no descending branch

04-Unexpected behaviour/errors
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manousos
Posts: 8
Joined: 01 Apr 2014, 12:38

no descending branch

Post by manousos »

i added R/C walls in a 3 storey building (in order to retrofit it) and i moduled it with seismostruct. i am trying to run pushover analysis but numerical difficulties do not allow the finish of the analysis. although it is not exist any failure that will challenge the total collapse of building, the capacity curve do not has descending branch. i have tried everything that you are suggesting in the forum in order to solve numerical difficulties but i can not find a solution. Anything thoughts?
huffte
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Location: Cookeville, Tennessee, USA
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Re: no descending branch

Post by huffte »

Hi manousos. I'm sure the SeismoSoft team will be able to do it much more quickly than I can, but until they are able to get to it I would be happy to try and take a look at your model to see if anything strikes me as unusual. Feel free to e-mail the model to me. Best of luck manousos.
Tim Huff
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z.gronti
Posts: 576
Joined: 16 Oct 2013, 08:14

Re: no descending branch

Post by z.gronti »

Dear Manouso,

You may find suggestions for solving the numerical difficulties in the "Solving Convergence Difficulties" video. You can watch it in the following links:
https://www.youtube.com/watch?v=p6tAyoofDog
https://www.facebook.com/video.php?v=22 ... =3&theater

You may also refer to the Help System (Theoretical background and modelling assumptions ->Convergence and Divergence flags).
Zoi Gronti
Seismosoft Srl.
huffte
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Joined: 22 Jul 2011, 10:19
Location: Cookeville, Tennessee, USA
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Re: no descending branch

Post by huffte »

I have a few comments manousos.

1. Upon trying to run your model as is, with no modification, I get the following message:

“Errors detected in Performance Criteria Module:

C19A is not a declared inelastic frame element (infrmDB, infrmFB, infrmFBPH or infrmDBPH). Performance Criteria checks can be carried out only for inelastic frame elements.”

I confess that this is a mystery to me as I find no evidence of an element C19A in your model. While you have defined a section C19A and an element class C19A, it appears to me that you never used this element class in the model.

2. I am not sure why you have so many different concrete materials, all with identical properties.

3. Your reinforcement patterns in many members are rather unusual. For example you have T-beams defined with reinforcing steel in one side of the flange only. And many columns have extremely light reinforcing. Are these actual members or made-up numbers?

4. I an unclear as to what you mean when you say “there is no descending branch”. Once I eliminate the performance criteria the model runs fine and is clearly softening as load increases.

This is a fairly complex model. If the results are not making sense to you, I would suggest a gradual build-up of the model with intermittent analyses at various stages of completion to try and pinpoint any perceived problem.

And, given the unusual reinforcement patterns in many members, you may wish to rethink your statement that there is nothing to challenge the structure.

Best of luck manousos.
Tim Huff
manousos
Posts: 8
Joined: 01 Apr 2014, 12:38

Re: no descending branch

Post by manousos »

Dear huffte,
1.i have used different concrete materials because i need to simulate the different confinement factor that elements have.the properties are comparable but i wanted to be accurate.
2. of course my reinforcement patterns are unusual, but the elements that i modules are aimed to simulate member that designed with the older seismic regulations of Greece.This building estimated that it constructed in 1967, so it has many unusual in geometry and in
reinforcement patterns.
3.do you mean that the capacity curve has a descending branch after the peak of the maximum load? Because when i am running the analysis, the analysis stops when the maximum load reached.
huffte
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Joined: 22 Jul 2011, 10:19
Location: Cookeville, Tennessee, USA
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Re: no descending branch

Post by huffte »

Agreed. Older structures can have light reinforcing. But I have never seen a T-beam with no reinforcement on fully one-half of the flange width. I could be mistaken, but suggest you review your reinforcements and make sure you haven't mis-typed a number or something similar.
Tim Huff
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seismosoft
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Joined: 06 Jul 2007, 04:55

Re: no descending branch

Post by seismosoft »

Manousos,

One potentional source of instability is the fact that you employ very lightly reinforced cross-sections without reinforcement at one side, combined with the fact that you employ zero tensile strength for your concrete material models. Though indeed there are members in older section without reinforcement at one side, the concrete tensile strength is always positive (it is only neglected in the analysis because it is on the safe side).

Seismosoft Support
simuni
Posts: 1
Joined: 13 Feb 2020, 19:24

Re: no descending branch

Post by simuni »

Dear Seismosoft,

I have to carry out a pushover of an existing steel structure, but first I'm trying to model a strange part of it: in the bay between two consecutive steel beams and columns, RC squat shear walls were realised, and inside this walls a couple of concentric steel braces was added.
So I'm trying to model my RC walls through a strut and tie diagonal model, using diagonal RC struts with, in the middle of the rectangle section, a reinforcement with the same area of the brace.

I've sent the .spf file to support@seismosoft.com referring to this thread.

The problem is that the pushover reaches the peak response but it doesn't follow the descending branch: at the peak there's an evindent convergence problem, so that "the model cannot withstand the defined load" (in my opinion the analysis fails at the response corresponding to the collapse of the compressed diagonal RC struct).
I've already tried to modify all the parameters provided by the Seismostruct Help, I even tryied to modify the material types, the position of the reinforcement (putting it at the corners of the struct section and adding trasversal reinforcement), modifying the number of Load Increments.

Do you have any advice? Anyway thank you in advance for your help.
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seismosoft
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Joined: 06 Jul 2007, 04:55

Re: no descending branch

Post by seismosoft »

If you go to the Convergence Problems of the post-processor you will see that problematic elements or locations in your structural model that lead to divergence. In particular in your model the convergence problems arise from the PUNT9_4_3_b element and the way it is reinforced (a large rebar in the centre).
The Convergence Problems tab is mentioned in the help of the program. Please read carefully to the particular section on convergence difficulties of the manual, because sending your model, since we obviously cannot provide personlised support to all our academic users.
Seismosoft Support
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