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RE: LRFD, ASD, and USD

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"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.