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RE: Pushover

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>Continue increasing the load and altering the model until you get a
>progressive collapse.
Actually you should do this with a single nonlinear model of everything. 
If you alter the model as you go along by removing pieces the cumulative 
effects don't accumulate properly because the results of a non-linear  is 
very much path dependent. You have to get the loading sequence right, 
too. And normally you can't simulate gravity loads with forces because 
the can't make the forces shift properly. Like if a floor were to 
collapse partly and dump all the furniture and ceiling tile and carpets 
and stuff onto a lower floor. Elastic-perfectly plastic assumptions won't 
do it either--work-hardening comes into play and makes members stay put 
longer. 

Not to open up old wounds, but this is one of my major beefs with LRFD. 
It isn't really a true limit analysis. You can't do a true limit analysis 
member by member--you have to do the whole thing at one time to get the 
interactions right. It isn't like a chain with only one weakest link. 

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



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