I know that in the NDS it says that I can use this factor for joists, rafters etc. where the members are 24" or less on center. I just don't recall it saying specifically that it can be used on members attached to one another.
I think you're all missing the point. The repetitive member factor applies when there are other adjacent members in close proximity that are capable of sharing the load because there is a load distributing element (usually decking) between them. If one becomes slightly overloaded, there are adjacent members to help share the load. This would not apply to a built-up, multi-ply beam as it is being designed as one member, and there are no other adjacent members to pick up any overload.
spats, Fastened together applies to 2x to 4x members
Per the NDS (1997 commentary) section 4.3.4-Repetitive Member Factor "...The adjustment recommended in ASTM D245 (18), is based on the increased in load-carrying capacity and stiffness obtained when multiple framing members are fastened together or appropriately joined by transverse load distributing elements..."
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
Woodman88: Learn something new every day, but I still don't buy it! So if I decide to use two 1 3/4" LVLs instead of one 3 1/2" PSL for a beam, I can take a 15% increase on the LVLs! Where is the load sharing/redistribution of load among framing members? The Commentary says the increase has been demonstrated by both analysis and testing. How would you demonstate that by analysis? Sounds like bunk to me.
The repetitive member factor is used to adjust the allowable design stress for the member you are designing. The allowable design stresses for wood are based on the 95th percentile of actual load capacity. So for every 100 joists loaded, 95 will have an actual strength greater than the design strength. When designing a single member, there is a probability that the member will fall into the lower 5th percentile. When you have 3 or more members that can share load, what is the likelihood that all three will fall into the lower 5th percentile? Through physical testing, a 15% strength increase was calculated to account for this probability.
Based on this premise, as long as the load can be shared among 3 or more members, the repetitive member factor can apply. Whether the 3 members are attached to each other or spaced up to 24", the members still share the load. A 3-2x10 beam has the same allowable bending stress as a 2x10 joist. The capacity is different because it is bigger, but the basic allowable stress does not change.
Note that this factor applies to sawn lumber only. Engineered Lumber does not allow increases to be taken because it is already factored into the design. Also, being an engineered product results in a smaller coefficient of variation for design stresses.
Thanks jaypartain! Now that's an explaination I can buy into... not the lame explaination in the Commentary: "It reflects two interactions: load-sharing or redistribution of load among framing members and partial composite action of the framing member and the covering material."
To be quite honest (I don't do a bunch of wood design), I wasn't aware that the repetitive member factor only applied to dimension lumber. It's curious that when I select Beam - LVL n-ply in Woodworks Sizer (v6.4), it activates and selects the repetitive member check box! It also does it for Lumber n-ply, regardless of the number of plies... kind of a fake-out.
