Modal analysis after the non-linear time history analysis 1

[Deeptiranjan]

Hello everyone, I have modelled a four storey building in SAP2000 and analysed by non-linear time history analysis. Initially I defined all the load pattern, then define the modal analysis. Also, I define the gravity load case as non-linear static load case type considering all the load patterns. After that non-linear time history load case is defined considering the gravity load case as the initial conditions. After the analysis, through out the whole building 43% hinges are formed.

The problem is, when I do the modal analysis again (say, modal-2 load case) taking the time history load case at stiffness to use section (Stiffness at end of nonlinear case), the building period is not changed at all. The 43% hinges formed in the building should reduce the stiffness, but I can't see any changes in the building period. Also I increased the scale factor, the building period is remains unchanged.

I have attached the Define Load Case file below.

Is there anyway this problem can be solved?

Thank you.

Deepti Ranjan Majhi
Research scholar, PhD
IIT Roorkee

 

[mronlinetutor]

Are you referring to the non-linear analysis as a push over analysis or a non-linear time history analysis? Secondly, with 43% hinges forms, would a collapse mechanism be formed and the analysis deem invalid? Thirdly, you did not mention whether concrete floor considered are flexible diaphragms or rigid diaphragm.

As I recall, the frequency of vibration, omega=sqrt(k/m) does depends on the stiffness of the structure. If the reduced section is used, that means the stiffness of the building should change unless the reduced section considered in the plastic hinge is not reduced. That means you are using the same stiffness for both of the analysis.

disclaimer: all calculations and comments must be checked by senior engineers before they are taken to be acceptable.