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Re: ASD vs LRFD

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> From: James_F_Fulton(--nospam--at)RohmHaas.Com (James F Fulton)

> for strength. To what extent does deflection end up controlling over strength
> and a redesign then become necessary. Based on an AISC seminar I attended, it
> looks like deflection checks are not only more important than for ASD, but
> the calcs themselves were time consuming and more complicated than just using
> a simple deflection equation. (With ASD, a member size based on allowable
> stress most of the time met the deflection acceptance citeria). But I defer
> to those that are practitioners in LRFD. 

Gasp! Service loads in LRFD are the same as service loads in ASD and the
deflection equations are identical!

When you state that, "a member size based on allowable stress most of
the time met the deflection acceptance citeria", do you base this
statement on design experience using A36 material or do you imply that
this is true in ALL cases.

If this is true in ALL cases, then this implies that ASD may save you
some design time but costs the owner for extra steel supplied for stress
reasons that may not have been required! Somewhere, you have to trade
between design dollars and volume of steel.

Deflection analysis is not, "more important," in LRFD but may be
required more often. This is, as ever, a judgement call that the
experienced LRFD practitioner makes.

-- 
Paul Ransom, P. Eng.
Civil/Structural/Project
Burlington, Ontario, Canada
<ad026(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>