Download PDF version (134.2k) Log In or Register to view the full article as a PDF document.
Q.There's a framing rule of thumb I've used for years that says it's okay to offset a second-story bearing wall from the first-story bearing wall below, as long as you don't offset it by more than the depth of the joist. We use this rule for standard dimensional lumber, but would it also apply to wood I-joists?

A.Gary Schweizer, PE, a senior engineer with iLevel by Weyerhaeuser, in Charlotte, N.C., responds: Wood I-joists actually need to be designed to account for the offset wall load. The rule of thumb you mention is a code provision (2006 IRC 502.4) for conventional construction and is applicable to all rectangular joists (sawn lumber, structural composite lumber, glulams, and so on), but it doesn't apply to I-joists.

Generally speaking, sawn-lumber joist spans are limited by bending or deflection in the middle of the span, rather than by shear or bearing at a support; the additional load due to the offset wall does not result in an overstress condition.

However, an I-joist by design is optimized for material usage and performance, which results in conditions where the spans may be limited by shear or bearing capacity.

Because the added load from an offset wall could result in an overstress condition, most I-joist manufacturers recommend stacking bearing walls directly over supports below and including blocking panels or squash blocks (vertical 2x4s) between the joists to transfer the load from the wall above.