic strengthening design for the underfloor cripple walls and sill bolting in compliance with the minimum seismic resistance standards of Division 92, Article 1, Chapter IX of the LAMC." or when applicable:
- "The Registered Deputy Inspector, required as a condition of the use of structural design stresses requiring continuous inspection, will be responsible to me as required by LAMC Section 91.1704."
|Number of Stories above Cripple Walls||Minimum Sill Plate Connection and Maximum Spacing||Amount of Wall Bracing|
|One Story||Adhesive or expansion anchors shall be 1/2-inch (12.7 mm) minimum diameter spaced at six feet (1829 mm) maximum center to center.||Each end and not less than 50% of the wall length.|
|Two Story||Adhesive or expansion anchors shall be 1/2-inch (12.7 mm) minimum diameter spaced at four feet (1219 mm) maximum center to center; or 5/8 inch (15.9 mm) spaced at six feet maximum center to center.||Each end and not less than 70% of the wall length.|
|Three Story||Adhesive or expansion anchors shall be 1/2- inch minimum (12.7 mm) diameter spaced at two feet eight inches (813 mm) maximum center to center; or 5/8-inch minimum (15.9 mm) diameter spaced at four feet (1219 mm) maximum center to center.||100% of the wall length.|
- 1. Plate washers for use with adhesive or expansion anchors shall be two-inch (51 mm) by two- inch (51 mm) by 3/16-inch (4.8 mm) for 1/2-inch (12.7 mm) diameter anchors and 2-1/2-inch (64 mm) by 2-1/2-inch (64 mm) by 1/4-inch (6 mm) for 5/8 inch (15.9 mm) diameter anchors.
- 2. Existing sill plate anchor bolts shall be permitted to provide all or a portion of the sill plate connection requirement if:
- the anchor bolt is cast in concrete and in sound condition; and
- the diameter size and maximum spacing meets or exceeds the requirements of LAMC Table 92-A; and
- a new plate washer conforming to footnote 1 is installed; and
- the sill plate is connected to a snug tight condition and torque tested per LAMC Subdivision 91.9204.3.1.
- 3. Anchor side plates shall be permitted when conditions prevent anchor installation vertically through the sill plate.