Inelastic Spectral Matching

02-Analytical capabilities
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adeelfaisal
Posts: 42
Joined: 29 Apr 2014, 15:42

Inelastic Spectral Matching

Post by adeelfaisal »

Hi
I've to perform inelastic spectral matching. The target spectrum is that of a design accelerogram (it is design accelerogram for the multi-span continuous deck bridge given in Example 1, chapter 4, SeismoStruct Verification Report). If I load the source accelerogram Cape Mendocino, come to step.2 "Define the Target Spectrum", it is not possible to set ductility factor when using either of the options "Use Spectrum from Loaded Accelerogram" and "Load spectrum from File". The ductility factor can be set only in the first option "Use EC8 Spectrum". Unless we set a ductility factor greater than 1 in Step.2 of defining the target spectrum, the software performs elastic spectral matching as described in the Help.

How can we perform Inelastic Spectral Matching in this case?

Regards
huffte
Posts: 978
Joined: 22 Jul 2011, 10:19
Location: Cookeville, Tennessee, USA
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Re: Inelastic Spectral Matching

Post by huffte »

Hi adeelfaisal. I don't believe that you can perform inelastic spectral matching. Consider the difficulty in trying to do so since the inelastic response depends on the hysteretic model, so one would need to define not only damping, but also the nature of the system hysteresis.

But, also consider that the goal of spectrum matching is to find a ground motion characteristic of that defined by an elastic spectrum. This ground motion could still produce inelastic response in the structure being designed. So, if you will dwell on it for a while, I think you may see that inelastic spectral matching is unnecessary.

Spectrum matching produces a set of ground motions characteristic of the site, which is defined by an elastic acceleration spectrum in current codes and specifications. Those ground motions are then used in a nonlinear analysis of a given structure, which may experience inelastic behavior when subjected to those ground motions.

I hope this makes sense adeelfaisal. Best of luck.
Tim Huff
adeelfaisal
Posts: 42
Joined: 29 Apr 2014, 15:42

Re: Inelastic Spectral Matching

Post by adeelfaisal »

Thank you huffte for your comprehensive response.
Yes you're right at at the moment seismic codes use elastic response spectrum for matching accelerograms which can actually cause nonlinear response in structures. I'm studying the effect of strong motion duration on the inelastic response of bridges. In order to eliminate the domination of spectral characteristics of the accelerograms on response, I am interested in spectrally matched accelerograms. As the response of interest is the inelastic response, I'm convinced to perform inelastic spectral-matching.

I have averaged the ductility factors for the three piers of my bridge model and intend to set the ductility factor to this average ductility for performing inelastic spectral matching.

Regarding the difficulty of using appropriate hysteretic model, I intend to use Bilinear Takeda Hysteresis Model. I think if in SeismoMatch, we set the correct ductility factor and Post-yield Hardening ratio, we can perform inelastic spectral matching as far as the target spectrum is a code-based spectrum.

However, i am unable to perform inelastic spectral matching if the target spectrum is the spectrum of design accelerogram.

Your valuable comments will be of great value to me.

Regards
Adeel Faisal
huffte
Posts: 978
Joined: 22 Jul 2011, 10:19
Location: Cookeville, Tennessee, USA
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Re: Inelastic Spectral Matching

Post by huffte »

Hi once again adeelfaisal. It is an interesting issue you are investigating.

I still see no problem with using spectral matching to an elastic acceleration spectrum.

How did you come up with the ductility factors for your piers? Somewhere along the line, I imagine you must have had an elastic acceleration spectrum as the basis of design. As long as your ground motions are matched to the basis of design, your study should be valid. You could pick a set of short duration records and a set of long duration records. Then match each to the target spectrum.

You would want to re-examine the durations of the spectrally matched records since spectral matching can significantly alter such characteristics.

So, in a sense, it seems counterproductive to attempt a match to an inelastic spectrum. This would artificially force the inelastic response to a certain level for both the short-duration and long-duration sets, while it seems your goal is to evaluate the effect on inelastic response of long duration matched records versus short duration matched records, with both matched to the same elastic spectrum.

Interesting work, adeelfaisal. I have rambled on long enough.
Tim Huff
adeelfaisal
Posts: 42
Joined: 29 Apr 2014, 15:42

Re: Inelastic Spectral Matching

Post by adeelfaisal »

Thank you very much again, huffte. Your comments have energized me a lot.

Yes I'm convinced that it is rather irrational to perform inelastic spectral matching as some of the ground motions may not be "strong" enough to actually cause inelastic response. Elastic spectral matching seems to be both necessary and sufficient in this case also.

I am thankful to SeismoSoft for providing this forum.

Adeel Faisal
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