Extremely different results between modeling with infrmFB element and infrmDBPH element

04-Unexpected behaviour/errors
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Koloubos
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Joined: 23 Dec 2019, 13:43

Extremely different results between modeling with infrmFB element and infrmDBPH element

Post by Koloubos »

Hello, I am currently modeling a simply reinforced one bay frame, using the two different types of elements in seismostruct. My goal is to compare any difference found between lumped and concentrated plasticity models. The results are extremely different though. Having read previous papers on the subject I find this very odd. Is there a probable reason or solution to my problem. Thank you very much in advance!

Koloubos
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Joined: 23 Dec 2019, 13:43

Re: Extremely different results between modeling with infrmFB element and infrmDBPH element

Post by Koloubos »

I failed to mention that I am conducting a static pushover analysis and my results greatly vary in terms of yielding force and ultimate displacement.

Koloubos
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Re: Extremely different results between modeling with infrmFB element and infrmDBPH element

Post by Koloubos »

The goal is to find differences between lumped and DISTRIBUTED plasticity models. Terribly sorry for the mistake in the first post!

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seismosoft
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Re: Extremely different results between modeling with infrmFB element and infrmDBPH element

Post by seismosoft »

The only very obvious reason we can see is that you are using very short plastic-hinge length, very different from the default 16.67% value. Can this be the reason?
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Koloubos
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Joined: 23 Dec 2019, 13:43

Re: Extremely different results between modeling with infrmFB element and infrmDBPH element

Post by Koloubos »

Dear seismosoft,
Thank you for your response. In the infrmDBPH element type, it is not required to assign an plastic-hinge length to your element, in contrast to the infrmFBPH element type. Analysis run using the infrmFBPH element type have no problem but this type of element uses a similar distributed plasticity formulation to the infrmFB element type, as stated in the manual. I am looking to replicate a lumped plasticity model for further comparison to the distributed one.

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seismosoft
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Re: Extremely different results between modeling with infrmFB element and infrmDBPH element

Post by seismosoft »

Sorry for this. By mistake, we thought that you meant the FBPH member type.
in DBPH the axial load is only considered at the beginning, in order to get the bilinear moment-curvature curve. On the contrary in the FB element the axial load is considered at every step to derive the bending moment (at that step). Can it be that you have large variations in the axial loading of the members. If this is the case, large discrepancies are to be expected.

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Koloubos
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Joined: 23 Dec 2019, 13:43

Re: Extremely different results between modeling with infrmFB element and infrmDBPH element

Post by Koloubos »

Dear Seismosoft,
My model consists of two columns and one beam connecting them, all reinforced with 4 corner bars(2cm diameter,) all sized 50cm x 50 cm. The only load i have, except the selfweight, is the incremental load i use on an upper node in order to conduct the pushover analysis so there are not great axial loads or variations of axial loads taking place. The height is 4m and the bay length is 6m. It's the simplest model possible. Are you sure there is not a bug it the DBPH element type? There are huge differences between the two elements analysis (DBPH and FB) in every kind of frame i tried it, not only the simpler case that I exhibit. Thank you!

huffte
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Re: Extremely different results between modeling with infrmFB element and infrmDBPH element

Post by huffte »

But if the only load other than self-weight is the lateral push load, it seems to me that the axial load variations would be quite large due to the overturning effect (large increase in compression on the 'leeward' column and large induced tension on the 'windward' column).
Tim Huff

Koloubos
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Re: Extremely different results between modeling with infrmFB element and infrmDBPH element

Post by Koloubos »

Dear Huffte,
Thank you for your response. You are quite right and I believe that your comment explains the difference in yielding moment/force due to possible increase in axial load, which is not considered. Would that lack of axial load consideration also explain why the frame develops very small inelastic displacement when modeling with the infrmDBPH?

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