I believe I have discovered an error that occurs when combining loads using ABS and SRSS. I've attached a STAAD file that demonstrates this error. Note that this error occurs in STAAD version Select Series 6 20.07.11.50 and not Select Series 5 20.07.10.65.
Essentially the model was run for four primary load cases:
Load Case 1 - X Direction Response Spectrum Analysis (Ex)
Load Case 2 - Y Direction Response Spectrum Analysis (Ey)
Load Case 3 - Z Direction Response Spectrum Analysis (Ez)
Load Case 4 - Accidental Torsion (AT)
There are then four load combinations:
Load Combination 10 - SRSS Seismic w/o AT (Ex^2 + Ey^2 + Ez^2)^0.5
Load Combination 20 - X Direction Response Spectrum Analysis |Ex| + |AT| (call this 'ExAT')
Load Combination 21 - Z Direction Response Spectrum Analysis |Ez| + |AT| (call this 'EzAT')
Load Combination 22 - SRSS with Accidental Torsion (ExAT^2 + Ey^2 + EzAT^2)^0.5
Below are the Fx member force results for Beam# 1 from Beam End Forces tab, Max Axial Forces tab, and Hand Calculating.
Max Axial Forces tab: (Postprocessing=>Beam=>Forces=>Beam Force Detail=>Max Axial Forces)
Beam End Forces tab: (Postprocessing=>Beam=>Forces=>Beam End Forces=>All)
| Beam# | Load Case | Max Axial Force | Beam End Force | Calculated Value |
| 1 | 1 LOAD CASE 1 (Ex) | 9.62 | 9.62 | - |
| 1 | 2 LOAD CASE 2 (Ey) | 1.032 | 1.032 | - |
| 1 | 3 LOAD CASE 3 (Ez) | 6.661 | 6.661 | - |
| 1 | 4 ACCIDENTAL TORSION (AT) | -1.692 | -1.692 | - |
| 1 | 10 SRSS SEISMIC (NO AT) = SRSS(1,2,3) | 11.747 | 11.747 | 11.746 |
| 1 | 20 X AND ACCIDENTAL TORSION | 11.313 | 11.313 | 11.312 |
| 1 | 21 Z AND ACCIDENTAL TORSION | 8.353 | 8.353 | 8.353 |
| 1 | 22 SRSS X, Y, Z = SRSS(2,20,21) | 20.698 | 14.1 | 14.1 |
Load case 10 is calculated by SRSS'ing load cases 1, 2, and 3. The results match for all three columns above.
Load case 20 is calculated by taking the absolute value of load cases 1 and 4 and adding them together. Load case 21 is calculated by taking the absolute value of load cases 3 and 4 and adding them together. The results match in all three columns.
Load case 22 is calculated by SRSS'ing 2, 20, and 21. Notice that the Beam Force Detail tab for load case 22 gives a much larger value than the Beam End Forces tab or Hand Calculated value. It appears that the value given in the Beam Force Detail tab is calculated by adding load cases 2, 20, and 21, not SRSS'ing them. We note that this error does not seem to occur in SRSS load combinations not preceded by an ABS combination (e.g., Load Combination 10 above).
This Max Axial force (20.698 kips)is then used when performing code checks on the members resulting in overly conservative results.
Based on the input file I have attached is the syntax for the load combinations correct for their intended purpose? Is there something I can do to correct this issue with the syntax? Is this a known STAAD error? Are there any workarounds?
Thanks,
Mike Moses
(Please visit the site to view this file)