Seismosoft | Earthquake Engineering Solutions

Does the fiber model automatically account for strength degradation, stiffness degradation, and pinching when a reinforced member is subjected to cyclic displacement? I tried to run a cyclic pushover for a RC beam-column subassemblage using the fiber model (steel-mp model; concrete core- confined concrete; concrete cover- unconfined model) up to 6% drift; however there was no strength degradation in beams, and the pinching was very minor. Could you give some hint hoe to correctly model it? Thanks a lot,

Difficult question. Fibre beam-column elements are not perfect (at least not yet!). Indeed, issues such as bond-slippage, shear deformations, non-perfect crack closure, etc, are not readily inherent to fibre formulations, and thus require ad-hoc additions (e.g. addition of a link/spring element, with an appropriate hysteretic rule, at the end nodes of the element). With columns, it is easier to see pinching and degradation, due to the effects of axial load (they introduce higher concrete degradation as well as p-delta effects). With beams, since axial load is "missing", pinching and degradation are less likely and/or evident. Having said that, if the beam is under-reinforced, one should still see some degradation etc, the degree of which would vary according to the beam's characteristics and loading.
Finally, as you would have seen from the newsletter we recently sent around, the next version of SeismoStruct will feature the presence of force-based beam elements, which are known to model pinching/degradation slightly better with respect to their displacement-based counterparts, which are the ones currently used in SeismoStruct.
Ok, we know the above is not really a straight answer, but hopefully the discussion will be of some use?..

SeismoSoft Support

Thanks for the prompt reply. I hope the new version could be issued soon. SeismoStruct has the best user interface I have ever seen.

Is there any other model or other approach in SeismoStruct which could be used to model strength degradation, stiffness degradation, as well as pinching? I am not sure if you know IDARC. It has a build-in option in which the user can adjust strength degradation, stiffness degradation, and pinching through a few parameters.

Thanks for developing such an excellent nonlinear analysis program.

Thanks for the kind words.

Yes, we know IDARC, of course, in the same way we know Drain2D, Ruaumoko, etc, all of which are plastic-hinge based programs, where the attainment of pinching/degradation is "simply" a question of "playing" around with the parameters of the hysteretic moment-curvature relationship associated to a given hinge. In SeismoStruct you could also do that for beams, where interaction between axial load and moments is not important, by adding a link at the element end nodes and associating a Takeda type of response curve to such links/hinges.

But, we note, one should not seek to obtain pinching/degradation simply for the sake of it. We mean, a well-designed flexure-dominated beam may not exhibit significant pinching/degradation, and hence so should not the corresponding computer model. As we stated before, reinforced concrete elements subjected to cyclic action should not necessarily exhibit pinching/degradation all the time. It depends on their characteristics and on those of the loading.

To conclude, the important issues in our opinion are if a given element is shear deformation-sensitive, or features bar-slippage, or is under-designed in terms of flexural reinforcement, etc. And then, if the answer to any of these questions is positive, one may try to understand if it is possible to model any of such response phenomena in the computer code being used, through appropriate considerations and reasoning, rather than through somewhat arbitrary tweaking of the parameters of a given hysteretic model.

Does it make sense? Anyway, apologies if we became excessively philosophical. Hope it all helps a little bit, somehow..

Thanks again.

SeismoSoft Support