Unexpected Interstory Drift Ratios

04-Unexpected behaviour/errors
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emreakin
Posts: 15
Joined: 08 Aug 2017, 13:55

Unexpected Interstory Drift Ratios

Post by emreakin »

I am performing nonlinear time history analyses of two different types of reinforced concrete frame buildings. One is ductile and designed according to the seismic design principles. The other one is nonductile and have certain deficiencies in terms of seismic response. I also use "number of story" as a variable. I have 3, 5 and 8 stories buildings which are designed as either ductile or nonductile. I apply Kocaeli Yarımca EQ motion (no scaling) and use lumped masses. The number of integration sections for the columns is 5 and number of section fibers is 100. As a result, I checked ultime inter-story drift ratio (IDR) at all story levels. And I saw that the ultimate IDR for the first floor may go up to 5%-6% for the ductile and 9%-12% for the nonductile building models. It was interesting to observe such high IDR values. Then I checked stress/strain values at the firsty story column ends for one building model. And what I saw was that all first story column reinforcements (at the top and bottom) has already yielded and plastic hinges formed long before these IDR values were reached. It is very interesting because collapse mechanism should have taken place before I saw these IDR values. It looks like Seismostruct does not stop the analysis when mechanism (plastic hinge formations at both ends of all first story columns) take place. Do I have to make any arrangement in analysis setup? Or is there anything that I am missing?
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seismosoft
Posts: 1184
Joined: 06 Jul 2007, 04:55

Re: Unexpected Interstory Drift Ratios

Post by seismosoft »

Hi emreakin,
No SeismoStruct does not stop the analysis automatically, if the stability of the solution is achieved. In this way users can check the structural performance in the highly inelastic region. Note that by employing performance criteria and/or code-based checks you can monitor parameters such as the shear force values, the shear demand against the shear capacity, the chord rotation demand and capacity, and the strains of the monitoring points. In each code-based check or performance-based criterion, users may decide either to stop or pause the analysis, or to simply get notified.

In any case IDRs of 10% are very high. Can it be that you are applying the earthquake records with a very high multiplication factor?
Regards,
Seismosoft Support
emreakin
Posts: 15
Joined: 08 Aug 2017, 13:55

Re: Unexpected Interstory Drift Ratios

Post by emreakin »

Thank you for your answer. The problem is that I am using academic license key which does not allow me to use code-based check or perfmance-based criterion. I should find a solution until I buy full version from a future project budget. I may manually check reinforcement strains at both ends of first story columns and define approximate plastic hinge formations manually. This may inform me about the instant of collapse mechanism. But I should also check shear demand/capacity for nonductile building models, which is also not easy to do manually for 25 columns. By the way, I am applying Kocaeli EQ record without any scaling (scale factor is 1). I may also decrease scale factor. Does anyone have any other suggestions?
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seismosoft
Posts: 1184
Joined: 06 Jul 2007, 04:55

Re: Unexpected Interstory Drift Ratios

Post by seismosoft »

Note that in the academic licenses there is no limitation regarding the performance based criteria, the limitation is just for code-based checks. You can check the "real" (i.e. without safety factors) shear capacity of the members using performance based criteria in SeismoStruct.

Regarding the record that you are using check the record units against the units you are using in your project.

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emreakin
Posts: 15
Joined: 08 Aug 2017, 13:55

Re: Unexpected Interstory Drift Ratios

Post by emreakin »

I did not know that we may use performance based criteria with academic license key. That solved my problem, thank you. I directly used the record which comes in seismostruct files, given in units of "g".
roshen.rj
Posts: 41
Joined: 10 Feb 2020, 07:57

Re: Unexpected Interstory Drift Ratios

Post by roshen.rj »

seismosoft wrote: 08 Aug 2017, 15:00 Hi emreakin,
No SeismoStruct does not stop the analysis automatically, if the stability of the solution is achieved. In this way users can check the structural performance in the highly inelastic region. Note that by employing performance criteria and/or code-based checks you can monitor parameters such as the shear force values, the shear demand against the shear capacity, the chord rotation demand and capacity, and the strains of the monitoring points. In each code-based check or performance-based criterion, users may decide either to stop or pause the analysis, or to simply get notified.

In any case IDRs of 10% are very high. Can it be that you are applying the earthquake records with a very high multiplication factor?
Regards,
Seismosoft Support
Hi

May I know how can we calculate IDRs from seismostruct? Thanks
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z.gronti
Posts: 824
Joined: 16 Oct 2013, 08:14

Re: Unexpected Interstory Drift Ratios

Post by z.gronti »

Dear roshen.rj,

If you carry out a search on this Forum for the term "interstorey drift ratio" you should find an answer on this matter.
Zoi Gronti
Seismosoft Srl.
huffte
Posts: 978
Joined: 22 Jul 2011, 10:19
Location: Cookeville, Tennessee, USA
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Re: Unexpected Interstory Drift Ratios

Post by huffte »

If you employ inelastic force-based elements (infrmFB)for the columns, and you do not subdivide said members (as it is typically unnecessary to do so with the infrmFB element), then you may simply go to the post-processor, 'Element Action Effects', and select the appropriate chord rotation. IDR is, strictly speaking, the column drift divided by the story height. For small angles, this is essentially equal to the chord rotation. I include the following text from the 'Element Class' - 'Inelastic force-based frame element type - infrmFB' section of the excellent Help System:

'As discussed in Material Inelasticity, element infrmFB is the most accurate among the four frame element types of SeismoStruct, since it is capable of capturing the inelastic behaviour along the entire length of a structural member, even when employing a single element per member. Hence, its use allows for very high accuracy in the analytical results, while giving users the possibility of readily employing element chord-rotations output for seismic code verifications (e.g. Eurocode 8, NTC-08, KANEPE, FEMA-356, ATC-40, etc).'
Tim Huff
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