structural integrity monitoring of concrete structures via optical fiber sensors: sensor protection...

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Structural Health Monitoring

DOI: 10.1177/1475921703002002004 2003; 2; 123 Structural Health Monitoring

G. F. Fernando, A. Hameed, D. Winter, J. Tetlow, J. Leng, R. Barnes, G. Mays and G. Kister Systems

Structural Integrity Monitoring of Concrete Structures via Optical Fiber Sensors: Sensor Protection

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123

Structural Integrity Monitoring of Concrete

Structures via Optical Fiber Sensors: Sensor

Protection Systems

G. F. Fernando,* A. Hameed, D. Winter, J. Tetlow, J. Leng, R. Barnes,

G. Mays and G. Kister

Engineering Systems Department, Sensors and Composites Research Group,

Cranfield University, Shrivenham, Swindon SN6 8LA, UK

This paper reports on the design requirements and a range of specific concepts for optical fiber-based

sensor protection systems that can be used in concrete structures. The designs range from sensor

protection systems that are manufactured involving stainless steel, fiber reinforced composites and

concrete. The feasibility of manufacturing the sensor protection systems using these materials is also

demonstrated. A detailed finite element analysis was carried out to optimise the stainless steel-based

sensor protection system for concrete. Experimental results involving the sensor protection system will

be presented in Part 2 of this series of papers.

Keywords sensor protection � Fabry-Perot � Bragg grating � concrete � structural integrity

1 Introduction

There is considerable global interest in the condi-

tion assessment of civil structures such as bridges

and highways [1]. Apart from the obvious safety

implications and economic considerations, this

interest in structural integrity monitoring of civil

structures is generally dictated by the need to

monitor the structure due to one or more of the

following: (i) increased traffic or payload;

(ii) deterioration brought about by de-icing

agents, specified chemical agents, atmospheric and

traffic pollution; (iii) damage caused to the struc-

ture by land subsidence, earthquake and vehicle

impact; (iv) the development of strategies for

maintenance schedules, refurbishment and reno-

vation; (v) compliance with new codes of practice

and legislation; and (vi) availability of new non-

destructive techniques and sensor systems.

The established technologies for the structural

integrity assessment of civil structures include,

visual inspection, ground radar, X-ray radiogra-

phy, ultrasonic scanning, acoustic emission,

shearography, stimulated infrared thermography,

vibration testing, radar, conductivity, etc [2,3].

Whilst these techniques are adequate and reliable

for specified applications, they are not capable of

providing real-time and on-line information on

the measurands of interest. For example, local

and global strain, temperature, vibration charac-

teristics and concentration of specified chemicals

that may cause deterioration of the structure.

With reference to the above mentioned struc-

tural integrity issues, the availability of an on-line

*Author to whom correspondence should be addressed.

E-mail: [email protected]

Copyright � 2003 Sage Publications,

Vol 2(2): 123–135

[1475-9217 (200306) 2:2;123–135; 10.1177/147592103034253]

Copyright � 2003 Sage Publications,

Vol 2(2): 123–135

[1475-9217 (200306) 2:2;123–135; 10.1177/147592103034253]


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monitoring system would be able to comply with

most of these needs. For example, significant

advances have been made in the last decade on

the utilisation of optical fiber-based sensor sys-

tems for health monitoring of engineering and

civil structures [4,5]. Fiber optic sensors (FOS)

are ideal candidates for remote and real-time

structural integrity monitoring as they offer the

following unique advantages over conventional

sensors and inspection techniques: (i) their rela-

tive small dimension and uniform cross-section

makes it easy for surface mounting or embedding

into materials such as polymers, fiber reinforced

composites and concrete; (ii) in general, when

embedded, they tend not to influence the quasi-

static tensile mechanical properties of materials;

(iii) they are immune to electro-magnetic inter-

ference and therefore can be deployed in areas

where electrical-based sensors would fail or require

expensive protection; (iv) FOS can be used to

infer the following measurements and in some

cases multiple properties may be accessed, for

example, strain, temperature, pressure, humidity,

vibration, specified chemicals, acoustic emission

and fracture [6–8]. Although a number of these

sensor systems have been demonstrated success-

fully in the laboratory under ideal conditions,

their exposure to on-site rigours is now being

given greater prominence and emphasis; (v) the

options of remote data telemetry and on-line data

acquisition are an extremely attractive feature of

FOS [9,10]; (vi) unlike conventional electrical-

based sensors, FOS can be multiplexed. In other

words, a number of similar or different sensors

can be attached along a single optical fiber. With

certain sensor design, distributed sensing can also

be achieved along the length of a fiber; (vii) the

cost of the fibers is relatively low and the overall

sensor systems costs can be modest if: (a) the

sensors are fabricated in-house; (b) telecommuni-

cation wavelengths are used (�1310 and

1550 nm); and (c) low-cost, off-the-shelf interro-

gation units can be used.

Fiber optic sensors have their limitations and

these need to be appreciated with specific refer-

ence to civil structures if they are to be exploited.

Examples of areas of concern in this industrial

sector are as follows. (i) The robustness and

survivability of the sensor system [11]: The low

attenuation characteristics of silica-based optical

fibers makes them ideal for data transmission

over long distances. However, these fibers and

sensors tend to be brittle and hence adequate

protection must be provided. In addition to this,

exposed silica fibers may be susceptible to hydro-

lytic degradation. With reference to optical fibers,

the protection generally takes the form of a

polymeric coating (acrylate, polyimide, fluoro-

hydrocarbon). Where additional protection is

required, reinforcing fibers are used in conjunc-

tion with a polymeric jacketing. Protecting the

sensing region is more problematic as it is

important to ensure that the sensing region is not

insulated or isolated from the measurand. This

first paper in this series will address the strategy

behind materials selection and design for surface-

mounted and embedded fiber optic sensors in

concrete. (ii) Transduction: This is a term that is

used sometimes to indicate the efficiency of the

transfer of the measurand in question to the

sensor. For example, for the structural integrity

strategies based on strain and/or stiffness method-

ology, it is essential for efficient strain transfer

from the substrate to the sensor. Factors that can

influence the strain transfer efficiency include the

relative moduli of the various components, the

type and degree of chemical bonding between

the sensor and the substrate, and the physical

protection rendered to the sensor system. The

homogeneity of the matrix can also have a sig-

nificant influence on the environment around the

sensor, for example, concrete. (iii) Interpretation

of the data from the sensor: The interpretation of

the data from the sensors may not be straight

forward because the output could be influenced

by a multitude of parameters such as axial, radial

and lateral strains, temperature, thermal expan-

sion of the structure, humidity, etc. With systems

consisting of multiple or multiplexed sensors,

some form of data reduction and fusion will be

needed. Data fusion in essence is the fusion of

multi-sensor data to increase the accuracy and

reliability of measurands which can be monitored

and characterised. This topic is of significant

relevance because of the complex and inter-

dependence of various parameters associated

with structural integrity monitoring and assess-

ment. The problem of identifying malfunctioning

124 Structural Health Monitoring 2 (2)



sensors needs to be addressed and may be

simplified by fusing the measurements with per-

formance and signal characteristics. By fusing

many measurands with a contextual database, it

may be possible to derive the following informa-

tion: (a) Material characterization. Identification

of material properties unobtainable through

single sensors. (b) Fault diagnosis. The data

fusion process may allow faults to be identified

and provide early warning of failures to reduce

maintenance costs. (iv) Influence of the embedded

sensor system on the long term integrity of the civil

structure: On the basis of the wealth of informa-

tion that is currently available on the use of

optical fiber-based sensor systems in concrete

structures, there is no evidence to suggest that the

embedded sensor has any detrimental effect on

the concrete structure. This is reasonable consid-

ering the relative volume fraction and dimension

of the optical fiber sensor system in relation to

the reinforced concrete. However, precautions

need to be taken to ensure that the entry and the

exit points do not provide a means for the

external environment to ingress into the concrete

structure. (v) Long-term stability and reliability of

the sensor and interrogation unit. As this is an

emerging technology, there is little information

available on the long-term performance of such a

sensor and interrogation unit. On the basis of the

information available thus far, the suggestion is

that this is not likely to be a cause for serious

concern. However, attention will need to be paid

to the relative magnitudes of the cross-talk

caused by the external environment (temperature,

humidity) and the signal generated by the mea-

surand of interest (strain, temperature, vibration,

humidity etc). (vi) Repair strategy for the

embedded sensor system. Given the constant

reference in the literature for the need to repair

and strengthen existing concrete structures such

as bridges, due consideration needs to be paid to

future renovation and repair strategies.

2 Sensor Protection System (SPS)

2.1 Requirements for the SPS

The primary requirements for the sensor protec-

tion system (SPS) are: (i) protection of the silica

fibers from the alkaline environment typical in

concrete; (ii) protection of the sensor system (a)

during the concrete pouring operation, (b) against

mechanical or abrasion damage caused by the

aggregates and against any aggressive chemical

environments; (iii) ability to secure the sensor

system in the required orientation during the

concrete pouring operation; (iv) protection of the

leading and trailing optical fiber into and out of

the structure; (v) appropriate protection to the

sensor system against physical damage and detri-

mental environmental effects when it is surface-

mounted; (vi) an appropriate repair strategy in

the event that the structure has to be repaired or

renovated. Due consideration must also be paid

to a redundancy strategy with regard to the total

number of sensors that will be installed. In other

words, as the technology is relatively new, an

extra numbers of sensors should be installed as a

precaution; (vii) the sensor system must ensure

good transduction for the measurand of interest;

and (viii) the sensor system must be relatively

low-cost, easy to fabricate and install.

In this current paper, the rationale for devel-

oping strategies for the embedded and surface-

mounted sensor protection system are developed

and discussed. The technology proposed in this

current publication is intuitive and is based on the

fulfilment of the criteria for the SPS set out in the

previous section. However, a selection of previous

publications that deal with sensor mounting and

protection systems can be found in previous

literature [12–19] and SPS designs proposed by

these authors/corporations are duly acknowl-

edged. An excellent coverage of the use of optical

fiber-based sensors and protection systems can be

found in the book authored by Measures [1].

2.2 Embedded Sensor Protection

Systems

With reference to the above mentioned criteria,

three classes of materials were selected: stainless

steel; carbon fiber prepreg-based composite and

concrete. Stainless steel is relatively inert in the

concrete environment except where chloride

attack is concerned. Here also, caution needs to

be exercised with designs that involve welding the

SPS on to metallic components on the reinforcing

Fernando et al. Structural Integrity Monitoring of Concrete Structures 125



bar (rebar) or structure as the use of dissimilar

metals can result in the formation of a corrosion

cell. Carbon fiber composites are used for reinfor-

cing concrete structures and are also used as pre-

tensioned tendons instead of steel in certain

applications. The carbon fiber composite is gen-

erally coated with a moisture-impermeable layer

that is alkali resistant. Concrete was evaluated as

a potential matrix for the SPS as it offers similar

mechanical, thermal and chemical properties.

2.2.1 Metal-Based Sensor Protection Systems

Stainless Steel: A preliminary series of experi-

ments were carried out to assess the suitability of

using a stainless steel tube to protect the FOS in

concrete. This was found to be unsuitable as no

anchorage points were provided. A finite ele-

ment study was undertaken to optimise the

shapes and dimensions of the anchorage points

or flanges. The details of this were reported in a

previous publication [20] and only a brief sum-

mary of the main findings are presented here.

A summary of the relevant material properties

used in the finite analysis is presented in Table 1.

The assumptions made in developing the FE

model were as follows: (i) the material was

homogenous and isotropic; (ii) the test specimen

was only subjected to elastic loading; (iii) prelimin-

ary analyses were performed with the following

type of contact between the concrete and the SPS

interface: (a) no separation – here the target and

contact surfaces are tied but they are allowed to

slide once the ultimate shear stress was achieved;

(b) rough – here perfect and rough frictional

contact was assumed; and (c) bonded – in this

case, the contact integration points were initially

inside the pinball region, the contact was attached

to the target surface along the normal and tangent

directions to the surface; this permitted sliding

when the ultimate shear stress was reached; (iv) in

the above-mentioned cases, debonding and slip-

page will occur when shear stress at the SPS–

concrete interface reaches the ultimate anchorage

stress of 2.2MPa, as calculated in accordance with

BS 8110 [21]; and (v) with reference to Figure 1,

the SPS was positioned in concrete matrix to

ensure that there is no direct compressive load on

the tube. In other words, the strain transfer was

purely by shear as shown in the inserts in Figure 1.

Figure 2 illustrates the three flange geometries

that were evaluated in the finite element analyses.

Figures 3 and 4 show the finite element mesh that

was used for the concrete columns, with and

without the flanges respectively.

With reference to the finite element analysis

package, ANSYS, CONTACT169 and

TARGET172 elements were used to model the

contact between the SPS–concrete interface.

Model symmetry was exploited and a quarter

section of the model was developed for the finite

element analysis using ANSYS PLANE42 ele-

ments as shown in Figures 3 and 4. The condition

imposed for this analysis was a compressive load

of 100 kN (12.73MPa) on the concrete cylinder

which in turn generated an interfacial shear stress

(at the concrete–SPS interface) that exceeded the

ultimate anchorage stress causing debonding. The

requirement for the ultimate anchorage stress for

concrete specimens with flanges is specified in

British Standard [21] BS 8110. For a bar

deformed in compression, the ultimate anchorage

bond stress was calculated [20] to be approxi-

mately 1.7–2.2 MPa. The accuracy of the FE

model was confirmed by mapping axial strain at

random selected nodes. The magnitude of the

axial strain that developed along the SPS, as a

consequence of the compressive loading, was

monitored experimentally using a surface-

mounted electrical resistance strain gauge that

was located on the surface of the SPS. The strain

magnitude corresponded to within 5% of the

experimental values and within 2% of the theore-

tical values.

With reference to the bonding conditions that

were stipulated between the SPS and the concrete

previously, Figure 5 presents a summary of the

computed strain along the SPS for a circular

flange with a diameter of 5 mm. The term

‘‘experimental’’ refers to experimental data that

Table 1 Summary of relevant materials data used infinite element analysis (ANSYS).

MaterialElastic

Modulus (GPa)Poisson’sratio

Steel tube 207 0.3Concrete 32.4 0.16




were generated by an electrical resistance strain

gauge that was bonded on the centre gauge

length of the SPS prior to embedment in the

concrete cylinder. On inspecting Figure 5, a

number of conclusions can be made: (i) irrespec-

tive of the nature of the bonding between the

concrete and the SPS, the strain transfer from

the concrete to the SPS is effective between

the flanges; (ii) the computed strain along the SPS

at the centre of the gauge length showed excellent

agreement with the experimental data; and (iii) in

the situations where a high bond strength is

assumed between the concrete and the SPS,

localised crushing is observed at the end of the

SPS. On the basis of these observations, the sub-

sequent FEA analyses were carried out assuming

Figure 1 Schematic illustrations of: (a) the concrete test specimen withthe embedded stainless steel SPS; (b) the relative positions of thesurface of the concrete test specimen and the location of the top of theSPS; and (c) the mode of load transfer from the concrete specimen tothe embedded SPS.

Figure 2 Schematic illustration of the three flangeshapes that were investigated in this current study: (A)disk; (B) cone; and (C) inverted cone. The dimensions arein mm.




bonded-contact between the SPS and the concrete.

In Figure 6, it is seen that flange diameters greater

than 5 mm were seen to generate shear stress in

excess of the limit calculated in accordance with

BS 8110; this is depicted by the two horizontal

lines. Details of the finite element analysis for the

other flange configurations were presented pre-

viously in [20].

Figure 5 Axial strain distribution in an embedded steel tube with flange; the analyses werecarried out as a function of the degree of bonding between the SPS and the concrete. Theexperimental data were obtained via a surface-mounted electrical resistance strain gauge thatwas located at the centre of the SPS.

Figures 3 and 4 Finite element mesh of protective tube with and without flange in a concrete cylinderblock.




With reference to the FE data presented

above, the following conclusions can be made. (i)

An SPS system based on flanges is capable of

providing good mechanical interlocking. (ii) High

shear stress develops in the vicinity of the flanges

for the three designs considered in this study. (iii)

For a specified flange diameter, the magnitude of

the shear stress at the concrete–SPS interface was

found to be independent of the shape of the

flange. (iv) The SPS was designed to accommo-

date an embedded optical fiber sensor approxi-

mately 35mm away from the flanges. The shear

stress and strain profiles in this vicinity were

within the specified limit and strain transfer

across the interface was good. (v) The shear

stress and the direct strain in the vicinity of

sensor are not affected significantly by the shape

and diameter of the flange.

On the basis of the FE analysis carried out in

this current study, a stainless steel based SPS was

designed for deployment in concrete structures

that are subjected to predominantly compressive

and tensile loading. A schematic illustration of

this SPS and photograph of it is presented in

Figures 7 and 8 respectively.

With reference to Figure 7, the items specified

in the schematic illustration correspond to the

following: (1) single mode optical fiber with a

Figure 6 FEA of the shear stress at the SPS–concrete interface for specified diskdiameters. The analyses were carried out as a function of the length along the SPS.The two horizontal lines represent the permissible band for the shear stressdistribution in accordance with BS 8110.

Figure 7 Schematic illustration of the embeddable stainless steel-based SPS. See textfor details.




polyacrylte or polyimide buffer inserted in a

PTFE tubing. The PTFE tubing is used to render

protection to the optical fiber protruding from the

stainless steel SPS. It does not extend into the

optical fiber sensing region; (2) silicone rubber

was used to protect the potted thermosetting resin

from fluids and to offer cushioning to the protrud-

ing optical fiber emerging from the stainless steel

tube; (3) a short section of a polyethylene heat

shrink tubing was used to create a moat for the

silicone rubber and has a secure housing for the

polyaramid-reinforced PTFE and polyamide

tubing; (4) the function of the flanges were to

enhance mechanical locking into the reinforced

concrete structure; (5) the stainless steel tube with

flanges provides the chemical and mechanical

protection and anchorage methodology for the

sensor protection system. (6) a Fabry-Perot or

fiber Bragg grating sensor that is located at

the mid-section of the stainless steel tube;

(7) the bore of the stainless steel tube was

potted with a thermosetting resin; (8) the reflector

fiber; and (9) end-fixture: This served as a location

device to secure the SPS in position when it was

located in the mould. This section of the SPS can

be selected to suit a specified application and

method for securing it in/on the structure. A

photograph of the SPS is presented in Figure 8.

In conclusion, finite element analysis was used

to design and optimise an SPS-based on a stainless

tube with two flanges. A circular flange with a

diameter of 5 mm was found to be cost-effective

and was selected for embedment and retrofitting

trials in concrete cylinders. Experimental results

using the stainless steel SPS will be presented in a

subsequent publication.

Rebar-based Protection Systems: In situations

where it may be necessary to weld the sensor

protection system to the rebar, a schematic

illustration of a possible approach is presented in

Figure 9(a). In this sensor design, a hole is drilled

in the longitudinal or transverse directions in a

short section of a rebar and this is then welded

on to the main rebar. The fiber optic sensor can

then be potted into the cavity in the rebar using

an appropriate resin system as discussed pre-

viously for the stainless steel-based SPS. A similar

FEA based modelling approach can be used to

model the load-transfer characteristics from the

potted rebar to the sensor. The option of thread-

ing the cavity could be considered as this would

permit a drilled-out bolt with an embedded

sensor to be screwed onto the welded rebar

section, see Figure 9(b).

2.2.2 Embeddable Fiber Reinforced Composite

Protection System Whilst the stainless steel

protection system is robust, it is time-consuming

to manufacture. Hence, the option of using

carbon fiber reinforced composites (CFRP) was

considered predominantly for their ease of manu-

facture using prepreg (pre-impregnated fibers).

The processing of all the prepregs was carried out

in an autoclave. Schematic illustrations of the

various sensor designs that were considered is

presented in Figure 10(a), (c) and (e) along with

the corresponding photographs in Figure 10(b),

(d) and (f). With reference to Figure 10(a) and

Figure 9 (a) Schematic illustration of a sensor protec-tion system involving the use of a short section of a rebarwith a central cavity to accommodate the optical fibersensor (protection of the optical fiber is not shown); (b)Schematic illustration of a nut that was constructed from asection of a rebar. Here the sensor is potted in a pre-drilled cavity in the bolt.

Figure 8 Photograph of the embeddable stainless steel-based SPS. See text for details.




(b) Unidirectional ‘‘rolled-up’’ prepreg with the

sensor being located at the centre of the compo-

site cylinder. (c) and (d) Dumb-bell protection

system. Here additional reinforcement is used to

create the ‘‘flanges’’ (e) and (f). Here the prepreg

is moulded over the rebar preform. This can then

be bonded on to the rebar on site.

2.2.3 Concrete or Resin-based Sensor Protection

System In the course of this study, the feasibility

of using concrete as the matrix to pre-fabricate

the SPS was investigated. In this instance, a thin

layer of a cement-compatible thermosetting resin

was applied to the fiber optic sensor. The coated

sensor was then encapsulated with concrete within

the confines of a cylindrical mould. The rationale

for doing this was to minimise the mismatch in

the mechanical and thermal properties of the SPS

in comparison to the concrete structure. SPS of

this design can mimic more closely the chemical

environment of the concrete structure. The

concrete SPS offers some interesting options here

as it can be pre-fabricated and then secured

onto the structure using cement-based grout or a

Figure 10 (a)–(f) Schematic illustration of specified embeddable sensor protection systems

manufactured using CFRP prepregs and the corresponding photographs. See text for details.




compatible resin as the bonding medium. A

schematic illustration of this design concept is

illustrated in Figure 11. (i) A rebar of suitable

dimension is selected and (ii) placed in a mould

containing uncured silicone rubber mould. After

curing (iii), the ‘‘female’’ silicone rubber mould is

ready for use (iv). (v) The FOS positioned

centrally within cavity. Finally, the potting

medium is then poured in and allowed to set (vi)

and then is subsequently removed (vii).

2.3 Surface-mountable and Retrofit

Sensor Protection Systems

The temptation here is to exploit existing techni-

ques and procedures that have been developed

for surface mounting electrical resistance strain

gauges. In other words, the procedures involved

are abrading and degreasing the surface of the

substrate, then using a primer to make the

surface receptive for the adhesive. However, in

the case of the electrical resistance strain gauge,

the contact area with the substrate is significantly

larger than that offered by the optical fiber

sensor region. Other relevant issues include the

visco-elastic properties of the adhesive will need

to be taken into account when interpreting the

data from the fiber optic sensors and the control

of the glue-line thickness.

The SPS discussed previously can be adapted

for retrofitting onto existing structures. As with

the examples of embeddable SPS shown pre-

viously in Figure 10, prepreg-based manufactur-

ing techniques can be used to manufacture high

quality SPS where the sensors are embedded in

the prepreg prior to processing. Schematic illus-

trations of such designs are presented in Figure

12(a)–(f). Figure 12(a) and (b) represent flat or

planar and curved composites with the embedded

FOS. In the latter case, the prepreg was moulded

over a cylinder of specified diameter. These

devices can be mechanically fastened and/or

bonded to the concrete structure. Figure 12(c)

and (d) are photographs of the flat and curved

prepreg-based SPS. Figure 12(e) represents the

case where the SPS illustrated in Figure 8 was

retrofitted onto a concrete cylinder. In this

instance an electrically powered grinding wheel

was used to create the necessary recess on the

concrete column to accommodate the SPS.

Figure 12(f) is a photograph where the flat and

curved prepreg-based SPS were surface-mounted

Figure 11 Schematic illustration of the production process for theproposed concrete-based SPS manufacturing methodology.




Figure 12 (a)–(f) (a) Schematic illustration of a rectangular prepreg-based SPS; (b) Here the prepregwith the embedded optical fiber sensor system was autoclaved using a curved mould; (c) Photograph of arectangular prepreg-based SPS; (d) Photograph of a curved prepreg-based SPS; (e) Photograph ofretro-fitted metal-based SPS (see Figure 8 for details); (f) Photograph of a concrete cylinder with a rangeof surface-mounted prepreg-based SPS and electrical resistance strain gauges.




on to a concrete test specimen. Surface-mounted

electrical resistance gauges were also used to

correlate the data obtained from fiber optic

sensors; the outcome from these investigations

will be reported in a subsequent publication.

4 Conclusions

This paper has outlined the major aspects asso-

ciated with the design and deployment of sensor

protection systems for concrete structures. A

number of designs were proposed for the SPS

and two of these were evaluated in detail: an

embedded stainless steel protection system and a

glass and carbon fiber-based protection for sur-

face mounting onto concrete structures. The

design for the metal-based SPS was based on

finite element analysis where the shape and

dimensions of the flanges were optimised.

Acknowledgements

The authors wish to acknowledge the funding provided by

EPSRC (GR/M56265 and GR M83605), the Institution of

Civil Engineers (Enabling Fund Ref: 9905) and the

Engineering Systems Department, Cranfield University.

The assistance and encouragement given by Professor Brian

Ralph, Mike Teagle, Jim Harber, Rodney Badcock,

Richard Rose, Julie Etches and Maggie Keats are duly

acknowledged. This project was carried out in collabora-

tion with colleagues from the Universities of Kent and City

under the remit of an EPSRC Structural Integrity research

grant.

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