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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: Michael Davis <michael.davis(--nospam--at)ferc.fed.us>
- Date: Tue, 28 Jul 1998 13:46:39 -0400
"Alex C. Nacionales" <anacio(--nospam--at)skyinet.net> wrote: >The big difference IMHO between USD and WSD in concrete is that in >USD the design assures that in case of failure of an RC member, a >beam for example the steel will fail first by yielding which will not be >abrupt unlike concrete failure. Max steel percentage allowed in USD is >75% of steel required for Balanced condition as in WSD (concrete and >steel fails simultaneously). Actually, the WSD does not result in the steel and concrete failing simultaneously even though they may reach their allowable stresses simultaneously. That is why the Strength Design Method (SDM) was adopted in the first place. They recognized that the behavior of reinforced concrete beams did not follow Hooke's Law beyond the working stress levels, and that additional capacity could be obtained above these levels that are not accounted for using the WSD. According to Wang and Salmon, even sections over-reinforced using the WSD have less tension steel than 75% of the amount in the balanced strain condition defined by the SDM. They also state that the ideally reinforced section using WSD is approximately one-half of the maximum permitted by the SDM. Therefore, sections designed using the WSD, even nominally over-reinforced, will still fail by yielding of the steel. Michael S. Davis, P.E.
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