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   (Amended by Ord. No. 171,175, Eff. 7/25/96.)
91.8811.1.  General.  (Amended by Ord. No. 171,939, Eff. 4/15/98.)  The requirements of this section are in addition to the requirements of Sections 91.8808 and 91.8809.  The relative stiffness and strength of a diaphragm governs the amount of amplification of the seismic ground motion by the diaphragm and, therefore, a diaphragm stiffness and strength-related check of the out-of-plane stability of unreinforced masonry walls anchored to wood diaphragms shall be made.  This section contains a procedure for the evaluation of the out-of-plane stability of unreinforced masonry walls anchored to wood diaphragms that are coupled to shear-resisting elements.
91.8811.2.  Requirements for Terms.  (Amended by Ord. No. 179,324, Eff. 12/10/07, Oper. 1/1/08.)  The requirements for the terms used in this Division shall be defined as follows:
   CROSS WALL is a wood-framed wall having a height-to-length ratio of:
   1.   Two to one for wood structural panels.
   2.   One to one for gypsum board, gypsum lath, cement plaster or diagonal sheathing.
   The total strength of all cross walls located within any 40-foot (12 192 mm) length of diaphragm measured in the direction of the diaphragm span shall not be less than 30 percent of the strength of the diaphragm in the direction of consideration.
   DEMAND-CAPACITY RATIO (DCR) is a ratio of the following:
   1.   Demand = lateral forces due to 33 percent of the weight of the diaphragm and the tributary weight of the walls and other elements anchored to the diaphragm.
   2.   Capacity = diaphragm total shear strength in the direction under consideration as determined using the values in Table No. 88-K or Table No. 88-L.
91.8811.3.  Notations.  (Amended by Ord. No. 171,939, Eff. 4/15/98.)
   D   =   depth of diaphragm, in feet (mm), measured perpendicular to the diaphragm span.
   h/t   =   height-to-thickness ratio of an unreinforced masonry wall. The height shall be measured between wall anchorage levels and the thickness shall be measured through the wall cross section.
   L   =   span of diaphragm between masonry shear walls or steel frames.
   Vc   =   total shear capacity of cross walls in the direction of analysis immediately below the diaphragm level being investigated as determined by using Tables Nos. 88-K and 88-L.
   vu   =   maximum shear strength in pounds per foot for a diaphragm sheathed with any of the materials given in Tables Nos. 88-K and 88-L.
   Wd   =   total dead load of the diaphragm plus the tributary weight of the walls anchored to the diaphragm, the tributary ceiling and partitions and the weight of any other permanent building elements at the diaphragm level under investigation.
91.8811.4.  Design Check Procedure.  (Added by Ord. No. 171,939, Eff. 4/15/98.)
91.8811.4.1.  General.  The demand-capacity ratio (DCR) for the building under investigation shall be calculated using the following equations:
   DCR = 0.33Wd /2vuD   (11-3)
   DCR = 0.33Wd /(2vuD + Vc)   (11-4)
91.8811.4.2.  Diaphragm Deflection.  The calculated DCR shall be to the left of the curve in Figure No. 88-A.  Where the calculated DCR is outside (to the right of) the curve, the diaphragm deflection limits are exceeded, and cross walls may be used to reduce the deflection.
91.8811.4.3.  Unreinforced Masonry Wall Out-of-Plane Stability.  The DCR shall be calculated discounting any cross walls.  If the DCR corresponding to the diaphragm span is to the right of the curve in Figure No. 88-A, the region within the curve at and below the intersection of the diaphragm span with the curve may be used to determine the allowable h/t values per Table No. 88-G.