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Section A111 Special Procedure
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The special procedures of this section may be applied only to buildings having the following characteristics:
- Flexible diaphragms at all levels above the base of the structure.
- Vertical elements of the lateral force-resisting system consisting predominantly of masonry or concrete shear walls.
- 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 parallel to each axis of the building (see Section A111.8 for openfront buildings).
Crosswalls shall meet the requirements of this section.
A crosswall is a wood-framed wall sheathed with any of the materials described in Table A1-D or A1-E or other system as defined in Section A111.3.5. Crosswalls shall be spaced no 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.
Exceptions:
- Crosswalls need not be provided at all levels when used in accordance with Section A111.4.2, Item 4.
- Existing crosswalls need not be continuous below a wood diaphragm at or within 4 feet (1219 mm) of grade, provided:
- Shear connections and anchorage requirements of Section A111.5 are satisfied at all edges of the diaphragm.
- Crosswalls with total shear capacity of 0.5SD1ΣWd interconnect the diaphragm to the foundation.
- The demand-capacity ratio of the diaphragm between the crosswalls that are continuous to their foundations does not exceed 2.5, calculated as follows:
Within any 40 feet (12 192 mm) 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.
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 diaphragms above and below.
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.
Other systems, such as moment-resisting frames, may be used as crosswalls provided that the yield story drift does not exceed 1 inch (25 mm) in any story.
A diaphragm is acceptable if the point (L,DCR) on Figure A1-1 falls within Region 1, 2 or 3.
Demand-capacity ratios shall be calculated for the diaphragm at any level according to the following formulas:
- For a diaphragm without qualifying crosswalls at levels immediately above or below:
- For a diaphragm in a single-story building with qualifying crosswalls, or for a roof diaphragm coupled by crosswalls to the diaphragm directly below:
- For diaphragms in a multistory building with qualifying crosswalls in all levels:
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 Equation A1-10. - For a roof diaphragm and the diaphragm directly below, if coupled by crosswalls:
An analysis for diaphragm flexure need not be made, and chords need not be provided.
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.
- 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.
Diaphragms shall be connected to shear walls with connections capable of developing the diaphragm-loading tributary to the shear wall given by the lesser of the following formulas:
using the Cp values in Table A1-C, or
The wall story force distributed to a shear wall at any diaphragm level shall be the lesser value calculated as:
but need not exceed
but need not exceed
The wall story shear shall be the sum of the wall story forces at and above the level of consideration
Shear walls shall comply with Section A112.
Moment frames used in place of shear walls shall be designed as required by the building code, except that the forces shall be as specified in Section A111.6.1, and the story drift ratio shall be limited to 0.015, except as further limited by Section A112.4.2.
The provisions of Section A110.2 are applicable, except the allowable height-to-thickness ratios given in Table A1-B shall be determined from Figure A1-1 as follows:
- 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.
When diaphragms above and below the wall under consideration have demand-capacity ratios in different regions of Figure A1-1, the lesser height-to-thickness ratio shall be used.
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:
- . Effective diaphragm span, Li, for use in Figure A1-1 shall be determined in accordance with the following formula:
- Diaphragm demand-capacity ratio shall be calculated as:
Related Code Sections
Section A111 Seismic Strengthening Provisions for Unreinforced Masonry Bearing Wall Buildings, Special Procedure
The special procedures of this section may be applied only to buildings having the following characteristics: Flexible diaphragms at all levels ...
Existing Building Code 2015 of Idaho > A1 Seismic Strengthening Provisions for Unreinforced Masonry Bearing Wall Buildings > A111 Special Procedure