steel model behaviour

[Newmark]

I use this soft to do pushover analysis and IDA, and I define the material as the steel bilinear(stl_bl), I want to know what the hardening mode is, Kinematic hardening or Isotropic hardening? Because in IDA analysis, for a given earthquake intensity, the strength is hardening in the base shear force-top displacement curve.
The second question is, in the force-displacement curve, the stiffness has deterioration, I don't know why this happens because I define the simplest hysteretic mode(stl_bl)?
The third is in cyclic pushover analysis, the force-displacement has negative stiffness segments(drop segments) when I consider the P-delta effect, but in IDA, the curve does not have negative stiffness segments, why this happens for the same structure?
Thanks very much.

[seismosoft]

- see the Help System page on the 'stl_bl' material model.

- is yours a Steel or RC structure? If the latter, then concrete cracking will explain the stiffness decay you have observed.

- is the loading regime completely identical in both analyses? If it is not, then that might explain the differences you have observed.

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[hrmarin]

Does seismostruct consider cracking and bond-slip?
Best regards,

[seismosoft]

As it is stated in our response above (and throughout the Help System) cracking is indeed explicitly modelled in SeismoStruct's fibre elements.

As stated in the Help System (and in several posts in this Forum), bond-slip can be modelled through the introduction of appropriately calibrated phenomenological elements (i.e. link elements) or through empirical changes to material models (i.e. rebars steel).

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