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Re: Pushover

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> The end reactions of a 'failed piece' remain 'locked' at the value at
> which the piece failed. What gets redistributed is the subsequent
>   incremental load.
Sounds like the ANSYS EKILL feature. That's one way of doing it, there 
are several others. You can get reasonable results using bilinear 
materials. Done properly it's probably a better simulation of reality, 
since members don't suddenly pop free diring a collapse until the 
deformations become enomous.

>Continue increasing the load and altering the model until you get a 
>progressive collapse. 
This takes a very comprehensive FEA capability involving multiple 
restarts. ANSYS can do this, but trying to figure our where to stop or 
set up APDL logic to stop the analysis isn't really a practical way to do 
analysis. It's doable, but not the sort of thing you'd do routinely, in 
any event. 
> I don't think a 'single nonlinear model of everything' is possible because
> non-linearity  implies iteration. 
It's done with ANSYS, LS-DYNA  and ABAQUS (to name only 3) every day. 
It's not for the newbie and requires a solid understanding of the physics 
involved, but that's the way non-linearities are done these days.

>'Locked' end reactions take care of cumulative effects.
It does for the elastic/perfectly plastic assumption, but it's a lot more 
straight-forward with bi-linear material assumptions.

>>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. 

>Luckily "debris falling non-linearity" isn't very popular yet. 
I hope someone lets all the buildings know. I'm sure they wouldn't want 
to behave in an unpopular fashion ;-> 

> Moreover, in pushover analysis terms, a 'failed piece' 
> isn't a disintegrated piece but one with zero or nearly zero value of 
tangent modulus. 
Hence the utility of a bilinear material assumption.

>Unlike steel, assumption of bi-linear stress-strain curve for concrete is 
>not valid so you can't get away with 'appropriately buggered stress levels'. 
That's how it seems to be done, though.

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

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