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- To: <seaint(--nospam--at)seaint.org>
- Subject: Glulam Roof Girders in a Tilt-Up
- From: "Gerard Madden, SE" <gmadden(--nospam--at)maddengine.com>
- Date: Tue, 28 Oct 2003 15:47:46 -0800
I’m working on everyone’s favorite type of project – New Rooftop Mechanical Units on an existing panelized wood roof on a 1 story tilt-up building. The system is comprised of 2x4 subpurlins @ 24” o.c., supported by 4x14 purlins @ 8’ o.c., supported by 25.5” deep glulams beams (simple span of 40 feet) at 24’ o.c., supported by glulam girders (a series of single cantilevered continuous beams). It creates 40’x45’ bays to columns.
I’ve checked the 4x purlins and glulams girders and have determined both are overstressed under just DL + LL without the new units. Accordingly, I am reinforcing all of the existing 4x purlins that have new units sitting on top of them.
For the glulam girders, it is a series (13 bays) of single cantilevered glulam beams with the cantilever of 10 feet and a backspan of 35 feet. Under pattern loading, the original design has some overstresses on the order of 25-30%. When I walked through the building, I noticed that someone along the line also came to the same conclusion I did and tried to reinforce the glulams. They placed simpson coil straps (or just a really long strap) on either side of the bottom of the glulams at the last lam. The strap is probably 20 feet long at about middle of the back span. The new units add about 4-8% more demand load to the glulams so the situation is getting worse.
Another odd thing they did (or maybe not so odd) was that the glulam beams spanning 40 feet frame in about 1.5 feet away from the column to the girders. This creates a huge shear stress in the glulam, but by taking advantage of the D from face of support clause, the glulam would not be overstressed.
Adding the straps (as the previous engineer did) helps the tension at the bottom, but does not increase the compression capacity at the top. It is difficult to strengthen the top due to the deep glulam beams that hang off the girders and would this interrupt any new side mounted reinforcing steel plates. Some of the overstress, I suspect, is that the glulams were design without the volume factor (Cv) for glulams – the original was designed under the 1976 UBC…. There’s a question here coming…
What are some solutions you would recommend or care to comment on. I was thinking adding more straps on the underside of the glulam for the tension increase, and at the top lam on either side. Another option would be to use a channel or lt. gage steel track lagged/screwed up to the bottom, and side mount channels to the top on either side just under the sheathing. How would you tackle the problem of an intersecting beam at the top every 24 feet and located near the highest stress points (over the support and at midspan)???
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