3D Correct Modeling
3D Correct Modeling
(OP)
I have been designing 3D structures(by STAAD) for powerplant facilities(i.e. piperacks, shelters, etc.) and i usually designate a pinned support as long as theres a grade beam in it. some engineers i noticed, they model it using fixed support(even with the presence of grade beam) and others, they model it with pinned supports (even w/o grade beams). in my point of view, for a 3D structure w/ a grade beam, i usually model my supports as pinned since the grade beams will take care of the moments and design my foundation from axial load only(smaller fdn). without grade beams, i model it using fixed supports and design my foundations from axial & moment (bigger fdn). Anyone who can suggest a better modeling so i can improve and at the same time rectify any mistake through your valuable comments. Thank you very much!






RE: 3D Correct Modeling
I agree that a properly designed grade beam could relieve the foundation of the moment. But, the moment is transfered to the grade beam only through the column at the bottom and the column need to be designed/checked for the moment. Further,you need the moment at the support to design the grade beam.
A pinned base does not provide this moment and hence does not represent the actual structure
Hence, the support shall be modeled as fixed and if you design the grade beam for the base moment, the foundation can be spared of the moment, but do not forget to add algebraically to the column axial force, the push-pull effect of the moment on the two columns connected by the grade beam.
Good luck
RE: 3D Correct Modeling
RE: 3D Correct Modeling
if there is no grade beam, then the support should be modeled according to the approximate condition of the actual connection.
good luck
RE: 3D Correct Modeling
To get around that, one must use the Fixed command and release all the components that you want based on the design. In other words, I can model in STAAD a pinned support using the Fixed command but releasing the moments and force components.
Be careful because pinned is not always true!
Good luck
RE: 3D Correct Modeling
The model I suggested does not include the grade beams. To avoid the anomaly that you have described, we can model the frame without grade beams with fixed support and design the foundation and the grade beams by taking moments on the grade beam and the axial force added with the push-pull effect on the foundation separately.
Alternatively, if you want to include the grade beams also in the model, do not provide the grade beams at the footing level. Provide the beams at a slightly higher level from the foundation and make the support pinned. In such case, the grade beams will experience the moments and also, the effect of the push-pull between the adjacent columns will be accounted for.
Again, one has to be careful to model the small bit of column between the grade beam and the support with sufficiently small stiffness. Otherwise, the short length of column will act as very stiff and draw more moment to itself at the junction with the grade beams, showing less moments on the grade beams. It will also show a large horizontal reaction at the support due to the small vertical lever arm.
Hope I am clear enough.
Good luck
Trilinga
RE: 3D Correct Modeling