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Re: Pushover

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My understanding of pushover analysis differs slightly. Please correct me if I am wrong.

>Modify the model by removing or altering the failed piece to mimic what
>will theoretically happen which re-distributes the load to other members

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.


>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.

I don't think a 'single nonlinear model of everything' is possible because non-linearity
implies iteration.


>>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.

'Locked' end reactions take care of cumulative effects.

>>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 saw just
one paper on the subject which simulated a masonry structure using statics,
dynamics and kinematics. Moreover, in pushover analysis terms, a 'failed piece'
isn't a disintegrated piece but one with zero or nearly zero value of tangent
modulus.


>Do I design all my concrete buildings using allowable stresses then?
>>As near as I can tell from the little I know about concrete, it's just
>>like LRFD. You base the design on what amounts to elastic assumptions
>>with appropriately buggered stress levels (or stress levels converted to
>>equivalent loads, which you can only do with linear response). Then you
>>change some terminology, call it limit analysis and pretend you're doing
>>something new and techy.

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'.


Rudra Nevatia
Structural Engineers' eBook

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