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Non-Linear Time History Analysis on ETABS

Non-Linear Time History Analysis on ETABS

Non-Linear Time History Analysis on ETABS

Q: How do we find the direction of forces from a nonlinear time-history ETABS analysis of a building model so that we can apply them to a SAFE model of a mat foundation?

We have run a nonlinear time history (TH) analysis for a model of a building with nonlinear elements (friction dampers)using 7 site-specific time histories.  The code says that we can take the average of the 7 TH's to get a design force.  To do this, we've taken the average of the maximum absolute values of the results.  This gives us one set of results, all positive.  

For the design of the mat foundation, we need to apply these loads in the right directions (with the right signs.) How can we separate the results to get EQ-x and EQ-y axial loads to be applied to the SAFE model, and how do we find the correct signs?  Our thought was to get the signs from the static analysis, but this does not address the problem of separating the x and y forces.  

I contacted CSI and they suggest the following:

Assuming you are working with AccelX
1) Find the average response of structure for time history analysis.
This can be done by looking at the maximum response of the structure say Base Shear in X-direction for each time history case. Average the base shear for all 7 cases.
2) Modify the scale factor for one-time history case so it can produce the average base shear.
3) Identify the mode shape which has maximum mass participation in X direction and add this Mode using Define > Load Combinations.
4) Look at the maximum over-turning moment due to averaged time history case and the modal case and re-scale the mode shape scale factor which can produce the similar results i.e. average over-turning moment due to averaged time history case.
5) Now look at the reaction for Mode shapes and use that reaction for foundation design.
Repeat the same steps for Y-direction.

This does not work!  The problem with this is that every time I "scale" the single time-history, the building behaves differently, and I get different base shears that do not equate.  I don't get beyond step 2!!

Are there any experts out there that can help me?      

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