(Amended by Ord. No. 171,175, Eff. 7/25/96.)
91.8808.1. General. (Amended by Ord. No. 171,939, Eff. 4/15/98.) Every structure within the scope of this division shall be analyzed and constructed to resist minimum total lateral seismic forces assumed to act non concurrently in the direction of each of the main axes of the structure in accordance with the following formula:
V = IKCSW (8-1)
The value of IKCS need not exceed the values set forth in Table No. 88-D based on the applicable rating classification of the building.
Fp = ICpSWp (8-2)
For the provisions of this Section, the product of IS need not exceed the values as set forth in Table No. 88-E.
EXCEPTION: Unreinforced masonry walls in buildings not having a Rating Classification I may be analyzed in accordance with Section 91.8809.
The value of Cp need not exceed the values set forth in Table No. 88-F.
91.8808.3. Anchorage and Interconnection. (Amended by Ord. No. 171,939, Eff. 4/15/98.) Anchorage and interconnection of all parts, portions and elements of the structure shall be analyzed and designed for lateral forces in accordance with Table No. 88-F of this Code and Formula (8-2) as modified by Table No. 88-E. Minimum anchorage of masonry walls to each floor or roof shall resist a minimum force of 200 pounds per linear foot (2.92 kN/m) acting normal to the wall at the level of the floor or roof.
91.8808.4. Level of Required Repair. Alterations and repairs required to meet the provisions of this division shall comply with all other applicable requirements of this Code unless specifically provided for in this division.
91.8808.5. Required Analysis.
91.8808.5.1. General. Except as modified herein, the analysis and design relating to the structural alteration of existing structures within the scope of this division shall be in accordance with the analysis specified in Division 16 of this Code.
In addition, the compatibility of the roof diaphragm stiffness with the out-of-plane stability of the unreinforced masonry bearing walls of the story immediately below the roof system shall be verified in accordance with the provisions of Section 91.8811.
EXCEPTION: Buildings with rigid concrete or steel and concrete roof diaphragms shall use the h/t values for “all other buildings” in Table No. 88-G.
91.8808.5.2. Continuous Stress Path. A complete, continuous stress path from every part or portion of the structure to the ground shall be provided for the required horizontal forces.
91.8808.5.3. Positive Connections. All parts, portions or elements of the structure shall be interconnected by positive means.
91.8808.6.1. General. Stresses in materials and existing construction utilized to transfer seismic forces from the ground to parts or portions of the structure shall conform to those permitted by the Code and those materials and types of construction specified in Section 91.8809.
91.8808.6.2. Connections. Materials and connectors used for interconnection of parts and portions of the structure shall conform to the Code. Nails may be used as part of an approved connector.
91.8808.6.3. Unreinforced Masonry Walls. (Amended by Ord. No. 181,758, Eff. 8/8/11.) Except as modified here, unreinforced masonry walls shall be analyzed as specified in the applicable parts of CBC Sections 2106 and 2107 to withstand all vertical loads as specified in Division 16 of this Code in addition to the seismic forces required by this Division.
Substantial changes in wall thickness or stiffness shall be considered in the analysis for out-of-plane and in-plane wall stability, and the wall shall be restrained against out-of- plane instability by anchorage and bracing to the roof or floor diaphragm in accordance with Section 91.8808.3 of this Code.
EXCEPTION: Variations in wall stiffness caused by nominal openings such as windows and exit doors need not be considered.
No allowable tension stress will be permitted in unreinforced masonry walls. Walls not capable of resisting the required design forces specified in this division shall be strengthened or shall be removed and replaced.
EXCEPTIONS:
1. Unreinforced masonry walls in buildings not classified as a Rating Classification I pursuant to Table No. 88-A may be analyzed in accordance with Section 91.8809 of this Code.
2. Unreinforced masonry walls which carry no design loads other than their own weight may be considered as veneer if they are adequately anchored to new supporting elements.
91.8808.7. Combination of Vertical and Seismic Forces.
91.8808.7.1. New Materials. (Amended by Ord. No. 179,324, Eff. 12/10/07, Oper. 1/1/08.) All new materials introduced into the structure to meet the requirements of this section, which are subjected to combined vertical and horizontal forces, shall comply with CBC Section 1605.
91.8808.7.2. Existing Materials. (Amended by Ord. No. 179,324, Eff. 12/10/07, Oper. 1/1/08.) When stresses in existing lateral force-resisting elements are due to a combination of dead and live loads plus seismic loads, the allowable working stress specified in the Code may be increased 100 percent. However, no increase will be permitted in the stresses allowed in LAMC Section 91.8809, and the stresses in members due only to seismic and dead loads shall not exceed the values permitted by CBC Section 1605.
91.8808.7.3. Allowable Reduction of Bending Stress by Vertical Load. In calculating tensile fiber stress due to seismic forces required by this division, the maximum tensile fiber stress may be reduced by the full direct stress due to vertical dead loads.
91.8808.8. Irregular Features. (Amended by Ord. No. 179,324, Eff. 12/10/07, Oper. 1/1/08.) All structures having any of the irregular features described in Table 12.3-1 or Table 12.3-2 of ASCE-7 shall be designated to meet the additional requirements of those sections referenced in the tables.