Section A111 Special Procedure
The special procedures of this section shall be applied only to buildings having the following characteristics:
- Flexible diaphragms at all levels above the base of the structure.
- Vertical elements of the seismic force-resisting system consisting predominantly of masonry or a combination of masonry and concrete shear walls.
- Except for single-story buildings with an open front on one side only, not fewer than two lines of vertical elements of the seismic force-resisting system parallel to each axis of the building (see Section A111.8 for open-front buildings).
A crosswall is a wood-framed wall sheathed with any of the materials described in Table A108.1(1) or A108.1(2) or other system as defined in Section A111.3.5. Crosswalls shall be spaced not more than 40 feet (12 192 mm) 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.
- Crosswalls need not be provided at all levels where used in accordance with Section A111.4.2, Item 4.
- Diaphragm forces at corners of openings shall be investigated and shall be developed into the diaphragm by new or existing materials.
- In addition to the demand-capacity ratios of Section A111.4.2, the demand-capacity ratio of the portion of the diaphragm adjacent to an opening shall be calculated using the opening dimension as the span.
- Where an opening occurs in the end quarter of the diaphragm span, the calculation of vuD for the demand-capacity ratio shall be based on the net depth of the diaphragm.
A diaphragm is acceptable if the point (L,DCR) on Figure A111.4.1 falls within Region 1, 2 or 3.
Demand-capacity ratios shall be calculated for the diaphragm at any level according to the following formulas:
- (Equation A1-7)
- (Equation A1-8)
- (Equation A1-9)
- (Equation A1-10)
Diaphragms shall be connected to shear walls and new vertical seismic forceresisting elements with connections capable of developing the diaphragm-loading tributary to the shear wall or new seismic force-resisting elements given by the lesser of the following formulas:
using the Cp values in Table A111.5, or
[BS] TABLE A111.5
HORIZONTAL FORCE FACTOR, Cp
|CONFIGURATION OF MATERIALS||Cp|
|Roofs with straight or diagonal sheathing and roofing applied directly to the sheathing, or floors with straight |
|Diaphragms with double or multiple layers of boards with edges offset, and blocked plywood systems.||0.75|
|Diaphragms of metal deck without topping:|
|Minimal welding or mechanical attachment.||0.6|
|Welded or mechanically attached for seismic resistance.||0.68|
The wall story force distributed to a shear wall at any diaphragm level shall be the lesser value calculated as:
but need not exceed
The wall story shear shall be the sum of the wall story forces at and above the level of consideration.
- In Region 1, height-to-thickness ratios for buildings with crosswalls may be used if qualifying crosswalls are present in all stories.
- In Region 2, height-to-thickness ratios for buildings with crosswalls may be used whether or not qualifying crosswalls are present.
- In Region 3, height-to-thickness ratios for "all other buildings" shall be used whether or not qualifying crosswalls are present.