I'm running a test STAAD model to practice performing a dynamic analysis of a 3 story concrete framed structure with concrete shear walls. The 1st and 2nd stories are modeled as rigid diaphragms while the roof is modeled as a flexible diaphragm. I've applied the dead loads as well as the dynamic masses as reference loads. When I run the model, I get a base shear of 514.79 kips using the ELF procedure but my dynamic base shear is only 263 kips. That's only ~50% of my static base shear (much less than the minimum 85% required per ASCE 7-10). Is this a common error with STAAD? My dead loads and dynamic masses are identical so I don't see how I can get that much of a difference in base shear. Is there something I'm doing wrong or is this common and I need to go in and scale my forces per ASCE 7-10 section 12.9.4.1 (basically 0.85*static base shear divided by dynamic base shear).
Here's a summary of my reference load cases:
DEFINE REFERENCE LOADSLOAD R1 LOADTYPE Dead TITLE DEAD LOADSELFWEIGHT Y -1 FLOOR LOAD_1ST_FLOOR FLOAD -0.1 GY_2ND_FLOOR FLOAD -0.1 GY_ROOF FLOAD -0.05 GYLOAD R2 LOADTYPE Mass TITLE DYNAMIC MASSSELFWEIGHT X 1 SELFWEIGHT Y 1 SELFWEIGHT Z 1 FLOOR LOAD_1ST_FLOOR FLOAD 0.1 GX_1ST_FLOOR FLOAD 0.1 GY_1ST_FLOOR FLOAD 0.1 GZ_2ND_FLOOR FLOAD 0.1 GX_2ND_FLOOR FLOAD 0.1 GY_2ND_FLOOR FLOAD 0.1 GZ_ROOF FLOAD 0.05 GX_ROOF FLOAD 0.05 GY_ROOF FLOAD 0.05 GZEND DEFINE REFERENCE LOADS
Copy of my model:
Dynamic Analysis test part 2.STD
I looked at the model and did not see any error in the input as such. I think the difference for this particular structure has to do with the fact that the dynamic response is not from a single dominant mode but is distributed over a range of modes. Scaling the dynamic response, as you indicated, would be the right approach in this case.
Thanks!