Gluelams (or other engineered beams like LVL) distribute stresses a bit differently than I-joists do. With I-joists, so much of the moment of inertia is concentrated at the flange that you can get away with a lot of holes in the web without impacting the strength of the beam by more than 2-3%. Since the cross-section of the gluelams are straight, you don't get the clear-cut flange/web, and a comparable hole will have a larger impact on the strength of the overall beam. Also, generally, these beams are used in situations where the shear stresses are significant, in which case little or no drilling may be acceptable. If it's a standard application (simple floor joist or floor beam) then the manufacturer's specifiers guide will have all the guidelines. (There are differences between the manufacturers, and between manufacturer's product lines- grade of 2x used, type of ply, thickness of ply, etc, so you have to be careful here.) But you really need to check with the engineer- if the member is borderline in strength for the application, the % reduction in strength caused by your hole could cause it to fail or to bounce uncomfortably.
Same goes for 2x members. Dead center in the beam is almost always the best place to drill as it has the least impact on a simply loaded joist. Stress for cantilevers and for continuous spans over a bearing wall is a special case and NOT centered mid-span, but 1/3 span, with a disproportionate amount of the weight of the floor borne by the mid-span support. So, if you've got a large floor held up by 2x4 studs in the middle, please don't go boring 2" holes through them! (Center-boring a 2x4 with a 2" hole decreases buckling strength by 20% and cuts bearing capacity by more than half- though once the drywall/sheathing is up, there is a composite action and the decrease in strength is less pronounced.)
Joists and rafters (and other simple beams) gain strength by the cube of their height, but lose strength by the 4th power of their length- as most people here know, the top is in compression and the bottom in tension. The middle of the beam is (generally) neutrally loaded. Drilling a 1.5" hole in the center of a 12" LVL will reduce the bending strength by less than 1%; drilling a 1.5" hole at the edge of that same reduces the bending strength by 40%! (Even more if it was a square-cut notch and not a round bore) In both cases, shear strength is reduced by about 12%. Of course, this is assuming a simple beam with continuous loading; the engineer will be easily able to point to the beam and tell you exactly where you can drill without causing problems.
[This message has been edited by SteveFehr (edited 06-12-2006).]
