Return to index: [Subject] [Thread] [Date] [Author]

Re: The roof is still up there

[Subject Prev][Subject Next][Thread Prev][Thread Next]
With regard to roof failures and recalcitrant tenants:  About 10 years ago,
we got a call from a building manager that the tenant (a clothing store) had
noticed that the ceiling lights were closer to the checkout stand than they
used to be.  A quick investigation showed that the roof in the act of
collapsing.

Construction was bowstring trusses with laminated 1x10 chords and closely
spaced 1x4 diagonals for webs.  The top chord was braced by the joists
spanning between trusses, but near the parapet the roof plane separated from
the top chord so as to form a gutter sloping to the back of the building.
Near the front of the building, the unsupported length of the top chord had
buckled, and the truss was being held together by a few 6d nails and shear
in the bottom chord.

While we were up there, things gave a creak and dropped a few more inches.
We scrambled back down, told the store manager to close the store, and get
everyone out.  Similar to Roger Turk's experience, the manager resisted
strongly, and we had to threaten to call the building official that second.
We got a contractor out there with shoring in about 20 minutes, and then
designed repairs and strengthening.

The interesting thing is that the building was about 70 years old, the day
was dry with no snow or rain accumulation, the last time any new loads had
been added was 10 years previously with a new layer of roofing.  A
microscopic investigation of the wood showed no dry-rot or similar damage.

A careful analysis showed that from the day it was built, that portion of
the chords had a factor of safety against buckling only slightly more than
1.0.  The additional roofing took it down below unity.  It took 10 more
years for the wood to finally give up.

Any time a contractor tells me that the structure is standing there so it
must be okay, I haul out this story.  It works like a charm.
-- 
Kris P. Hamilton, P.E.
Geiger Engineers
1215 Cornwall Avenue
Bellingham, WA  98225
Ph: 360 734 7194
Fx: 360 734 7399