Bearing plate under a tank

[vesselguy]

Does anyone know or heard of using a "bearing plate" for tanks to distribute the UDL to the soil?

I was totally puzzled a few years ago when a Civil/Structural Engineer, who is responsible for the foundation of the tank I designed and purchased, to add bearing plate (ring) to the underside of my tank to distribute the edge uniformly distributed load. I have never heard of such a thing and is (and still now) not aware of anyone use such a thing under a tank.

She explained to me the reasoning is that the bottom edge of the bottom shell course has to distribute the UDL from the shell weight + roof weight + wind load to the soil and this load has exceeded the max soil-bearing pressure for the site. My main specialty is design of pressure vessels so I quickly understand her reasoning to be same as that used for designing the vessel skirt base plate on a vertical pressure vessel. Now, API-650 do not address this issue. The bottom annular plate per Table 3-1 and paragraph 3.5 sizes annular plate to support a column of water above this plate and that's it. I don't see anywhere in the Standard where my issue is addressed. Note that NO other Civil/Structural Engineer whom I worked with in many EPC have asked for a bearing plate. The bearing plate ended up being a thick ring that is welded directly under the floor plate at the shell the shell to floor junction so that the shell load can be distributed to the soil by the bearing plate.

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Can anyone here tell me if they have encountered this before and discuss the technical merits? Have you provided a bearing plate under your tank before? How does API 650 address this shell line load to the foundation?

 

[MJCronin]

vessel..

As I understand your question, your structural engineer is simply not in agreement with Figure B-1 and B-2 of Appendix B of API-650.

Paragraph B4.2.2 states: "When a concrete ringwall is designed, it shall be proportioned so that the allowable soil bearing is not exceeded"..

Was this done for your tank or was there some change in the design at a late stage ?

Since this is fundamentally a structural/concrete/soils issue, I suggest that you repost in another forum...

regards


-MJC

 

[JStephen]

I haven't run accross that particular issue before. I agree that Fig. B-2 doesn't seem to require this.

More often, if the tank is large enough that this would be an issue, it's on a ringwall anyway, and money spent on that bearing plate would probably be better spent on a better foundation.

Normally, you have an allowable bearing given at a specific depth, but seldom an allowable bearing at the surface- so in most cases, you don't have the information needed to size this ring.

 

[JoeTank]

The use of bearing plates was in API-12C until some time in the late 1950s.

Joe Tank

 

[vesselguy]

Thanks to JoeTank, JStephen and MJC for the reply to my post. I particularly like JoeTank's response.

Since I posted my question, I have talked to a Structural Engineer I worked with at Fluor about this issue. With his input, I have since come to the conclusion that the Engineers that asked me to have the vendor provide a "bearing plate" have not been putting in either a concrete ring wall nor a build up "ring" of crushed rocks compacted to an acceptable bearing pressure. He's been just using a compacted soil foundation!!!! That is why the soil bearing pressure is low and force me to get the vendor to put in a plate to spread the bearing load after the PO was signed. That was 5 years ago, and now I have to work with this guy again. I am going to deal with this issue by specifing a paragraph in the Company Standard to say "a suitable compacted rock ring wall or concrete ring wall shall be provided per API 650, Appendix B, Fig. B-1 and B-2 to support the tank...." I'm Mechanical and I'm not going to argue with a civil/Structural Eng about foundation design, but I do not want to put in a silly looking bearing plate under our shop and field fab tanks.