And at the same time, fibers stretch along the bottom edge. As a beam bends the outermost (extreme) fibers are compressed along the top edge. Loads cause beams, joists and rafters to bend. Fb value or extreme fiber stress in bending.For example: No.2 grade eastern white pine has an E value of 1,100,000 and No.2 hem-fir has an E value of 1,300,000. A material with a higher E value is stiffer. E is a ratio that relates the amount a given load causes a material to deform. E value or modulus of elasticity of the individual elements.will provide a stronger and stiffer floor assembly than 2×8 joists of the same grade and species that are spaced 16-inches o.c. It is important to realize that the way you select and use materials will control costs and performance. Many factors influence how a system responds to loading. For example, Appendix A indicates that one type of clay tile roof system has a live load value of 20 psf and a dead load value of 15 psf. AWC’s Appendix A lists a variety of live and dead load combinations for floors, ceilings and rafters. Simply reference Span Tables for Joists and Rafters published by the American Wood Council (AWC). But for most cases there is a cookbook solution. For a floor system you can find the individual weights of drywall, strapping, floor joists, subfloor, underlayment and carpet in an architectural handbook like Architectural Graphic Standards. To determine the dead load value for a given floor or roof system, the weight of all permanently installed materials in a given component are added together. Unlike stiffness, live loads and dead loads are added together to determine minimum design values for strength. Joists, and rafters must be strong enough not to break when loaded. Strength of a material is obviously important.
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