Machine Foundation Design - An Introduction

Machine Foundation Design | Lawrence Galvez
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MACHINE FOUNDATION DESIGN
Lawrence O. Galvez, LM.PICE, M.ASEP
ABSTRACT: This lecture paper serves as an introduction in the complex analysis and design
of machine foundation which involves familiarity in areas of structural dynamics, structural
engineering and geotechnical engineering. This paperis aimed for graduate or practicing civil
or structural design engineers who are new to the subject but is familiar with the areas of civil
engineering knowledge cited above; although this paper can also be a handy reference for
those already doing machine foundation design. It discusses a brief review of structural and
soil dynamics which both play important roles in response of a machine-foundation system
due to vibratory loadings induced by the operation of the machine. Current state-of-the-art
approach in creating mathematical models using finite element method is discussed together
with a recommended step-by-step procedure in doing the analysis and design to satisfy
strength and serviceability requirements. An example design of a block foundation is shown
to help the reader familiarize in the design process and use rational assumptions in getting a
good preliminary geometry of foundation before starting with a detailed analytical model.
Main references for this paper were taken from the course syllabus of Advance Foundation
Engineering & Design in Graduate School of Engineering of University of the City of Manila
but other salient references were gathered from internationally recognized codes, standards
and practices.
KEYWORDS: machine foundation; resonance; rocking vibration; soil dynamics, block
foundation
1. INTRODUCTION
Foundations supporting vibratory loadings are often present in the industries of oil & gas,
petrochemicals, heavy manufacturing and power generations. These vibratory loadings are caused by
the unbalanced machine forces as well as the static weight of the machine. If these vibrations are
excessive they may damage the machine or cause it not to function properly. It should be noted that the
initial cost of construction of a machine foundation is generally a small fraction of the total cost of the
machine, accessories and installation, but the failure of the foundation as a result of poor design or
construction can translate to heavy dollar losses (Prakash and Puri, 1988). For example, a turbo-
generator unit in a power plant may cost around $20 to $ 50 million and an estimated loss of $250,000
per day if it malfunctioned; it is therefore crucial for a foundation engineer to give special attention to
the foundation design of vibrating machines.
Historically, analysis and design of machine foundations were done by approximate rule-of-thumb
procedure which consist of sizing the foundation based on the weight and type of vibrating machines
being supported or by strengthening the soil beneath the foundation by using piles. These procedures
generally works; however, this often results to considerable overdesign (Bowles, 1997).
With the advent of improved manufacturing technology, machines now are of higher ratings which give
rise to considerably higher dynamic forces and thereby higher stresses which demands improved
performance and safety (Bhatia, 2008).
The analysis and design of foundation and structures subjected to vibratory loads is considered a very
complex problem because of the interaction of structural engineering, geotechnical engineering and the
theory of vibrations (Arya, 1979); added to these, from author’s experience, is that a designer should
be keen in the details of machine’s (equipment’s) loadings, configurations or generalarrangements and
design operations which are already a basic part of mechanical engineering. However,due to practical
availability of commercial finite element analysis software nowadays, a designer can create a good
mathematical model of machine-foundation system with reasonable time to have a reliable prediction

P a g e 2 | 43
of the dynamic response of the system and create necessary changes in the model to mitigate unwanted
amplitudes of vibrations i.e. to control the vibrations in the system.
2. STRUCTURAL DYNAMICS
2.1 Static vs Dynamic Analysis of Foundations
Design of concrete foundations supporting static or pseudo-static loadings usually involves
computation of internal forces (e.g. axial, shear, moments, etc.) by satisfying the equations of
equilibrium (FH=0; FV= 0; FM= 0) and then check for strength and serviceability requirements.
Static loadings are dead loads or permanent loads while the pseudo-static are occupancy live loads,
wind and earthquake loads. (Note that earthquake loads mentioned denotes the use of approximate
equivalent static lateral force procedure per building codes). For static analysis the material and
geometric properties needed are density, Young’s modulus of elasticity, stiffness, thermal coefficient
and Poisson’s ratio.
For foundations supporting large vibratory a.k.a. “dynamic” loadings, a dynamic analysis is required
to determine the dynamic response of the structure. The response of the structure is measured in the
form of vibration amplitudes (e.g. displacements, velocity or accelerations); these are done by
satisfying the equation of motions based on Newton’s second law of motion (i.e. F = m a). In dynamic
or vibration analysis, in addition to material and geometric properties needed in static analysis;
damping coefficient and natural frequency of the machine-foundation system are required.
2.2 Theory of Vibrations
In the analysis and design of machine-foundation system, a designer should be first familiar with
fundamentals of theory of vibration. As mentioned previously, traditional approach in controlling
vibrations in design of foundations was to increasing the mass (or weight). It was only in the 1950s
where foundation engineers begin to use vibration analyses which was based on a theory of a surface
load on elastic half-space (i.e.the portion of supporting soil supporting the foundation which is assumed
to behave elastically), see Figure 1.
Figure 1. Foundation base in equilibrium position just prior to being displaced slightly
downward by a quick push
(Source: J.E. Bowles, 1997)
According to Bowles (1997), Figure 1 is similar to a beam-on-elastic-foundation case except the beam
uses several static soil springs in the analysis but here the foundation uses only one spring and it is a
dynamic soil spring i.e. dependent on time. Note that since the usual value of unbalance loads is

P a g e 3 | 43
relatively small compared to the weight of the machine-foundation system the assumption that the soil
behaves elastically is acceptable. The soil spring in (a) has been compressed in an amount of zs =
W/Kz wherein zs and Kz are both static values; in (b) the footing experiences a quick push in the z
direction and a quick release which makes the block moves up and down (i.e. it vibrates), hence,the zs
and Kz at this time are dynamic values.
Displaced or compressed value:
In Figure 1a, we can write the differential equation to describe the motion in a form of F=ma from
Newton’s second law of motion to be:
Solving by the methods given in differential equation textbooks afterdividing through by the massterm
m and defining 2
n = Kz / m,also known as the angular frequency, we can obtain the period of vibration
T as:
And the natural frequency fn as any one of the following:
From Eq. (b) it would appear that the vibration will continue forever; we know from experience that
this is not so. There must be some damping present,so we will add a damping device termed a dashpot
(analog = automobile shock absorbers) to the idealized model. To maintain symmetry we will add half
the dashpot to each edge of the base as shown in Figure 1b. Dashpots are commonly described as
developing a restoring force that is proportional to the velocity (ź) of the moving mass being damped
(i.e. being restrained). Knowing this conceptof the dashpot (also damping or viscous force) cz,a vertical
force summation gives the following differential equation:
Equation (c) is the equation of motion for a Single Degree of Freedom System (SDOF) with damping.
The solution of this equation yields the dynamic characteristic of the system e.g. natural frequency of
the system.
From the solution of the differential equation (c), the critical damping (in units of force/velocity) is
defined as:
Critically damped system happens when,

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When a forcing function, F, is introduced to the system, the equation becomes
According to Arya (1979), vibrations developed by operating machinery produces several effects that
needs to be considered in the design aside from the usual static loads. The usual procedure in doing a
vibration analysis is to establish a mathematical model of the realstructure.The structuralconfiguration
of machine foundation is generally determined by geotechnical consultant and machine manufacturer,
these initial configurations may change to suit design criteria or avoid interference of other fixed objects
such aspipelines and other ancillary items. The common configurations or types of machine foundations
are shown in Figures 2a and 2b.
Figure 2a. Types of machine foundations (a) Block foundations (b) Box or caisson
foundations (c) Complex foundations
(Source: S. Prakash, 1991)
F (t) (d)

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Figure 2b. Types of foundations for vibrating machines
(Source: S. Arya, 1979)
The following basic definitions are important in the vibration (dynamic) analysis:
Vibration - an oscillation of the partsof a fluid or an elastic solid whose equilibrium has been disturbed
Period (T) - if motion repeat itself in equal intervals of time, it is called a periodic motion and the time
elapsed in repeating the motion is called its period of vibrations. It is the time needed for one
complete cycle of vibration to pass a given point. As the frequency of a wave increases the period of
the wave decreases, i.e. frequency is the reciprocal of period
Figure 3. Graph of Period of vibration along time, t
Cycle - motion completed during a period is referred to as a cycle
Frequency (f) - the number of cycles of motion in a unit of time is called the frequency of vibrations
Natural frequency (fn) - if an elastic system vibrates under the action of forces inherent in the system
and in the absence of any externally applied force, the frequency with which it vibrates is its natural
frequency
Forced vibrations - vibrations that occur under the excitation of external force are termed forced
vibrations
Degrees of freedom (n) - number of independent coordinates necessary to describe the motion of a
system specifies the degreesof freedom of the system. When a system have severaldegrees of freedom,

P a g e 6 | 43
the system is called multidegree of freedom system (MDOF). Figure 4a shows degrees of freedom a
rigid block foundation. Figure 5 shows systems with different degrees of freedom.
Figure 4a. Degrees of freedom of a rigid block foundation
The rigid block foundation has six degrees of freedom (also called modes of vibrations)
1. Translation along Z axis
2. Translation along X axis
3. Translation along Y axis
4. Rotation along Z axis
5. Rotation along X axis
6. Rotation along Y axis
(Note: Each degrees of freedom of the system will produce different natural frequencies)

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Figure 4b. Types of motion of a rigid foundation due to unbalanced forces of
reciprocating machines: (a) pure vertical translation; (b) pure rocking; (c) simultaneous
horizontal sliding and rocking (d) pure torsional oscillations

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Figure 5. Systems with different degrees of freedom (a) One degree of freedom (n=1);
(b) Two degrees of freedom (n =2); (c) Three degrees of freedom (n=3); (d) Infinite
degrees of freedom (n=∞)
Resonance - if the frequency of excitation force coincides with any one of the natural frequencies of
the system, resonance will occur. This is a phenomenon that is being avoided in machine foundation
design because it will the amplitudes (e.g. displacement, velocity or acceleration) of vibrations to be
excessive and cause damage to the system.
Normal mode ofvibrations - when the amplitude of some point of the system vibrating in one of the
principal modes is made equal to unity, the motion is called the normal of vibrations


n = 1
Simple pendulum
(a)
(b)
n = 2

Equilibrium
Position
(c)
(d)
n = 3
m1
m2
m3
n = ∞

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Damping - damping is associated with energy dissipation and opposes the free vibrations of a system.
If the force of damping is proportional to its velocity, it is called viscous damping. If it is not dependent
on its material property and it is contributed by geometry of the system it is called geometrical damping.
2.3 Harmonic Motion
The simplest form of motion is harmonic motion,which is represented by sine or cosine functions. The
excitation in structures or foundations caused by the unbalanced force in the machines are generally in
the form of harmonics under steady-state condition.
Rewriting equation (d) with a harmonic force;
where; Fo = unbalanced mass of the machine; t = time
Fo sin (t) (e)

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In dynamic system, the excitation force present arises out of unbalances in the rotating masses. Shown
below are the forces generated for reciprocating and centrifugal machines.
Figure 6. (a) Crank mechanism of a reciprocating machine; (b) Forces from a
centrifugal machine (rotating mass excitation)
For reciprocating machines:
For centrifugal machines:

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Figure 7a. Rotating vector representation of a harmonic function x = A sin t
Figure 7b. Harmonic motion representation of displacement (x), velocity (ẋ) and
acceleration (ẍ)

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3. VIBRATING (DYNAMIC) MACHINES
There are many kinds of machines that generates periodic forces; the three most important categories
are:
a. Reciprocating machines: machines that produced unbalanced force (such as compressor and
reciprocating engines). The operating speeds of such machines are usually less than 600 rpm.
For the analysis of their foundations, the unbalanced forces can be considered to vary
sinusoidally.
Figure 8a. Single cylinder crank mechanism (a simple reciprocating machine)
(Source: Prakash and Puri, 1988)
Figure 8b. Reciprocating machine diagram
(Source: ACI 351.3R, 2004)
b. Rotary machines: high-speed machines like turbogenerators or rotary compressors may have
speed of more than 3,000 rpm and up to 10,000 rpm. High speed rotary machines are well
balances and the eccentricity e is generally very small. However, due to their high speed of
rotation, the magnitude of exciting loads may be significant and may cause the effective
eccentricity to increase due to wear and tear.

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Figure 8. Unbalanced forces due to rotary machines, (a) single rotor; (b) two rotors with
equal unbalanced forces in phase; (c) two rotors with equal unbalanced forces but with
a phase difference of 180o; (d) two rotors with equal unbalanced forces at any phase
(Source: Prakash and Puri, 1988)
Dynamics loads during operations are caused by unbalanced loads (created when the mass centroid of
rotating part does not coincide with the center of rotation) with a frequency corresponding to machine
speed. These loads are usually given by the machine manufacturer. If not, they may be calculated via
the balance quality grade G of the rotor.
G = e 
where; e is in (mm) and  is in (rad/s)
From ISO 1940/1 specification the usual value of G is 2.5 mm/s

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Figure 9. Rotating machine diagram
(Source: ACI 351.3R, 2004)
Figure 10. Simple cycle gas turbine
(Source: General Electric)
c. Impact machines: machines that produce impact loads like forging hammers. Their speeds are
from 60 to 150 blows per minute. Their dynamic loads attain a peak in a very short interval and
then practically die out.
3.1 Typical assembly of machine
Machine would necessarily include:
- A drive machine
- A driven machine
- A coupling device

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3.2 Machine - Foundation Loading Data
Loadings due to self-weight of machine and unbalanced (dynamic) loading is provided by machine
manufacturer. This is sometimes called vendor data. Other loadings due to machine operation are listed
including loading due to temperature operation, accidental loads due to malfunctioning, loss of blades,
emergency torques, etc. A sample of foundation loading data is shown below.

P a g e 16 | 43

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3.3 Machine - Mechanical Outline
A designer should review the mechanical outline (drawings) of the machine to properly model later the
machine in the structural analysis. Shown below is a sample of machine drawings from vendor.

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3.4 Machine - Anchor bolt layout and details
The layout of anchor bolts should be carefully reviewed by the foundation designer to make sure all the
location of loadings will match on what is considered in the analysis. Shown below is a sample of
anchor bolt layout and details of a machine.
3.5 Machine - Dynamic Loads from Vendor Data
The rated (operating) and critical speeds of the machine and unbalance loadings are taken from vendor
data.

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3.6 Machine - Suggested Load Combinations from Vendor => there are specific load combinations
usually recommended by vendor to protect the machine during operation and accidental situation. The
designer should include this in his analysis and design.
3.7 Machine - Dynamic Design Criteria
Sample of dynamic design criteria for rotating machine is shown below

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Allowable amplitude of vertical vibration (note: 1 cps = 1 Hertz)
4. SOIL DYNAMICS
Definitions
Soil Mechanics - coined by Dr. KarlTerzhagi who is recognized as the “father of soil mechanics”.
Soil mechanics deals with engineering properties of soil under stress.
Soil dynamics - is that branch of soil mechanics which deals with engineering properties of and
behavior of soil under dynamic stress.
Types of dynamic loads that may act on foundations and structures:
- earthquakes
- bomb blasts
- operation of reciprocating and rotating machines and hammers
- construction operation (such as pile driving)
- quarrying
- fast-moving traffic
- wind

P a g e 21 | 43
- wave action of water
Earthquake constitutes to the single most important sources of dynamic loads on structures and
foundations. Every earthquake is associated with a certain amount of energy released at its source
and can be assigned a magnitude (M) which is just a number (Richter, 1958).
Definition of earthquake terms
Shown below is a sample of soil behavior during the Canterbury earthquake in NZ.

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Figure 11. Sample Damage of Niagata earthquake 1964
Automobile sunk in during Niagata
earthquake 1964
Sewage treatment tank floated to surface during
Niagata earthquake 1964 (after Idriss, 1967)
Tilting of building during Niagata earthquake
1964 (After Seed and Idriss, 1967

P a g e 23 | 43
Due to ground motion during an earthquake, footings may settle, building may tilt, soils may liquefy
and loses ability to support structures, and light structures may float.
Below are the sample ground acceleration readings of past earthquakes captured in accelerogram.
Compared to the graph of dynamic loading for reciprocating and rotary machines, the machine’s
dynamic loadings are sinusoidal in nature.Although in reality it is not perfectly sinusoidal e.g.the peaks
in any two cycles may be different.

P a g e 24 | 43
Figure 12. Trace of vertical acceleration of ground due to pile driving
Natural undamped frequency of point bearing piles on rigid rock

P a g e 25 | 43
4.1 Soil behavior during dynamic loading
During earthquake, the ground may be susceptible two categories:
4.2 Dynamic Soil parameters needed for dynamic analysis of machine foundation
Dynamic loading of soils are divided into small and large strain amplitude responses. In a machine
foundation, the amplitudes of dynamic motion and, consequently, the strains in the soil are usually low,
whereas a structure that is subjected to an earthquake may undergo large deformations and thus induce
large strains in soil.
The “elastic halfspace model” is usually used in determining the response of the soil that is subjected
to harmonic loading. In this model, it presumesthat a circular footing restsupon the surface of an elastic
halfspace (the soil) extending to an infinite depth, which is homogeneous and isotropic and whose
stress-strainproperties canbe defined by two elastic constants,usually shearmodulus (G) and Poisson’s
ratio (v).
Dynamic soil parameters to be taken from field and/or laboratory test or from published correlations of
static properties of soil:
- Dynamic shear modulus, G
- Poisson’s ratio, v
- Soil Damping ratios, D
4.3 Geotechnical investigation to determine dynamic soil properties
Note: all of these soil investigation should be reported in detailed in geotechnical report.

P a g e 26 | 43
Sample schematic of cross-hole test on field

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Figure 13. Crosshole Test
Sample schematic of down-hole test on field

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Sample schematic of spectral analysis of shear wave
Figure 13. Surface Oscillator Test

P a g e 29 | 43
Sample of Downhole Test Results
Borehole plan
Bore
Hole
No.
Soil
Dept
h (m)
Type
of
Soil
N-
Valu
e
Seismic
Wave
Velocity
VP
VS
Poi-
sson
Ratio
Dynamic Soil
Properties (MPa)
G Ed
Kd
Silty Sand
and
Gravel
Silty Sand
and
Gravel
Sandston
e
Sandston
e
Sandston
e
Sandston
e
Sandston
e
Marl
Marl

P a g e 30 | 43
4.4 Computation of Equivalent Soil Springs
Table 1. Equivalent Spring Constants (Source: S. Arya, 1979)
Figure 14. Coefficients z, x and 

P a g e 31 | 43
Table 2. Equivalent damping ratios (Source: S. Arya, 1979)
5. STRUCTURAL ANALYSIS AND DESIGN (for block foundation)
Two Stages:
Static Analysis  includes check for strength of foundation, stability of foundation and
check for soil bearing capacity (actual soil bearing should be less than 50~75% of
allowable bearing capacity)
Dynamic Analysis  includes determination of natural frequencies of the machine-
foundation system and checking of response vibration amplitudes
5.1 Foundation Trial Sizing Check
- Footing thickness shall be greater than the maximum of ( 0.60 m ; the rigid criterion 0.60
+ L / 30) , provide additional 0.60m on both sides for maintenance purpose
- Footing width shall be greaterthan 1.0~1.5 x vertical distance (foundation base to machine
C.G.) + 0.60 m on both sides for maintenance purpose
- Mass ratio check (for rotary machines: foundation mass = 2 to 3 times of machine mass;
for reciprocating machines: 3 to 5 times of machine mass)
- Foundation eccentricity check  shall not be greater the 5% on both plan dimensions (i.e.
the centroid of mass of machine-foundation system wrt base contact area)
Note: Table-top type foundation will have different requirement in trial sizing.

P a g e 32 | 43
5.2 Structural Modeling
5.2.1 Finite element modeling (Note: Modeling is a critical part in the structural analysis)
5.2.2 Modeling of supports
(Note: Static soil spring is different from dynamic soil spring; dynamic modulus of elasticity, Ec’
of concrete = 1.60* static modulus of elasticity, Ec)

P a g e 33 | 43
5.3 Time-History Analysis
5.3.1 Dynamic input in analysis software
Sample of input in analysis software (operating speed)
Sample of input in analysis software (emergency speed)
5.3.2 Unbalanced loads model

P a g e 34 | 43
5.3.3 Determine machine-foundation system natural frequencies, fn

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5.3.4 Investigate the shapes of observed critical modes

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P a g e 37 | 43

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5.3.5 Resonance check
Sample of resonance check

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5.3.6 Vibration amplitudes check
Sample of velocity amplitude check

P a g e 40 | 43

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5.3.7 Show plots of governing amplitudes

P a g e 42 | 43
6. SUMMARY OF DESIGN CHECKING
Static Design  check for serviceability and strength requirements, update accordingly; follow
best practices in reinforced concrete detailing; check seismic requirements if project is located in
high seismic zone area.
Dynamic Design  verify if vibration amplitudes are below allowable values, review and/or adjust
accordingly; check mode shapes of the system, check if natural frequencies are outside the resonance
range, check other requirement checking required by vendor e.g. deflection of bearing points,
misalignment of rotor.
Below is a schematic flow of dynamic design process.
Figure 15. Schematic diagram of a machine-foundation system subjected to dynamic
loads
(Source: K. Bhatia, 2008)

P a g e 43 | 43
REFERENCES
J.E. Bowles (1997) “Foundation Analysis and Design 5th
Ed. - Chapter 20 Design of foundation for
vibration controls”
S. Prakash (1991) “Soil Dynamics - Chapter 9 Machine Foundation”
S. Prakash (1991) “Soil Dynamics - Chapter 1 Introduction and 2 Theory of Vibration”
Prakash and Puri (1988) “Foundation for Machines - Analysis and Design”
Prakash and Puri (2006) “Journal on Foundation for Vibrating Machines”
S. Arya et al. (1979) “Design of structures and foundations for vibrating machines”
K.G. Bhatia (2008) “Journal on Foundations for industrial machines and earthquake effects”
American Concrete Institute ACI 351.3R (2004) “Foundations for Dynamic Equipment”
Technical Paperon“Seismic Design of Buried Structures in PHandNZ” presentedby Lawrence Galvez
in 17th
ASEP International Convention” (2015)
Lecture notes on Advance Foundation Engineering and Design Subject in University of the City of
Manila - Prof. Rolando D. Rabot, ASEAN Engr, M.Engg, Ph.D-Te
OTHER SUGGESTED REFERENCES FOR READING
German Standards
DIN 4024 Part 1 Machine Foundations; flexible structures that support machines rotating machines
DIN 4024 Part 2 Machine Foundations; rigid structures that support machines periodic excitation
British Standards
CP 2012 Code of Practice for Foundations for machinery
American Standards
American Society of Civil Engineers (ASCE) Task Committee on “Design of large steam turbine-
generator foundations”
ABOUT THE AUTHOR
Lawrence “LG” Galvez is a licensed civil engineer professional involved in structural engineering and
design of industrial and infrastructure projects for local and international clients. His experience in civil
and structural design are from oil & gas, petrochemicals, mining, telecommunications, water &
wastewater, power generation, bridge & railway and land development industries.
LG is a PICE Life Member and a Regular Member of ASEP. He can be reach thru email:
wrence23_ph@yahoo.com.

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