Prelude: You might want to see this flow chart [svg], if you’re new to performing seismic analysis.

There’s a new undocumented feature in SACS (5.2) to create equivalent static loads from time history earthquake analysis.

Add ESL in column 33-35 of THLOAD line. Note that ESL option is not recognized in Data Generator 6.0.0.13—SACS’s Input data generator and viewer; and if you use the wizard or data generator again, it may well wipe the unrecognized option out.

Here’s a sample input file, which uses time and displacement—for history.

1. DROPT TIME 20 +Z
2. SDAMP 5.
3. LOAD
4. *--------------------------------------------------
5. * Add "ESL" in column 33-35 of THLOAD line.
6. * Replace MXMMXS with ALL to generate equivalent
7. * static load for each time point.
8. *--------------------------------------------------
9. THLOAD CARD SDO CU ESL MXMMXS
10. *--------------------------------------------------
11. * All factors are taken as 1.0 because the factors:
12. * 1.0, 1.0, 0.5 (for X, Y and Z resp.) would have
13. * already been captured in the Time-History series.
14. *--------------------------------------------------
15. THFACTOR 1. 1. 1.
16. TIME 24.912
17. THBEGIN 3STD C-3 D
18. * time (s) disp (cm)
19. * x-dir y-dir z-dir
20. *THDATA 0. 0. 0. 0.
21. THDATA 0.010 0.065 0.018 -0.317
22. THDATA 0.100 0.645 0.170 -3.115
23. THDATA 0.200 1.265 0.253 -6.088
24. THDATA 0.300 1.858 0.175 -9.209
25. THDATA 0.410 2.457 -0.062 -12.928
26. *----------------------/---------------------------
27. * THDATA continues... the entire time-history is
28. * not listed here for the sake of brevity.
29. *----------------------/---------------------------
30. THDATA 24.512 27.460 2.350 -31.287
31. THDATA 24.612 27.506 3.095 -31.283
32. THDATA 24.712 27.210 3.368 -31.385
33. THDATA 24.812 26.748 3.249 -31.417
34. THDATA 24.912 26.143 2.885 -31.344
35. THDATA
36. END
37.  
38. Download this example: /inputfiles/th-seis-dyr.txt

I am generally comfortable with SACS's spectral seismic analysis capabilities. In this case however, I had no spectra, but I had USFOS time-history results of the host structure on which the structure I intended to analyse was mounted. From support points, I averaged history (displacement)— for each direction—and used it in the input above.

Nodal displacement v. Time — an USFOS generated plot.

Update [13.Feb.2008]: There is a SACS bug in time-history generated seismic (inertia) loads.¹ The generated seismic (inertia) loads do not match the generated base shear. I believe the base shear is about right, but the seismic loads (Fx and Fy) do not add up to become:

√(Fx² + Fy²) = Base shear .

A workaround until this is fixed—if you have no choice but to perform a time-history seismic analysis—would be to factor up the seismic (inertia) loads to match the generated base shear for a code check. You risk underestimating seismic loads if you do not.

1. This bug does not exist if you perform a spectral seismic analysis. [←]

analysis, earthquake, input, sacs, usfos

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