Rectangular Conc. Tanks WWTP Boundary Conditions
Rectangular Conc. Tanks WWTP Boundary Conditions
(OP)
Designing a WWTP with rect conc tanks:
It basically is 4 tanks together (4 cells). Each tanks is 100' x 67' x 18' tall. How should I model the foundation boundary constraints. I did: Vertical soil springs on interior. Around perimeter of group of tanks, I also did vertical subgrade springs, but two other directions were fixed (rotations not fixed).
Am I doing this right?
How thick should walls be? My 200k-ft horiz. corner moment gives 30-36" thick.
Plan view:
_____________
| | | 67'
|_____|_____|
| | | 67'
|_____|_____|
100' 100'
Adam
It basically is 4 tanks together (4 cells). Each tanks is 100' x 67' x 18' tall. How should I model the foundation boundary constraints. I did: Vertical soil springs on interior. Around perimeter of group of tanks, I also did vertical subgrade springs, but two other directions were fixed (rotations not fixed).
Am I doing this right?
How thick should walls be? My 200k-ft horiz. corner moment gives 30-36" thick.
Plan view:
_____________
| | | 67'
|_____|_____|
| | | 67'
|_____|_____|
100' 100'
Adam





RE: Rectangular Conc. Tanks WWTP Boundary Conditions
In the absence of better data (and with a project this size you need better data), I'd use shear cosntraints on each external wall, not normal ones, in the horizontal plane.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Rectangular Conc. Tanks WWTP Boundary Conditions
RE: Rectangular Conc. Tanks WWTP Boundary Conditions
Yes there is a bursting tendency. I'm not sure what this means: "you have suppressed it with your constraints"
I put no boundary conditions on the wall itself, just at the base of the wall. That is, I modeled that slab as about 1000 plate elements. Then the walls are made up of thousands of 5ft*5ft plates. I only put boundary conditions on the nodes at the bottom of the tank.
Regarding the "shear constraints," again I am not putting any boundary conditions on the walls as they are not attached to anything. Just the foundation where it "attaches" to the soil.
Adam
RE: Rectangular Conc. Tanks WWTP Boundary Conditions
I think I get what your saying: I dont need as many translational boundary restaints as I have on the exterior. Currently I have one every 5ft or so around the perimeter ((100ft+100ft+67ft+67ft)/5ft = total 70 perimeter restaints).
Your saying I really only need 2 nodes in the entire model to be horizonal restrained. One should be restained in both directions...the other should be only restained parralell to the wall.
Am I understanding you?
Another question:
I am beginning to think that there should be no rotational restraints at the base of the exterior walls. I am thinking this because the wall plates are fixed to the foundation plates. Therefore there is no need to rotationally fix them.
Am I going in the right direction?
Last question:
Do I have the right idea about every node at the foundation level having vertical subgrade springs (about 56kips/inch)?
RE: Rectangular Conc. Tanks WWTP Boundary Conditions
RE: Rectangular Conc. Tanks WWTP Boundary Conditions
Adam
RE: Rectangular Conc. Tanks WWTP Boundary Conditions
Regards.
RE: Rectangular Conc. Tanks WWTP Boundary Conditions
The tanks have 3 rigid body movements on the soil : 2 translational movements and one rotational movement.
To fix them you need at least 3 constraints as explained by vonlueke.
The recipe for 3D Solid elements without rotational stiffness is as follows :
- Fix two translational dofs of one node. It lacks the rotational movement to fix.
- Choose one another node. The two nodes make a line.
- Fix the translational dof normal to this line to fix the rotation.
It is just a method to fix rigid body movements without adding constraints.
Does it reflect the reality?
Regards,
Torpen.
RE: Rectangular Conc. Tanks WWTP Boundary Conditions
I understand much better now.
-Adam