critical analysis of the britannia · pdf file · 2012-09-17a critical analysis of...

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S. M. Collingwood – [email protected]

A CRITICAL ANALYSIS OF THE BRITANNIA BRIDGE, WALES

S. M. Collingwood1

1 Undergraduate Student - University of Bath

Abstract: An overview of the re-design and construction of The Britannia Bridge following the deterioration of the

original structure due fire damage. Emphasis is placed upon the difficulties in constructing a new bridge around an

existing one, whilst trying to keep the bridge open for use. The aesthetic properties of the bridge are analysed by

Leonhardt’s 10 rules. The loading on the main steel arch spans are considered in respect to BS 5400 during various

stages of the reconstruction.

Keywords: Britannia, arch , truss, railway

1 Introduction

In 1838 George Stephenson proposed the

extension of the London-Chester railway line to

Holyhead. In order to reach Holyhead the railway line

had navigate the Menai Straits in North Wales. The

route surveyed by Stephenson suggested the most

suitable bridging point across the Menai Straits would

be where Britannia Rock lay in the middle of the

channel to provide a location for a central pier, shown

in Fig. 1.

Figure 1: Map of Location of Britannia Rock

(Ref. [1])

The main design criteria influencing early design

was that the admiralty required the channel beneath the

proposed bridge to be navigable both during and after

construction. The design chosen in 1845 to do this was

that of Robert Stephenson’s, a 4 span wrought iron box

girder bridge (Ref. [1]).

The bridge consisted of two central spans of 460ft

(140m) and two approach spans of 230ft (70m). The

bridge was constructed out of eight separate wrought

iron tubes that were joined together in fours, to make

two parallel tubes each 1,513ft ( 461m) long, that act as

a continuously supported beam. The railway lines ran

through the inside of the tubes, one direction in the up

tube, one direction in the down tube. These tubes

consist of malleable iron plates that are riveted to L and

T sections of iron to form a rectangular section with

celled top and bottom flanges (Ref. [2]). These

wrought iron tubes sat upon five abutments and towers

constructed out of Anglesea marble, shown in Fig 2.

Figure 2: Artistic impression of original Britannia

Bridge (Ref. [1])

Until the construction of the Britannia Bridge the

longest spanning cast iron beam bridge was the rail

bridge over the River Arno on the Florence and

Longhorn line, which was also designed by Robert

Stephenson, at 18.3m long, although this was later

increased to 31.7m by Edwin Clark through the use of

wrought iron rods to form a truss beam (Ref. [1]).

Therefore it can be seen that the development of the

box girder beam for the Britannia Bridge is of

significant historical importance, assisting in the

advancement of possible bridge spans at the time (Ref.

[3]).

Between the 23th

-25th

May 1970 there was a fire

caused by the accidental ignition of a highly flammable

Proceedings of Bridge Engineering 2 Conference 2010 April 2010, University of Bath, Bath, UK

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timber joint, that was covered in hessian tar, that joins

the wrought iron tubes with the stone entrance. The fire

spread vertically and set alight the roof canopy

designed to protect the tubing (Ref. [4]). Severe

damage took place with structural continuity of both

the up and down tubes broken over the central 3 towers

and severe sagging was observed. There were also

signs of vertical corrugations of the plates on the side

of the bridge.

The bridge was considered unsafe and all rail

traffic was suspended. (Ref. [5]).

2 Design Considerations

One of the main problems with the Britannia

Bridge was the breaking of the cast iron races guiding

the roller and ball bearings carrying the tubes. There

were grave concerns that the tubes would slip off its

bearings and into the channel. The engineers involved

with the redesign had the objectives of restoring the

rail traffic at the earliest opportunity and the safe

removal of the damaged wrought iron tubes, but

without generating unnecessary construction costs

(Ref. [5]).

The removal of the damaged tubes was a key

consideration. In order to remove the tubes in their

entirety at least two of the largest floating cranes

available at the time would have been required and the

operation would have to be conducted in an area of

water with greatly fluctuating tides. It was decided that

this method posed too many risks and costs. The option

of reverse jacking the tubes back down the slots in the

tower used to raise them was considered. But it was

calculated that the tubes in their present condition

would be unable to withstand the process (Ref. [5]).

It was concluded that the new structure must be

able to fully support the existing tubes, so that they

may be dismantled and removed in sections.

The Welsh Office for the Department of the

environment had shown interest in the redesign

incorporating an upper road deck carrying a relief road

for the over congested Menai Bridge, shown on Fig.1.)

The selected method for achieving the above

considerations would be to build arches underneath the

main spans, making use of the undamaged piers and

towers. The approach spans would be replaced with

new box girder spans, shown in Fig. 3.

The main design consideration left was to be the

construction material of the arch, be it steel or

concrete. Steel was preferred on the basis that it could

be prefabricated off-site and floated in for quick

construction. Steel would provide a lighter design that

would allow more economical abutments to be

designed. At the time British Steel also promised to

heavily prioritize steel construction for the bridge (Ref.

[5]).

Figure 3: Photograph of the New Steel arch truss

design (Ref. [6])

3 Design

It was decided that the bridge would consist of a

two spandrel arch. The arch would be constructed out

of steel box sections and the truss elements out of H-

sections. For ease of construction all the arches would

be constructed out of 22 inch flange lengths and 42

inch web lengths, the thickness out of the steel would

vary between 1.25-1.75 inches thick. The railway

would run a single track on one side of the lower deck

where the original tubes once were, with a concrete

upper road way for the road traffic, see Fig. 4.

Figure 4: Section of arch at mid-span (in direction of

Britannia Tower) (Ref. [5])

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4 Aesthetics

One of the factors also affecting the design of the

new bridge was its aesthetic qualities. The engineers

working on the project felt that due to the fire damage,

they would be unable to restore the bridge to a similar

outward appearance. One of the important aesthetic

considerations of the original bridge was the increasing

depth of the box girder sections from the sides towards

the center (Ref. [5]).

The original bridge had been set in the landscape

for 100 years and it was important that the new

Britannia Bridge was sympathetic towards the old,

maintaining the character it had developed.

In the 20th

Century Fritz Leonhardt developed ten

areas of aesthetics that need to be considered when

designing a bridge to ensure that it is not considered

objectionable (Ref. [7]).

4.1 Fulfillment of Function

Leonhardt proposed that the bridge should clearly

be shown to fulfill its function. The purpose of the new

Britannia Bridge was to re-establish the rail

connections across the Menai Straits and provide a new

road crossing. The bridge clearly has achieved this, it is

plain to see that the vertical loads are transmitted

through the bridge deck to the truss elements that

transfer the load to the steel arch. One misconception

could be that the horizontal thrust from the arches is

resisted by the masonry towers. This is not the case and

the load is transferred via reinforced concrete down

into the bedrock.

4.2 Proportions of the Bridge

It was also put forward by Leonhardt that a bridge

required balance between its masses and voids, light

and shadows, but also in its spans and depths. The

Britannia Bridge has open spandrels that are in stark

contrast to the masonry towers. These masonry towers

look heavy by modern standards as they are the

original towers from the original bridge .The arch

sections and the lower deck also looks heavy in

relation to the other elements in the structure. This may

be because they have the additional redundant capacity

to carry the excess loading of the original wrought iron

tubes. But as they are the main loading bearing

elements it doesn’t look incongruous. There is a good

balance between the shadow creating by the different

structural layers and the light that it allows through.

4.3 Order within the structure

If a bridge has a lot of lines and edges it can

become a mental disquiet to the beholder (Ref. [7]).

This is a drawback of the truss type construction. When

it is viewed from an angle it has a high order with lots

of edges and lines intersecting making it look untidy.

4.4 Refinement of Design

Subtle extra details can greatly improve the overall

aesthetics of the structure. In this case the use of

tapering columns to stop the illogical impression that

the top of a column is wider than the base. The

masonry towers taper from 62ft by 52ft 5 inches and

the bottom to 55ft by 32ft underneath the level of the

original tubes (Ref. [2]) in order to create this

impression. There is also detailing at the top of the

towers, as shown by fig 5, designed to make the top of

the towers look more refined. There are two limestone

lions at both ends of the bridge, purely for decorative

purposes, and an example of Victorian grandeur.

Figure 4: Artists View of the abutment area (Ref. [1])

The additional new structure contains little refinement

as it focus was more upon speed and ease of

construction

4.5 Texture

The variety of textures created by material choices

can be beneficial to the appeal of a bridge. The

weathered masonry piers have a rough and complex

texture that makes them interesting. Similarly the truss

contains a large number of bolts that upon inspection

provide novel textures but when viewing from a

distance it isn’t possible to see these subtleties. The

concrete elements appear smooth and uninteresting

from a viewing distance.

4.6 Colour

Colour can be used to highlight or detract from

elements of a bridge to make them appear thinner or

more noticeable. The arch and lower bridge deck, as

discussed earlier, appear heavy but this is somewhat

offset by the whiter appearance of them that makes

them appear lighter.

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4.7 Character

The original Britannia Bridge had a lot of

character due to its historical value. The loss of the

original box section appearance and continuous beam

function could therefore be described as diminishing

the bridge’s character. However, the final design

chosen for the rebuild bears a striking resemblance to

an initial design proposed by Thomas Telford and

Rennie, Fig. 5. The proposal anticipated spanning the

two main sections with cast iron arches, although this

was rejected on the basis that it impeded the navigation

criteria set out by the admiralty (Ref. [1])

Figure 5: Artistic Impression of Telford and Rennie

(Ref. [1])

The bridge as it stands now can be viewed as a

composite of many different ideas across 150 years and

this gives it a tremendous amount of character.

4.8 Integration into the Environment

The modern bridge upholds many of the original

aspects of the bridge which has been part of the

environment for a long as there is living memory. The

bridge now serves the needs to the local and national

communities to a higher level with the addition of the

road. It is sufficiently in keeping with its previous

incarnation to be considered integrated with its

environment.

4.9 Complexity

In order for a bridge to be visually stimulating it

needs a degree of complexity which may appear

contradictory to other principles such as having low

order. It is important that a balance is met so that a

bridge requires time to appreciated without appearing

chaotic. The Britannia Bridge has many different

elements and can be seen as complex with different

structural mechanisms used on different spans. It is

also a double decker bridge that makes it more

complicated. Though interesting as discussed in the

order sections it can be confusing with too much going

on.

4.10 Nature

Over time nature has developed some of the most

beautiful and brilliant structural designs. Leonhardt

proposed that by incorporating elements of nature into

the structural design you would arrive at an elegant

solution. However, this is not the case for the Britannia

Bridge, which lacks distinctly in a natural theme but

rather pays homage to the efficiency and functionality

necessary for continuing the second industrial

revolution occurring at the time.

4.11 Aesthetics Summary

The rules outlined by Leonhardt are not a

comprehensive set of absolute principles that will

produce a beautiful bridge every time aesthetics are

subjective to the critical beholder. The Britannia Bridge

follows several of Leonhardts rules but is truer to

function and its original heritage which was created

before these rules of aesthetics were derived and

documented.

4 Construction Method

As mentioned previously the arch units were

constructed out of steel and prefabricated offsite. The

arches were prefabricated at the harbor of Port

Dinorwig, which lies 2 miles away. At the harbor 8

erection bays were constructed so that the arches could

be constructed on heavy gauge bogies and transferred

onto purpose built pontoons that could be attached to

the dock wall at low tide.

These pontoons were then transported to site by

being pulled by 2 tugs. It was timed so that they arrived

onsite during slack water to try and mitigate the effects

of the strong tide, which would make accurate

construction difficult. Once onsite, the pontoons were

moored into position and attached by mooring lies to

the bridge itself and purposely positioned buoys.

On the next slack tide a lifting gantry raised the

arches into place. Construction of the arches used a

cantilever method of construction. The half arches

attached to Britannia Tower were tied back to together

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with steel rods to each other so not to produce any

moments. The half arches on the landward towers were

tied to the approach spans.

The arches were designed so they could both be

closed into 3 pin arches at the same time. If one of the

spans was completed first and its tying back force

released it was calculated that the horizontal force

generated would cause a high shear load at the mid

span and this would cause the failure the bottom

bracing cord bearings that were designed to only rotate

sufficiently to allow both 3 pin arches to form 2 pin

arches as shown in Fig. 6 (Ref. [8]).

Figure 6: Construction sequence of the arches (Ref.

[5])

In order to produce suitable dry conditions within

which to build the concrete foundations for the arches,

sheet pile cofferdams were constructed around the

bases of the Britannia and Anglesey towers and the

water pumped away. Simpler trench sheeting was then

deemed suitable to have the same effect at Caernarvon

Tower due to different ground conditions.

Whilst the arches were being constructed the

openings in the towers were being increased to make

space for the upper road deck and conform to new

British rail clearances. The deck for the steel arch was

laid in prefabricated units.

Flat jacks where used to attempt to uniformly

transfer the load of the existing tubes on to the new

structure. It was determined that once this was

completed that the bottom section of the existing tubes

had sufficient bearing capacity as to be able to carry

the railway. The railway was temporarily resumed

through the down tube.

Meanwhile the up tube was being dismantled. A

railway line was also run through the up tube that

carried a heavy trestle tower driven by a locomotive,

see Fig.7. The tubes were cut into 15 ft long pieces and

attached to the locomotive trestle tower. The roof and

sides of the tubes were then cut free of the bottom

flange, whereupon the weight transferred to the trestle

tower. The loose components were then transferred

along the track to the ends where they could be safely

removed.

The up tube could then be replaced with a new rail

deck. The railway was then transferred to the newly

completed track and the down tube was dismantled in a

similar fashion.

Once the railway was complete and the old tubes

removed, the upper road deck was constructed out of

concrete (Ref. [8]).

Figure 7: Picture of the dismantling of wrought iron

tubes (Ref. [5])

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5 Loading

During the construction of the new bridge the

arches would have to resist a variety of loads acting in

different combinations. The load cases considered to be

most adverse during both construction and completion

by the design team are shown in Fig.10 ( Ref. [5]).

The new structure must be able to withstand its

self-weight, the load from the existing wrought iron

tubes, rail traffic and vehicle traffic loading, as well as

the wind loading and secondary stresses caused by the

variation in differential temperature. When initial

designs of the new structure took place they were

working from loading under BS: 153 Part 3 (Ref. [5]).

However as BS 5400 has superseded this now and shall

be the basis for the loading analysed on the structure

(Ref. [9]).

All the loads being applied to the structure must

first be multiplied by two partial factors. �FL a partial

load factor and �f3 a factor to account for any

inaccuracies that may occur during analysis. For steel

bridges �f3 = 1.00 for Serviceability limit state (SLS)

and 1.10 for Ultimate Limit state (ULS) (Ref. [9]).

5.1 Loading Combinations

British Standards requires that five load

combinations are verified for both SLS and ULS. The

combinations are (Ref. [7]):

1. All permanent loads, plus any primary

live loads (also secondary live loads if a

rail bridge)

2. Combination 1, with the addition of wind

loading and temporary erection loads

3. Combination 1, with the addition of the

effect of temperature and temporary

erection loads

4. All permanent loads, secondary live loads

and their associated primary live loads

5. All permanent loads plus loads due to

friction at the supports.

5.1 Dead Load

Under all load cases the bridge must resist its self-

weight. For the design of steel bridges �FL = 1.05 for

ULS and 1.00 for SLS which will be applied to the

dead load.

The estimation of the dead load is shown in fig. 8.

Deadload Steel Reinforced

Concrete

Ballast Track

Main

Arches

(including

parapets &

inspection

walkways

38.29 11.61 5.88 1.26

Upper

Road deck

11.66 39.07

Lower

Road

access way

2.87 9.77

Figure 8: Approximate Total Dead Loads for both

main spans Mega Newton’s (Ref. [5])

5.2 Primary Live Loads

The bridge will experience a variety of primary

live loads during its construction. During the early

stages of construction the bridge will have to support

the weight of the wrought iron tubes with a loading of

17.2 MN each (Ref. [5]). Initially, both tubes will have

to be supported; following the removal of one tube, the

remaining tube will act on the arch with an eccentric

load which may cause torsional problems.

There will also be primary live loads due to the

railway loading. The Britannia Bridge has been

redesigned so that that the bridge will run with a single

track across it. BS 5400 typical load case for standard

railway loading, RU, is shown in Fig.9. This loading is

applied to each of the two rails at the same time. Once

again, as the position of the railway track varies during

construction, (shown in Fig. 10) there is a possibility

that large torsional forces will be developed in the arch

spans.

Figure 9: Standard RU loading to be applied per

rail according to BS 5400 (Ref. [9])

The above loading represents the static load case based

on the weight of the trains and the point loads produced

by the wheel contact with the rail. However, a train

load is not static it is a dynamic load, hence the load

case needs to be adjusted in order to account for

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Figure 10: Loading cases considered on the main

spans (Ref. [5])

impacts, oscillations and other dynamic effects the

loadings in

Fig. 9 must be multiplied by a dynamic factor, BS 5400

Clause 8.2.3. The dynamic factor can be seen in Figure

11 .

Figure 11: Dynamic factors according to BS 5400

(Ref. [9])

The length of the influence line for an arch

structure is half of its span (Ref. [9]). As the span of the

main arches in the Britannia Bridge is 134m,

evaluating fig. 8 produces a dynamic factor of 1.00.

The partial load factor for the rail primary and

secondary live loads, �FL, for the ULS under load

combination 1= 1.4, under load combination 2= 1,2

and under load combination 3= 1.2. For all SLS �FL

=1.0

The bridge will also be required to resist the

primary live loads of the road traffic. During the initial

design of the bridge the main traffic loading considered

was that of HA loading. The ratio of loading between

the road live loads and the dead load and the railway

live load combined was in the order of 1:7 and

therefore it was considered suitable to apply just HA

loading (Ref. [5]). The carriage way was considered to

have 3 notional lanes, as shown in Fig. 10.

HA Loading consists of a uniformly distributed

load (UDL) acting across the notional lanes as well as a

knife-edge load (KEL). For a HA UDL acting over the

arch span of 134m, the required magnitude is 20.9

kN/m per notional lane. The KEL per notional lane is

120kN placed in the most adverse conditions.

5.3 Secondary Live Loads

The railway loading will also produce several

secondary live loads such as lurching. Lurching is the

transfer of the live load from one rail to the other and is

taken into account by the dynamic factor.

Nosing is another secondary live load associated

with rail loadings. Nosing considers the lateral loading

a train may impart on a rail. It is represented by a

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single point load of 100kN acting perpendicular to the

direction of the rail (Ref. [9]).

If there is curvature in the track the train may also

cause a loading due to centrifugal force, but this is not

the case as the track across the Britannia Bridge is

straight.

The longitudinal loading caused by braking and

traction forces can be evaluated from Table 18 in BS

5400 Part 2. The longitudinal load due to traction is

750kN applied in the direction of travel (Ref. [9]). The

braking forces can be evaluated from equation 1.

Braking Force = [20(L-7) + 250] (1)

= 2960kN

Therefore the maximum longitudinal load applied is

3710kN.

There are also secondary live loads associated with

HA loading. The longitudinal loading associated with

trucks is represented as a 8kN/m UDL across the width

of the notional lanes and a single 200kN point load.

The skidding of a vehicle is considered as a single 250

kN point load in any direction in one notional lane

only. Once again, as with rail loading, centrifugal

loading will not be applicable. Impact loading

considerations would also be investigated.

5.4 Wind Loading

The wind pressures will cause additional loading

on the structure that needs to be considered. The wind

pressures can cause transverse, longitudinal and uplift

loading. As shown by Fig. 9 the main concerns in this

design were those of transverse and longitudinal

loading, as uplift is unlikely due to the high self-weight

of the bridge.

5.4.1 Transverse wind loading

The maximum wind gust can be calculated from

equation 2.

Vc = vK1S1S2 (2)

Based upon factors taken from BS5400 vc for

Britannia Bridge can be evaluated as 68.9 m/s. The

transverse wind load can then be evaluated by equation

3.

Pt=qA1CD (3)

With

q=0.613vc2

(4)

The tranverse wind load is 3.2 MN for each of the main

spans.

5.4.1 Longitudinal wind loading

The longitudinal wind loading can be given by

equation 5.

Pv = qA3CL. (5)

By obtaining the factors from BS 5400 for the

Britannia Bridge this can be evaluated at 59kN.

5.5 Temperature Effects

The variation in effective temperature will cause

elements in the bridge to expand or contract. As the

members are held in place by one another this will

generate unwanted stresses and strains.

There are also stresses produced by a difference in

temperatures between elements. During construction of

the new bridge a variation of up to 20oc was measured

between elements in direct sunlight and shade. The

effect of temperature has been mitigated by the use of

expansion joints in both the rails and road deck above

the landside towers and Britannia tower (Ref. [5]).

5 Strength

The ULS and SLS for the above loadings were

considered for all stages of construction by the use of a

Finite Element package, Electronic Calculus

Incorporated (ECI) programs 201 and 631. Performing

initial 2D and the 3D analysis (Ref. [5]).

As mentioned previously the arches used in the

main spans were designed to act as a 2 pin arch. As the

dead load of the structure combined with rail and road

live loads are higher than any other load, it is possible

to estimate the present bending moments in the arch by

using load combination 1. As shown in Fig. 9, this load

combination will be applied both to the full-span and

half-span to see which produces the more critical

bending moment.

When the factored combination loading 1 was

considered on the arch structure loaded as a whole. The

maximum sagging moment that the arch has to resist

was 34.4 MNm, with a higher hogging moment of

38.8MNm. The bending moment diagram for the

structure is shown in Fig.12.

S. M. Collingwood – smc25@bat

Figure 12: Bending Moment diagram with Load

Combination 1applied across whole span

It was interesting to see that the loading on the

structure became more critical when the live loading

was only applied to half the structure. In this case the

sagging moment increased to 49.1 MNm

hogging moment to the most critical 67.4 MNm, as

seen in Fig.11.

Figure 13: Bending Moment diagram with Load

Combination 1applied with permanent

whole span and live loads across half the span

6 Natural Frequency

The natural frequency of a bridge of this

magnitude is very important. If the dynamic loading of

the bridge from the railway track is similar

the natural frequency, unwanted oscillations can occur.

The same effect can occur from the variation in wind

forces acting on the structure. These oscillations

magnify the effects of a load and can cause

catastrophic failure.

9

th.ac.uk

Bending Moment diagram with Load

across whole span

It was interesting to see that the loading on the

structure became more critical when the live loading

was only applied to half the structure. In this case the

sagging moment increased to 49.1 MNm and the

hogging moment to the most critical 67.4 MNm, as

Bending Moment diagram with Load

loads across the

and live loads across half the span

bridge of this

very important. If the dynamic loading of

the bridge from the railway track is similar to that of

unwanted oscillations can occur.

ariation in wind

These oscillations

magnify the effects of a load and can cause

7 Durability

The steel Britannia Bridge was con

risk to attack by corrosion d

bridge to the sea and the high winds

further increased. The designers thought that there

might also be an issue that some of the steelwork

would be unable to be accesse

completed to perform any maintenance required.

All the steel work used in the bridge underwent

high quality grit blasting to ensure all

the steel mill was removed. The steel was then als

treated by being sprayed with aluminum.

There are a large number of bolted connections in

the Britannia Bridge. These bolted connections are

subject to fatigue. The repeated cyclic loading of the

bridge, due to its dynamic live loading, will constantly

place the connections under a varying amount of stress.

This can cause the bolts to loosen and allow more

flexibility than has been allowed for. The result of this

is that the deflections at serviceability limit state can be

higher than predicted causing dif

track alignment.

8 Foundations

The foundations for the arch springings are

designed to transmit the thrust from the arch into the

bedrock. The foundations are heavi

concrete blocks that transfer

mixture of shear and bending. The

bases of the masonry towers was

This was done by exploratory drilling. Any cavities

discovered were filled by pressure grouting. The

bearing capacity of the bedrock

[5]) .

9 Cost

It is estimated that the reconstruction of the bridge

to make it safe for rail traffic was £5.5 million. The

vast majority of that was spent on the contractor’s

building cost. It cost an addition £4.75

construct the upper road deck and build the new

approach ways to divert the traffic

10 Future Changes

During 2006 Atkins ltd was tasked by the Welsh

Assembly Government to determine different options

for increasing the road traffic carrying capacity of the

Britannia Bridge., which they undertook in November

2007.

ritannia Bridge was considered to be at

due to the proximity of the

bridge to the sea and the high winds this likelihood is

The designers thought that there

might also be an issue that some of the steelwork

would be unable to be accessed once the bridge was

completed to perform any maintenance required.

All the steel work used in the bridge underwent

ality grit blasting to ensure all rust and dirt from

the steel mill was removed. The steel was then also

treated by being sprayed with aluminum.

a large number of bolted connections in

the Britannia Bridge. These bolted connections are

subject to fatigue. The repeated cyclic loading of the

due to its dynamic live loading, will constantly

ace the connections under a varying amount of stress.

This can cause the bolts to loosen and allow more

flexibility than has been allowed for. The result of this

is that the deflections at serviceability limit state can be

higher than predicted causing difficulties with the rail

The foundations for the arch springings are

designed to transmit the thrust from the arch into the

bedrock. The foundations are heavily reinforced

that transfer the load down by a

mixture of shear and bending. The condition of the

bases of the masonry towers was also investigated.

This was done by exploratory drilling. Any cavities

discovered were filled by pressure grouting. The

bearing capacity of the bedrock was 1180kN/m2 (Ref.

It is estimated that the reconstruction of the bridge

make it safe for rail traffic was £5.5 million. The

vast majority of that was spent on the contractor’s

building cost. It cost an addition £4.75 million to

construct the upper road deck and build the new

approach ways to divert the traffic

During 2006 Atkins ltd was tasked by the Welsh

Assembly Government to determine different options

for increasing the road traffic carrying capacity of the

Britannia Bridge., which they undertook in November

10


Atkins reported that they had 3 main options, which

were, firstly to try and widen the existing carriage way

to increase the capacity of the existing structure.

Alternatively, to construct a new multi-span structure

out of concrete alongside the existing bridge or

construct a new single span cable-stayed bridge next to

Britannia Bridge.

These options were than subject to a public

consultation exercise to investigate how the public felt

about the matter. It was interesting to see the public’s

primary concern was that the reduction of current

congestion and that safety was the next most important

factor. The history and heritage of the bridge was only

the third most important factor to the public in design

considerations. 22% of people preferred the option to

widen the existing bridge. 70% of people preferred the

option of constructing a new bridge alongside the

existing one (Ref. [10]).

Recommendations

Given the already extensive history of the Britannia

Bridge it seems illogical to construct a new bridge

alongside it to deal with the issues of congestion. If the

bridge can feasibly be widen as proposed by Atkins it

would add to the already extraordinary history of the

bridge rather than conflicting with it aesthetically.

References

[1] RYALL, M.J, 1999. Britannia Bridge: from

concept to construction, Proc. Instn Civ. Engrs,

Civ. Engng 132, May/August 132-146, Paper

11736.

[2] DEMPSEY G. DRYDALE, 1864. Tubular and

other Iron Girder Bridges, particulary describing

the Britannia and Conway Tubular Bridges, Virtue

Brothers and Co. London. Reprinted 1970,

Redwood Press Limited, London.

[3] INTITUTE OF CIVIL ENGINEERS, 2008. ICE

manual of Bridge Engineering, Second Edition,

Thomas Telford Ltd, London.

[4] CAERNARVONSHIRE COUNTY COUNCIL

FIRE BRIGADE & ANGELESEY COUNTY

COUNCIL FIRE DEPARTMENT, 1970. A Joint

Report on A Fire in The Britannia Tubular

Bridge, Menai Straits On Saturday – May 23rd

1970.

[5] H C HUSBAND, 1975. Reconstruction of the

Britannia Bridge Part 1: Design, Proc. Instn Civ.

Engrs, 58, Feb 25-66.

[6] Britannia Bridge In: travel web shots [Online]

Available from URL:

http://travel.webshots.com/photo/23132172800862

15107EfXcIY [Accessed 5th March 2011]

[7] TIM IBELL. Bridge Engineering, Department of

Architecture and Civil Engineering, University of

Bath.

[8] R W HUSBAND, 1975. Reconstruction of the

Britannia Bridge Part 2: Construction, Proc. Instn

Civ. Engrs, 58, Feb 25-66.

[9] BS5400: 2006. Steel , Concrete and Composite

Bridges. BSI

[10] WELSH ASSEMBLEY GOVERNMENT, 2008.

A55 Britannia Bridge – Release of the results of

the recent public consultation exercise.

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