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.

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