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- To: seaoc(--nospam--at)seaoc.org
- Subject: Failures
- From: BVeit <BVeit(--nospam--at)aol.com>
- Date: Wed, 3 Dec 1997 23:13:31 EST
I noticed a few previous posts referred to various bldg failures of some sort or another -- the posts of Kate O'Brien, Tim McCormick, and Drew Norman come to mind. I'd appreciate first hand accounts of failures that various engineers have witnessed. Drew's example of improperly braced trusses was particularly instructive. Perhaps those with a lot of experience can share it with regard to this topic. For example, I imagine City inspectors would be more likely to have seen failures and have a greater catalog to report. These might also spark some code debate. I'll start, for what it's worth: I did a repair job to a golf driving range in San Francisco. It's visible from 280. The existing class H1 wood utility poles had been designed to an ANSI standard for wood telephone poles to a height of approx. 70 feet. They support 1" mesh netting. The owner had extended the poles with sched. 40 pipe another 20 feet. During those great big winds of winter, 1995/96, one whole row snapped like matchsticks about ten feet up from the ground. Luckily no one was hurt, but nearby power lines and cars were damaged. I had been telling my clients that they should have snapped near the ground -- I was surprised that they broke at ten feet up, since I'd assume maximum moment was at the base. (It could be due to the taper of the poles.) And they all broke at this height, ruling out the odd check, void, or flaw. On the opposite side of the range, the poles were more sheltered from the wind, and no poles broke. You can imagine that the owner chose to replace only the side that had broken. I suppose failure was due to two main causes: winds in excess of design windspeed, and extension of the poles without regard to engineering. Due to the increased lever arm, this addition doubled the moment demand on the pole. In addition, wood has a brittle failure mode here, and is not a good choice for this application. The steel replacements have a reinforced concrete core up thirty feet -- I convinced the owner to design for a higher windspeed based on risk/return and cost/benefit considerations. But I still couldn't talk him into doing the other side.... Any more out there? Thanks, Brian Veit, P.E.
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