Hi all,
We are currently working on a project requires Russion SNIP code. In order to speed up things we decided to try Robot Structural analysis which is compatible with SNIP codes. But we are very new to Robot Structural, so we don't know the inner workings and assumptions that is made during the analysis.
So we decided to do a simple study to learn it. However, we have encountered a problem in ROBOT with modal analysis results (different periods with SAP2000). Which we cannot figure out why it happens.
We have modeled the same simple 3D structure in SAP2000 with all material, section properties and loading being identical. So we hoped to see very close results in terms of support reactions, M diagrams, and modal periods.
Support reactions are identical for both ROBOT and SAP2000, M diagrams (for BEAMONLY model) are quite close, still I would expect closer results. But modal analysis results are different.
Building Models:
- BEAMONLY: This is a model with concrete beam-column frames. Loading is a little different, simply all beams have DL2 loading with value 0.5t/m, and LL loading with value 0.75t/m. No floor diaphragm is defined.
Since this structure is very simple, one can expect closer results from both programs.
- BASICFLOOR: This model is same with other model but beams does not have loading instead defined floor members are loaded with DL2=0.2t/m², and LL=0.5t/m². However, in ROBOT we have chosen rigid-floor option so no finite element and no contribution to stiffness matrix. Only simple load distribution algorithm is used to calculate beam loads transferred from floors. On the other hand, SAP2000 does not have such option (at least i don't know how to) therefore we have chosen membrane type element in order not to change lateral stiffness, added diaphragm for rigid floor. And loads on floor is transferred to column tops directly (no beam loading in SAP). Therefore, M diagrams will not agree while total reactions are. But still mass matrix should be similar as a result periods too for both program.
Model T1 T2 T3 ROBOT_BEAMONLY 0.71 0.51 0.48 SAP_BEAMONLY 0.59 0.45 0.40 ROBOT_BASICFLOOR 0.48 0.23 0.23 SAP_BASICFLOOR 0.62 0.45 0.43
Some thoughts:
- Since M diagrams are quite close between Robot and SAP models, I think analytical models are similar. No auto rigid end zone for ROBOT or any other automated model modification affecting the results. Materials definitions should be correct too.
- May be mass definition is somehow different. I actually defined in both program to convert loads to mass in the same manner. Or I think I did it correct.
- Also results of two ROBOT models, BEAMONLY and BASICFLOOR, are unexpected. For model BEAMONLY, total load is less than BASICFLOOR model. SAP results show an increase in period for basic floor model. However, ROBOT results goes other way. It gives higher period values for BEAMONLY model. I am not sure but when I created BEAMONLY model from BASICFLOOR model, I deleted floors and also loads defined for floors. Somehow mass conversion table was deleted. So I redefined them. May be mass is doubled even though I cannot see it on the table. Is there a way to output total MASS in ROBOT?
Any help would be much appreciated, since ROBOT is quite foreign to me at the moment. Thanks.
Best Regards,
Ergin Ceyhan
P.S.: I have provided SAP2000 models in s2k files, and ROBOT 2013 files as attached. M diagrams of BEAMONLY model is also attached.