Repetitive Member Increase

Repetitive Member Factor for IBC

Many wood structures have a series of closely-spaced parallel members, i.e. joists in a floor, rafters in a roof and studs in a wall. For a problem to qualify as repetitive members it must have the following attributes as defined by NDS, section 4.3.9 Repetitive Member Factor:

There must be three or more parallel members.
Members are spaced not more than 24 inches on center.
Members are connected together by a load-distribution element such as roof, floor, or wall sheathing.

In this arrangement, the performance of the system does not depend solely on the capacity of an individual member. This can be contrasted to an engineered wood structure with relatively large structural members spaced a greater distance apart. The failure of one large member would essentially be a failure of the system. The repetitive member factor recognizes system performance. If one member should become overloaded, the load is distributed by sheathing to adjacent members which share the load.

This factor is applied to the bending stress capacity of wood members which means it is useful in increasing moment carrying capacity. No other capacity is affected by this factor. The published bending stress capacity of wood is based on the lowest performance of a given test group, i.e multiple pieces are tested and the bending value selected is below average. This is a conservative approach that prevents a low-performing wood member from being overloaded in moment when used as a main framing member. For repetitive members, the chance that a low performer is flanked by other low performers is low so the repetitive member factor allows us to increase the bending capacity of these side-by-side members.

For dimension lumber, NDS allows for a 15% increase in bending stress capacity. Weyerhaeuser's Microllam® LVL, TimberStrand® LSL and Parallam® PSL: are allowed a 4% increase. The TJI® joist series is not allowed this increase. Dimension lumber is subjected to great variation in its bending capacity due to natural imperfections such as knots and splits, and the subjective nature of visual grading performed by humans, so NDS allows for a greater gap between what is considered average and low. This variation becomes smaller for rectangular engineered lumber with much higher quality control during manufacturing thus requiring the smaller 4% correction. For the TJI® joist series, whose flanges are subjected to rigorous manufacturing and quality control regimes, the repetitive member increase factor isn't even used.

When is the Repetitive Member Factor Applied?

Forte software automatically applies this factor when the member type is joist and spacing is 24" on center or less. It applies to all building codes and to all structural systems (floor and roof). If you are designing a member type joist where the spacing is 24" o.c. or less but there is no sheathing or decking nailed to the top of the joists, and the product is not a TJI®, i.e. a ceiling system, please contact a Weyerhaeuser representative as the repetitive member increase is not applicable.

Note: this information is loosely based on the repetitive member factor section in the Design of Wood Structures -ASD, 5th edition, by Breyer, Fridley, Pollock and Cobeen.