Old Wood Truss Failure
Old Wood Truss Failure
(OP)
I've got a task to study some old (1940's) wood trusses which have been undergoing various levels of distress for the last 20 years or so.
The trusses are bowstring (bottom chord flat/level) spanning 160' with a depth of about 20' at midspan. They are made up of 2-6x12 bottom chords and two similar sized glu-lams at the top chord. The webs are 6" nominal that extend between the chords.
What is happening is the 6 splices that occur across the bottom chord have been failing. These are made up of about 6 feet length of lapped members which are through-bolted with 4" diameter shear plates at each bolt/interface.
It appears that the wood isn't checked or split generally (except for the splits where it failed) but it looked quite dry. No sign of rot. These trusses are about 28 feet above a concrete floor within a building that is very long.
Does wood tend to loose strength over time? This is in a non-air conditioned space so it probably gets a little hot in the summer (midwest US). I know all about duration of load factors and moisture strength loss factors. What I was wondering was, does wood simply get dried out and "brittle" or something after 60 years?
The trusses are bowstring (bottom chord flat/level) spanning 160' with a depth of about 20' at midspan. They are made up of 2-6x12 bottom chords and two similar sized glu-lams at the top chord. The webs are 6" nominal that extend between the chords.
What is happening is the 6 splices that occur across the bottom chord have been failing. These are made up of about 6 feet length of lapped members which are through-bolted with 4" diameter shear plates at each bolt/interface.
It appears that the wood isn't checked or split generally (except for the splits where it failed) but it looked quite dry. No sign of rot. These trusses are about 28 feet above a concrete floor within a building that is very long.
Does wood tend to loose strength over time? This is in a non-air conditioned space so it probably gets a little hot in the summer (midwest US). I know all about duration of load factors and moisture strength loss factors. What I was wondering was, does wood simply get dried out and "brittle" or something after 60 years?





RE: Old Wood Truss Failure
Likewise, the same book states, "In relatively dry and moderate temperature conditions where wood is protected from deteriorating influences such as decay, the mechanical properties of wood show very little change with time."
Was there anything unusually heavy hanging or hung from the bottom cords at the splices? What's the condition of the shear plates and bolts?
RE: Old Wood Truss Failure
Visually, the wood just looked very dry and dusty and appeared (visually anyway) to be less dense than I would have expected.
No acid that I'm aware of as the trusses are 28' in the air and the entire facility was used in the 40's to build aircraft. Currently a fitness center.
No unusual loading either....there are at least 50 other trusses that are OK.
RE: Old Wood Truss Failure
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RE: Old Wood Truss Failure
RE: Old Wood Truss Failure
I am working on an investigative report for a bowstring truss with a bottom chord failure. The failure initiated at a knot. The building is in Chicago. The truss actually failed during the summer when no snow loads were present. The only documented loads were moderate winds during the week that the truss failed.
If you find any good information on deterioration of protected wood over time, please let me know.
Thanks!
RE: Old Wood Truss Failure
We found that a number of the bottom chord splices were split in areas originating at the bolts (split ring connectors). Our calculations showed that the ring connector capacities were less than the chord forces that the original engineer showed on the plans.
The most unique issue, though, was that there were only two areas (of about 6 trusses each) that had significant problems. These were discrete areas along a very long building. We kept asking "why just in these areas - the conditions are the same throughout".....then we noted that these two banks of trusses were actually supported at one end by a steel support truss spanning over two side doors (each 100 feet wide). Apparently, the side (lintel) truss was more flexible than the other supports (concrete columns at each truss) and this may have been the straw that broke the camels back.
Our repair: we didn't use the post-tensioning rods shown in the article, but did do, basically, the same thing as we added steel brackets to the bottom chord along the length and attached steel tube sections, one on each side of the bottom chord, to ensure adequate strength should a splice fracture. We could live with a little extra deflection in this roof, but had to have a safe roof. Post-tensioning was determined to be too difficult as access was very limited at each end of the trusses.
thanks again for the reference.
RE: Old Wood Truss Failure
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RE: Old Wood Truss Failure
About 10 years ago I investigated a similar truss failure in Richmond, California, where all of the trusses failed in a ripple effect. The trusses were approximately 50 years old and had been badly abused - ie: hanging stuff off of the bottom chords, non-engineered repairs to various members etc. I believe the final fix was steel tension rods. This particular design had been done by an architect and had lots of eccentric joints. He was still alive and verbally defending his design.
RE: Old Wood Truss Failure