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Re: ASD vs. LRFD

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> The discussion began with the question of what makes LRFD any better or
> more useful than ASD. So far we've been wave-of-the-future-ed to death; I

Although seemingly tangential to the question of whether LRFD is better, if
someone can show that LRFD will definitely replace ASD, then the question
becomes much less interesting unless someone has a plan to preserve ASD and
hopes to use the comparison of the two as a spring-board for this
preservation effort.

> recollect one comment about uniform design factors world-wide, several
> posts pointed out that the arithmetic effort is little different and one
> said that LRFD is more correct, although it's hard to imagine how this
> could be true given the assumptions involved in going from a true
> non-linear, large deformation analysis to something practicable for
> design office use. So let's have it--why is LRFD better? One thing does
> spring to mind: have I missed any comments about superior service
> performance? Is there any such experience?

Since you don't seem to place much credence in the defenses so far, how about
this one... since LRFD factors LL more heavily than DL, designs with large LL
compared to DL should come out somewhat more heavily designed in LRFD
compared to ASD, which just lumps LL and DL together into a service load.
This could be expected to result in lower floor vibration in an LRFD building
compared to an ASD building.  So, in this case, I would argue that LRFD would
result in better performance and happier tenants.  The predictability of LL
and DL are different, so in general I think that you would get a better
performing design by treating them differently rather than lumping them
together, and placing some extra strength where it is needed, and not where
it is not required.  The weight of a beam in plf is well established and not
too variable; the same is true for a metal deck floor.  Applying 100psf for
an area of public assembly or 125psf for light storage can get a bit sketchy
(especially when a tenant will not know the difference between "light" and
"heavy" storage).

> >If you
> >think that ASD should live on, then action needs to be taken to turn the
> >academics and code writer soon, or for better or worse it will "starve."
> There's no question that ASD will live on, if only in the Nuclear Codes
> and lifting equipment design codes, simply because the design of such
> things is governed by service loading.

These codes will necessarily be ASD in the future because they are ASD now?
A load factor could not be found for nuclear or lifting equipment?  While I
have no idea whether these codes will ever change and don't care to
speculate, I'm not sure that I agree with the premise behind the statement.

Paul Crocker