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.

Necessity, as they say, drives us to create things that don’t exist yet. These recent times have been such. I’ve been using USFOS² a lot more recently, and I realized that my old syntax file was awfully out of date. Compounding the matter, I had but forgotten all control parameters. This was forcing me to refer to the manual every now and then.

Thought it’s time to get rid of the annoyance of looking-up. So, I wrote these two files:

• Syntax highlighting file—I love highlighted code, highlighted input files. They make understanding much easier by simply highlighting in color. (A command for example, is in Blue, whereas a comment is either in Green or in Grey.) With the syntax file, any available command—within USFOS command library—is now highlighted (or color coded in Textpad).
• Clip library—This second file takes care of my short memory of USFOS control parameters. Open Textpad and double-click on any USFOS control parameter listed in the USFOS clip library, and it pastes a set of USFOS command lines (together with related parameters) at cursor position in the editor.

Installation instructions are in the download file.³ Enjoy.

Update [14 Sep]: The files are now officially available from Textpad site—thanks to Stephen MacDonald, Helios Software Solutions for hosting them.

1. I am great fan of Textpad—for more reasons than one. I have been guilty of using it in its fully functional evaluation mode for years, and with an exception of a rare pop-up, it has never bugged me. A software like this deserves to be bought, and it’s worth every penny. If you’re a regular user, please consider buying it—this is an honest fan’s plug.
2. USFOS is an excellent computer program for nonlinear static and dynamic analysis of space frame structures.
3. I’ve put these files under a Creative Commons License. Free to use academically, professionally, and commercially. If you think this license is restrictive or feel uncomfortable using within your domain, let me know, I am willing to change it.

Whoever knows he is deep, strives for clarity; whoever would like to appear deep to the crowd, strives for obscurity.

So for the rest of us, there’s a nice Oilfield Glossary by Schlumberger, and hopes to make the translation appear less lossy.