Built Up Wood Beam - Beam Stability Factor
Built Up Wood Beam - Beam Stability Factor
(OP)
Given a 3-ply sawn lumber bending member (face width oriented for strong axis bending), what width value, "b", would you use when evaluating the Beam Stability Factor, CL, in NDS 2015 3.3.3?
For example: Let's say you have a 3-ply 2x10 bending member. Ignore the possibility that you may have continuous lateral restraint or that you may have something preventing rotation at the member's ends (so CL doesn't default to 1.0). Do you evaluate the CL factor using b=1.5" for one 2x10 and multiply your design capacity by 3? Or rather, do you take b=4.5" and determine the bending capacity of the overall 3-2x10?
Thanks, team.
-Cody
For example: Let's say you have a 3-ply 2x10 bending member. Ignore the possibility that you may have continuous lateral restraint or that you may have something preventing rotation at the member's ends (so CL doesn't default to 1.0). Do you evaluate the CL factor using b=1.5" for one 2x10 and multiply your design capacity by 3? Or rather, do you take b=4.5" and determine the bending capacity of the overall 3-2x10?
Thanks, team.
-Cody






RE: Built Up Wood Beam - Beam Stability Factor
I might have missed something but if I assume 'b' is 1.5 and work out the bending capacity of a single 2x10 and multiply by three, I arrive at he same answer as using 'b' as 4.5 and working the three beams has one.
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: Built Up Wood Beam - Beam Stability Factor
RE: Built Up Wood Beam - Beam Stability Factor
RE: Built Up Wood Beam - Beam Stability Factor
Desert fox - im not sure thats true. You get a greatly reduced CL for a single 2x10 than you would for all 3 together. CL reduces the allowable stress. The 3-ply will have a higher allowable stress than three single plies due to reduced LTB.
You also get the 15% repetitive member factor for 3-ply members and greater but thats separate from CL.
RE: Built Up Wood Beam - Beam Stability Factor
I calculated the 'I' value of the beam based on a single beam with a width of 1.5" x 10" and multipling by 3 and then on a beam having a width of 4.5" x 10" using the standard formula b*d^3/12 and found no difference, I don't have access to the standard mentioned so I guess I could easily have misunderstood the OP post.
regards
desertfox
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: Built Up Wood Beam - Beam Stability Factor
BA
RE: Built Up Wood Beam - Beam Stability Factor
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Built Up Wood Beam - Beam Stability Factor
I can't find a thesis or anything to back up my memory of the discussion, but he mentioned that there was generally too much slop and crushing at the fasteners to develop full composite action. Instead, you got an intermediate state -- not fully composite, but not just 3x individual capacity either.
I'd imagine that in the real world, it falls into the category of "usually works, but for different reasons"
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The name is a long story -- just call me Lo.
RE: Built Up Wood Beam - Beam Stability Factor
Sure, it's an intermediate state with any built up composite section no matter the material. Heck, it's really an intermediate state even in solid sections once the effect of shear deformation is considered. But sure, if somebody's put pen to paper on this, then I'll happily defer to it for the global strength and/or stiffness case. I think that it's worth noting that fastener demand for stability concerns are an order of magnitude less than they are for global strength and stiffness however. If our 2% steel numbers are to be believed, perhaps closer to two orders of magnitude. Find that thesis man!
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Built Up Wood Beam - Beam Stability Factor
Desertfox: For purposes of bending stress checks using basic mechanics, you're correct in your approach. However, the National Design Specification for Wood (NDS) requires application of several adjustment factors. One of these is called the "Beam Stability Factor" (CL), which is dependent upon member depth-to-breadth ratio as well as some other considerations like support conditions, compression edge restraint, etc.. I think the CL factor ultimately boils down to mechanics with a lot of curve-fit equations to cover up some real-life behavior.
Regarding the fastener requirements to reach composite behavior in bending: I'm not sure how I'd go about really calculating the requirements and I'm not sure I'd want to, either (like KootK). But to just say minimum fastening is sufficient seems a little sketchy: the built-up compression member fastening requirements per NDS2015 Section 15.3 get pretty intricate to the point where I'd be concerned that workers won't really achieve the proper nailing/details/construction. The caveat, though, is that's specifically for column-type compression members and we're discussing bending... But this CL factor does carry over into the combined bending and compression checks.
I think for most applications the difference in capacity isn't going to be a game changer. I'll probably determine capacity based on individual ply capacity, multiplied by "n" number of plies as well as the repetitive member adjustment factor mentioned by Jerehmy (thanks! forgot about that little guy).
RE: Built Up Wood Beam - Beam Stability Factor
RE: Built Up Wood Beam - Beam Stability Factor
RE: Built Up Wood Beam - Beam Stability Factor
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Built Up Wood Beam - Beam Stability Factor
RE: Built Up Wood Beam - Beam Stability Factor
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
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RE: Built Up Wood Beam - Beam Stability Factor
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Built Up Wood Beam - Beam Stability Factor
RE: Built Up Wood Beam - Beam Stability Factor
1. In Woodworks, the default setting is to calculate the KL (Lateral Stability) factor based upon a single ply, for multiply beams that are laterally unsupported. If your engineering judgement dictates otherwise, there is a menu option to toggle this assumption off.
2. Excerpt from WoodWorks help file for multiply beams: "6. For lateral stability calculations for the lateral stability factor KL in CSA O86 7.5.6.4.4, Sizer offers you the a Design Setting choice of whether to use the full member width or the single ply width as b in the expression for the slenderness ratio and in the limiting ratios for which the stability factor can be assumed to be 1.0. Research has shown that nailed and bolted beams have at most 30% composite action effect in terms of resisting torsional buckling, so. It is recommended to use single ply width unless adhesives are used to laminate the members together."
3. Keep in mind that this discussion mostly disappears if you have lateral support, which is more often the case.
4. There are provisions in CSA-O86-14, cl. 7.5.6.3 and cl. 7.5.6.4 that are pertinent to the discussion as they provide ways to consider a multiply beam as a single unit for calculating the lateral stability factor.
More digging needed to identify the source research material.
RE: Built Up Wood Beam - Beam Stability Factor
I do that too when it works. I was just reviewing a competitors detail yesterday where there was much ado about how lamination was accomplished. The plies had to be separated by packing etc rather than tight together. Clearly gunning for composite stiffness.
30%? How disappointing. I was in the wood design industry before I got my formal engineering education. One of the first neat things we hit in mechanics of materials was how to fasten a wood box beam together based on VQ/It etc. I remember thinking that I couldn't wait to apply it in the wild. I'd not have been so enthusiastic if I'd known it was a bunch of hooey.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Built Up Wood Beam - Beam Stability Factor
RE: Built Up Wood Beam - Beam Stability Factor
1. I often see built-up “beams” of the type described by Oldestguy. That is, “[a] builder using several 10 ft. planks for a built up beam measuring 18 feet long, with a few joints near the middle.” I frequently see them as header beams above garage openings in residential construction (not on my projects). I can't see how it would work and yet, that's how LVL beams are constructed, though, granted, not nailed together. But what say ye esteemed folk?
2. In the past I’ve seen existing lumber joists strengthened by installing a 2x6 flange at the bottom of them, gluing and screwing them to the existing (designed using VQ/It, as KootK noted) making inverted T-joists. Per the research referred to by Timberworks it sounds like gluing makes the difference. But what say ye esteemed folk?
RE: Built Up Wood Beam - Beam Stability Factor
1a. I cannot count the times I have come across multiply wood beams with incorrect splice locations.
1b. The prescriptive code I work under (OBC) allows a splice within 6" of the quarter points of the spans.
1c. If I need to splice, I generally adopt an approach where, if a 3 ply beam is required, than I spec a 4 ply beam and make sure I stagger the splice locations by a reasonable distance, plus include a nailing/bolting pattern (of course, then you get to site and find out the contractor did whatever they wanted and are surprised when you show them the drawings).
2. I have also increased the depth of multiply wood beams or full size timber beams by installing a 2x flange on the bottom. I specify the number of fasteners required by calcs, then throw in the glue as an extra.
RE: Built Up Wood Beam - Beam Stability Factor