question the validity of adding straps to the bottom. We talk about
deformation compatibility often when we discuss seismic, what about the
straps? How much will the GLB need to deform before the nails or screws
engage the strap? However, that doesn't solve your problem. I also
second Bill's comment. Make sure you are using the correct allowable
stresses. If necessary, you may even be able to regrade the dimensional
lumber to improve your capacity.
solution I used recently was to add kickers to the columns and reinforce the
columns for bending. You will have unbalanced loads, so you get moments
in the columns. The kickers can shorten the span of the GLB's. Be
careful, you create tension stress in the top and most simple-span GLB's have
only half the tension capacity in the top as compared to the bottom. You
might also try creating moment continuity with the short GLB along with adding
kickers to the columns. But by the time you do all that, Patrick's idea
of a new frame may make sense.
Madden, SE [mailto:gmadden(--nospam--at)maddengine.com]
Sent: Tuesday, October
28, 2003 4:48
Subject: Glulam Roof Girders in a
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)???