STRUCTURAL HEALTH MONITORING
OF STEEL STRUCTURES
USING ELECTRICAL STRAIN GAUGES
by
AMIT SETHI 2002003
Submitted in partial fulfillment of the requirements of the degree of
Bachelor of Technology to the Department of Civil Engineering
Indian Institute of Technology Delhi
May, 2006
i
ABSTRACT
Man’s imagination and creativity has led to the construction of vast framed structures of
very complex nature. Due to their unique design and construction, rigorous structural
health monitoring (SHM) programmes are required during the construction and operation
processes. Their continuous monitoring ensures better performances and facilitate in
depth understanding of the overall structural behaviour. This project aims to investigate,
by conducting experiments on Steel Structures, the feasibility of adopting smart sensors,
particularly Electrical Strain Gauges, in the development of automated, real-time and
online structural health monitoring systems, which can provide a cost effective alternative
to conventional monitoring systems, such as visual inspection. In this project, Strain
Gauges, are attached at critical locations, on the steel specimen to enable experimental
verification of the pros and cons of using Strain Gauges in structural health monitoring.
ii
CERTIFICATES I do certify that this report explains the work carried out by me in the Courses CE491S
Project-Part 1 and CE492S Project–Part 2,under the overall supervision of Prof. Ashok
Gupta and Dr. Suresh Bhalla. The contents of the report including text, figures, tables,
computer programs, etc. have not been reproduced from other sources such as books,
journals, reports, manuals, websites, etc. Wherever limited reproduction from another
source had been made the source had been duly acknowledged at that point and also
listed in the References.
Amit Sethi
This is to certify that the report submitted by Amit Sethi describes the work carried out
by him in the course Courses CE491S Project-Part 1 and CE492S Project–Part 2, under
our overall supervision.
Prof. Ashok Gupta Dr. Suresh Bhalla
iii
ACKNOWLEDGEMENT
I would like to sincerely appreciate my guide Prof. Ashok Gupta, and co-guide Dr.
Suresh Bhalla, who have been patiently guiding and enlightening me for the completion
of this research. I also express my gratitude to the Department of Civil Engineering,
Indian Institute of Technology, Delhi for providing such research opportunity. Special
thanks to all technicians in the Structural Laboratory, without them, the experiments
would not have been conducted smoothly.
Amit Sethi
iv
TABLE OF CONTENT
ABSTRACT I CERTIFICATES ii ACKNOWLEDGEMENT iii TABLE OF CONTENT iv-vi LIST OF FIGURES vii-ix LIST OF SYMBOLS x-xi LIST OF TABLES xii
CHAPTER ONE 1-2 1.1 Introduction 1
1.2 Objectives and Scope of Project 2
CHAPTER TWO 3-17
2. LITERATURE REVIEW 3
2.1 Overview of structural health monitoring (SHM) 3
2.1.1 Sensor systems for SHM 4
2.1.2 Commonly used sensor systems for SHM 5
2.1.3 Figure showing the three types of strain gauges 10
2.1.4 Emergence of More Advanced Methods 11
2.2 Piezoelectric Transducers 12
CHAPTER THREE 18-24
3. THEORY AND METHODOLOGY 18
3.1 Introduction 18
3.2 Theoretical Formulations And Computational Approach 18
3.2.1 Determination Of Axial Load 18
3.2.2 Determination Of Bending Moments 20
3.2.3 Determination Of Bending Moment Diagram 21
3.2.4 Determination Of Deflections 22
v
3.2.5 Determination Of Deflction Profile 23
CHAPTER FOUR 25-41
4. EXPERIMENTAL METHODOLOGY 25
4.1 Introduction 25
4.2 Portal Frame 25
4.2.1 Analysis Of The Structure 27
4.2.2 Finding The Moment Capacity Of The Structure 31
4.3 Dimensions Of Specimens And Locations Of Strain 34
4.3.1 Frame 1 34
4.3.2 Frame 2 36
4.4 Instrumentation Of Specimens 37
4.5 Experimental Proceedings 38
4.5.1 Loading Pattern 38
4.5.2 Observations 39
4.5.2.1 Frame 1 39
4.5.2.2 Frame 2 40
CHAPTER FIVE 42-77
5. RESULTS AND DICUSSIONS 42
5.1 Introduction 42
5.2 Data Analysis 42
5.2.1 Frame 1 46
5.2.2 Frame 2 48
5.3 Determination Of Actual Load 52
5.3.1 Frame 1 52
5.3.2 Frame 2 54
5.4 Determination Of Bending Moment Diagrams 56
5.4.1 Frame 1 56
5.4.2 Frame 2 59
5.5 Determination Of Deflections 65
5.5.1 Frame 1 65
vi
5.5.2 Frame 2 66
5.6 Determination Of Deflection Profiles 69
5.6.1 Frame 1 69
5.6.2 Frame 2 74
CHAPTER SIX 78-80
6. CONCLUSIONS AND RECOMMENDATIONS 78
6.1 Conclusions 78
6.2 Recommendations 80
REFERENCES 81-82
vii
LIST OF FIGURES
Figure 2.1 A vibrating wire strain gauge 10
Figure 2.2 (a) An electrical strain gauge foil; (b) wheatstone bridge circuit. 10
Figure 2.3 Fabrication and principle of FBG based sensors. 11
Figure 2.4 (a) A PZT patch bonded to structure. (b) Interaction model of one half of the
PZT patch and the host structure. 14
Figure 2.5 Conductance signatures of a patch at various stages of severity of damage
introduction (Bhalla and Soh, 2003)
Figure 3.1 Member XY with a pair of strain gauges at point A 19
Figure 3.2 Member XY with two pairs of strain gauges 20
Figure 3.3(a) Member XY with two pair of strain gauges at points A and B 21
Figure 3.3(b) Measured values of moments at points A and B 21
Figure 3.3(c) Linear extrapolation to calculate moments at all points 21
Figure 4.1 (a) A portal frame with hinged ends. 26
Figure 4.1(b) Cross section A-A’(thickness= 4mm) 26
Figure 4.2 A Portal Frame 27
Figure 4.3 Free body diagram of the beam 28
Figure 4.4 Free Body Diagram of the frame 30
Figure 4.5 Moment Diagram of the frame 30
Figure 4.6 Cross section showing direction of Ixx 31
Figure 4.7 Frame 1 34
Figure 4.8 Details and position of strain gauges for frame 1 35
viii
Figure 4.9 Details of position of all strain gauges for frame 2 36
Figure 4.10 data logger (front view) 37
Figure 4.11(a) Wires attached to strain gauges coming out of the frame. 37
Figure 4.11(b) Wires from the frame attached to the data logger(rear view) 37
Figure 4.12 Complete experimental setup 38
Figure 4.13 Failure of the frame due to compression 39
Figure 4.14 Tension failure 39
Figure 4.15 Compression failure of the frame 40
Figure 4.16 Closer view of failure 40
Figure 5.1. Load vs Strain for Strain Gauge 9 of Frame 1 43
Figure 5.2 Location at which strain gauge 9 was bonnded 43
Figure 5.3(a) Load vs Strain for Strain Gauge 7 of Frame 1 44
Figure 5.3(b) Load vs Strain for Strain Gauge 8 of Frame 1 44
Figure 5.4 Condition of strain gauge no.7 and 8 after failure 45
Figure 5.5(a) Load vs Strain graphs for strain gauge no.1 to 6 46
Figure 5.5(b) Load vs Strain graphs for strain gauge no.7 to 12 47
Figure 5.5(c) Load vs Strain graphs for strain gauge no.13 to 15 48
Figure 5.6(a) Load vs Strain graphs for strain gauge no.1 to 10 49
Figure 5.6(b) Load vs Strain graphs for strain gauge no.11 to 20 50
Figure 5.6(c) Load vs Strain graphs for strain gauge no.21 to 29 51
Figure 5.7 locations of strain gauges on frame 1 52
Figure 5.8 Graph between actual load and theoretical load vs deflection 53
Figure 5.9 locations of strain gauges on frame 2 54
Figure 5.10 Graph between actual load and theoretical load vs time 55
ix
Figure 5.11 Bending Moment Diagrams for Member PQ at different loads 57
Figure 5.12 Bending Moment Diagrams for Member QR at different loads 58
Figure 5.13 Bending Moment Diagrams for Member PQ at different loads 59
Figure 5.14 Bending Moment Diagrams for Member QR at different loads 60
Figure 5.15 Bending Moment Diagrams for Member RS at different loads 61
Figure 5.16 Bending Moment Diagrams for Member ST at different loads 62
Figure 5.17Bending Moment Diagrams for Member UT at different loads 63
Figure 5.18 Bending Moment Diagrams for Member QT at different loads 64
Figure 5.19 Deflection profile at load 4.833 KN 70
Figure 5.20 Deflection profile at load 5.833 KN 71
Figure 5.21 Deflection profile at load 7.5 KN 72
Figure 5.22 Deflection profile at load 9.333 KN 73
Figure 5.23 Deflection profile at load 5.667 KN 74
Figure 5.24 Deflection profile at load 6.583 KN 75
Figure 5.25 Deflection profile at load 9.416 KN 76
Figure 5.26 Deflection profile at load 11.667 KN 77
x
LIST OF SYMBOLS f Natural frequency of vibration of a wire
F Tension in a wire
la Half-length of PZT patch
wa Width of PZT patch ha Thickness of PZT patch
Z Mechanical impedance
Electro-mechanical admittance
d31 Piezoelectric strain coefficient
Complex Young's modulus of the PZT patch at constant electric field
Complex electric permittivity of the PZT material at constant stress Za Mechanical impedance of the PZT patch
ω Angular frequency κ Wave number G Conductance B Susceptance Gj
1 Post-damage conductance at the jth measurement point
Gj0 Corresponding pre-damage value.
PA Axial load acting at point A
εT Measured strain in tension
εC Measured strain in compression
Ax Cross sectional area of member XY
E Young’s Modulus of elasticity of steel
MA Moment acting at point A
εTA,,εTB Measured strains in tension at points A and B
εCA,,εCB Measured strains in compression at points A and B
I Moment of inertia of member XY
2y Separation between the strain gauges
FEM Fixed End Moment
θ Deflection
xi
Ψ Side sway
MN,MF Moments at near and far ends respectively
Δ Displacement
x Distance from the end
K Constant of integration
DF Distribution Factor
K Member stiffness factor
P Load capacity of the frame
L Length
I Moment of Inertia
Ixx Moment of Inertia about x- axis
Iyy Moment of Inertia about y- axis
A Area of cross section of the frame
Fbt Allowable bending stress
fy yield strength of steel
Z Section Modulus
ryy Radius of gyration about y- axis
Leff Effective length of a member
R Reaction force acting on the frame in horizontal direction
fa Actual stress in axial compression
Fa Allowable stress in axial compression
fb Actual stress in bending
Fb Allowable stress in bending
xii
LIST OF TABLES
Table 2.1
Salient features of some common sensors used in SHM
5
Table 5.1
Values of all the deflections for4 different loads for frame 1
65
Table 5.2
Change in angles PQR and P with load for frame 1
66
Table 5.3
Values of all the deflections for4 different loads for frame 2
67
Table 5.4
Change in angles QRS, RST,U and P with load for frame 2
68
Table 5.5
Actual and calculated displacements with load
69
Chapter 1
1
1.1 INTRODUCTION
The research of Structural Health Monitoring (SHM) and damage detection has recently
become an area of interest for a large number of academic and commercial laboratories.
This kind of technique allows systems and structures to monitor their own structural
integrity while in operation and throughout their life, and are useful not only to improve
reliability but also to reduce maintenance and inspection cost of systems and structures. It
is a system devised to allow the testing for structural damage without interfering directly
with the structure itself.
Over the past two decades, many types of Structural health monitoring (SHM) techniques
have been reported in the literature, based on either the global or the local interrogation
of structures.
In Global dynamic techniques, the structure is subjected to low frequency excitations
and, from the structural response, the first few mode shapes and their corresponding
natural frequencies are extracted. Global static response of structures: The limitation of
the static response techniques is that their application on real life-sized structures is not
practically feasible.
Local methods: which rely on the localized interrogation of the structures. Some
common methods in this category are the ultrasonic wave propagation technique, acoustic
emission, magnetic field analysis, electrical methods, dye penetrate testing, impact echo
testing and X-ray radiography.
Chapter 1
2
Various sensor systems have been used in SHM like Extensometer, Accelerometer,
Pressure transducers, Temperature sensors. But each of these is attributed to certain
drawbacks, most of them highly susceptible to ambient noise frequency, inaccessibility to
remote areas, fragile nature, or is equipped with only manual and visual readouts. The
large size and complex nature of the civil structural system render the conventional visual
inspection very tedious, expensive, and sometimes unreliable.
The aim of this project is to study the feasibility of using Electrical Strain Gauges for
load and health monitoring of steel structures.
The development of intelligent structures capable of monitoring their in service structural
health and performing automatic damage detection is gaining increased attention of the
researchers and the industry.
1.2 OBJECTIVES AND SCOPE OF PROJECT
The intention of the presented study is to demonstrate experimentally the feasibility of
using Electrical Strain Gauges for monitoring of Steel structures.
The behaviour of the strain gauges attached to the surface of steel frame, subjected to
point load is studied. The measured strains are used for load monitoring and determining
bending moments, deflections and deflection profiles of the structure.
In addition, the project aims at continuous monitoring of critical positions of a structure.
This was achieved by analyzing the relationship between the intensity of damage (Visual
Inspection) and corresponding measurement from the strain gauge.
Chapter 2
3
2.LITERATURE REVIEW
2.1 OVREVIEW OF STRUCTURAL HEALTH MONITORING (SHM)
Health monitoring of civil infrastructures has achieved considerable importance in recent
years, since the failure of these structures can cause immense loss of life and property
which can be attributed to the inability to timely detect incipient damages present in the
structures.
Many structures require long-term monitoring of external loads, stress distributions,
deflections and occurrence of damages in a continuous manner, thereby ensuring a high
level of safety. At the same time, it can serve as means for design validation as well as a
database for economizing future constructions. This becomes more relevant in
circumstances where unfamiliar construction technologies are used or when a known
technology is extended beyond the normal range of application
Structural Health Monitoring (SHM) can prevent catastrophic loss of lives by
providing early warning to the engineers. At the same time, it can serve as means for
design validation as well as a database for economizing future constructions. This
becomes more relevant in circumstances where unfamiliar construction technologies are
used or when a known technology is extended beyond the normal range of application
Chapter 2
4
SHM is defined as the measurement of a structure’s operating and loading environment,
as well as critical responses to track and evaluate any symptoms of operational incidents,
anomalies and deteriorations/damages, which might affect its smooth operation,
serviceability or safety reliability (Aktan et al., 2000).
Deteriorations/damages may result from changes in material properties, geometrical
configuration, boundary conditions, system connectivity and loading environment.
Hence, comprehensive SHM calls for close monitoring of each aspect.
2.1.1 SENSOR SYSTEMS FOR SHM
Comprehensive structural monitoring can be realised only if the structure to be monitored
is instrumented with arrays of sensor systems at all critical locations. At the same time, it
is implausible to expect that any one type of sensors alone would be able to track down
the complete structural behaviour as well as to detect all possible structural abnormalities.
Hence, comprehensive monitoring calls for deploying complementary sensor systems
with sufficient redundancy, so that a few of the sensors could be permitted to fail without
triggering total collapse of the monitoring system (Boller, 2002). In addition, the sensors
and the associated data retrieval systems should be capable of withstanding the harsh
conditions encountered during construction and operation.
In general, SHM sensors can be classified as surface-bonded type and embedded type.
The surface-bonded sensors can be replaced if they develop fault at any stage. However,
there is very limited possibility of repair or replacement for the embedded sensors.
Chapter 2
5
The following parts of this section cover the operating principles of various sensing
systems, which could possibly be deployed for monitoring structures.
2.1.2 COMMONLY USED SENSOR SYSTEMS FOR SHM
A summary of the various sensors used in SHM is presented in the table below:
Table 2.1 Salient features of some common sensors used in SHM
S.no. Device Purpose Remarks
1 Strain gauge
Measuring strains which are
caused by member deformations
resulting from bending, torsion,
shearing and elongation/
contraction.
The gauge should be stable
with respect to time and
temperature. In addition, it
should exhibit linear
response over the strain
range of interest
Chapter 2
6
S.no. Device Purpose Remarks
(a)
Vibrating wire
strain gauge
(VWSG)
As shown in Figure 2.1, it is a
sensor coil, positioned above a
pretensioned stainless steel wire,
when energized, plucks the wire
and measures the frequency of the
resulting vibrations. From the
theory of vibrations, the natural
frequency of vibration, f, can be
related to the tension F in the wire
They are only suitable for
measuring static strains since
they require plucking of
wire. They are also
susceptible to extraneous
noise in the form of ambient
vibrations. If installed
externally, special protection
is required to prevent
damage from routine
construction activities
(b)
Electrical
strain gauge
(ESG):
Figure 2.2 (a) shows an ESG.
They are based on the principle
that under mechanical stress,
electrical resistance of a conductor
varies in proportion to the load
induced strain. Thus, it is attached
to one arm of a wheatstone bridge
with other three resistances known
as shown in Figure 2.2 (b). As the
resistance of ESG changes, current
ESGs demand considerable
care during installation due
to their fragile nature.
Electrical noise is very
frequently associated with
ESGs. ESGs are very prone
to deterioration by water
Chapter 2
7
S.no. Device Purpose Remarks
from the wheatstone bridge
changes. In this, thin metallic foil
grids is bonded to polyimide
plastic film which can be
adhesively bonded to the test
surface. The polyimide film
provides electrical insulation
between the gauge and the
monitored component
(c)
Optical fibre
bragg grating
(FBG)
Optical fibres, which are thin
fibres (few μm to few hundred μm
in diameter) of glass and silica,
utilise fibre properties to generate
optoelectronic signals indicative of
the external physical parameters to
be measured. Fabrication and
principle of FBG is shown in
Figure 2.3.
Utilize fibre properties to
generate optoelectronic
signals indicative of the
external physical parameters
to be measured. They are
very fragile. For this reason,
efforts to install FBG sensors
on civil structures often
result in high rate of sensor
failure due to the presence of
harsh environment. In
addition, the measurement
Chapter 2
8
S.no. Device Purpose Remarks
system and the sensors
themselves are relatively
expensive as compared to the
conventional sensor systems.
1 Extensometer
(a) Tape
extensometer
Measurement of convergence /
divergence (relative displacement
between two points), e.g. in a rock
caverns after construction
Both extensometers entail
manual recording, which
proves tedious Slight
loosening of convergence
pins could severely affect
measurement accuracy
(b) Borehole
extensometer
Measurement of relative
displacements between several
points. Can provide displacement
distribution in large rock volumes
2 Accelerometer
Measurement of dynamic
response, either harmonic (e.g.
vibration tests) or transient (e.g.
Drawbacks include
bulkiness, small bandwidth,
high cost and susceptibility
Chapter 2
9
S.no. Device Purpose Remarks
earthquakes) to mechanical and electrical
noise
3 Pressure
transducers Measurement of pressure
When used in concrete,
shrinkage of concrete often
causes problems
(a) Diaphragm
type
Measurement range between 200
and 700 MPa, up to 10 kHz
frequencies
(b) Quartz based Measurement range 700 MPa, up
to 200 kHz frequencies
4 Temperature
sensors Measurement of temperature
(a) Expansion
type
Bulky, generally provide
visual readouts only, not
suitable for automated
monitoring
Chapter 2
10
S.no. Device Purpose Remarks
(b)
Resistance
temperature
detectors
(RTD)
Miniaturised, temperature
measurement can be
automated; but tend to be
unstable near upper limit
(c) Thermistors
More miniaturised, sensitive
and stable as compared to
RTDs
(d) Thermocouple
Demand maintaining
constant temperature at one
terminal
2.1.3 FIGURE SHOWING THE THREE TYPES OF STRAIN GAUGES
Figure 2.1 A vibrating wire
strain gauge
Figure 2.2 (a) An electrical strain gauge foil; (b)
wheatstone bridge circuit.
Chapter 2
11
Figure 2.3 Fabrication and principle of FBG based sensors.
2.1.4 EMERGENCE OF MORE ADVANCED METHODS
Owing to the deficiencies in currently available methods, a more rigorous and practical
method equipped with following features should be developed.
1) The capability of the technique to perform automated, on-line, real time,
frequent structural health monitoring, i.e., monitor the integrity of the structure while
it is in service with high speed and with limited access to the structure and attempt to
detect damage in the early or incipient stages.
2) Ability to locate and quantify damages even in the remote areas, which are
difficult to access, i.e. it should have remote sensing capability.
3) Economical, simple, easy-to install, amenable to retrofit market.
Chapter 2
12
One of the promising approaches to satisfy the above requirements would be the
employment of smart piezoelectric sensor/actuator based on high frequency dynamic
response theory.
The piezo-impedance transducers are relatively new type of sensors. They do not
measure any direct physical parameter like stresses, strains or temperatures. Rather, they
extract a signature of the host structure to analyse for the presence of any structural
damage.
2.2 PIEZOELECTRIC TRANSDUCERS
Piezo-impedence transducers are made up of piezoelectric materials such as Lead
Zirconate Titanate (PZT), often referred to as piezoceramic patches or PZT patches.
These materials generate surface charges in response to applied mechanical stresses.
Conversely, they undergo mechanical deformations in response to applied electric fields.
This unique capability enables the material to be used both as a sensor and as an actuator,
and eventually as an mechatronic impedance transducer (MIT) for SHM (Park, 2000).
In the electro mechanical impedence (EMI) technique, a PZT patch is bonded to the
surface of the monitored structure by means of high-strength epoxy adhesive and
electrically excited by an impedance analyser. In this configuration, the patch behaves as
a thin bar undergoing axial vibration and mechanically interacting with the host structure,
as shown in Figure 2.4 (a). The PZT patch-host structure system can be equivalently
Chapter 2
13
represented by a mechanical impedance Z connected to an axially vibrating thin bar, as
shown in Figure 2.4 (b). In this figure, the PZT patch has half-length la, width wa and
thickness ha, and it expands/contracts dynamically in direction ‘1’ due to an alternating
electric field in direction ‘3’. It can be assumed to be infinitesimally small and possessing
negligible mass and stiffness as compared to the host structure. Hence, the two end points
of the patch can be assumed to encounter equal mechanical impedance Z (from the host
structure). Under these conditions, the patch invariably has zero displacement at the mid-
point, irrespective of its location on the host structure. Liang et al., 1994 solved the
governing one-dimensional wave equation for the generic system comprising one half of
the patch and the structure, as shown in Figure 2.4 (b), using the impedance approach.
Using Liang's generic derivation, the following expression can be written for the complex
electro-mechanical admittance (inverse of electrical impedance), of the coupled system
shown in Figure 2.4 (a).
(2.1)
where d31 is the piezoelectric strain coefficient, the complex Young's modulus of the
PZT patch at constant electric field, the complex electric permittivity of the PZT
material at constant stress, Z the mechanical impedance of the structural system, Za the
mechanical impedance of the PZT patch, ω the angular frequency and κ the wave
number.