Wood is somewhat elastic, so it depends on how many allowable factors you care to investigate to determine its maximum span.Įven the same grade and species can have its “allowable stresses” increased based on “duration of load.” That is to say that if you put a heavy short duration load on a beam it will act the same as a smaller long term load. Grade of beam can be upgraded if more of the beam (edges of beam) can be observed during grading. That’s why carpenters like to hand pick beams, (they know they have an extra safety factor so the beams won’t twist, Bend, etc.)īeams are calculated using the “working stress design” or “ultimate stress design”. This is based upon the span of the rafter (horizontal span, not total rafter length), live and dead loads. Working stress leaves a BIG safety factor because they know a home owner will be using it. To size joists and rafters use the Maximum Span Tables for southern yellow pine. This Rafter Span Calculator is species specific for Douglas Fir, hemlock, etc. Ultimate stress design figures exactly when the beam will fail and then uses an allowable safety factor.sometimes 1.5, or 1.8%. I like a very safe size rather than figuring everything too tight. Span Calculator for Wood Joists and Rafters available for the iPhone and iPad. (Maximum spans for Douglas fir-larch, hem-fir, southern pine and spruce-pine-firb and. You never know when the client will call and want to add an air conditioner on the roof, or they’re going to invite you to their 50th anniversary out on their deck. Simple clicks on the drop down boxes will allow variable inputs to calculate a.
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