Pump Foundations 2

[Althalus]

I've looked up old posts on this topic and I can't tell if there is consensus or not. So, let me ask this in my own way.

1) A typical pump foundation satisfies the dimensional requirements for an unreinforced pedestal.
2) API 686 RP requires temperature steel of at least 0.0018 ratio. I'm assuming this is in the horizontal direction only.
3) The same practice does NOT state anything about vertical reinforcement. But it does say to follow any other requirements of ACI 318. It may be prudent to consider temperature steel in the vertical direction as well and use a 0.0018 ratio there as well. But it is not clearly stated there.
4) As a column, we would only need to reinforce it at 0.005 ratio. But I do not believe this should be necessary.

This in no way behaves as a column. The aspect ratios in no way resemble a column. There is zero potential for buckling. There is never sufficient bending along any axis to create any tension force in any portion of the concrete.

Is there any requirement anywhere that states such a concrete mass needs to have some minimal vertical reinforcement?

 

[delagina]

small pump foundation only has #5 @ 12" each way at the top. no vertical or bottom reinforcement.

 

[Althalus]

While this may be true for a "small" pump. I'm currently dealing with a very large pump with reciprocating action that requires a 5.5' depth and has multiple layers of reinforcing to meet the 0.0018 ratio requirement horizontally.

I've placed verticals at about 12" centers around the perimeter mostly for bracing purposes. This results in a ratio of about 0.0009 (half of the 0.0018 usually required for temperature steel).

To make matters worse, it is about 27' x 10' x 5.5'. I'm afraid such a mass may require type IV cement.

 

[penpe]

How would one confirm that #5 @ 12" each way meets the ACI code requirements?

 

[rjenyoy12]

Pump foundation are designed as:
1) The foundation should satisfy 3 X the Pump Total Weight
2) Dynamic analysis to check resonance & eccentricity. If the size of pump foundation satisfied the requirement, check using static analysis
3) Calculate pump foundation rebars as a footing wherein Shear and Moment due to pump loads are used
4) Minimimum rebar shall be as per ACI-318 on footing, i.e.,0.0018bt using half for each direction giving 0.0009 ratio.
5) All faces shall have the same rebars as calculated. Note that for pump foundation, reinforcement on all faces are considered.

You may want to check the book Design of Structures and Foundations for Vibrating Machines by Suresh C. Arya.

 

[Althalus]

rjenyoy,

I had a chance to look at that book a year ago when I worked with someone who owned it. It now sells for over $400. It appears to be out of print. At the time I went through it, it didn't have much information on rebar detailing. It went into the theory of vibration just fine. But not a lot on rebar ratios and such.

Please clarify your item #5 for me.

Thanks,

Althalus

 

[delagina]

This seem more like a compressor foundation than pumps.

For pumps, some clients require reinforcing to be placed on all faces. In this case, you will provide U-bars each way at top and bottom enclosed with U-ties.

 

[EngineeringEric]

The past few pump foundations that i have designed have been governed by floating. at 20' below water table you will see some 1300psf on the foundation, then for bearing you will see the weight... so that alone requires reinforcement on both faces. Now i have not done any 5' deep pads. mostly 18-24 inches and then we had 1-2ft extensions to keep my pump from floating.

The one requirement that i have experienced is that the walls and all are to be designed for a concrete strain of .0015 no .003 as typically done with concrete.

Now i may have read this wrong and just interpreted 'Pump Foundation' as what i refer to.

 

[rjenyoy12]

Althalus,

Item #5 means that rebars should be placed on all faces of the pump foundation. So if you came up with a minimum rebar for the top and bottom of the foundation block, then same will apply to all the four remaining sides/faces.

delagina,

Actually, the book applies to both reciprocating or rotary masses (such as compressors, pump and turbines.)

 

[Althalus]

Here's the problem I'm having with using the same rebar everywhere.

The 10' dimension is still good with 0.0018 ratio.
The 27' dimension requires a 0.003 ratio for crack control.
The 5.5' dimension -- I'm not even sure if it requires any concrete except for bracing the rebar in the other directions.

To use 0.003 for the 10' dimension or for the vertical bars does not seem warranted. Is that recommendation from Suresh Arya?

 

[delagina]

I dont design compressor foundation because it is a huge foundation that requires dynamic analysis. We have an "expert" in our office that does that.
I do have the Arya book and it doesnt talk about reinforcement. Most of it are about dynamic analysis.

 

[rjenyoy12]

Suresh Arya only deals with the sizing of concrete blocks using dynamic analysis. Rebar spacing or ratio will be the engineer's discretion based on his analysis and on governing codes/specs such as ACI. So if you came up with the rebar ratio based on the concrete design and found that only minimum rebars are needed then use these.

Rebars should be the same in all faces as the direction of the dynamic forces are in all 3 axis (x, y, z) and that the foundation is designed as a block. If your horizontal rebars governs, then this steel ratio applies to all faces. Reinforcing bars on all faces should be spaced at maximum 12" vertical and horizontally per ACI 207.1R Mass Concrete. The reinforcing bars are relatively small in reference to the concrete block due to its massive size, so the ACI 318 minimum footing/slab requirement of 0.0018 ratio governs.

Also, intermediate horizontal rebars of two-way reinforcing is needed for concrete block/foundation greater than 5ft and spacing between intermediate horizontal layers shall not exceed 3.25 ft meter. This intermediate rebars would need verticals also.

Also check ACI 207.1R Mass Concrete on concrete mixes during construction to prevent hydration.

 

[Althalus]

Thank you, Rjenyoy.

 

[delagina]

rjenyoy,

what reference did you get this?

"Also, intermediate horizontal rebars of two-way reinforcing is needed for concrete block/foundation greater than 5ft and spacing between intermediate horizontal layers shall not exceed 3.25 ft meter. "

also what is the rule of thumb regarding reinforcing for table top?

 

[rjenyoy12]

delagina,

Design considerations for my previous post were from Saudi ARAMCO and Exxon-Mobil GP references.

By the way, my previous post should say "Also, intermediate horizontal rebars of two-way reinforcing is needed for concrete block/foundation with thickness greater than 5ft, and spacing between intermediate horizontal layers shall not exceed 3.25 ft____." Sorry for some typo error & missing words.

For machinery on table tops,the flexibility of the entire structure are to be considered. So the analysis is different from that of a mass concrete foundation as on the case of the OP. Human response to building vibrations are also taken into account. The following guides for table tops are from Exxon-Mobil GP (I just re-worded some).

a) Upper table and the foundation slab should be rigid in the horizontal plane.
b) Weight of the upper table plus 1/2 of the weight of the columns shall be greater than the weight of the machinery.
c) The foundation slab shall not weigh less than the combined supported weight of the upper table, column or walls and the machinery including its base plate/s.
d) Frequencies, degrees of freedom,etc shall be considered where dynamic analysis also come in.
e) Concrete design shall be as per ACI 318 where rebar requirements shall be determined.

 

[Althalus]

I just took that to mean 3.25 ft (1 m).

Since 1 m = 3.2808 ft