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Question DetailsAsked on 7/7/2016

SSS
How much cup is allowed in a 16 inch LVL?

I have been supplied with the second set of LVL, to be used for a 24 foot ridge beam, my local lumber yard says we should bolt it together to pull the cup out of it. I put 1/2 inch bolts every 12 inches and could not budge any cup out of it. I need to fasten 2x12 rafters with plum cuts to this beam, but I only have about a 1/2 inch of contact.

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You would have to look on the beam for the certification stamp - could be any one of the following, and each have different standards - usually available on their website or you could eMail their technical assistance department for specifics on your size beam:

APA - American Plywood Association, for plywood used in plywood joists


TJI - Timber Joist Institute - for plywood joists and mixed timber joists - plywood or OSB (shudder) web but solid wood flanges, or in a few manufacturers plywood/SOB web and built-up plywood flanges


SPB - Southern Pine Bureau, for solid wood laminated beams made of Southern Pine


AITC - American Institute of Timber Construction (or something real close to that, without looking it up) - for dimensional and solid-lumber built-up joists and beams except Southern Pine (though some of their tables include southern pine along with other junk woods)


CWC - Canadian Wood Council - for Canadian dimensional and built-up and veneer beams and joists


Unfortunately, the meaning of LVL has changed over the years, so to some people like me it means plywood truss joists, for others built-up beams made of laminated solid stock (not "veneer" at all - which people like me call glulam, for glued-laminated). For your use, sounds to me like this could be either depending on your selection of a system - likely plywood truss joist if TJI rafters, could be either if dimensional wood rafters but most likely plywood truss because unless carrying real load for some reason a ridge beam would not normally be glulam - usually just 2x8 or 2x10 dimensional lumber.


Since you could not pull it into shape, sounds like you have cupping of the web of a joist, not bowing of the entire beam, because reasonable bowing should pull out pretty easily.


Generally - and only generally - dimensions on any one element can vary 1/16" on thickness of plywood, 1/8" in any other dimension from "normal" dimensions for that size lumber. Out-of-plane can "generally" run 1/4 in the web of a joist, or 1/2" out on bowing or camber of the entire thing - but those standards vary by the rating group and sometimes by length, and also by the load rating of the beam as being out-of-true affects the strength of the member.


Normally, if such a beam was in place and bowed (say got wet during construction), reasonable lateral bowing would be taken out in fastening the rafters to it because normally a ridge beam has little lateral strength.


Cupping would be taken care of, normally, with dimensional lumber, by custom cutting the contacting end of each rafter to roughly fit up to the shape of the cupped web - then cut off the rafter tails (at the eave end) after all are in place to get a straight fascia surface. This is old-school where the rafters were designed to be about 2-4 inches shorter than the standard dimensional lumber length, so there was a lot of play in the rafter length without risking shorting the tails. If you pre-cut the rafters to length first, you don't really have that option unless you shorten the rafter tails to less than planned.


Of course, if you custom cut the rafter ends to fit, you have to be sure to keep the tops of the rafters in a flat plane, not inadvertantly drop any down out of the roofing plane because of the cut.


Strength-wise, for a conventional ridged roof, the cupping is inconsequential with respect to the ridge beam - because it is only a means to tie the ends of the rafters down so they stay opposed to each other on the two sides of the ridge and provide lateral support so the rafters stay at the correct spacing. The "beam" itself does not, in normal "bare ridge" construction, carry any vertical load at all because the rafter load at that point is all horizontal (from rafter against rafter), and carries only such lateral load as the rafters put on it before the sheathing is on because once the sheathing is on the sheathing nails will act as lateral bracing. Basically, once the roof is sheathed, the ridge beam only holds the ends of the rafters steady and prevents twisting of the beams as they dry out - the "structural strength" or load-carrying capacity of the roof is in the rafters carrying the load and opposing each other at the ridge - all the vertical load (for a true rafter roof without supporting posts under the ridge beam) carry down the length of the rafters to the walls (and any vertical support posts or cross-bracing or walls mid-span). In fact, in the old days, we just nailed 1x2's or 1x4's to the bottom edge of the rafters to hold them in place till the rafters were "married" - nailed into the opposing rafter from the other side of the roof. Sometimes these were left, sometimes after the sheathing or roofing boards were put on then the temporary tie pieces were removed, leaving bare rafters with the only other wood being sway-bracing 2x4's to carry longitudinal wind load and prevent the entire roof from pancaking like a stack of dominos. Some modern roofs still have the sway bracing at the ends, some use the joist hangers (if rated for twisting restraint) to prevent end-sway, particularly on roofs over fully living space attic where sway-bracing angled braces are not acceptable.


Unfortunately, if this has major cupping, you would have deal with the person at the lumber yard you talked to about a replacement - especially since it presumably now have 1/2" holes in it from the bolts, though he did tell you to do that. He may have been thinking you means lateral bowing, not cupping of the web. And of course you have the issue of when this was delivered - if you complained to him immediately upon delivery (would have been nice to have just rejected it upon delivery) you might have a good case - if he can argue it has been sitting out in the rain for days, your return/credit case is weaker.


Obviously, if you have an Architect or Structural Engineer on this job (doing the design ?) they can help look up the specific requirements or tolerances, and maybe help with some documentation to help your argument with the hyard - particularly if this is going to mean them having to send a boom truck to swap it out with another beam.

Answered 2 years ago by LCD




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