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Q.I’ve heard that engineers give no structural "credit" to gypboard, but I know it greatly stiffens partitions when I nail it up. How much shear strength does drywall really have, and why not credit it in the design?

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 gypboard is combined with plywood sheathing on the exterior (or with diagonally braced structural steel studs), then the strength of the gypboard is discounted. In the latter case, the plywood is considered the primary lateral-force-resisting material because of its greater strength and stiffness. In both instances, the designer must make certain that the primary lateral-force-resisting material is sufficiently fastened to the framing to resist the total lateral load despite the presence of other secondary materials.

In reality, it is the combination of all the primary and secondary materials that will resist the applied lateral loads. However, should the loading persist, the repetitive cycles of load/release will cause fatigue of the weaker materials (like gypboard) until essentially only the primary lateral material remains functional. If we were to credit the strength of the gypboard towards the total lateral load (and reduce the plywood nailing accordingly), our structure would lack critical capacity after the time when the gypboard had yielded. This is the reason gypboard receives no credit for its strength.