How is pressure under tank bottom plate distributed？

[tmgczb]

I am a young, but not very young, civil engineer.
Last month it was the first time I had designed a tank ringwall foundation.
In our calculation report, it is assumed that, weight of shell is transferred to ringwall only, while weight of product on bottom plate and bottom plate itself will be transferred to ringwall and backfill inside ringwall by the area proportion, meaning pressure under bottom plate is uniform expect for its perimeter under shell where load will just be transferred to ringwall.
What kind of support or constraint conditions are provided under tank?
How is pressure distributed under tank?
I'd appreciate if any civil engineer or mechanical engineer could provide some reference.

 

[HTURKAK]

I am a young, but not very young, civil engineer.

How old are you ? If you keep being hungry to learn something new , you are a young engineer . Never be satisfied, and always push yourself.

In our calculation report, it is assumed that, weight of shell is transferred to ringwall only, while weight of product on bottom plate and bottom plate itself will be transferred to ringwall and backfill inside ringwall by the area proportion, meaning pressure under bottom plate is uniform expect for its perimeter under shell where load will just be transferred to ringwall.

Your approach is excellent for gravity loads only. If the tank is located at high seismic zone , this will be another story.

What kind of support or constraint conditions are provided under tank?
How is pressure distributed under tank?

The bottom plate is filmsi when compared with stiffness of the tank shell so the content load will be uniformly distributed . You can model the bottom plate as supported on elastic foundation ( use winkler springs ) and see .
The following best practice doc. is useful for young engineers .

 

[LittleInch]

What kind of support or constraint conditions are provided under tank?
Those sufficient to bear the forces imposed on them by the weight of the product and bottom plates without subsiding the same as any foundation for a UDL.
Commonly well compacted sand on top of ground which has the required bearing capacity.

Poor ground needs piles or extra stone or removal of poor material and replacement with stronger more cohesive material and possibly a concrete slab.


How is pressure distributed under tank?
By having material under the tank floor which doesn't move or subside so the pressure is then evenly distributed. Gaps, voids, tunnels or subsidence lead to collapse or movement of the floor causing failure of the tank in the worst case.

Tank design and construction is just a different type of foundation engineering from the civil side of things. Settlement needs to be very carefully controlled.

 

[tmgczb]

Thank you for sharing the practice.

 

[JStephen]

"while weight of product on bottom plate and bottom plate itself will be transferred to ringwall and backfill inside ringwall by the area proportion, meaning pressure under bottom plate is uniform"

That would be the normal assumption.
It may be possible to have minor voids at laps or edge of padding or grout, which are typically neglected.
The effect of the crown in the tank bottom is typically neglected.

 

[Snickster]

Attached is an old BP spec which may help you.

 

[IFRs]

Have you looked at API 650 Annex B?
Did you account for the hoop stres sin the ringwall?

 

[tmgczb]

Have you looked at API 650 Annex B?
Did you account for the hoop stres sin the ringwall?

Thank you for your reply. I have not read Annex B API 650 yet. Just now I had a brief look, maybe it is what I am concerned with.
My question is why pressure under tank bottom plate, Wp, is uniformly distributed? Backfill and ring wall is different support.
Wp is product load on tank bottom.

 

[tmgczb]

The comparison of tank bottom plate to a film is very vivid. If possible kindly help look for a book or some articles about it.
Which software could be used to model tank bottom plate with Winkler spring elastic support?
Saudi Arabic Code is the same with PIP code. Now I am reading PIP code.

 

[LittleInch]

One thing liquid is very good at is spreading itself out very evenly and creating a uniform pressure.

Fluid pressure acts in all directions at the same time. So say a 5m deep water tank will result in a uniform distributed pressure at the base of 50 kPa.

The tank bottom is very thin in relation to this pressure and will simply deform until the reaction load from the foundations equals the load from the liquid above it.

You can get small voids which the tankfloor can span, but lack of general support or subsidence of the material under the floor will cause the floor to bend and possibly fail.

 

[tmgczb]

The backfilling will settle, that is why cone-up bottom plate is used. Usually we provide a 1:100 slope to reach a 1 inch: 10 foot slope under bottom plate.
But ringwall is concrete. Elastic spring is different around perimeter and in the center.

 

[IFRs]

Yuu can include a small ring of liquid pressure into your ringwall design, it will add some moment, more if the ringwall is side inside. Don't forget that as the soils are compressed vertically, they try to expand horizontally, inducing tension in the ringwall.