A contractor is installing shear walls in a residence located in Seismic Design Category D. The engineer's plans call for 15/32-inch structural sheathing with 8d nails at 6 inches on center at panel edges. The contractor wants to substitute 7/16-inch OSB with the same nailing. Is this substitution acceptable?
Correct Answer
A) No, the sheathing thickness cannot be reduced
In Seismic Design Category D, the specified sheathing thickness is critical for achieving the required shear values and cannot be reduced without compromising structural performance.
Why This Is the Correct Answer
In Seismic Design Category D, structural sheathing thickness is critical for achieving required shear values and lateral force resistance. The 15/32-inch thickness specified by the engineer provides specific structural capacity that cannot be achieved with thinner 7/16-inch material. Reducing thickness compromises the shear wall's ability to resist seismic forces, potentially leading to structural failure during an earthquake. Engineer specifications for seismic design cannot be arbitrarily reduced.
Why the Other Options Are Wrong
Option B: Yes, but only with engineer approval
While engineer approval might seem logical, this substitution fundamentally reduces the structural capacity below what's required for Seismic Design Category D. Even with engineer approval, the thinner sheathing cannot provide the same shear values as the specified thickness. The engineer's original specification was based on seismic requirements that cannot be met with reduced thickness.
Option C: Yes, if nailing is increased to 4 inches on center
Increasing nail spacing to 4 inches on center cannot compensate for the reduced sheathing thickness. The structural capacity of shear walls depends on both the sheathing material properties and thickness. While closer nailing improves connection, it cannot overcome the fundamental reduction in shear capacity caused by using thinner material in high seismic zones.
Option D: Yes, if the OSB is rated for structural use
Even if the OSB is rated for structural use, the reduced thickness (7/16-inch vs 15/32-inch) cannot provide the same shear values required for Seismic Design Category D. Structural rating indicates the material meets certain standards, but it doesn't compensate for the reduced thickness and corresponding reduction in lateral force resistance capacity.
Memory Technique
Remember 'Seismic Specs Stay' - in high seismic zones (Category D), structural specifications like sheathing thickness must stay as designed and cannot be reduced, regardless of other modifications.
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