Moody chart v/s FEM analysis for u/g tank
Moody chart v/s FEM analysis for u/g tank
(OP)
I am designing an underground tank with dim 10x15x5m deep.
With normal soil loading (using triangular distribution) i used Moody chart for short wall (5x10) with three edges fixed. At the same time i modelled this tank in STAAD with FEM mesh size of 0.5x0.5x0.4m thk. My problem is, the moments at the slab and vertical wall junction doesn;t match at all.
One thing which comes to my mind is that in Moody chart the edge conditions are fixed where as in actual STAAD model it is in continuation with base slab.
In staad analysis i have used soil springs based on the modulus of subgrade reaction of 15000KN/m3.
These springs have been desfined in vertical dir and in horizontal dir i have released the moments (i guess this is the closest approx of a subgrade).
Can any body tell me why these moments are so different?
With normal soil loading (using triangular distribution) i used Moody chart for short wall (5x10) with three edges fixed. At the same time i modelled this tank in STAAD with FEM mesh size of 0.5x0.5x0.4m thk. My problem is, the moments at the slab and vertical wall junction doesn;t match at all.
One thing which comes to my mind is that in Moody chart the edge conditions are fixed where as in actual STAAD model it is in continuation with base slab.
In staad analysis i have used soil springs based on the modulus of subgrade reaction of 15000KN/m3.
These springs have been desfined in vertical dir and in horizontal dir i have released the moments (i guess this is the closest approx of a subgrade).
Can any body tell me why these moments are so different?






RE: Moody chart v/s FEM analysis for u/g tank
If you were to model just one wall with fixed edges, you should get the Moody results.
RE: Moody chart v/s FEM analysis for u/g tank
can there be a mismatch of this level......moreover this moment varies if i change my spring stiffness...
how exactly spring stiffness controls this moment...?
RE: Moody chart v/s FEM analysis for u/g tank
RE: Moody chart v/s FEM analysis for u/g tank
But still how does spring stiffness control these moments.
If i change spring stiffness, it affects all momemts.
RE: Moody chart v/s FEM analysis for u/g tank
Model a square tank with very stiff springs, and you should be able to mimic the tables. (That won't work with a rectangular tank because of corner rotations.) Then soften the springs to see the effects.
RE: Moody chart v/s FEM analysis for u/g tank
Make some simple models in STAAD to verify your results.
Start with a simple slab that should give you w*L^2/8, etc.
One good intermediate model would be to take your current model and actually fix the bottom and edges. See if that matches your theoretically exact results--it should be close.
Will your deflected shape have the bottom of the slab going upward? If so, will the soil springs be correct or should they be compression only? I think it depends on your situation.
RE: Moody chart v/s FEM analysis for u/g tank
RE: Moody chart v/s FEM analysis for u/g tank
Yeah, I do that a lot myself. Nothing really bothers me here (no headaches or stomach aches), unless I post something I've put some thought into and it disappears, presumably red flagged. I wish I could challenge the red flags.
RE: Moody chart v/s FEM analysis for u/g tank
RE: Moody chart v/s FEM analysis for u/g tank
model is behaving in the same manner as you have said (or the other way round ;) )...
this has really improved my knowledge on underground tank design.....so now i am not worried about the difference in the moments.........
RE: Moody chart v/s FEM analysis for u/g tank
Seeing that your moments are a function of your spring constant, you might consider performing two solutions, with a maximum and a minimum spring constant.