X Bracing for retrofitting

[Ralazem]

Is it better to model braces as truss or elastic frame elements?

[seismosoft]

1) The 16.67% value is a reasonable average value for the inelastic force-based plastic hinge element type. For a reference see for instance:Plastic hinge integration methods for force-based beam–column elements, MH Scott, GL Fenves - Journal of Structural Engineering, 2006 - ascelibrary.org

2) The residual strength is the remaining strength AFTER fail has occurred. It does not determine when the failure of a particular element will happen. A small residual strength is always better for stability than removing the element altogether.

Seismosoft Support

[Ralazem]

What about this issue?

"Also it seems that the fundamental time period is sensitive to changes in PH, but shouldn't it be based on elastic properties? When compared the time period of this model with another software, I would have to use a PH of 1-2% to get a matching value, but ofcourse that value is unreasonable."

[seismosoft]

1-2% is definitely unrealistic. The fact that you get different eigenvalues for normal plastic hinge lengths, is definitely unrelated to the plastic hinge lengths. Further, because of the kinematics of the model, but also because the elastic section stiffness can be slightly different from the section stiffness that is derived through the integration of the section monitoring points, some small changes in the eigenperiods should be expected.
Seismosoft Support

[Ralazem]

Is it better to model braces as truss or elastic frame elements?

[seismosoft]

It depends if they fail or not.
Seismosoft Support

[seismosoft]

In the models database on our website, there should be a model of a RC building strengthened with steel braces.
Seismosoft Support

[Ralazem]

Yes, I saw that model. They were modelled as infrmFBPH elements which is strange considering the bracing should not yield..

[seismosoft]

Note that you can model buckling in the braces, by introducing imperfections at the mid node of the brace.
Seismosoft Support

[Ralazem]

What do you mean by imperfections? What does that mean in terms of modelling?