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AW: Boring through GLBs

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Hi Donald,


I would say, a hole of this size is not a real problem, wherever (as long as it is not at the support point). Usually a hole in the center of the cross section is more likely to become a problem, if it gets nearer to the support point, because you have a bigger problem with the shear forces, at such a hole, than with the bending moments. So if it gets nearer to the point of max moment, the problem gets smaller, as long as you don’t take half of the cross section away. Naturally, the size of these holes is stringently limited (at least in Germany), because bigger holes cause also bigger problems with the bending moments (in addition to the shear problems).






Von: Donald Bruckman [mailto:bruckmandesign(--nospam--at)]
Gesendet: Samstag, 8. Dezember 2007 18:05
An: seaint(--nospam--at)
eff: Boring through GLBs


I have a building I designed being constructed here in SoCal which has a clear-spanned GLB floor girder system about 35 feet long with 36” deep beams.  A few weeks ago I got a call from the GC telling me that the electrician, for reasons known only to the electrician, drilled a single 1 ¼” diameter hole through the GLB to fish some conduit through to an elevator room.  After quizzing the GC about where the holes were, I found that they were bored about 10 feet from the point of support and near the center of the depth of the beam. 


I was a bit miffed that it happened, but figured a hole that small in the center 1/3 of the depth of the beam away from the area of max moment wasn’t going to be a big deal. 


Well, my engineer was extremely upset about it and threatened to make them replace the beam. He finally agreed to approve it but I could not get any impression out of him whether it really was a big deal structurally or if he was just putting on a show to teach the GC a well deserved lesson.  Meanwhile, the building inspector gets into the act and is asking for a letter from the manufacturer approving the holes, an idea I found very odd.


My question:   What’s going on here?  What do I not know, after all these years in the biz, about holes in GLBs?  From a reduced section standpoint, away from the point of max moment and near the center of depth, such a small hole seems relatively unimportant to me. Am I missing something?