Strain hardening parameter

[OmidFaghih]

Hello
I have steel frame with ASTM A992 material.
my strain in fracture Eult is 0.2.( is it correct??)
i put this value in the Esp=(fult-fy)/(Eult-fy/Es), and my Strain hardening parameter is 0.0026.
so is not in the ranges from 0.005 to 0.015.
what is the problem???
help me please

[huffte]

Hi OmidFaghih.

Your calculation seems to be correct for a strain hardening parameter of 0.0026. So, I think you need to decide the goal of your model.

Are you attempting to estimate the force in a steel element during strong ground shaking? Then perhaps a larger strain hardening parameter based on a reduced ultimate strain - say, 50 to 75% of the fracture strain - would be appropriate because this would result in a higher force, generally speaking.

Are you attempting to estimate the displacement of a steel structure during strong ground shaking? Then perhaps your computed strain hardening parameter of 0.0026 would be appropriate because this would result in a higher displacement, generally speaking.

Are you trying to win a prediction contest? Then perhaps a sensitivity study on the effect of the strain-hardening parameter upon the response you wish to predict would be in order.

So I believe that the answer to your question is best answered by you and depends on the goal of your analysis.

It's an interesting question OmidFaghih. Best of luck with your work.

[OmidFaghih]

I attempting to estimate the max drift of a steel structure during strong ground shakin.
and with this value i reach to the wrong and huge displacement.
even 2m in 1 story frame!!!
i think my ultimate strain is wrong for this work.

[seismosoft]

A maximum drift of 2m might be an indication of structural failure and collapse. Can it be that the structure cannot withstand the ground motion?

SeismoSoft Support

[OmidFaghih]

Thanks seismosoft & huffte
I am so dizzy for this parameters!!!!
can you help me to find true value of fracture strain and strain hardening paramete??
evrytime i reach to the different RESOLVENT!!!

[huffte]

OmidFaghih, did you do the sensitivity analysis? Have you tried changing the strain hardening parameter to a higher value? If so, does the highe value for this single parameter dramatically effect the results?

I am not certain that this single parameter is the entire problem, if in fact, there is a problem.

It may be, as noted by seismosoft, that the structure simply cannot withstand the loading you are imposing on the structure.

It is not clear what you mean by the phrase "evrytime i reach to the different RESOLVENT".

The fracture strain for A992 is correct 18 to 20%. And A992 has a relatively small spread between ultimate and yield stresses. These are the reasons for the strain hardening parameter being lower than "typical".

So, OmidFaghih, I am not completely convinced that the strain hardening parameter is the sole culprit here.

Can you elaborate on why you believe your results are in error?

[OmidFaghih]

Thanks dear huffte.
with this value (ultimate strain = 0.2) my steel moment frame have unreasonable and large drifts (displacements) and my frame still show resistance.(by increasing base shear!!)
and i cant find collapse point(lateral dynamic instability)
the place dynamic analysis results as evidenced by excessive lateral displacements.

[seismosoft]

Can it be that you have geometrical non-linearities deactivated (Analysis tab in the project Settings)? If it is indeed deactivated, it is very likely that you cannot get a descending branch in the capacity curve. Further, have you modelled correctly the vertical/gravity loads of the structure?

SeismoSoft Support

[OmidFaghih]

Thanks dear seismosoft
geometrical non-linearities is active.
i think the the vertical/gravity loads of the structure is correct.
you think that max drift of a steel moment frame how much can be??

[seismosoft]

A maximum interstorey drift of 2% is reasonable for steel frames. Note however, that if you have simultaneously (1) a low value of strain hardening in the steel material (2) reasonable gravity loads that include both the frame and the floor loads (G+Q) and (3) geometric nonlinearities, then at some stage in the pushover analysis, you should get a descending branch.
We suggest that you check all these a bit further.

SeismoSoft Support