Pushover curve's shape

[nitadorakis]

Hi all.

I have a question regarding the pushover curve I'm getting as a result of the static pushover analysis of a 4-strorey plane RC frame.
Although PDelta geometric transformation is used for the column elements, the curve is increasing up (could be approximated as a bilinear) to the target displacement (1m) , without a 'backbone' curve shape (degradation after the maximum base shear), i.e. the max base shear occurs at the target displacement.
Forcebeamcolumn elements with fiber sections (with Concrete01+Steel01) have been used.
The lateral loads are based only on the first mode shape.

I would like to ask if this is normal, if it is due to the materials that have been used, or if there is something else I can't think of.
The pushover curve of an 8-storey frame with the same elements, materials and similar sections, has the 'backbone' shape I described above.
Looking forward for your help.

Evangelos

[vesna]

What $b did you use for Steel01?In case it is high, you will have to push your frame to a very large displacement so that P-delta takes over strain-hardening slope of Steel01.

[nitadorakis]

Thanks Vesna

I used the expession:
[expr ($fu-$fy)/($eu-$ey)/$fy*$ey] where
fy 500000
fu 600000
ey 0.0025
eu 0.025
So, b= 0.022.
I think it's low.Isn't it?

[vesna]

The value of $b is not too big but it still gives significant strain hardening to your steel. If you are confident with that value of $b, stick with it and accept the pushover curve as it is. However, for purpose of testing your model I would suggest you to set $b to 0.0 and check if you are going to get negative post-yielding slope on your pushover curve. In case you do not get it, it might mean that your P-delta is not working well.

[nitadorakis]

What do you mean that the P-delta might not work well...?
Apart from the Steel01 and the transformation, is there somenthing else I should check, or leave it as it is?

[vesna]

No, there is not something else to check. I just suggested you to do this analysis to make sure you defined your transformation well.

[nitadorakis]

thank you Vesna

I guess you are right.
I was wondering if it is more appropriate to use corotational transformation in pushover analysis instead of PDelta, due to the large displacements.

[vesna]

I would use corotational transformation in your case.