Hip roof rafter span table3/16/2024 In other words, how much a joist or rafter bends under the maximum expected load. Stiffness of structural members is limited by maximum allowable deflection. Perhaps the joists were strong enough if they didn’t break! But lack of stiffness leads to costly problems. For example, first-floor ceiling plaster would crack as occupants walked across a second-floor bedroom that was framed with bouncy floor joists. Strength and stiffness are equally important. Beams, studs, joists and rafters act as a structural skeleton and must be strong enough and stiff enough to resist these loads. The house acts as a structural system resisting dead loads (weight of materials), live loads (weights imposed by use and occupancy), like snow loads and wind loads. This article will focus on how simple beams like joists and rafters react to loading. If, when the loads of the house are combined, the house weighs more than the soil can support – the house will sink until it reaches a point at which the soil can support the load. Remember when your science teacher said: every action has an opposite and equal reaction? Well every building load has an equal “reaction load”. The structural goal of a house is to safely transfer building loads (weights) through the foundation to the supporting soil. A complete analysis of wood’s mechanical properties is complex, but understanding a few basics of lumber strength will allow you to size joists and rafters with the use of span tables. Wood is naturally engineered to serve as a structural material: The stem of a tree is fastened to the earth at its base (foundation), supports the weight of its branches (column) and bends as it is loaded by the wind (cantilever beam). Using span tables to size joists and rafters is a straight-forward process when you understand the structural principles that govern their use. Some information contained in it may be outdated. Neither SFPA, nor its members, have knowledge of the quality of materials, workmanship or construction methods used on any construction project and, accordingly, do not warrant the technical data, design, or performance of the lumber in completed structures.Please note: This older article by our former faculty member remains available on our site for archival purposes. The conditions under which lumber is used in construction may vary widely, as does the quality of workmanship. The design values contained herein are based on the Southern Pine Inspection Bureau’s Standard Grading Rules for Southern Pine Lumber (2002 edition with supplements) and modified as required by AWC’s National Design Specification® (NDS®) for Wood Construction (2012 edition).Īccordingly, SFPA does not warrant that the design values on which the span tables for Southern Pine lumber contained herein are based are correct, and specifically disclaims any liability for injury or damage resulting from the use of such span tables. SFPA does not grade or test lumber, and accordingly, does not assign design values to Southern Pine lumber. The maximum spans provided herein were determined on the same basis as those in the code-recognized Span Tables for Joists and Rafters(2012 edition) and Wood Structural Design Data (1986 with 1992 revisions), both published by the American Wood Council (AWC). The primary purpose of this publication is to provide a convenient reference for joist and rafter spans for specific grades of Southern Pine lumber. Forest Products Machinery & Equipment EXPO.Headers & Beams : Allowable Load Tables.Headers & Beams : Size Selection Tables.
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