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Re: ASD vs. LRFD

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>I'll give you a few examples of situations when "LRFD is better."
Appreciate your taking the time to provide a couple of 'f'rinstances. I 
think that's a first. I do have a couple of comments though. (I realize 
I'm sticking my neck way out. Before the LRFD thread started I'd always 
figured keeping well clear of building design was only a good career 
move. Now I'm wondering if it might be a matter for prayerful thanks. ;-> 
)

I'll agree that seismic loading is a gray area, and that plastic response 
often needs to be assessed. I daresay playing around at the margins of  
real honest to god non-linear dynamic analysis isn't going to help much. 
Not unless someone chooses to risk his life and publically propose time 
history seismic analysis. So we're left with response spectra methods and 
a sort of quasi-linear approach. Like we've always used. 

The wind loading is puzzling because it seems like you're just playing 
around with the ratio of lateral load to dead load, neither of which 
should be affected by the method of analysis. Hurricane Andrew blew just 
as hard for structures designed by limit methods and for elastic 
analysis. Or have I missed something. Seems to me that I should be 
designing against the same thing with linear elastic methods, simply by 
using the same proportion of lateral to dead load.

And the comment about service loads really doesn't address 'better' as 
much as 'different.' seems like you could accomplich the same thing by 
doing the design based on service loading and then checking for collapse 
mechanisms afterward, rather than figuring collapse mechanisms first and 
then doing the deflections. Besides the very real cumulative effects of 
service loading, my guts are telling me that we're still using all the 
linear asumptions (notably superposition) and pretending that they hold 
for non-linear response. 

I'm a little reluctant to get more and more involved in this since 
service loads govern the stuff I do, and it'll be ever thus. Moreover the 
problems I hear about on this list seem service based--very few 
discussions involving ultimate collapse. I wonder if my uncle felt the 
same way when he came back from France in 1919 to find a popular majority 
had somehow passed a law so wildly unpopular (and culturally irrelevant) 
as prohibition.

Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw