(Added by Ord. No. 171,259, Eff. 8/30/96.)
91.9201.1. Purpose. The provisions of this division are intended to promote public safety and welfare by reducing the risk of earthquake-induced damage to existing wood-framed residential buildings. The voluntary minimum standards contained in this division shall substantially improve the seismic performance of these residential buildings but will not necessarily prevent all earthquake damage. When fully followed, these standards will strengthen the portion of the structure that is most vulnerable to earthquake damage.
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.
91.9201.2. Scope. (Amended by Ord. No. 185,587, Eff. 7/16/18.) The provisions of this division may be applied to light wood frame Group R Occupancies with no more than four dwelling units when they contain one or more of the structural weaknesses specified in LAMC Subsection 91.9203.1.
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.
1. Buildings with a lateral force resisting system using poles or columns embedded in the ground.
2. Cripple walls that exceed four feet (1234 mm) in height.
3. Buildings exceeding three stories in height and any three-story building with cripple wall studs exceeding 14 inches (360 mm) in height.
4. 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.
5. Buildings where the Superintendent of Building determines that conditions exist that are beyond the scope of the requirements of this division.
The standard details approved by the Superintendent of Building and these prescriptive provisions are not intended to be the only acceptable strengthening methods permitted. Alternate details and methods shall be permitted when approved by the Superintendent of Building. Qualified Historical Buildings shall be permitted to use alternate building regulations of LAMC Section 91.8119 in order to preserve their original or restored architectural elements and features.
91.9201.3. Alternative Design Procedures. (Amended by Ord. No. 179,324, Eff. 12/10/07, Oper. 1/1/08.) When analysis by an engineer or architect is required or provided for a building within the scope of this division, that analysis shall be in accordance with all requirements of this Code except as provided in this division. The design shall provide strengthening for any structural weakness listed in LAMC Section 91.9203 that is at least equivalent to that provided by the prescriptive requirements of this division with respect to strength, deflection, and capacity. The Superintendent of Building may require that sufficient evidence be submitted to substantiate that equivalence. The base shear may be determined in accordance with the following:
V = 0.1375 W (92-1)
Where:
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.