heat straightening repairs to a steel road bridge

Simon K. ClubleyBridge Engineer,Winchester Bridges Group,Mott MacDonald, Winchester, UK

Stephen N. WinterSenior Bridge Engineer,Winchester Bridges Group, MottMacDonald, Winchester, UK

Keith W. TurnerDesign Team Leader, WinchesterBridges Group, Mott MacDonald,Winchester, UK

Proceedings of the Institution ofCivil EngineersBridge Engineering 159March 2006 Issue BE1Pages 35–42

Paper 14232Received 07/10/2004Accepted 29/06/2005

Keywords:bridges/stress analysis/thermaleffects

Heat-straightening repairs to a steel road bridge

S. K. Clubley PhD, S. N. Winter MA, CEng, MICE, MIStructE and K. W. Turner BSc(Hons), CEng, MICE

On 16 July 2002 a low-loader lorry carrying heavy

machinery travelling eastbound on the A27 struck the

underside of Brockhampton Road new bridge. The bridge

suffered structural damage to the main beams and

bracing. From an initial emergency inspection, the damage

was found to be located close to the point of contraflexure

in an area where the bottom flange was in tension. It was

not considered necessary to impose immediate closures

or traffic restrictions. A heat-straightening repair of the

damaged beams was proposed. Detailed analysis using

both hand calculations and the finite-element method

confirmed the structural adequacy of the bridge to

undergo the heat-straightening process. These checks

encompassed the temporary conditions prior to, during

and after the heating. The major heat-straightening

process was successfully completed on site in ten days.

Impact damage to the beams was repaired within

acceptable tolerances and the straightened steel was

subjected to hardness and magnetic particle investigation

testing. The repairs to Brockhampton Road new bridge

were concluded without undue disruption to the travelling

public or local residents.

1. INTRODUCTION

Brockhampton Road new bridge replaced an earlier bridge on the

same alignment. It carries a busy local road over the A27 Havant

bypass in Hampshire, UK. The structure was designed by Scott

Wilson and constructed in 2001 by Costain under a Highways

Agency design-and-build contract.

On 16 July 2002, Mott MacDonald as agent for the Highways

Agency was informed by the Hampshire Police in Netley that a

vehicle travelling eastbound on the A27 had struck the underside

of the bridge. The bridge had suffered structural damage to the

main girders and bracing, resulting from the impact of

construction plant carried on the rear of a low-loader lorry.

Following an initial emergency inspection by Mott MacDonald in

July 2002, the structure was inspected in detail by Fairfield

Mabey using access equipment and traffic management in

December 2002. From the emergency inspection, the damage was

found to be located close to the point of contraflexure in an area

where the bottom flange was in tension. It was not considered

necessary to impose immediate closures or traffic restrictions.

The horizontal force from the impact was redistributed by the

bottom flange to the cross-bracings. The reaction generated in

Bridge Engineering 159 Issue BE1 Heat-straighte

the ‘K’ bracing caused a punching-type deformation in the web of

the first beam. This force was relieved once it reached a

magnitude large enough to shear the M24 bolts attaching the web

stiffener to the bracing. At this moment, the bottom chord of

the bracing buckled. The lorry continued to scrape the underside

of the next three beams before being pushed up into the fifth

beam causing damage to the bottom flange. Fortunately, the

reaction generated in this bracing was insufficient to fail either

the web or connection bolts.

Brockhampton Road new bridge was built with a square,

three-span, composite steel plate girder and concrete slab

structure. This is supported on reinforced concrete bank seats at

the north and south ends, and by two intermediate leaf piers

which are 600 mm in thickness. Transverse concrete deck

diaphragms are built into the supports making the bridge fully

integral. The piers are protected from traffic on the A27 by safety

fencing. The span of the centre section is 34.60 m, and the side

spans are 14.90 m, to the centreline of the supports. There are two

3.65 m carriageways and two combined cycleway/footways of

2.40 m in width.

The deck consists of six grade S355 steel plate girders supporting

a 210 mm thick solid reinforced concrete slab. Each main beam is

continuously braced in pairs and is 1000 mm deep with 480 mm

wide bottom flange and 300 mm wide top flange. The plate

thickness is reduced between splices in the main span

approximately at the point of contraflexure. A bottom flange

thickness of 25 mm and web thickness of 12 mm occurs at the

location of impact.

A major concern was how the structure could be restored to its

original condition or at least within the geometrical tolerances

given by BS 5400 part 6.1 It was clear from the inspection that

sections of steel would have to be cut out and replaced from the

web. Although the A27 at this location is a dual two-lane

carriageway, it is heavily trafficked by up to 82 000 vehicles per

day (average annual daily traffic (AADT) two-way flow).

Although it was fortunate that the damage occurred at the point

of contraflexure, this was offset by the fact that access for repairs

would necessitate total closure of the eastbound carriageway.

Any diversions would have to be through Havant town centre.

This was acceptable between 2100 and 0500 h, but not during the

daytime and in particular in the peak rush hours. Any delay or

disruption to the road would have been very unpopular.

To reduce the work to manageable sections which could be

achieved overnight, Mott MacDonald suggested that heat

ning repairs to a steel road bridge Clubley et al. 35

36

straightening be used to repair damage to the bottom flanges and

restore the plan straightness of the first and fifth beams. In

conjunction with the heat straightening, a series of additional

repairs were required to reinstate buckled stiffeners, ‘K’ bracing,

web and cracked welds near the impact zones. New protective

paint coatings were also needed on all affected areas.

The heat-straightening method provided the most cost-effective

solution for repairing the damage to the bridge beams. Heat

straightening has many advantages over beam replacement with

respect to the issue of reduced delay and disruption to the travelling

public. The justification for the use of heat straightening was based

on the knowledge that the mechanical properties of steel return

unchanged for temperatures up to 6008C. Advice was sought fromthe Highways Agency regarding previous repairs using

heat straightening. The mitigation for this method was based upon

the following estimated added value.

(a) Monetary: heat straightening, £90 000 versus beam

replacement, £450 000.

(b) Non-monetary: reduction in delay and disruption when

using heat straightening.

This equated to an added value of £360 000.

Figure 1 shows a recent photograph of the structure illustrating

the general arrangement and the position of the A27 dual

carriageway.

2. THE HEAT-STRAIGHTENING PROCESS

Heat straightening is a relatively new method of repair for

highway structures in the United Kingdom. By comparison, the

technique has been used quite successfully for many years in the

United States, combining both ongoing research and practical

experience.

The heat straightening of Brockhampton Road New Bridge was

carried out in accordance with the guidance of US Department of

Transportation—Federal Highway Administration document:

Heat Straightening Repairs of Damaged Steel Bridges—A

Technical Guide and Manual of Practice.2 Two formal departures

from standard were required to cover the use of this document

and the method of heat straightening structural steelwork.

Concentrated local heating of certain areas will produce

expansion of the steel and a lowering of the yield stress.

Fig. 1. General view towards southern abutment

Bridge Engineering 159 Issue BE1 Heat-straightenin

If surrounded by cooler steel, a restraining action will occur

which will cause the heated area to yield if a sufficiently

high temperature has been used. This restraint produces a

contraction as the metal cools, the effect of which is to yield

and deform its profile. By using carefully designed heating

patterns, this process of deformation can be used to return

damaged steel to its original profile. The careful placement of

additional external jacking forces can improve the recovery

during the cooling phase.

The most critical aspect of the heat straightening is the

through-thickness temperature of the metal. Any applied heating

must remain clearly below the lower transition temperature

(approximately 7238C), to prevent permanently changing the

physical properties. A maximum temperature between 600 and

6508C should not be exceeded for steels manufactured in the UK—

a ‘cherry red’ colour will be visible on the steel surface if the

temperature is too high. The attainment of the correct

temperature is indicated by a silver colour under the flame,

surrounded by a brown and blue area. During this process the

yield stress of the material will reduce temporarily by

approximately 50%.

A molecular phase change in steel occurs when the temperature

exceeds the lower transition point. At this temperature, the

body-centred cubic structure of the steel begins to assume a

face-centred cubic form. This composition carries a larger

percentage of carbon in solution. A careful cooling process is

required to return the molecular structure successfully to the

body-centred cubic arrangement. Uncontrolled cooling of the

steel may not allow this reversal to occur, which will produce

a hard but brittle material known as martensite. This material

has low fracture strength and is particularly susceptible to

cyclic loads.

There are several possible methods for monitoring the

temperature during the heat-straightening process. Typically,

this is most easily achieved by observing the colour of the steel,

which is a skill that is largely dependent upon experience.

Unfortunately, this method can be significantly erroneous.

Research by Roeder3 and Avent et al.,4 has shown that even

experienced practitioners when judging temperature consistently

misjudged by 568C and in some instances in excess of 1008C.Alternative methods of surface temperature measurement are

possible using special heat-sensitive crayons, thermocouples or

digital infrared pyrometers. Although it is not advisable to

overheat the steel above the transition temperature during heat

straightening, it does not immediately necessitate the

replacement of the member. Further engineering judgement is

always required.

During the heat straightening of Brockhampton Road new

bridge, digital infrared pyrometers were used. This form of

temperature monitoring proved to be very convenient as the

pyrometers were used from remote distances on the carriageway

below.

3. IMPACT DAMAGE

As previously mentioned, Brockhampton Road new bridge

suffered structural damage to two main beams and ‘K’ bracing,

following the impact of a low-loader lorry. Both global and local

g repairs to a steel road bridge Clubley et al.

deformation of the bottom flanges and web were immediately

visible after the accident.

The structure was inspected in detail which established,

fortunately, that the impact was located close to the point of

contraflexure. Fig. 2 illustrates the bending moment and shear

force profile at the location of impact. At the damage point, the

bottom flange of the beam is in tension. The flexural stresses in

the steel were calculated to be low.

The low-loader lorry struck the first main beam on the

eastbound carriageway, subsequently scraping the bottom of

the next three beams and finally striking the bottom flange

of the fifth main beam. Despite a satisfactory loaded trailer

height, impact occurred due to improperly secured machinery

bouncing upwards as the vehicle moved. All of the damage to

the structure had occurred before the driver could stop the

lorry in time. Fig. 3 shows the damage to the first and fifth

main beams.

A summary of the localised and global damage is given in the

following subsections.

3.1. Beam 1

At 8.0 m from the face of the north pier, the outer (west) half of

the bottom flange was bent upwards by 70 mm. This deformation

occurred over an 800 mm length. The web adjacent to the

damage was found to be out of verticality by 30 mm over a

Fig. 2. Location of vehicle collision (dimensions in mm)


600 mm height. The bottom flange adjacent to the damage was

found to be out of level by 12 mm over its 480 mm width.

At 9.0 m from the face of the north pier, the web was pushed

outwards at the nearest bracing. There was a large crack in the

web parallel to the bottom flange just above the web-to-flange

fillet weld. The ‘K’ bracing between the first and second beams

closest to the point of impact buckled at the bottom chord level.

This bracing was fabricated from 120 � 120 � 12 mm rolled

steel angle. The end of the bracing on the first beam was no

longer attached to the web stiffener. A tapered global

deformation of 50 mm lateral deflection was recorded. This

global displacement ceased at approximately 8 m either side of

the impact point.

3.2. Beam 5

At approximately 3.5 m from the splice plate, the west side of the

bottom flange was bent upwards by 20 mm. This deformation

occurred over an 800 mm length—the web adjacent to the

damage was found to be still vertical. The bottom flange was

found to be horizontal. There was no visible damage to the

bracing between the fifth and sixth beams. Permanent formwork

used during construction prevented inspection of the concrete

deck soffit. There were no visible defects in the formwork or

signs of distress.

Damage to the beams in between comprised of superficial paint

scraping only. The minimum headroom of the structure was

maintained at 5.4 m.


Fig. 3. Damage to the main beams: (a) beam 1, west elevation;(b) beam 5, view towards south abutment

38

4. ANALYSIS AND MODELLING OF THE

STRUCTURE

A heat-straightening repair of the damaged plate girders required

the application of selective heating patterns. These comprised vee

heats for global deformation and line heats to correct

local distortion in the bottom flange. Passive restraint by

using hydraulic jacks was proposed in order to improve the

straightening during cooling. The heat-straightening process

was to be applied on the bottom flanges of the first and fifth

beams, and to a lesser extent, the web of the first beam.

In conjunction with the design of the main heat-straightening

process it was necessary to assess the bridge in its temporary

condition during the additional repairs. These repairs fell into two

categories. First, it was necessary to investigate a possible 50%

reduction in strength due to the heat-straightening process and

second, supporting calculations were required to confirm that

repairs could be conducted subject to dead and superimposed

dead loading without temporary supports to the damaged beams.

These repairs included the removal of web stiffeners, lateral

bracing and a section of buckled web to be cut out.

The maximum stresses in the bottom flange and web were

checked in accordance with an expected 50% reduction in the

yield strength of the steel beam when heated to a maximum of

6008C. Traffic management over the bridge was required to

prevent live loads from acting on each beam during the repairs.

This dictated the position of traffic flow so that all vehicular loads

were moved to the opposite side of the deck.

Fig. 4. Construction of the finite-element model

4.1. Frame analysis

The temporary condition defined the effects on the structure

during the initial repair process and in preparation for the

heat-straightening works. An analysis of the temporary

condition was required to investigate the following.

(a) Flexural stresses in the flanges incorporating a potential

temporary reduction in yield strength.

(b) Combined bending and shear in the web plate, including

the web cut-out.

(c) Removal of vertical web stiffeners.

(d) Yield and buckling of the web panel.

The bridge was modelled using the software Lusas Bridge Plus.

A portal frame was constructed to represent the integral


arrangement. The frame was supported on elastic springs

modelling the thrust and retardation of the soil strata.

A transition between cracked and uncracked section properties

occurred at the main span splices, approximately 5.2 m after

the leaf pier support. The analysis was linear elastic.

Stresses in the flanges and web plate were calculated using staged

composite construction. The factored tensile design strength for

Grade S355 steel plates subjected to heat straightening was

reduced by 50% to 161 N/mm2, including the partial factor for

material properties, gm, in accordance with Federal Highway

Administration document: FHWA-IF-99-004 1998.2 Fig. 2

illustrates the bending moment and shear force profile for

reference.

For dead and superimposed dead loading the analysis found that

stresses in both the damaged first and fifth main beams did not

currently exceed 70 N/mm2, a satisfactory value less than the

factored tensile design strength of 161 N/mm2. Results were

output between a location 1 m either side of the impact point.

This 2 m region encapsulated the expected area of heat

straightening.

Loading on the beam comprised only dead and superimposed

dead loads, as before. The structural integrity was checked for

each stage of repair. This included the addition of new cruciform

stiffening and finally, removal of the damaged web section.

The modified steel section with web cut-out was checked in

accordance with BS 5400 Part 3: 20005 for yield and buckling.

However, this guidance does not directly incorporate an

allowance for deformed or damaged sections when checking

these criteria. A plate panel check for an undamaged web, using

BS 5400 Part 3: 20005 produced a structural adequacy of 2.6

and 4.8 for the yield and buckling criteria, respectively.

4.2. Finite-element analysis

A finite-element model of the damaged beam was constructed to

confirm the results derived using the frame analysis, particularly

the effects of localised damage on yield and buckling. Using

volumetric meshing the stress conditions in the steel were

examined in detail. In accordance with the portal frame analysis,

the model represented the first beam as that was the most

severely damaged. This was subjected to identical dead and

superimposed dead loads. The entire composite beam over the

full length was modelled with restraints representing the bracing.

Fig. 4 shows the finite-element model.

To improve the numerical accuracy of the solution in the region

of the heat straightening, further refinement of the mesh was


Impact damage

(a) (b)

Fig. 5. Local stress analysis: (a) in-service flexural stresses, (b) shear stresses with web cut-out

incorporated. The web cut-out was conservatively approximated

without a smooth corner radius. As a result, a small stress raiser is

shown at the corners. The analysis type was again linear elastic.

Figure 5(a) illustrates the flexural stresses in the first damaged

beam subject to identical loading and shows the point of

contraflexure using a white colour. The finite-element model

exhibited better load distribution with a maximum bottom flange

stress of 50 N/mm2. Introduction of a 500 mm � 500 mm

web cut-out and cruciform stiffening produced stresses

approximately 30 to 40 N/mm2 higher. Fig. 5(a) confirmed

the fortunate location of the lorry impact near the point of

contraflexure.

Figure 5(b) visually illustrates the concentrated shear stress in the

web panel with the damaged steel removed. The finite-element

model was modified to approximate the localised deformation

recorded during the detailed inspections. Both the hand

calculations and finite-element analysis confirmed a satisfactory

web plate buckling factor of 0.21.

The finite-element analysis was further extended to examine the

same beam given a localised failure of the bottom flange due to a

hypothetical gross mis-heating. During the heat-straightening

process the applied temperature is restricted to below 6008C.Should the maximum permissible temperature be exceeded, the

material properties of the steel will become permanently altered.

This would potentially have serious consequences on both the

immediate and future in-service performance, particularly

fatigue. A section of the bottom flange was removed from the

finite-element model at the location of heat straightening.

The analysis conservatively showed the stresses in the web

and flanges did not exceed 130 N/mm2, still below the

reduced factored tensile design strength.

Beam 1:globaldamage

Beam 1:local damage(plan view)

Beam 1:local damage

(elevation view)

Beam 5:local

damage

Strain ratio, m 5.56 30.06 25.07 3.85Assessment status Acceptable Acceptable Acceptable Acceptable

Table 1. Damage strain ratios

5. DESIGN OF THE HEAT

STRAIGHTENING

Design of the heat straightening was

based upon advice from experienced

contractor Fairfield Mabey and

information gathered from desk

studies. The structure was inspected

in detail using access equipment and


traffic management in December 2002. This report included

magnetic particle inspection (MPI) testing to all impacted areas

plus MPI and hardness tests to all yielded steel.

The decision whether to use the method of heat straightening

was based upon a damage strain ratio calculation. Federal

Highway Administration document FHWA-IF-99-004 19982

recommends heat straightening can be carried out on North

American steel products if the damage strain ratio is less than

100. As the manufacturing process of steel in the UK differs from

that used in North America, the Highways Agency has currently

adopted a maximum strain ratio of 50. The strain ratio is

calculated using equations 3.1, 3.2 and 3.9 in Federal Highway

Administration document FHWA-IF-99-004 1998.2 For brevity,

the damage strain ratio calculation is summarised in equation

(1).

Damage strain ratio m ¼Eymax

Rf y1

where E is the modulus of elasticity of steel; ymax is the distance

from the centroid to the extreme fibre of the element; R is the

radius of curvature; and fy is the yield stress.

The strain ratio was calculated for local and global damage

considering both the first and fifth beams. A damage strain ratio

not exceeding 30 was determined based upon the detailed survey

measurements. This confirmed an acceptable assessment status

for all the proposed heat-straightening repairs. A summary is

shown in Table 1.

A series of passive jack restraints were designed for use during

the heat-straightening process. The careful placement of

additional jacking forces can substantially improve the

recovery during the cooling phase, therefore reducing the


40

number of heating cycles. A different arrangement was required

for local and global straightening. Fig. 6 shows the set of

inclined jacks restraining global movement while a single

vertical jack assists local flange straightening. It was very

important that none of the jacks was over-pressurised

causing cold mechanical straightening of the steel. A strict

monitoring programme was designed to prevent any damage

occurring in this manner.

The permissible jack restraint forces were calculated in

accordance with FHWA-IF-99-004 1998.2 To prevent

mechanical straightening, they were required to satisfy two

key criteria.

(a) First two heating cycles: loads should not produce bending

moments greater than 16% of the plastic moment capacity

of the member.

(b) Subsequent heating cycles: loads should not produce

bending moments greater than 33% of the plastic moment

capacity of the member.

Based upon the plastic moment capacity of the bottom flange, the

jacking loads at 16 and 33% were 33 and 67 kN, respectively.

5.1. Heating patterns

There are four main damage categories that can be repaired using

heat straightening.

(a) Category S: bending about the ‘strong’ or major axis.

(b) Category W: bending about the ‘weak’ or minor axis.

(c) Category T: torsion or twisting about the longitudinal axis.

(d) Category L: localised flange or web buckling, web crippling,

small bends or crimps in the plate.

Heat straightening should only be applied in the vicinity of a

yield zone. A yield zone is an area in which inelastic deformation

has occurred. Any permanent bulges or thickening found in the

heated zone after cooling is called ‘upsetting’.

The different heating patterns available for selection were: vee

heats, line heats, spot heats, strip heats and edge heats. Repair of

the local and global damage to the beams utilised a careful

combination of line heats and vee heats, applied to both sides of

the bottom flange. Vee heats are the most fundamental heating

pattern used to straighten Category S bending about the major

axis. Direct heating causes an expansion which increases the

deflection but cooling causes a subsequent larger contraction

correcting the original damage. The largest contraction occurs at

Fig. 6. Repair of damage to beam 1: (a) passive jacking restraints to(b) jacking restraint to correct local damage (concentrated vee heatrepair stiffening


the open mouth of the vee causing the section to move in the

direction of the vee. If the initial expansion is restrained, the

overall movement is increased.

Importantly, the welding torch passes across the vee shape in a

snake ‘S’ shape on both sides of the plate. With care, this

technique helps prevent an undesirable concentrated heating

effect.

Line heats are a heating pattern used to repair Category W

bending about the minor axis. This method of straightening is

conducted using a straight transition of the welding torch

between two points. Line heating can in some circumstances

require an external jacking restraint to accelerate the

straightening. Successful heat straightening is largely dependent

upon operator skill and for line heats the speed of the torch

over the metal is crucial to success.

Figure 6 shows the pattern of heating used to correct both local

and global damage of the first beam. A combination of vee heats

and line heats together with the raking restraints guided the

global correction of the bottom flange. A vertical restraint and

concentrated vee heat on both sides of the flange was used to

straighten the localised damage.

6. SITE OPERATIONS

All site operations were conducted at night during the closure

and temporary diversion of the eastbound A27 carriageway.

Work commenced on 17 November 2003 and by 29 November

2003 the site had been cleared and all aspects of the project had

been completed with the exception of the application of a full

protective coating system to the steel.

Following the removal of the buckled bracing and web

stiffeners, a series of trial heat cycles were carried out at

ground level on sections of steel plate. These were of similar

thickness and grade to those occurring in the bridge where

heat straightening was required. The trial sections were

heated, using vee heat patterns, until the colour of the steel

below the flame was silver, with a brown and blue surround.

An infrared digital pyrometer was used to confirm that

temperatures in the range 550 to 6208C were attained. All

operatives working on the bridge carried out the trial. One

trial section was marked up such that tensile and Charpy

impact tests could be carried out on both heated and unheated

steel. The results of these tests confirmed that no significant

correct global damage (alternate vee heat pattern shown);pattern shown); (c) local restraint and web cut-out; (d) cruciform


Fig. 7. Repaired bottom flange following heat straightening

changes in tensile strength or ductility occurred during the

trial heat cycle.

Global heat straightening of beam 1 began after the completion

of the trial heat cycles. Blast cleaning of the steel was carried out

and the lateral restraints were jacked to the design loads before

the heating started. Once a series of clearly marked vee heat

patterns had been marked on the bottom flange the first heat

cycle was carefully applied. The steel was subsequently allowed

to cool to below 1208C before beginning the second cycle.

Temperatures were monitored by observing colour and by using

infrared digital pyrometers.

After the second heat cycle, it was ascertained that the maximum

horizontal deflection had reduced from 52 to 31 mm. Three more

heat cycles were applied, reducing the maximum horizontal

deflection further to 21 mm. This was considered within

tolerance and it was decided to carry out no further global

straightening. In contrast, a series of nine heat cycles using line

patterns and vee heats were required to straighten the local

damage in the bottom flange to within an acceptable tolerance.

Restraining jacks were utilised between the top and bottom

flanges to assist the corrective movement during the cooling

phase.

Following the global and local straightening of beam 1, work

began to rectify the buckled web plate. This involved the removal

and reinstatement of the damaged section using flame cutting

followed by manual metal arc welding. Prior to cutting out the

deformed web, a cruciform arrangement of four web stiffeners

was welded in position to restrain the adjacent web areas in the

temporary condition (see Fig. 6). After repair of the web, some

cruciform stiffeners were removed to allow the installation of the

new vertical web stiffener. Accordingly, the replacement ‘K’

bracing system was then attached to the vertical web stiffener by

HSFG Part 1 bolts. Fig. 7 shows the final straightened profile

of the first beam.

The heat straightening was completed by the repair of beam 5.

The distortion caused by impact damage was straightened using

passive restraints between the top and bottom flanges, and a

series of four line heating cycles. Overall vertical distortion was

reduced from 22 to 8 mm. This marginally exceeded the target

value of 6 mm but, after consideration of the likelihood of further

heating improving the distortion by a final 2 mm, it was decided

to accept the correction value.

7. MATERIALTESTING

Magnetic particle investigation testing was carried out on all

areas of steel where heat cycles were applied, prior to and after

certain stages of the work. A final MPI test was carried out on

completion of the heat treatment. No defects were recorded. All

new fillet welds were tested by MPI and all new butt welds were

tested by MPI and ultrasonic testing. Again, no defects were

recorded.

Hardness testing was carried out on the distorted bottom flange

areas prior to the heat straightening and after certain heat cycles

for both beams. It was noted that, following the heat

straightening, the hardness test results (Vickers) reduced from

approximately 165 to 125 HV where the maximum heat had


been applied. Using comparisons with BS 970,6 this would

indicate a reduction in tensile strength at ultimate limit state

from 550 to 400 N/mm2. The ultimate tensile strength (UTS)

requirement to BS EN 100257 for grade S355 steel of this

thickness is 490 N/mm2.

It should be noted that the application of one heat cycle to the

trial sections did not cause the UTS to be reduced when tested

by destructive methods, producing a value of approximately

580 N/mm2. Accordingly, the estimated UTS using hardness

values was considered to form an indicative guide. In view of the

apparent reduction in hardness readings resulting from the

application of multiple heat cycles, it was recommended that for

future similar projects, the number of cycles applied to trial

panels is increased to six. Destructive testing of the trial sections

prior to starting work would thereby be more representative of

the heat straightening.

After consideration of the location of the repairs, which was close

to the point of contraflexure, a reduction in UTS to 400 N/mm2

would not constitute a theoretically overloaded section of girder.

The maximum factored tensile stresses resulting from 30 units

HB and associated HA loading at the locations where heat cycles

were applied did not exceed 90 N/mm2. If the factored tensile

design strength for grade S355 steel is reduced from its normal

value of 307 N/mm2 in proportion to the apparent decrease in

UTS, a maximum stress of 256 N/mm2 is obtained, which

exceeds the applied stresses by a considerable amount.

8. SUMMARY

Heat-straightening repairs to structural steelwork on

Brockhampton Road New Bridge were successfully completed,

pending new protective coatings, in ten days. Impact damage

to the beams was repaired within acceptable tolerances and

the straightened steel was subjected to hardness and MPI

testing. The implementation of the repair works reinstated the

affected members to their original designed strength

capacities.

This case study illustrates the future potential of the

heat-straightening method for structural steelwork repairs in

the UK. The mitigation for the scheme demonstrated an added

value of approximately £360 000. Repairs to Brockhampton Road


42

New Bridge were completed without undue disruption to the

travelling public or local residents.

9. ACKNOWLEDGEMENTS

The assessment and analysis of Brockhampton Road New Bridge

was conducted by Mott MacDonald on behalf of the Highways

Agency Area 3 commission. Gratitude is extended to the

Highways Agency for permission to publish this paper.

REFERENCES

1. BRITISH STANDARDS INSTITUTION. Code of Practice for the Design

of Steel, Concrete and Composite Bridges—Specification for

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