P-Delta Effect

04-Unexpected behaviour/errors
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Roberto.tartaglia
Posts: 2
Joined: 25 Jan 2018, 12:12

P-Delta Effect

Post by Roberto.tartaglia » 25 Jan 2018, 14:26

Goodmorning everyone
I have a problem with a planar model of steel MRF .
In particular, in addition to the normal MRF structure, I introduced the leaning columns where I applied: the concentrated masses, the incremental forces (for a PushOver analysis) and the concentrated vertical forces (to take into account the gravity loads for the P-delta effects). The model workswell with the one exception for P-delta effects. In fact, there is no degradation structural capacity curve (Base shear vs top displacement), moreover also changing the level of the concentrated force, the results seem to be insensitive.

Do you have any idea what could be the modeling error? For an easier understanding of the problem, attached you will find the model

Thank you
Roberto

P.S. I have already set the geometric nonlinearity.

Dropbox Link:
https://www.dropbox.com/s/aougvq599c2a8 ... r.spf?dl=0

huffte
Posts: 838
Joined: 22 Jul 2011, 10:19
Location: Cookeville, Tennessee, USA
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Re: P-Delta Effect

Post by huffte » 25 Jan 2018, 16:33

Roberto, the first thing I note in your model is that there are no end moment releases in the beams. It appears to me that the far right frame is intended to be a moment frame with all other bays intended to be standard shear connections with no moment at the end of the beams.

This is just my assumption since you mention leaning columns.

If my assumption is correct, then you have modeled a much stiffer structure than intended by making all bays moment resisting frames instead of just the right-most bay. You could release the end moment for all beams not part of the moment frame. Given that this would be 5 of the 6 bays, this would be a dramatic decrease in overall stiffness and would render P-Delta effects much more significant.

This could be a starting place. But as I say, I may be misinterpreting the intention of the model. I hope it helps.
Tim Huff

Roberto.tartaglia
Posts: 2
Joined: 25 Jan 2018, 12:12

Re: P-Delta Effect

Post by Roberto.tartaglia » 26 Jan 2018, 15:56

Dear Tim
First of all thank you for your quick reply, unfortunately I didn't explain my problem well.
The MRF structure consists of 5 bays to which the leaning columns are connected (on the right). The leaning elements are modeled as truss elements without weight, in order to not influence the structural behavior. On the nodes of the leaning columns (connected to the MRF by the diaphragm constraints) I applied the concentrated masses of each floor, the horizontal forces and the vertical forces .
When I run the model, the leaning column works well without influencing the structure and at the same time: the concentrated masses ensure a perfect dynamic behavior, the horizontal forces are perfectly transferred to the model, but the concentrated loads have no influence on the response which makes no sense to me.
This is my problem: why the concentrated vertical loads introduced to model the P-Delta effect don't work? Are there any another ways to model the P-Delta effect in SeismoStr.?
The only way that gives reasonable results is modelling the elements of the leaning elements with a infrmFB and assign to the elements extremity a “link element” with a very low (0.1) stiffness coefficient in M2 and M3 directions.
Attached you will find the Seismostruct model and few slides to better explain the main model features.
Thank you
Best regards
Roberto
Links:
https://www.dropbox.com/s/aougvq599c2a8 ... r.spf?dl=0
https://www.dropbox.com/s/qd1u5sub8ighw ... .pptx?dl=0

huffte
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Joined: 22 Jul 2011, 10:19
Location: Cookeville, Tennessee, USA
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Re: P-Delta Effect

Post by huffte » 26 Jan 2018, 19:12

Ok Roberto. That's a bit more clear. Now my second inclination is to wonder about the rigid links. Would you get better results if you eliminated the rigid links and used rigid offset lengths when you define the elements? It seems that might be worth looking into.
Tim Huff

rashid
Posts: 26
Joined: 05 Apr 2014, 16:27
Location: Christchurch, New Zealand

Re: P-Delta Effect

Post by rashid » 05 Sep 2018, 23:47

Hi Seismosoft team,

I'm aware that Seismostruct can account for global p-delta effects (geometric non-linearity) but do we need to include the leaning pinned columns with the concentrated loads in our model in order for Seismostruct to account for geometric non-linearity ?

Thank you.

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seismosoft
Posts: 594
Joined: 06 Jul 2007, 04:55

Re: P-Delta Effect

Post by seismosoft » 13 Sep 2018, 16:08

All SeismoStruct frame elements have by default formulations to account for second order effects (provided of course that such option is selected in the program settings). Note however that second order effects become important in the structural response with large deformations.
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prestige
Posts: 8
Joined: 21 Jan 2019, 12:01

Re: P-Delta Effect

Post by prestige » 06 Mar 2019, 09:49

hi please help
Is it necessary to consider leaning columns (with pinned connection) for p-delta effect?
why somebody use this method?
Many scholars only consider geometric nonlinear effects.

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seismosoft
Posts: 594
Joined: 06 Jul 2007, 04:55

Re: P-Delta Effect

Post by seismosoft » 07 Mar 2019, 12:32

As mentioned in the previous post, the second order effects are accounted for by automatically, if the corresponding option is selected in the program settings.
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