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Foundation Design Philosophy for Rotating Equipment

References has been taken from,

1. Design of structures and foundations for vibrating machines by S. Arya, M. O'Neill and G. Pincus

2. Foundation analysis and design by J. E. Bowels

3. Dynamics of bases and foundations by D. Barkan

4. Design of Machine Foundations - Lecture Notes of Professor M.H. El Naggar, Department of Civil Engineering,

The University of Western Ontario, London, Ontario, Canada, N6A 5B9

In this page I will talk about the rigid block foundation for Centrifugal (Pump) and Reciprocating machines

(Compressor). We are considering the concrete block is infinitely rigid and thus a lump mass model can be

considered in computer 3D modelling. To start the design of a block foundation, we need to follow the following

steps to collect the design data:

Step-1 : Review of pump / compressor drawing (Vendor Equipment Drawing)

The machine data pertinent to the dynamic analysis and design of the block foundation should be obtained

from vendors.

Plan dimension of pump / compressor base frame

Height of rotor / shaft center line from the bottom of skid

Anchor bolt location, size and embedment depth

Weight of machine parts and the rotor parts (pump / compressor rotor and motor rotor)

Location of center of gravity both vertically and horizontally

Operating speed of machines and power rating of motor (RPM)

Magnitude and direction of unbalanced forces.

For reciprocating machines both primary and secondary unbalanced forces and couples and respective

CG locations needs to be checked.

Limit of deflection and vibration amplitudes at center line of rotor.

Step-2 : Collection of Geotechnical / soil data (Pl discuss with soil consultant and look into project design criteria)

The Geotechnical data are used for evaluating the soil / pile stiffness and damping coefficients, and are

required for both static and dynamic design and analysis of of block foundations. Following soil parameters

are required:

Soil weight density

Poisson's ratio

Dynamic shear modulus (G)

Shear wave velocity (vs)

Dynamic modulus of sub-grade reaction (ks)

Allowable soil bearing pressure or pile load carrying capacity for design of foundation

Step-3 : Categorization of rotating machines based on machine speed:

The rotating machines are categorized based on machine speed. Following are different categories of

machines:

Low Speed machine: The low speed machines operate at a speed range of less than 500 RPM. High

tuned foundations, having first natural frequency more than machine's operating speed, should be

designed for this type of machines. In this case machine do not pass the resonance during machine start

up and coast down condition.

Intermediate speed machine: The intermediate speed machines operate at a speed range 500 RPM to

1000 RPM. Foundations should be designed for this type of machines high tuned or low tuned side

whchever more practical. If the foundation is low tuned, dynamic amplitude shall be checked during start

up and coast down condition.

High Speed machine: The high speed machines operate at a speed range of more than 1000 RPM. Low

tuned foundations, having first natural frequency less than machine's operating speed, should be

designed for this type of machines. In this case machine will pass the resonance during machine start up

and coast down condition. Dynamic amplitude shall be checked during start up and coast down

condition. You need to ensure that there is no adverse effect to machine operation during the resonant

conditions.

Variable Speed machine: The variable speed machines operate at a speed range as prescribed by

vendor. Foundations should be designed for this type of machines high tuned or low tuned side whchever

more practical. A detail dynamic analysis of foundation is required for a range of machine operating

speeds to ensure that the dynamic design criteria are met.

Step-4 : Preliminary sizing of foundations:

A block foundation consists of massive concrete blocks, piers and mat foundation. The preliminary sizinng of

block should be based on the following:

Weight of the block foundation should be at least 4 times the weight of reciprocating machines and 3

times the weight of centrifugal machines.

The width of foundation should be at least 1.5 times the vertical distance from the bottom of foundation to

the center line of the shaft / rotor.

The center of mass of machine foundation (machine+foundation system) should coincide with the

centroid of the soil foundation or pile group resistance. Horizontal eccentricity should be limited to 5% of

the corresponding foundation dimension.

For a rigid mat, following criteria to be followed:

Minimum thickness of the mat will be 600 mm or 1/5 th of least foundation dimensions or 1/10 th of

largest foundation dimensions, whichever is greater .

Maximum thickness of the mat will be 1500 mm

Minimum thickness of mat, t = 0.0012 x (ks x (a)4)1/3 ft, ks = soil dynamic modulus of subgrade

reaction, lbs/in3, from soil report, a = maximum cantilever projection (inches), measured from

face of block (Refer: Foundation analysis and design by J E Bowles)

Step-5 : Requirement for dynamic analysis of foundations:

Dynamic analysis of concrete foundations are not required for all the foundations supporting rotating

equipment. You need to refer your project design criteria for the conditions for dynamic analysis. Following are

the general criteria for not performing any dynamic analysis of foundation supporting rotating equipment:

Dynamic analysis is not required if the weight of machine is less than 25kN.

Dynamic analysis is not required if the power rating of motor is less than 200hp.

If you are not doing any dynamic analysis of concrete block foundation, then follow Step-4 for foundation sizing

and put it into 3D model for any interference check.

If you are doing the dynamic analysis of concrete block, then follow the following steps.

Step-6 : Calculation of un-balanced forces for dynamic analysis of foundations:

If unbalance force is not mentioned in the Vendor equipment drawing, then you will calculate the force as

follows:

Un-balance force for pump: Fpump = mp-rotor x e x w2

Un-balance force for motor: Fmotor = mm-rotor x e x w2

Where, mp-rotor = weight of pump rotor, mm-rotor = weight of motor rotor

w = circular frequency = 2 x pi x (f / 60), f = speed of machine from vendor drawing (RPM).

e = rotor eccentricity, depends on machine speed

e = rotor eccentricity, depends on machine speed

Eccentricity Table ( Refer reference -1)

Machine Operating Speed (f in RPM) Eccentricity e (mils)

Pump / compressor f < 3000 (1.8-107) / (f)2

Pump / compressor f > 3000 (12000/f)1/2

Motor f < 1500 1.5

Motor 1500< f < 3000 1

Motor f > 3000 0.5

Now you are having all the information to start the foundation analysis and design. You can put all the above data in

any computer software program (say - Dyna5) or use any text books to calculated the natural frequencies of

foundation. You can also use the different tables that I have attached here (click for the table).

Natural frequency analysis of foundation:

This rigid block has six degree of freedom. So, you will calculate all the following uncouple natural frequencies:

1. Sliding Frequency along horizontal X-direction

2. Sliding Frequency along horizontal Y-direction

3. Sliding Frequency along vertical Z-direction

4. Rocking Frequency about X, rotational mode

5. Rocking Frequency about Y, rotational mode

6. Rocking Frequency about Z, rotational mode

When the CG of foundation system is far above the foundation base, coupling effect needs to be considered to

calculate the foundation natural frequency. In this case sliding mode and rocking mode frequencies overlap each

other and as a result foundation dynamic analysis may be more critical. You can calculate the coupled natural

frequency using the formula mentioned in the table.

frequency using the formula mentioned in the table.

Coupled condition: 1. Sliding along X & Rocking about-Y and 2. Sliding along Y & Rocking about X

Once, analysis is completed, please check the foundation for the following conditions:

Resonance Frequency Check:

Calculate resonanace frequency and check that the ratio of machine frequency vs resonance frequency (f /

is either less than 0.8 or greater than 1.2 in all six degrees of freedom.

Resonance frequency can be calculated as follows: fd = fn / (1-2 x D2)½

where, fn = foundation natural frequency, D = Damping ration (see table 5 and 12)

Soil Bearing Pressure / Pile Capacity Check

Soil bearing pressure or pile load should not exceed 75% of the allowable. Please avoid any foundation

upliftment in seismic / wind condition.

Maximum Velocity check:

Maximum velocity should fall in "Good Condition" per table-1 of attached table

Environmental condition

Maximum displacement amplitude of vibration at foundation level should lie within or below "Zone-B" of figure -

1 and it should fall below the Zone "Troublesome to persons" of figure -2 in the attached table.

Reinforcement:

Reinforcement shall be provided per project approved design code. However, you can use minimum

reinforcement as follows:

0.2% rebar on all face of concrete block and mat.

1% rebar for all concrete pedestal.

Rebar spacing should not be more than 300 mm.

Anchor Bolt:

Anchor bolt shall be checked for start-up and coast down contion.

For a typical pump foundation drawing click here

I hope this page will be very helpful to you to understand the basic design of a Pump foundation.

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Copyright 2009. All rights reserved. Please do not print or copy of this page or any part of this page without written permission

from Subhro Roy.

Disclaimer: This page is prepared based on experience on Civil Engineering Design. All definitions and most of the

explanations are taken from different text books and international design codes, which are referenced in the contents. Any

similarity of the content or part of with any company document is simply a coincidence. Subhro Roy is not responsible for that.