I recently ran a fairly basic 3D steel frame analysis and design problem in STAAD. Members were grouped and SELECTed and then CODE CHECKed. I know that the SRF for LRFD3 is 0.85 and for AISC UNIFIED is 0.9. The design results from both runs is similar. Both end up selecting the same sizes for the 4 member groups and the RATIO calculations agree with my manual (spreadsheet) calcs.
What surprised me a lot was the difference in analysis results for the COLUMNS.
I made no changes to the loading, yet the ANALYSIS results for axial compression in the two analyses vary by almost 2:1. The LRFD3 analysis produces axial loads approximately DOUBLE of the values produced by the AISC UNIFIED analysis. Why should ANALYSIS results change? The only thing I can think of differences in live load reduction, but I did not touch those sections in the input file.
I would not have bothered too much if the difference were subtle (BEAMS end up with almost identical values in both), but a 2:1 difference in axial load on columns, especially since the beams don't seem to be affected, seems very strange to me.
Any ideas?
Indranil (Maryland, USA)