Dynamic Analysis of an Elevated Frame Foundation for Pumps

[oengineer]

I am working on the design of 3 pumps siting on top of a circular slab of a wet well.

In PIP REIE 686, it states that "A dynamic analysis of an elevated frame foundation(table-top foundation) shall be required to demonstrate that the natural frequencies of the foundation do not coincide with and are separated from the operating speed range of the equipment by at least 20 percent. The foundation design for variable-speed equipment will require that the foundation be checked for resonant frequencies through the entire range of operating speeds."

Would anyone happen to know a technical document that has an example of performing a dynamic analysis for a table top foundation?

PIP REIE 686 states: A “table-top foundation” is an elevated three-dimensional reinforced concrete structure that consists of large beams or a thick slab connecting the tops of the supporting columns. The mechanical equipment is supported by the large beams or the slab located at the top of the structure.

Suggestions/comments are appreciated.

 

[SlideRuleEra]

...3 pumps siting on top of a circular slab of a wet well.

Likely not a "table-top foundation".

Table-top foundations typically look like this:

http://www.SlideRuleEra.net

 

[WARose]

Would anyone happen to know a technical document that has an example of performing a dynamic analysis for a table top foundation?

The book: 'Design of Structures and Foundations for Vibrating Machines', by Arya, et al has such an example. (By STRUDL.)

But i agree with SLE: likely not a table top for such a situation.

 

[JoshPlumSE]

You might be making this a little more difficult than it needs to be. Typically pumps are pretty well balanced, so they don't impart large forces on their support structures.

Some thoughts:
1) Run an eigenvalue analysis for your slab. Solve for sufficient modes that you get above the operating frequency by at least 50%.

2) Make sure that there aren't any vertical modes within 20% of the operating frequencies of the pumps.

If there are, then you might add more mass, or you might try a more rigorous analysis (time history).

There are some hand calc methods that can calculate the approximate "dynamic ampflication" of a static load based on how close you are to your natural frequency. So, if you're within 19% and don't want to change your design, you could check to see how bad the amplification would be.

 

[r13]

joshplum,

Would multiple pumps on the same support complicate the issue/analysis?

 

[oengineer]

I misread the vendor drawings.

The 3 pumps are mounted to a skid that is sitting on top of the wet well.

The skid will be anchored to a foundation slab around the circular wet well. The reference I mentioned from PIP REIE 686 does not apply.

I need to treat this situation as a foundation for equipments/pumps on skid.

Thank you all for your input.

 

[JoshPlumSE]

If you're doing a complex analysis (i.e. time history), then having multiple pumps would complicate the analysis. You'd have to analyze the response for when the pumps are in-phase vs out of phase.

How heavy are these pumps? Typically, if the equipment is less than about 6 kips and 200 HP then dynamic analysis isn't really needed. And, you just need to make sure the total weight off the support slab is about 3 times the weight of the equipment. Now, that "rule of thumb" is (I believe) more intended for equipment supported on grade. But, at least it should put your situation into perspective.

 

[r13]

And, you just need to make sure the total weight off the support slab is about 3 times the weight of the equipment.

I concur, we were a little more conserve at 4 times

 

[JoshPlumSE]

I should have said "at least" 3 times the weight of equipment rather than "about".

 

[r13]

It's only a convenient/rule of thumb first guess, anyway.

 

[Blackstar123]

Likely not a "table-top foundation

Could you elaborate this point for those of us to whome the reason is not obvious?

 

[r13]

Blackstar,

The OP was going to put the pumps on a "wet well" (partially/fully underground structure), that is a different support type as the table top type support shown in the picture.

 

[Blackstar123]

Oh! Got it.

 

[SlideRuleEra]

Blackstar123 - The wet well the OP described is a buried vertical cylinder with a circular slab on top. The three pumps are sit on the slab.

The "table-top foundation" (TTF) described in the referenced document is an elevated slab sitting on several legs which likely sit on a mat foundation. The slab, legs and mat (which can be pile supported) are usually massive. There will be only one machine (prime mover) on a table-top foundation since a TTF has two purposes:

1) Elevate the mechanical equipment.

2) The entire TTF, including mat and any piles are designed as "tuned" to tend to cancel, or at least not compound mechanical equipment operating vibration. If there was more than on prime mover on a TTF, tuning to specific frequencies needed would not be practical.

TTFs are normally used for very specific purposes, not "general purpose". The ones I'm familiar with are "turbine-generator pedestals" used at electric generating stations:

http://www.SlideRuleEra.net

 

[ldeem]

oengineer I had a project recently where the vendor supplied support had dynamic load issues. I would suggest verifying with the vendor that their skid is specifically designed for the support conditions you are planning. To my thinking when I hear skid I think of a frame designed to be fully supported. It sounds like in your case the frame will be a supported on the ends outside the wet well.

Depending on the vendor they may or may not have used the skid in a similar situation.

 

[JoelTXCive]

How many horsepower are the pumps? And do they run full time. And are there workers that are going to be there full time subjected to the vibrations?

If they are less than 200hp, and the answer to the other questions are 'No', then it's probably easier to skip the dynamic analysis and use ACI's "Rule of Thumb Method (yes, that's the real name). Thicken the top slab to get a 5 to 1 mass ratio of concrete to your equipment.

 

[JoelTXCive]

 

[Blackstar123]

If the vibration is engaging the out of plane stiffness of the top slab only, would it make any difference whether the column are partially burried or not?
Won't the behaviour of a slab on a wet wall and table top foundation be same?
Ignore all the in-phase and out-of-phase loading scenarios and assuming there's only a single vibrating body on the top slab for the sake of simplicity.

 

[JoshPlumSE]

Blackstar -

In my experience, when we use the term, "table top foundation" we're usually talking about some pretty massive equipment and slab thickness on the order of 10+ ft.

For relatively light pumps, the analysis is usually much simpler. Some aspects of the behavior may be similar. But, you get to cut a lot of corners with the smaller pumps as opposed to large turbine generators.

 

[r13]

The table top looks simple, but the number and magnitude of the loads, and the dynamic aspects can drive one to lose sleep, becomes nut.