I had a
VERY similar situation happen to me regarding a
Governmental building framed with glu lam beams and wood decking. The
original designer made several design errors which were very obvious once the
calculations were made.
Several floor beams were
way oversized, but many were way undersized and sagging causing problems with
the second floor offices. The same beam was used everywhere regardless
of span or condition.
I came clean with the
client as quickly and diplomatically as I could. My firm was then hired
to make any and all corrections to the building.
I was asked to provide repair plans
and specifications for an existing roof damaged by a vehicle (sic). The
problem now became not only structural, but ethical, too.
The building is apparently a former
post office (1970s?), with 5 1/8"x 24-3/8"glulam beams @26' o.c.
cantilevering 16' off brick wall along one side. Further
into the building, the glulams also bear on wood columns, and then cantilever
and support other glulams spanning toward the opposite brick
Sawn purlins span about 26' between
the glulams at 8-to-9 feet on-center, supporting 2x4 @24", some insulation,
lots of electrical conduits and ducts, plywood diaphragm, and the composition
The "fascia" (still 6x12,
nominal/typical) purlin of the overhang was apparently hit and broken by the
u-turning big rig, with subsequent damage (delamination with lateral offset)
to one of the cantilevering glulams.
I ran the analysis based upon the
current codes as well as upon the 1960s and 1970s codes (UBC). The
analysis shows that the undamaged subpurlins, purlins, glulams, and columns
are inadequate against DL+LL (70-to-100% overstressed), and even against DL
only (with 0.9 factor, overstressed 15%).
For the sawn lumber, I tried even
select structural (dense select structural) - did not help...
Some of the purlins are visibly
sagging. The glulams look OK.
I can just replace the damaged
purlin (these are the only ones that are adequate
due to 50% tributary area) and to
repair only the damaged glulam to the "preexisting condition" (I can do it with lag screws etc.). After
all, the building may be OK solely by the fact of it existing like that for
about 30 years, right?
If I am right in my assumptions -
how that could happen (don't answer that)?
If I am wrong in my assumptions -
what am I missing?
What should I do within the
limitations of common sense, structural analysis, and professional
V. Steve Gordin, PhD