Staad.Pro Seismic Question

[cawunsch]

I'm using the program to check a pedestrian bridge approach structure for lateral seismic loading. The section is basically a tall, narrow hollow box (20' tall and 10' wide) and is supported by 24" drilled shafts. The lateral seismic loading produces reactions at the drilled shaft supports (which I modeled as springs), but they are always of the same sign. How can this be? Shouldn't the lateral load produce compression in one spring and tension in the other? Staad is telling me that I have no uplift, which doesn't make sense to me.

 

[BridgeEI]

You may want to recheck your support configuration. I know the springs are kind of tricky to get right. For example, you may have specified all of the springs but if the spring isn't in compression the support isn't doing any good. I think you may need to assign a tension or compression and then check your results. I don't have STAAD in front of me at the moment so I'm doing this all off the top of my head so I may be dead wrong on this. Long story short, check the supports.

 

[SethGuthrie]

If you are working with a dynamic response spectrum analysis, that will also produce all positive results (or results without sign more specifically), unless you set up the dynamic load case with SIGN or DOMINANT options. For details, see the Help: Section 5.32.10.1.1 Response Spectrum Specification - Generic Method.