Offset Loading of Silo

[ldeem]

I am working on a project to add a new circuit to the top of a 50’ diameter 110’ tall concrete silo. Client has asked about installing a stone ladder (material is hydrated lime) on the side wall. The original design is center loading of the silo and I think moving the loading to the side wall will cause bending moments in the wall since the material will be much higher on one side than the other. Angle of repose is around 45 degrees so loading on the side wall will mean a material height difference of 50’.

I can’t find a simple equation in silo design references to address this issue so my plan is to make an FEA model to find the side wall tensions and bending moments. My question to the group is if anyone has done this before on similar silos and did it work out? The analysis will be pretty involved and I don’t want to waste the clients’ money if there is no chance this will work. Reinforcing the silo is not an option.

The silo walls are 10” thick with rings of reinforcing on both faces. So it has some bending moment capacity.

 

[MotorCity]

Can you post a sketch? Not sure what you mean by "a new circuit" and "stone ladder"

 

[ldeem]

I don't really have a sketch but new circuit is additional screening equipment and gates.

The stone ladder is just a vertical series of boxes or ledges so when stone is dropped in the silo it doesn't fall straight to the bottom.

 

[JStephen]

You are correct that such loading will introduce large bending (and shear) forces that were not there previously- also an additional overturning moment on the base. Simplest solution is, just don't do it.
Be aware that, unlike a liquid storage tank, there are large variations in the design assumptions used for loading silos, and you may find that the original design seems very conservative or very unconservative on this basis alone. IE, if you do a FEM analysis of the existing silo as currently operating, there are no guarantees that design will check out, even without the change in loading.

 

[ldeem]

JStephen, I have seen the issue before where I go back to look at something and it seems under designed. I have done a several hoppers and silos in the past and I am familiar with the wide range of values you can get when calculating side loads. It would be nice if the original design included some of those assumptions in the general notes.

 

[JStephen]

Long ago, my employer bid on a project to take the steel plates from two old molasses tanks and build them into one large silo (welded steel). The silo was to be unloaded through a garage door via front-end loader. So filling was symmetrical, unloading was not. Roughly 60' diameter x 40' high tanks, 60' diameter by 80' silo.
For the bid, I went through and designed some large stiffeners to try to stabilize everything. And we were way high on the project.
A year or so later, I drove by and saw what the other contractor had done. No stiffeners. And there was a big ol' buckle up at the top of the silo. I guess nobody cared as long as it didn't fall down.
And, we've got a silo in a town up north of here with a big elephant's-foot bulge about halfway up.
So the moral, these things are NOT always adequate to begin with.