(A) General. Every structure within the scope of this section shall be analyzed and constructed to resist minimum total lateral seismic forces assumed to act nonconcurrently in the direction of each of the main axes or the structure in accordance with the following equation:
V=ZIKCSW Formula (1)
The value of IKCS need not exceed the values set forth in Table D based on the applicable rating classification of the building.
(B) Lateral forces on elements of structures. Parts or portions or structures shall be analyzed and designed for lateral loads in accordance with division (A) of this section and Uniform Building Code § 2312(e) but not less than the value from the following equation:
Fp= ZICp SWp Formula (2)
For the provisions of this subsection, the product of IS need not exceed the values as set forth in Table E. Exception: unreinforced masonry walls in buildings not having a rating classification of I may be analyzed in accordance with § 15.64.100. The value of Cp need not exceed the values set forth in Table F.
(C) Anchorage and interconnection. Anchorage and interconnection of all parts, portions and elements of the structure shall be analyzed and designed for lateral forces in accordance with Table F of this code and the equation Fp=ZICpSWp and modified by Table E. Minimum anchorage of masonry walls to each floor or root shall resist a minimum force of 200 pounds per linear foot acting normal to the wall at the level of the floor or roof.
(D) Level or required repair. Alterations and repairs required to meet the provisions of this chapter shall comply with all other applicable requirements of the Uniform Building Code unless otherwise specifically provided for in this chapter.
(E) Required analysis.
(1) General. Except as modified herein, the analysis and design relating to the structural alteration of existing structures within the scope of this chapter shall be in accordance with the analysis specified in Chapter 23.
(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.
(3) Positive connections. All parts, portions or elements of the structure shall be interconnected by positive means.
(F) Analysis procedure.
(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 Uniform Building Code and those materials and types of construction specified in § 15.64.100.
(2) Connections. Materials and connectors used for interconnection of parts and portions of the structure shall conform to the Uniform Building Code. Nails may be used as part of an approved connector.
(3) Unreinforced masonry walls.
(a) Except as modified herein, unreinforced masonry walls shall be analyzed as specified in Uniform Building Code § 2406, 2407 and 2408 to withstand all vertical loads as specified in Uniform Building Code Chapter 23 in addition to the seismic forces required by this chapter. The 50% increase in the seismic force factor for shear walls as specified in Uniform Building Code § 2407(h)4F may be omitted in the computation of seismic loads to existing shear walls.
(b) No allowable tension stress will be permitted in unreinforced masonry walls. Walls not capable of resisting the required design forces specified in this chapter shall be strengthened or shall be removed and replaced.
(c) Exceptions:
1. Unreinforced masonry walls in buildings not classified as a Rating Classification I pursuant to Table A may be analyzed in accordance with § 15.64.100;
2. An unreinforced masonry wall which carries no design loads other than its own weight may be considered as veneer if it is adequately anchored to new supporting elements.
(G) Combination of vertical and seismic forces.
(1) New materials. 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 Uniform Building Code § 2303(f).
(2) Existing materials. When stresses in existing lateral force resisting elements are due to a combination of dead loads plus live load plus seismic loads, the allowable working stress specified in the Uniform Building Code may be increased 100%. However, no increase will be permitted in the stresses allowed in § 15.64.100 and the stressed in members due only to seismic and dead loads shall not exceed the values permitted by Uniform Building Code § 2303(d).
(3) Allowable reduction of bending stress by vertical load. In calculating tensile fiber stress due to seismic forces required by this chapter, the maximum tensile fiber stress may be reduced by the full direct stress due to vertical dead loads.
(`78 Code, § 15.64.090.) (Ord. 1967 § 1 (part), 1989.)