Big slope in linear part for force-displacement curve

03-Analytical/modelling capabilities
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simon1000
Posts: 15
Joined: 02 May 2017, 17:05

Big slope in linear part for force-displacement curve

Post by simon1000 »

I want to verify a reinforced concrete beam. The experimental beam created in half scale in laboratory (span=1.5m ,Width=350 mm, height=450 mm).
This beam pushed in a mid-span in vertical direction until failure. The results of the experimental were: the final displacement=24mm, the maximum load (push) which can be carried by the beam was 1200 KN in displacement=10 mm.

I create this simple beam in seismostruct based on: infrmFB element, static pushover analysis, applied load type: displacement, Target displacement=24 mm, Response control, and based on the same properties materials which were reported in the experimental paper.

I have a problem with the results:
The results of a static pushover analysis of this beam showed a very big slope in force-displacement in linear part (before yielding point). The slope reached to a displacement equal to 6 mm with load=1350 KN and after that, the results slowly decreased. I want to decrease the pre-yielding slope in this response curve. I think I should change a parameter such as applying a coefficient to Ec in section tab because the experimental beam has a half scale. I don't know the reason of this slope (slope of pre-yeilding) which is very bigger than experimental results. I would be happy if the seismostruct team guide me to solve my problem. Thank you for supporting the users.
simon1000
Posts: 15
Joined: 02 May 2017, 17:05

Re: Big slope in linear part for force-displacement curve

Post by simon1000 »

Infact, this poibt (1350 KN, 6 mm) should be 1200 KN and 10 mm based on experimental results.
simon1000
Posts: 15
Joined: 02 May 2017, 17:05

Re: Big slope in linear part for force-displacement curve

Post by simon1000 »

I create the model again by seismostruct v.7.0.6. All of defined properties were the same with that model which was created by seismostruct 2016. The results of these two version were very different. Also, they didn't match with the experimental again. I sent these two files and also their results to z.gronti@seismosoft.com. Please let me to know why I can't verify this simple RC beam and why the results of these two version were different.
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seismosoft
Posts: 1184
Joined: 06 Jul 2007, 04:55

Re: Big slope in linear part for force-displacement curve

Post by seismosoft »

Have you tried to open the v7.0.6 file with SeismoStruct 2016? Did the results match? The chances are that there is a difference in two models that you created.

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z.gronti
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Joined: 16 Oct 2013, 08:14

Re: Big slope in linear part for force-displacement curve

Post by z.gronti »

Dear simon1000,

I took a quick look at your models and realised that there are differences between the two models such as the assigned restraints. Please check your models carefully.

I also noticed that you have not assigned the self weight of the beam section in both of your models.
Zoi Gronti
Seismosoft Srl.
kaysisig
Posts: 7
Joined: 13 May 2021, 15:29

Re: Big slope in linear part for force-displacement curve

Post by kaysisig »

Hello Seismosoft team,

I am having trouble with the result of the pushover curve of the experimental column I am trying to validate. Although the ultimate force and ultimate strain somehow matches, the slope of the curve for the pre-yielding section is very much different. Mine is much steeper compared with the result of experimental.

I tried to modify my column by considering stiffness modifier ( 70% of uncracked ) but to no luck, the curve is still the same.

Any advice?
huffte
Posts: 978
Joined: 22 Jul 2011, 10:19
Location: Cookeville, Tennessee, USA
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Re: Big slope in linear part for force-displacement curve

Post by huffte »

What type of element are you using? Force-based (infrmFB) or displacement-based (infrmDB) or some other? If the elements are relatively short, you should probably be using DB. If you are using a single, relatively long element for the column, then you should probably use FB. Element type can really affect theses results. If nothing obvious arises from this comment, I'd suggest reading up on the various element types in the Help System to try and determine which type might be most appropriate for your model conditions.
Tim Huff
kaysisig
Posts: 7
Joined: 13 May 2021, 15:29

Re: Big slope in linear part for force-displacement curve

Post by kaysisig »

I am using a combination ( infrmDB - for the 455mm length of element where couplers are supposedly placed ) and ( infrmFB for the remaining length fo column ). The whole length of column is 2.4m. I tried changing both to infrmDB but the force-displacement curve does not have any significant change.

Also, I am curious about z.gronti's response to simon1000 regarding assigning of self-weight of the section. is it not automatically considered by the software? should I manually define it in the Mass Element type?

Thank you for your time :)
huffte
Posts: 978
Joined: 22 Jul 2011, 10:19
Location: Cookeville, Tennessee, USA
Contact:

Re: Big slope in linear part for force-displacement curve

Post by huffte »

Hi kaysisig.

My advice would be to double-check all settings (geometric non-linearity, convergence criteria, etc.) and to study the verification problems which might be somewhat similar to what you are trying to accomplish with your model. It is not surprising that a change in stiffness modifier did not affect results - I believe that setting would only affect eigenvalue and response spectrum analysis results.

I am not sure how complex your model is. If it is a simple cantilever, this may not have much affect. But if it is more complex, then adaptive pushover, rather than standard pushover, might help.

This is challenging work, even for simple structures. So don't give up. Maybe try infrmDB for all elements, but increase the subdivisions? Just thinking out loud.

As to the response of z.gronti regarding self-weight, it is 100% correct. Users have the option of how mass and self-weight are converted to loads, either automatically or not at all. Refer to the Project Settings --> Gravity and Mass feature.
Tim Huff
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