A. As you suspect, properly fastened gypboard does have the capacity to resist racking and/or lateral forces. The 1997 Uniform Building Code (Table 25-1) gives shear values for both gypsum wallboard and gypsum sheathing. In fact, the allowable lateral force on a wall with fully blocked 5/8-inch gypboard on both sides nailed at 4-inch centers (350 plf) actually exceeds that of a wall with 1/2-inch Structural I plywood fastened with 10-penny nails at 6-inch centers (340 plf). Be careful, though: If you are working in seismic Zones 3 or 4, note that even with fully blocked edges you must reduce the allowable lateral load on gypboard by 50%.
As to crediting the design for the strength of the gypboard, this decision is based on the materials selected for the particular structure. If you build a house with rigid-foam insulation panels on the exterior (under finishes) and gypboard on the interior, then the gypboard is the lateral force-resisting material. However, if the interior...
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