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RE: LRFD, ASD, and USD[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: LRFD, ASD, and USD
- From: "Bill Allen, S.E." <Bill(--nospam--at)AllenDesigns.com>
- Date: Mon, 27 Jul 1998 20:22:53 -0700
I'll jump in here. USD in concrete certainly reduces the amount of reinforcing required and overall sections. However, IMO it is too easy to overlook service performance (deflections and cracking) for certain elements in certain conditions. Also, some engineers I have had experience with (admittedly, the younger ones) don't have a clue about the strain transformation as the load increases, first starting elastically, cracking and still elastic then into plastic deformation. I have reviewed some calculations of foundations (sized for soil pressure) for example and, even with factored loads, the strain on the section is way below 0.002. However, the engineer still used the USD formulae. Without having a good knowledge of this transformation or taking the time to check crack control, invariably a design could end up with too large of bars too far apart. Further, I feel that it is a "numbers game" unless one is designing a frame for example and using limit state design or plastic hinging analysis. Using factored loads and distributing them elastically is still too conservative when checking for an "ultimate" condition, whatever that is. It doesn't seem like too many engineers (especially us old geezers) are too "warm and fuzzy" about LRFD in steel, although I just read a study in Modern Steel Construction whereby one could save about 15-20% in tonnage when using Gr. 50 steel and LRFD vs. Gr. 36 steel and ASD. Regards, Bill Allen -----Original Message----- From: CarlS95(--nospam--at)aol.com [mailto:CarlS95(--nospam--at)aol.com] Sent: Monday, July 27, 1998 7:58 PM To: seaint(--nospam--at)seaint.org Subject: LRFD, ASD, and USD Christopher Wright wrote (concerning the distinction between LRFD and ASD): << It seems like a distinction without a difference. >> Good quote. I'm going to save this one. Would you say the same about Ultimate Strength Design for concrete? We have three decades of experience to go on. So, here's Question No. 1: Has Ultimate Strength Design made a difference? Thanks. Carl S.
- LRFD, ASD, and USD
- From: CarlS95
- LRFD, ASD, and USD
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