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.