Repetitive Member Factor Explained:
Once plywood is laid on this roof, Cr will be applicable
Similar to the idea that to break one stick is easier than breaking a bundle of sticks (remember that childhood fable), the sawn lumber repetitive member factor (Cr) is used in cases where multiple sawn lumber members act compositely to properly distribute a load amongst themselves. The wood members must be closely spaced, and connected by either a sheathing or a decking (e.g. roof, outdoor deck, wooden stage) so that one member cannot cause a single failure point, but rather the system would have to fail. The idea is that if one member were to become overloaded, other parallel members would come into play.
This is the opposite of a wood post and beam structure where one member could be able to exceed capacity without properly distributing the loads to other nearby (or not so nearby) parallel members.
Note: If the wood does not meet the conditions to be considered a repeating member, then this factor can be ignored, or taken as unity (1.0).
Repetitive Member Factors [1]:
If all of the following are true:
- The lumber is 2-4" (5.1-10.2cm) thick.
- Spaced no more than 24" (61cm) on center.
- No less than three parallel members acting in the system.
- All members must be joined together by some load distributing element (plywood, sheathing, flooring, roofing, etc.) in order to distribute the load amongst the various members.
Then a 15% increase in the capacity of the members can be used (Cr = 1.15)
Non Sawn-Lumber Systems:
The repetitive member factor is used two additional times aside from the above sawn lumber application. Those instances are briefly explained here:
- For Wood I-joists the repetitive factor is present, but when used Cr = 1.0. Therefore in my own opinion it can be ignored.
- For Structural Composite Lumber, when the above conditions are true (#1-#4), then Cr = 1.04.
References:
- American Forest and Paper Association, "National Design Specification for Wood Construction", 2005