element class specification (integration section and section fibers)

02-Getting started with the modelling
mehdiharsami
Posts: 11
Joined: 01 May 2020, 15:20

element class specification (integration section and section fibers)

Post by mehdiharsami »

Hi
I need to run an IDA analysis for three different type of lateral system (EBF, SCBF, SMF ) in steel structures that are modeled in 8 and 12 story structures. the problem is I don't know what are the optimum numbers for integration section and section fibers for members of each model in order to make sure about the accuracy of nonlinear analysis and also avoid over needed analysis time and analysis complication. I'm already so grateful for advices I've had in this forum and I'd really appreciate so much if you can introduce me the optimum numbers for below members to me :
special moment frame : beam and columns
special braced frames :braces, beam and columns in braced bays, gravity columns and beams
EBF: short link beams of braced bays, beams outside the link in braced bays , columns in braced bays.
sincerely yours.
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seismosoft
Posts: 904
Joined: 06 Jul 2007, 04:55

Re: element class specification (integration section and section fibers)

Post by seismosoft »

The optimum number of fibres and integration sections does not depend only on the structural configuration. It is mostly affected by the loading applied on the members and the level of inelasticity that is introduced in the members.
Typically, for the infrmFB element type 4 or 5 integration sections are enough. If you do not have inelasticity in the middle of the members (but only at the edges) the infrmFBPH is faster and more stable. Finally, for short members (and only for short members) the infrmDB is also a good alternative.
You might also want to carry out some sensitivity analysis, in order to correctly determine these parameters.

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mehdiharsami
Posts: 11
Joined: 01 May 2020, 15:20

Re: element class specification (integration section and section fibers)

Post by mehdiharsami »

thanks a lot seismosoft support
the problem is I'm willing to run an Ida analysis and members will experience high levels of inelasticity in some runs for sure. for example when I try northridge record by a scale factor equal to 3 on the EBF model the beams outside the links (in braced bays) which have distributed load assigned on them take the analysis to an end caused by elm-inv flags for these beams, I have tried different load steps and convergence tolerances to avoid divergence of the analysis but the problem still happens, these beams are 2.25 meter long, can I use infrmDB for them?

another problem is that for taking shear deformations into account I have implemented shear springs at to ends of EBF short link beams and nodes at these ends experience exceedance of displacement from allowed displacement norms at some steps and these steps repeat for many times (32) until it reaches it's maximum number and adp-conv happens, this happens in some stages of the analysis and make the run time so much longer, is there any thing I can do about this?
best regards
mehdiharsami
Posts: 11
Joined: 01 May 2020, 15:20

Re: element class specification (integration section and section fibers)

Post by mehdiharsami »

one more question : since the records will be scaled by a scale factor that leads to collapse of the models in IDA analysis, I guess there might be inelasticity for gravity members as well eventually, how would you see using infrmFBPH or infrmDBPH for gravity beam and columns in SCBF and EBS models? I had convergence problems when I used infrmFB elements for them. some of my models have more than 2000 elements.
kind regards
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seismosoft
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Joined: 06 Jul 2007, 04:55

Re: element class specification (integration section and section fibers)

Post by seismosoft »

For the convergence problems with the infrmFB element type, carry out a search in this forum for 'convergence' and you will find numerous tips. One thing to start from is to check the Convergence Details tab on the post-processor that shows the elements and/or locations where the program encounters convergence difficulties.
As we mentioned above, if you do not have inelasticity in the middle of the members (this is almost always the case for beams and columns) the infrmFBPH is faster and more stable. and for short members the infrmDB is also a good alternative.
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mehdiharsami
Posts: 11
Joined: 01 May 2020, 15:20

Re: element class specification (integration section and section fibers)

Post by mehdiharsami »

hi seismosoft team
I almost read all posts that refer to convergence and ran into some suggestions about assigning links instead of releases in beams with simple connections. that didn't helped as well and made the analysis to diverge even sooner. I've tried any thing I could have think of and my deadline is so close. can I ask you to do me a favor and check my model please ?
https://ufile.io/pno7gfqp
in my former model links were assigned only for shear links (and not instead of releases) and the analysis lasted longer, I know i'm using large scale factors for the time history record but the drifts are not large enough to think of structural collapse.
best regards
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seismosoft
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Joined: 06 Jul 2007, 04:55

Re: element class specification (integration section and section fibers)

Post by seismosoft »

Even under static loading, the vertical drift in some columns is almost 3cms. There is something not right in the vertical stiffness of your columns. What is even more worrying is the difference between the vettical drift in the external and the internal columns (almost 2cm in some cases). One should probably start from this.
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mehdiharsami
Posts: 11
Joined: 01 May 2020, 15:20

Re: element class specification (integration section and section fibers)

Post by mehdiharsami »

thanks a lot for your attention
I'm sorry what do you mean by vertical drift ? I thought you meant s.thing corresponding to axial deformation of columns caused by pressure and I checked the model under static loading, but I didn't see any axial deformation equal to 2 or 3 cm for columns(from element action effects, frame forces).
plus, I used the material and sections from the software library and if there is s.thing wrong with the stiffness of columns, were else can I search for it.
kind regards
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seismosoft
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Joined: 06 Jul 2007, 04:55

Re: element class specification (integration section and section fibers)

Post by seismosoft »

If you check in the deformed shape viewer to see the displacements (from the menun on the left) you will notice that in some of the columns the vertical displacements at the top are excessive.
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mehdiharsami
Posts: 11
Joined: 01 May 2020, 15:20

Re: element class specification (integration section and section fibers)

Post by mehdiharsami »

thanks again
I checked the deformed shape menu and i'm afraid you might be wrong, because the displacements that you mentioned are cumulative integers, and displacement caused by each of the columns is nearly 2-3 millimeters. so at the top of the building there is a 3 cm vertical displacement relatively to base level, which is the summation of every column's axial deformation under the gravity loads. and if you check the displacement of any of the columns relatively to the upper story column displacement it wouldn't be more than 3 mm.
can you tell me how can i know if my model doesn't converge or it simply fails under the applied dynamic loads?
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