STAAD TANK ANALYSIS - STIFFNESS
STAAD TANK ANALYSIS - STIFFNESS
(OP)
I'm analyzing a large, multi-cell concrete tank in STAAD. I've "cut" the tank into a quadrant to make the model small enough to analyze. I feel I need to place "fixed-but" supports at the wall edges that would have been connected to adjacent walls. I think I need to provide a spring stiffnes for all degrees of freedom to make the model think it is still connected to other elements. Has anyone ever calcualted this spring stiffness before? I've used this eqation: K = (Ec/48)*(tw/h)^3 which I took for a uniform cantilever wall from ACI 350.3. I'm not getting correct results in my model. Any thoughts?
Thanks,
Jason
Thanks,
Jason






RE: STAAD TANK ANALYSIS - STIFFNESS
RE: STAAD TANK ANALYSIS - STIFFNESS
You could try a smaller model of similar proportions and develop an empirical stiffness that you could apply to the larger model. You may have to do some iterations but I think this approach sounds reasonable.
RE: STAAD TANK ANALYSIS - STIFFNESS
I know high end FEM programs (ansys and such) use a symmetrical boundary condition.
Below are responses the responses I got from Bentley
Dear User,
Further to the response below, one of my senior colleagues had the following suggestion:
Actually, if the Y-Z plane is the plane of symmetry, the symmetrical supports are
FIXED BUT FY FZ MX
An anti-symmetrical model with anti-symmetrical loads can be defined as well. Then Symmetrical plus anti-symmetrical becomes the response of the structure in the plus X half of the complete structure; and Symmetrical minus anti-symmetrical becomes the response of the structure in the minus X half of the complete structure.
And from a previous email.
Dear User,
In case, the structure is symmetric (and the loadings are symmetrical as well), you can indeed model only half of the structure with the line of symmetry being adequately supported (supports being FIXED BUT and free in the Y direction and restrained in the other directions).
However, these methods are appropriate if the algorithms are old and slow. But STAAD has the advantage of possessing the advanced analysis engine and hence, from the analysis point of view, it would not be a problem to model the full structure.
From the modeling point of view as well, if you model half of the symmetrical structure, you can easily model the other half using the "Mirror" facility within a very short interval.
I hope this helps.
RE: STAAD TANK ANALYSIS - STIFFNESS
how would you change the support condition for different load combination..
RE: STAAD TANK ANALYSIS - STIFFNESS
That being said replacing half the structure with a symmetric Boundary condition is very easy in just about any program (RISA or STAAD) as long as the symmetry is about one of the global axes. If the symmetry is about an inclined axis then it would get a bit more complicated. But, that's just because you have to define an "inclined support".
RE: STAAD TANK ANALYSIS - STIFFNESS