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American Wood Council

Are there span tables for cantilever beams?


No. An engineer or architect should design cantilever members. Design of cantilever beams involves many variables including load, cantilever span and interior or back span. Often the load is not a single uniform distribution over the length of the member, and other additional loads are present, such as point loads at the end of the member. Because the system is composed of two pieces: the cantilever span, and the back span, the placement and magnitude of load on these sections singly or combined will cause different stresses to develop in the member. The designer seeks to find the worst combination of loading that will impose maximum shear, bending, or deflection in the member. 

American Wood Council's 2001 Wood Frame Construction Manual has engineered and prescriptive provisions that may give guidance for typical cantilever cases. For example, the Engineered Design provisions for sawn lumber floor joists in 2.3.1.6 state the maximum overhang length is limited to the depth of the joist if the end of the cantilever supports a load bearing wall or shear wall (Figure 1). When designed for additional loads, cantilevers are limited to 4 times the depth of the joist (Figure 2). The cantilevered joist must be located directly over studs unless the top plates are designed to carry the loads. If the end of the cantilever supports a non-loadbearing non-shear wall, then the maximum overhang length is limited to one-fourth of the joist span (Figure 3). Consult manufacturer's recommendations if using I-joists. Prescriptive Design provisions found in 3.3.1.6.1 for sawn lumber floor joists are the same except that for roof live loads and ground snow loads less than or equal to 20 psf and 30 psf, respectively, cantilevers shall not exceed one-eighth of the joist span for lumber joists supporting only a roof with a clear span of 28 feet or less.

 
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