Design of Foundation for Steel Silo storing wood chips

[Karlos80]

Hi,

I am designing a foundation for 27meter high steel silo, 15m in diameter, full of wood chips (density of green wood chips approx. 350kg/m3). The silo is supported on a braced steel frame structure, 10 meter high on a raft foundation approx. 1.2m deep. I have sized the steel frame and raft based on the wood chip density and wind loads.

However, I was told that the silo and foundations should be designed for the density of water, in the event of fire (deluge system). This would mean that the actual weight would be 3 times more than I assumed in my design.
The ground conditions are reasonable, could get allowable bearing approx. 150 to 180kPa but if I need to assume a silo full of water this could go up to 400kpa. Also the steel tonnage will increase significantly, about twice the tonnage I have now.

I suppose my question is to anybody who has experience in designing silos for wood or similar material. Is this a common approach? Or maybe the silo should have some kind of relief pressure valves?

Thx
Karlos

 

[BAretired]

I cannot claim any experience in this type of design, but the idea of designing for a full head of water seems totally ludicrous to me.

BA

 

[JStephen]

It seems an unlikely requirement, but no specific experience with that application.
A couple of questions. "I was told"- is this the customer/owner telling you this is a requirement for their operation? If so, design accordingly. If it's just somebody else in the office speculating, run the idea past the owner/process designer to see if that is a realistic expection.
Do they have enough water available to actually flood the silo if they wanted to?
Is the bottom of the silo and the attached equipment actually sealed where it would hold water if they did start flooding it?
Is it the intent that if they have a fire, flood the silo, that the silo would then still be usable?

By the way, I have read of them keeping piles of coal wet at power plants to reduce the fire hazard. I have heard that bales of green hay in a big stack can generate heat and spontaneously combust. So it wouldn't be unreasonable that they would have a fire hazard there.

 

[dhengr]

Karlos80:
You might try filling a bucket with wood chips, weighing it; and then filling the bucket and chips further with water, and weighing it again. These would seam to be the extremes, and it would be unlikely that you could have a silo full of water. The chips still take up the majority of the silo volume, and even when drenched they weigh less than water. I agree with JStephen that the silo would likely drain, or should drain. In case of fire all you want is to saturate the wood chips, not to fill the tank with water.

 

[paddingtongreen]

This is a difficult assessment. You need to know what kind of fire suppression system will be provided. If it is an automatic deluge system, you could get the loads you discuss. I tried to fight off similar problems involving oil filled transformers. There is no guarantee that someone will come along and turn the system off before you have the full load.

Someone else on the design team must know what system is to be used.

Sorry, that's not much help is it.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[IDS]

This seems to be a situation where a limit state approach would be beneficial. The absolute maximum vertical load would be with the silo full of water, so the applicable load factor would be 1.0. The bearing capacity should be the design ultimate value, rather than based on an allowable pressure. Does that approach bring the design back to something similar to that required for the other loads?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[JStephen]

Just happened to think...if they fill the silo with wood chips, then flood it with water, does all that floating wood push the roof off the silo?

 

[Archie264]

If you really have to accommodate that I think the foundation will be the least of your worries. Unless you design the structure as a water tank I would think the silo would burst long before the foundation had issues. I would think a better approach would be to design it with drains near the bottom, as noted above, even if the "drains" are simply panels designed to blow out in localized areas in the event of flooding.