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RE: ConcreteOne Way Joist and Beam[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: ConcreteOne Way Joist and Beam
- From: William Keil <WJK(--nospam--at)brph.com>
- Date: Thu, 16 Dec 1999 08:04:06 -0500
My favorite reply to contractors is "Rip it out and do it correctly this time" which their response to that is something that I should not repeat. I then proceed to tell them that this is the last option. If the concrete "looks" different from the others then it probably is. So I would suggest analyzing the cylinders and cores to find any deleterious materials which may have made it into the subpar concrete. Odds are you will find something and if it is detrimental to concrete then the concrete has to be removed. If nothing odd is found then the possibility of post-tensioning and/or carbon fiber reinforcing could be considered to help the flexural stresses. Load testing would be my second to last option to consider ... which reminds me of a story. A large university was constructing an addition to their football stadium. The designers chose to straddle the existing construction with a cantilevered concrete structure. All went well. One of the parties decided to load test the structure to three times the design LL. My only guess is they did not trust that cantilever thingy or because some rowdy students/alumni might try to jump up and down during a game. Surprise, surprise! The concrete cantilever developed a hairline crack at the support. Of course it would, concrete does crack when subjected to loads higher than design loads; however, the structure does not collapse after it develops a hairline crack. Does not matter though, the university decides the structure is unsafe and must be post-tensioned to close the cracks and "prevent failure." This "fix" goes smoothly and opens on time to get the lucrative ticket revenue flowing in. Then, the university sues the original design engineer to recover the costs of having to "fix the deficient structure." No hard feelings, Mr. Engineer and former Board of Director member, it's just business. What's a few million dollars between friends? It is strange situations like the one above that makes me wonder ... are all of the safety factors we use enough? When I first started working, I was teamed up with some engineers that were about to retire (what other business would a wet-behind-the-ears graduate be teamed up with a professional with 45 years experience, nearly twice my age?). Their common method of design was to perform the calculations and determine whatever property they needed to select a member ... then double it. Or precisely calculate the load per the building code ... then triple it. Just something to think about before you make your decision. Hope this helps. Happy Holidays to all! BTW, does any one know something about this Y2K event? :o) Happy New Year! William J. Keil, P.E.
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