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RE: Deflection Crteria[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaoc(--nospam--at)seaoc.org>
- Subject: RE: Deflection Crteria
- From: "Mark A. Scott" <MAScott(--nospam--at)SGN-Eurisys.com>
- Date: Mon, 20 Jul 1998 10:15:16 -0700
The values of L/360 and so on are recommended minimums. I recently experienced the review from hell regarding deflection control. The building was a composite concrete deck on steel, design/build contract office building. The designer followed code allowables for sress and deflection, but did not ponder the recommended guidelines for span to deth ratios. The building utilized 50ksi yield steel and minimum steel sections. The resulting 34 to 37 foot beam spans with 16 inch deep steel sections . The great flexibility of this system led to the overfilling of the deck which resulted in a poor performing structure with reduced live load capacities. Additionally, the high beam end rotations and the resulting cracks in the concrete deck at the face of the girders resulted in poor performance of the girder composite action. Even though the design live load deflections met some initial criteria, the actual live load deflections at mid span resulting from beam/girder deflections resulted in partitions pulling out of their channel restraints attached to the suspended cieling, deflection induced cracking and separation of wall panel systems and roller coaster like deflections visible in the long corridors. Structure performance requires more than just evaluating to a standard. It involves evaluating how the magnitude of the deflections will be handled by the architectural components and the motion perception of those sitting at the desk on a bouncy floor. Serviceability is the key to acceptable performance and that is a function of perception not code performance. Mark A. Scott, SE, Washington St.
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