Friday, May 27, 2011

Repetitive Member Factor: An Introduction

An increase in the reliability of wood members when placed into assemblies with repetitive use components has been recognized for many years by the wood industry.  The term assembly is typically defined as 3 or more members spaced at 24” on center or less and attached with load distributing elements capable of supporting the design loads.  Examples include floors, walls, roofs and three-ply beams with load distributing elements of wood structural sheathing or transverse mechanical fasteners. 

Floors and walls are typically solid-sawn members attached with plywood or OSB sheathing.  Roofs, however, are solid-sawn members or prefabricated trusses that have the top chord and/or bottom chord connected with structural wood sheathing.  The sheathing creates load sharing between the members of the assembly.  Three-ply beams share load through the mechanical fastening system used to build the member. 

The capacity of the assembly benefits from an increase in lateral stability of the members, partial composite action where the load is distributed lengthwise along the member, and load sharing where the load is distributed across parallel members that have different stiffnesses.  The parallel members having different stiffnesses result in varying amounts of deflection under uniform loads.  The load distributing element reduces the effects of various deflections by distributing the load away from more flexible members to stiffer members.  Members that are more flexible tend to be weaker than stiffer members, resulting in load being distributed away from weaker members and into stronger members.

Load sharing is accounted for through the use of repetitive member factors.  Since 1968, the National Design Specification for Wood Construction (NDS) has specified a repetitive member factor of 1.15 for bending design values of dimension lumber.  Also, since 1970, ASTM D245 has recommended a load sharing factor of 1.15 for bending stresses in multiple member systems.

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