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411B.1 Limits for Application. The Special Procedure of this section may only be applied to buildings with the following characteristics:
1. The building is not an essential or hazardous facility.
2. Wood or plywood diaphragms at all levels above the base of structure.
3. A maximum of six stories above the base of the building. The base shall be the ground level, and basement or basements shall be excluded from the story count.
EXCEPTION: An otherwise qualifying building of greater than six stories may also be retrofitted using the Special Procedure, provided the building is not located on poor soil as defined in Section 403A or does not contain any Group A Occupancies with an occupant load of 300 or more, or Group E, or Group I Occupancies.
4. Except for single-story buildings with an open front on one side only, a minimum of two lines of vertical elements of the lateral force resisting system complying with Section 412B parallel to each axis. At least one line in each direction shall be a masonry or concrete shear wall. Requirements for open front buildings are contained in Section 411B.8.
5. In buildings containing one or more party walls, the Special Procedure shall not be used unless each building sharing a party wall individually complies with all of the limitations set forth above, and the owner of each such building consents to the use of the procedure in writing.
411B.3 Crosswalls. Crosswalls when used shall meet the requirements of this section.
411B.3.1 Crosswall definition. A “crosswall” is a wood-framed wall sheathed with any of the materials described in Table 4B-D or 4B-E or other system as defined in Section 411B
.3.5. Spacing of crosswalls shall not exceed 40 feet (12.19 m) on center measured
perpendicular to the direction of consideration and shall be placed in each story of the building. Crosswalls shall extend the full story height between diaphragms.
EXCEPTIONS:
1. Crosswalls need not be provided at all levels in accordance with Section 411B.4.2(4).
2. Existing crosswalls need not be continuous below a wood diaphragm at or within 4 feet (1.219 m) of grade, provided:
(1) Shear connections and anchorage requirements, Section 411B.5 are satisfied at all edges of the diaphragm.
(2) Crosswalls with total shear capacity of 0.08åWd interconnect the diaphragm to the foundation.
(3) The demand-capacity ratio of the diaphragm between the crosswalls that are continuous to their foundations shall be calculated as:
DCR = (0.332Wd + Vca)/2vuD (4B-4)
and DCR shall not exceed 2.5.
(Amended by Ord. 65-19, File No. 190136, App. 4/12/2019, Eff. 5/13/2019)
411B.3.2 Crosswall shear capacity. Within any 40 feet (12.19 m) measured along the span of the diaphragm, the sum of the crosswall shear capacities shall be at least 30 percent of the diaphragm shear capacity of the strongest diaphragm at or above the level under consideration.
411B.3.3 Existing crosswalls. Existing crosswalls shall have a maximum height-to-length ratio between openings of 1.5 to 1. Existing crosswall connections to diaphragms need not be investigated as long as the crosswall extends to the framing of the diaphragm above and below.
411B.3.4 New crosswalls. New crosswall connections to the diaphragm shall develop the crosswall shear capacity. New crosswalls shall have the capacity to resist an overturning moment equal to the crosswall shear capacity times the story height. Crosswall overturning moments need not be cumulative over more than two stories.
411B.3.5 Other crosswall systems. Other systems, such as moment resisting frames, may be used as crosswalls, provided that the yield story drift does not exceed 1 inch (25.4 mm) in any story.
411B.4 Wood Diaphragms.
411B.4.1 Acceptable diaphragm span. A diaphragm is acceptable if the point (L,DCR) on Figure 4B-1 falls within Regions 1, 2 or 3.
411B.4.2 Demand-capacity ratios. Demand-capacity ratios shall be calculated for the diaphragm at any level according to the following formulas:
1. For a diaphragm without qualifying crosswalls at levels immediately above or below:
DCR = 0.332Wd/ΣvuD (4B-5)
2. For a diaphragm in a single-story building with qualifying crosswalls:
DCR = 0.332Wd/(ΣvuD + Vcb ) (4B-6)
3. For diaphragms in a multi-story building with qualifying crosswalls in all levels:
DCR = 0.332ΣWd /(ΣΣvuD + Vcb) (4B-7)
DCR shall be calculated at each level for the set of diaphragms at and above the level under consideration. In addition, the roof diaphragm shall also meet the requirements of Formula (4B-6).
4. For a roof diaphragm and the diaphragm directly below if coupled by crosswalls:
DCR = 0.332ΣW d /ΣΣvuD (4B-8)
411B.4.3 Chords. An analysis for diaphragm flexure need not be made and chords need not be provided.
411B.4.4 Collectors. An analysis of diaphragm collector forces shall be made for the transfer of diaphragm edge shears into vertical elements of the lateral force resisting system. Collector forces may be resisted by new or existing elements.
411B.4.5 Diaphragm openings.
411B.4.5.1 Forces. Diaphragm forces at corners of openings shall be investigated and shall be developed into the diaphragm by new or existing materials.
411B.4.5.2 Demand-capacity ratio. In addition to the demand-capacity ratios of Section 411B
.4.2, the demand-capacity ratio of the portion of the diaphragm adjacent to an opening shall be calculated using the opening dimension as the span.
411B.4.5.3 End quarter of diaphragm. Where an opening occurs in the end quarter of the diaphragm span, vuD for the demand-capacity ratio calculation shall be based on the net depth of the diaphragm.
411B.5 Diaphragm Shear Transfer. Diaphragms shall be connected to shear walls with connections capable of developing a minimum force given by the lesser of the following formulas:
V = 0.2CpWd (4B-9)
using the Cp values in Table 4B-C, or
V = vuD (4B-10)
411B.6 Shear Walls (In-Plane Loading).
411B.6.1 Wall story force. The wall story force distributed to a shear wall at any diaphragm level shall be the lesser value calculated as:
1. For buildings without crosswalls:
Fwx = 0.132 (Wwx + Wd /2) (4B-11)
but need not exceed
Fwx = 0.132Wwx + vuD (4B-12)
2. For buildings with crosswalls in all levels:
Fwx = 0.1 (Wwx + Wd /2) (4B-13)
but need not exceed
Fwx = 0.1 [Wwx + ΣWd (vuD/ΣΣvuD)] (4B-14)
and need not exceed
Fwx = 0.1W wx + vuD (4B-15)
411B.6.2 Wall story shear. The wall story shear shall be the sum of the wall story forces at and above the level of consideration.
Vwx = ΣFwx (4B-16)
411B.6.4 Moment frames. Moment frames used in place of shear walls shall be designed as required in San Francisco Building CodeChapter 16 except that the forces shall be as specified in Section 411B.6.1 and the story drift ratio shall be limited to 0.005, except as further limited by Section 412B.4.2.
411B.7 Out-of-Plane Forces – Unreinforced Masonry Walls.
411B.7.1 Allowable unreinforced masonry wall height-to-thickness ratios. The provisions of Section 410B.2 are applicable except the allowable height-to-thickness ratios given in Table 4B-B shall be determined from Figure 4B-1 as follows:
1. In Region 1, height-to-thickness ratios for buildings with crosswalls may be used if qualifying crosswalls are present in all stories.
2. In Region 2, height-to-thickness ratios for buildings with crosswalls may be used whether or not qualifying crosswalls are present.
3. In Region 3, height-to-thickness ratios for “all other buildings” shall be used whether or not qualifying crosswalls are present.
411B.7.2 Walls with diaphragms in different regions. When diaphragms above and below the wall under consideration have demand-capacity ratios in different regions of Figure 4B-1, the lesser height-to-thickness ratio shall be used.
411B.8 Open Front Design Procedure. A single-story building with an open front on one side and crosswalls parallel to the open front may be designed by the following procedure:
1. Effective diaphragm span, Li, for use in Figure 4B-1 shall be determined in accordance with the following formula:
Li = 2 [(Ww /Wd )L + L]x (4B-17)
2. Diaphragm demand-capacity ratio shall be calculated as:
DCR = 0.332 (Wd + W w ) / [(vuD) + Vcb ] (4B-18)