Figure 1: First Severn Suspension Bridge

CRITICAL ANALYSIS OF

THE FIRST SEVERN BRIDGE

Siobhan Gordon1

13

rd Year MEng Civil Engineering Undergraduate, University of Bath

Abstract: This paper provides a critical analysis of the First Severn Bridge between England and Wales over

the River Severn Estuary. With a high emphasis on construction methods and loadings that occur on the bridge

as well as looking into how the bridge has been maintained and improved since its original opening in 1966.

The First Severn Bridge was the first aerodynamically designed deck on a suspension bridge and marked a new

economic era and gateway to South of Wales.

Keywords: Severn Crossing, Suspension Bridge, Lightweight, and Aerodynamic Deck

1 Introduction

1.1 Brief background

Opened September 8th 1966 the design of the

Severn Bridge was a revolutionary concept, being the

longest single-spanning suspension bridge in the world

on its opening, and praised as the mark of an new

economic era, due to opening up a fast link between

England and South Wales.

It is sited at a mile wide crossing between England

and Wales, where ferries had previously been in

operation for over 800 years. The Severn is renowned

for its treacherous crossing with the second largest tidal

range in the world.

In 1945 a report on the Severn Barrage

Scheme was prepared for the Ministry of Fuel and

Power this was primarily a power scheme to provide

electricity but also featured a road crossing. As a result

the Ministry of Transport assumed responsibility for

the project to bridge the Severn under the 1936 Trunk

Roads Act, and in November of the following year

Mott, Hay& Anderson and, Fox & Partners were

engaged as Joint Consulting Engineers. Ref [1]

However construction of the bridge was

delayed due to the start of the Second World War in

1937, and priority given to the Forth Road Bridge

resulting in the original Severn Bridge designs being

implemented on the Forth instead.

In May 1962 a contract for the superstructure

was finally approved so work on the redesigned main

structure could commence. With the pier foundations

and anchorages already begun construction in 1961.

The Severn bridge was officially opened on

the 8th

September 1966, carried the M4 until the

opening of the Second Severn Crossing in 1996, when

the crossing road was renamed the M48 and resultantly

becoming a more minor crossing.

Proceedings of Bridge Engineering 2 Conference 2010

April 2010, University of Bath, Bath, UK

Figure 2 & 3: Severn Bridge location in proposed motorway

system

Figure 4: Severn Bridge from the shore

Figure 5: View from the tower

Figure 6: View through tower at sunset

1.2 Bridge Components

The Severn Crossing consists of 4 components, the

Aust Viaduct, Beachley Viaduct, Wye Bridge and the

Severn Bridge.

The Aust Viaduct is a twin box girder with a

concrete deck. It bridges from the Aust cliff to the

beginning of the Severn bridge.

The Severn Suspension bridge is 1597m long and

is the main component of the crossing and so is the

focus of this paper.

Beachley Viaduct is of a similar box girder

construction as the Aust Viaduct and is supported by

steel trestles, bridging the Beachley peninsular.

The Wye bridge is of cable stay design with

orthotropic box girder deck similar to the Severn

Suspension and crosses the border of England and

Wales over the Wye.

2 Aesthetics

The aesthetics of the Severn Bridge are based

on the renowned Fritz Leonhardt ten basic rules on the

principals of bridge aesthetics. Whilst a bridge’s

appearance is very subjective these rules allow viewers

to critique the bridge in many areas which affect the

ultimate appearance of the bridge. This is a big factor

on whether the bridge appears safe to users, therefore

affecting the overall success of the bridge.

One of the first rules is the bridges function.

The Severn suspension bridge clearly shows how it

works with a sense of stability given by its well defined

structural form. Structural hierarchy conveys safety to

users, with its obvious strength in the main cables and

substantial towers holding the cables and deck. This is

achieved in both elevation and cross section with the

bridge appearing rigid. There is clarity of how the

bridge works and is supported.

The features of the bridge look in proportion

to one another, with a balance between the mass and

void components of the bridge seeming in ratio. The

suspended deck looks in balance, both in depth and

size to its supporting towers and cables, with efficient

spacing between these elements. To the naked eye the

proportions, in simplistic terms, look like they work

and look right.

Design simplicity gives it order. With the

suspension bridge making sense from all view points

and angles. There are no unnecessary lines or edges

thus keeping the suspension bridge’s appearance sharp

and ordered. This is achieved by sitting the fascia

beams onto the outermost side of the towers so an

unbroken line is achieved. While the cables run parallel

to the deck almost unnoticeable at first glance helping

to detract attention away from the main structure.

There are many subtle refinements used on the

There are many refinements in Severn

Bridge’s design such as tapering toward the top of the

towers. This not only saves on material costs but also

emphasizes the slenderness of the piers. An approach

which can be seen throughout history, particularly in

many Greek designs.

Smaller spans were used in the approach

sections to keep aspect ratios between the ground, piers

and deck making it more logical in appearance. Hence

keeping the spacing throughout the bridge uniform.

The bridge has been painted white helping it

to stand out from its surroundings of the estuary. It

also conveys a sense of lightness of form. With its

simplistic form it suspends over the estuary in a

Figure 1: Severn Bridge in

proposed motorway system

Figure 2: Severn Bridge in

proposed motorway scheme

Figure 7: View of deck

striking effortless way without over powering the

surrounding area. It stands out but only with elegance

and grace.

3 Design

Despite completion just 2 years after the Forth

Road Bridge and built using the same consortium and

consultancy, even though it is of comparable size and

appearance the design of the Severn Bridge is

markedly different.

3.1 Deck

The development of the Severn Suspension

Bridges deck came about by accident. It was originally

proposed to use a stiffened truss lattice, similar to that

used on the Forth Road Bridge, however during wind

tunnel testing the model broke. Whilst a new model

was being constructed alternative shapes where tested,

leading to the development of the aerodynamic box

deck. Ref [2]

The re-modified deck shape led to the removal of

the impractical deck kerb, and placement of open

handrails and crash barriers to reduce wind effects as

much as possible to continue the lines of the deck and

flow of wind over it.

Testing and calculations showed that the deck was

stable in all wind speeds and angles of inclination.

However wind tunnel test predicted that oscillation

might occur under certain condition if structural

damping coefficient was less than 0.05.

3.2 Inclined Hangers

Due to the box deck being completely welded no

dampening effects could be expected in the deck. So

inclined hangers were incorporated into the design in

order to absorb the excess energy built up in the deck

thus reducing vibrations and resulting in only a 5-10%

in deck bending stiffness.

Two parts of each hanger are inclined to form a

triangulated system. The hangers situated near the

centre span have separate connections to the deck due

to the overstress that could be caused as a result of

longitudinal movement. The inclined angles are

specific to each section so that each hanger is equally

stressed and capable of taking the required loads.

3.3 Towers

Each tower leg is designed as simple rectangular

tubes formed from 4 stiffened plates, a step away from

the large box component towers of the Forth Road

Bridge. This design greatly reduces wind forces on the

towers, and results in a more efficient use of materials

with towers weighing 50% less than comparable

suspension bridges of similar spans.

3.4 Summary of Design

Like any suspension bridge the load bearing

elements are hung from the suspension cables which

are anchored to the ground. The development of the

box deck enables it to carry only bending forces

providing stiffness to the system, with the closeness of

the hangers allowing concentrated loads to be spread

between them.

The final design centred on the box girder deck,

has the advantage of lightness, more efficient area for

services and bridge components and wind effect where

1/5 of that experienced on the Forth Road Bridge. The

lightness of the suspended structure meant a reduction

in materials so causing a decrease in cable tension and

stiffness in the system as a whole.

4 Geometry

The suspension bridge spans a total distance of

1598m, with the main suspended deck spanning 988m

as seen in figure 9, making it the longest spanning

suspension bridge of its time.

Figure 8: Model of deck in wind tunnel

Figure 9: Elevation of the Severn Bridge

One of the main features of the Severn Bridge that

makes it stand out compared to other suspension

bridges built around the same period i.e. The Forth

Road Bridge is the construction of its suspended deck.

The deck in figure 10 consists of a hollow steel

box 3m deep and 23m wide with feathered edges and a

cantilevered footway and cycle track on each side. This

shape results in a smooth aerodynamic flow of air and

resistance to wind

The steel towers shown in figure 11 consist of 2

rectangular legs joined by three crossing members. The

rectangular towers taper gradually along the 122m high

distance they reach from their concrete piers. Each leg

is comprised of 4 stiffened high tensile steel plates 17m

in length. The design of the towers allows half the

weight of steelwork to be used in the Severn towers

than in the Forth.

5 Site conditions

Situated on a 1 mile wide stretch of the Severn it is

sited at a bottle neck section of the river. With areas

down and upstream of the site widening to 2 miles,

creating currents capable of reaching 4.1ms-1

during

spring tides. In addition the Severn experiences the

second largest tidal range in the world with neap tidal

ranges of 5.5m, and ordinary spring tides reaching

13m, and 14.6m on extreme spring day. These

conditions of flow have the advantage of stopping the

formation of any alluvial deposits at site; however

make construction in the area very difficult and heavily

dependent on the tidal periods and flow of the river.

A 21 borehole study was carried out to indicate the

geological makeup of the site shown in figure 13, it

showed that lots of carboniferous limestone was found

on the surface of the South Eastern Side of the Severn

site, on the North Eastern side the rock was overlain by

Keuper Marl up to 55m thick in places. Ref [2]

The location for Aust Pier was chosen to be

centred on this South Eastern outcrop of limestone

rock, which is uncovered in extremely low tides, with

the Beachley Pier located on the Keuper Marl rock to

the North East 988m across from the Aust pier.

Additional bore holing was carried out once the

site was decided, up to 6m below foundation depth on

the Aust side and 15m on the Beachley pier site, to

check the geological stabilities of the chosen site. i

6 Construction

6.1 Material and Cost

In table 1is a breakdown of the steel that make

up the main superstructure and the cost to the value of

the materials and labour at the time of construction.

Table 1: Quantity of steel work in superstructure and

cost

Section Amount Cost

Tower 2360 tones £476000

Splay, saddles +

anchors 312 tones £77000

Suspended

structure 11300 tones £1394000

Protective

treatment external 162000sq yards £336000

Protective

treatment internal 231000sq yards £587000

Cable wrapping

wires, cable bands,

suspenders

4710 tones £47000

Total £2,917,000

Figure 11: Cross section of tower

Figure 10: Cross section of deck

Figure 12: Site plan of Severn Estury

Figure 13: Geological section through centre line

of site

Figure 14: Beachley pier Figure 15: Aust pier

From left to right;

Figure 16: Climbing structure in Aust tower

Figure 17: First tower plates for bridge erection

Figure 18: Completed Beachley tower erection

Total construction cost amounted to £5,097,000

and took 4 years. The resulting build surmounted to

£8,014,000, making the new design for the Severn

Suspension Bridge £800,000 less than the original

design. Ref [2]

Savings were also made on maintenance costs due

to the reduced area of exposed steel in the box design

and better access to repainting. Further money could

have been saved on the foundations and anchorage due

to the reduced dead loads of the new design; however

these where implemented before the final box structure

designs were finished.

6.2 Anchorage

Due to the physical conditions of the site

tunnelling anchorage was deemed uneconomical, so

gravity anchorage was used. The gravity anchors are

composed of reinforced concrete walls enclosing a

splay chamber where 19 cable strands fan out to a

rectangular pattern. Ref [3]

The Aust anchor due to its position gave a tidal

range which resulted in 6 workable hours at low tide.

The foundations were therefore constructed by

excavation of the area to 2.5m removing all unsound

rock, which was then back filled with concrete. An

external shell was then built to a height 9m above high

tide levels creating a gravity dam. The anchorage was

then cast insitu in 54 sections forming the final anchor.

Ref [3]

The Western Anchor main foundations consisted

of 2 trenches 42m long by 10.6m wide at 23m centres.

They were dug through 6m of gravel and 12m Marl to

the Limestone base. The anchors where lined in the

trenches casting the concrete insitu and then inserting

the reinforcement throughout the anchorage and back

filling with the remaining concrete.

6.3 Towers

The towers were built as free standing structures

tied to the base by wire rope. To erect each tower a

climbing structure was devised carrying a 32 ton

capacity crane. The structure was built in stages with

17 metre sections of steel work added, until the

required 122m was reached. The crane remained for

the rest of the bridge construction.

The winch of the structure was achieved by an

arrangement of hydraulic jacks enabling the structure

to hoist itself, climbing 17m in 6 hours. Rollers were

needed in the tower faces to counteract the unbalanced

force resulting from the winching of the tower

components.

Lifts where installed as soon as possible enabling

framing to be extended as each tower progressed, and

to carry workers safely to various points of the tower.

The towers steelwork was fabricated to a fine

specification off site and then transported to site on

specialist barges. These methods was chosen in order

to avoid double handling of components and reduce the

amount of materials needing to be stored on site,

ultimately reducing costs and making the construction

sequence more methodical and time saving.

Due to tides and water level variations in the

estuary the erection sequence had to be timed carefully

to avoid interruption of work. At later stages in the

tower construction it was also necessary, for safety, to

halt work when wind speeds where more than 10mph.

When the towers where finally erected deflection

at the top of the towers, due to erecting loading,

amounted to 0.1m shoreward, this increased to a

further 0.9m due to tensions in the suspension wires.

This was rectified so extra deflection was calculated to

give the towers a vertical position on final completion.

Ref [1]

6.4 Cables

Cable spinning was used to produce the main

suspended cables. This is a process whereby a number

of single wires are laid parallel to each other then

compacted into a large bundle, when in the correct

position. Due to site difficulties the main cables were

brought across by lying out on the riverbed, under

water, and then hauled to the tower top. Set tides were

needed for this process taking 3 day to be completed

once hauling commenced.

After the first cables where stretched out and

attached to the anchors the remaining cables were

pulled into place by haulage wire and adjusted to the

Figure 19: Cable measurements prior to banding

Figure 20: Cable spinning wheel

Figure 19: Cable measurements prior to banding

Figure 20: Cable spinning wheel

Left to right:

Figure 23: Deck section being lifted

Figure 24: First section being manoeuvred into position

Figure 25: Underside of main span

correct sag ratio and then fixed in the supporting frame

of the tower and anchorage.

An overhead tram system was erected on the

supporting cables and unreelers were then laid 2 wires

at a time over a series of counter weights shelves

finishing in the anchor pits where the cables where

fixed. Counter weights were used to keep an even

tension and maintain the necessary sag in the wire

using both live and dead wires throughout the

superstructure. At the saddles the wires are adjusted to

the correct position using a winching system.

On completion compacting machines mounted

the cables forcing the wires into circular sections with

temporary galvanised steel bands placed around the

cables as they are compacted together, in order to

retain their shape and exclude the weather.

Suspenders were then attached to the

underside of the cable bands and to the suspended

structure diagonally.

6.5 Suspended Spans

Due to the deck shape it was able to float on

water. The 88 sections, which make up the spanning

deck, were assembled on the riverbank and sailed in

the from a local shipbuilding yard.

A special launching barge was developed for

the tasks of towing the deck sections to within 1m of

the designated position. It then held them there, for

approximately 10minutes, in the fast flowing estuary,

as they were winched into place. Getting the sections

into the required positions was heavily dependent on

the tides, with neap tidal range giving the ideal criteria

for the launching. Two week intervals were taken

between each major launching phase managing to erect

3-6 section in a period. [Ref 1]

The sections were lifted from the two main

suspension cables using a winching system sited on

either tower platform. To avoid unequal lifting only

two lift positions were available on each deck section.

However the two end sections couldn’t be winched

directly upwards due to sections being present on either

side. They were therefore slid into place at an angle.

When the weight of the section was taken by

the winch it was raised 7.5m, for washing, by a power

hose on the barge below. It was then winched the

remaining 30.5m to its required position and attached

and welded to the previous section.

Erection began at the centre of the main

suspension span and commenced toward each tower

equally. Initially the main spanning cables were 10m

above their final position, but as the erection

progressed the cables lowered and the steelwork

straightened into their final positions.

6.6 Finishing items

Once the main structure was erected and

welded, the road surfacing began along with the

erection of crash barriers, parapets, and lighting. All

external surfaces were then shot blast sprayed with zinc

and given two coats of finishing paint.

The construction took three years to complete.

It ran 5 months ahead of schedule, despite adverse

weather conditions during construction.

Figure 21: Transporting cable parts on gantry

Figure 22: Cables being compacted

Figure 26: Final painting of main cable

7 Loadings

Loading has been calculated to BS5400 part 2

standards despite it being designed to the older BS153

(2) standards. This is done in order to analyse the

bridge performance to the increase in max HGV

vehicle size from 24 tonnes to 44 tones. It will also

highlight the durability issues present in the bridge

today, which will be discussed in a later section of

changes.

BS5400 codes use the limit state philosophy

checking it to Ultimate limit state (ULS) and

serviceability limit state (SLS).

The most important loads to consider are:

Dead loads

Super-imposed dead loads

Live traffic loads

Wind

Temperature

The following loading cases are applied to the

partial load factors and safety factors to ensure the

bridge is structurally sound.

Table 2: Applicable Partial factors. Ref [4]

SLS ULS

Loading case fl f3 fl f3

Dead 1.00 1.10 1.05 1.10

Super-imposed load 1.20 1.10 1.75 1.10

Live 1.00 1.10 1.30 1.10

Wind 1.00 1.10 1.10 1.10

Temperature 1.00 1.10 1.00 1.10

The following loading combinations need to be

checked at both SLS and ULS;

All permanent load + primary live loads

Combination 1 + wind, and if erection

considered, temporary erection loads

Combination 1 + temperature and if erection

considered, temporary loads

All permanent loads plus secondary live loads

and associated primary live loads

All permanent loads plus loads due to friction

at supports Ref [5]

7.1 Dead and Super-imposed dead loads

Dead loads are in reference to the

superstructure of the bridge. Table shows the total

uniformly distributed load, which has then been

factorised with safety, ULS and SLS factors.

Table 3: Dead loads of bridge

Elements Load

(kN/m)

SLS Load

(kN/m)

ULS load

(kN/m)

Steel work 64.8 71.3 74.8

Cables 24.6 27.1 28.4

Hangers 0.6 0.7 0.7

Total Dead Load 90 99.0 104.0

Superimposed dead loads is comprised of the

deck surfacing, finishes and parapets these once again

the results are factorised with the appropriate factors.

Table 4: Superimposed loads of bridge

Elements Load

(kN/m)

SLS Load

(kN/m)

ULS load

(kN/m)

Road + Footway

surfacing

19.2 25.3 37.0

Service ducts, paint

etc

2.1 2.8 4.0

Handrail + crash

barriers

1.2 1.6 2.3

Total

Superimposed

Dead Load

22.5 29.7 43.3

7.2 Live traffic loads

There are 3 types of live traffic loading that need

to be considered;

HA loading

HB loading

Pedestrian loading

HA loading is formula loading representing

normal traffic loads in Great Britain. To BS 5400-2:

2006 standards this consist of a uniformly distributed

load (UDL) acting over national lane together with

knife edge load (KEL) Where the UDL is given by

equation

1.0)1

(36l

w (1)

The loaded length ( ) is assumed as the main

span (988m). This gives 18.06kN/m load per notational

lane. The KEL per notational lane is taken as 120kN.

These notational loads are factored to give

ULS loads of 24.8kN/m and 165kN respectively, and

for SLS 19.9kN/m and 132kN.

HB loading represents an abnormal vehicle

loading, taken as an equivalent to 45unit per axial

equating to 112.5kN per wheel. Variations of the inner

axial spacing of 6, 11, 16, 21, or 26m should be applied

to produce the most serve loading effects on the bridge.

In the case of the Severn Bridge the worst loading case

is when the inner axial is spacing is 6m. Ref [6]

HA, KEL and HB loadings are applied to the

bridge together to give the worst case loading, each

treated separately and given different factors. The

worst-case live loading scenario is shown in figure 28.

Figure 27: HB vehicle layout

Figure 28: Worst case live loading scenario

HA UDL and KEL are multiplied by factor

applied to each notational lane along with the partial

factors. The factors are 1=1, 2=0.67, 3=0.6, and

4=0.6.

Table 5: Loads due to live loads in each lane

Notational

lane

Load

(kN/m)

ULS load

(kN/m)

SLS Load

(kN/m)

1 18.1 24.8 19.9

2 12.1 16.6 13.3

3 10.8 14.9 11.9

4 10.8 14.9 11.9

Total

Superimposed

Dead Load

51.8 71.3 57.0

Due to the cantilevered pedestrian and cycle

paths on the bridge pedestrian loading is also obtained

using equation 2.

)270

10HA Nominal(5

lw (2)

Resulting SLS and ULS load due to

pedestrians is 0.47kN/m3 and 0.61kN/m

2, respectively.

Which are minor when considering the other loading

that is occurring on the bridge.

7.3 Wind loading

Due to the exposed situation of the suspension

bridge over the Severn the wind pressures acting upon

the bridge impartially the deck are very important.

Maximum wind gust speed, , on a bridge without

live load is calculated as;

sgd VSV (3)

Where Sg is gust factor, Vs is site hourly mean

wind speed, which is found using equation 4.

dapbs SSSVV ... (4)

Basic hourly mean wind speed (Vb), is given

as 22m/s, obtained from isotach maps. Probability

factor (Sp), is 1.05 for a highway for 120-year return.

Altitude factor (Sa) is 1.05 and direction factor (Sd) is

0.93 due to the dominant wind coming from the South

West due to the proximity to the sea. These factors

result in an basic hourly mean speed for the site of

22.5m/s.

Gust factor (Sg) is dependent on terrain of the

site, which is defined by;

hgbg STSS .. (5)

Where Sb is calculated by bridge terrain factor

(Sb’) and (Kf) the fetch correction factor giving a figure

of 1.7. Reduction factor (Tg) is taken as 1 due to no

towns sited 3km up wind from bridge site, and

topography factor (Sh’) is 1, thus Sg=1.7, so giving a

maximum gust wind speed of 38.3m/s in the area.

Nominal transverse wind load Pt acting on

the deck is found by;

dt CAqP .. 1 (6)

Where dynamic pressure head, q, is given in

equation 7, resulting in a value of 1230.3N/m3

Projected area, A1, is 5434m2, drag

coefficient, (Cd), for the deck is 1.7, hence the nominal

transverse wind load acting on the deck is 11.4kN/m.

Nominal transverse wind load (Pt) acting on

piers of towers also needs to be accounted for. With

dynamic pressure staying the same, projected area

decreases to 2817m2 per tower and drag coefficient

reduces to 1.5. Resulting nominal transverse wind load

is 5.2kN/m per pier.

Nominal vertical wind loads or uplift is

another important force to calculate. The nominal force

is giving by equation 8;

Lv CAqP .. 3 (8)

Where lift coefficient is given equation 9

below;

))2.01).(20

(1(75.0 d

bCL (9)

Lift coefficient equates to 0.57, and dynamic

pressure remains the same as was found in transverse

wind loading, the projected area A3 is 31418m2. So the

uplift for the bridge is 22.0kN/m.

7.4 Temperature effects

Temperature fluctuations are an important

consideration during bridge design. Impartically

overall temperature increase or decrease which can

create stresses in the bridge deck.

The variation of temperature between day and

night will have a significant effect on expansion and

contraction of a deck on this scale.

Expansion joints are sites on the deck at each pier,

thus the effective length of the deck is 988m,with the

total effective temperature of 50C, and steel expansion

coefficient of 12x106C, the change in deck length can

be calculated

Tl .. (10)

This results of an expansion of 592mm in the

deck.

Amount of stress induced to the bridge due to

temperature difference is given by which could occur

when expansion joint become blocked is calculated

with;

ET .. (11)

2 (7)

25m 25m 9.6m

No load No load HB

HA β1 HA β1

HA β3 HA β3

HA β4 HA β4

KEL

HA β2 HA β2

Figure 30: Fuss cable strength diagram

Figure 30: Forces acting on tower

Which gives stress of 126N/mm2, this is a

manageable figure and would not result in significant

damage to the bridge if not left in this state for a

substantial length of time.

Temperature difference loading can also

occurs when the top surface of the deck is warmer than

the bottom surface, creating temperature variation in

the deck, thus causing the deck to expand and contract

non-uniformly and adding stress to the deck.

7.5 Natural frequency

Rayleigh Ritz equation for natural frequencies

states for the natural frequence of a bridge to feel

comfortable for users of the bridge it should be

between 5Hz and 75Hz. The equation is;

4

20 )(

ml

EIlf n (12)

Where (βnl)2 is 22.37 for a bridge of this type, E

for steel is 200x106, I for the deck is 25.6m

4, l is width

of the deck 31.8m, and m is dead and superimposed

load only in giving 15000kg/m. The natural frequency

of the bridge is 12.9Hz. Although within the

parameters after construction movement was felt on the

bridge and further strengthening was added to reduce

these vibrations.

8 Strength

8.1 Longitudinal bending

Calculating the bending moment in the deck it is

assumed that it is acting as a simply supported beam.

With each support sited at the tower sites, and the main

length taken as the central span so 988m.

8

2

max

wlM (13)

Where w is taken as the dead and superimposed, so

giving a Mmax as 18.0x106 kNm.

However cables support the deck roughly every

23m, so the bending moment diagram will actually be

composed of a series of sagging and hogging moments

as shown below.

The max shear coeffiecient, sagging moment

The simplified method Ref [7] for finding

maximum shear, Smax, and maximum sagging and

hogging moments, Msag, Mhog, are shown below in

equation 14,15 and 16.

Smax1.143F (14)

FlMsag 77.0 (15)

FlMhog 107.0 (16)

This gives maximum shear as 168kN, and the

maximum hogging and sagging moments as 501kNm

and 3607kNm respectively. The bridge acts as

expected with the main moment caused in sagging due

to the stiffness of the deck.

8.2 Cable strength

Due to the suspension of the deck off the

cables the strength of the cables are imperative. Fuss

developed the following equation (17) to solve this

problem.

f

wLH

8

2

(17)

Where l is the span 988m, f is cable sag which

is 82.3m and, w is takes as the uniformly distributed

load, thus loading includes the weight of the cables

themselves and worst case HA and HB loading case. H

therefore equals 324MN.

Main cables are 500mm diameter consisting of 19

strands of galvanised wire 8,322 with 50mm diameter. 2rnArea (18)

Area=3.73x106

Therefore stress in each cable is;

A

F (19)

This is reasonable for a bridge of this span

86.9N/mm2

8.3 Tower Strength

Compression in the tower can be found by

resolving forces. To ensure translation of cables across

tower saddle doesn’t occur, cable tension forces must

be equal between main span and approach spans.

f

L/2 wL/2

H

L/2

Sag V

H

MMax

988m

Figure 28: Bending Moment diagram

Figure 29: Hogging and Sagging bending moment

diagram

Resolve forces so;

V=120.8MN

Rough area of each tower is taken as the average

thickness and width due to tapering in the tower

varying thickness between 25-14mm and width 5.2-

3.7m. Thus average area of the tower is 0.44m2.

Therefore stress, equation 19, in towers is

273N/mm2

Euler’s buckling load for the tower is also found as

it is the most likely mode of failure it will experience,

given equation 20.

2

2

'

)(

l

EIPcrit

(20)

Where the towers I value is 2.2mm4, and effective

length is;

(21)

Giving an effective length of 115.6m and so Pcrit

as 325MN.

9 Strengthening and Remedial work

Shortly after the completion of the Severn Bridge

in 1966 there was a dramatic change in the vehicle

loading in road transport in Britain. Between 1962-

1977:

Goods moved by road measured by ton/ km

virtually doubled

Goods vehicles with gross weight over 28 tons

rose from an insignificant number to 90,000

With the Severn Bridge originally designed to HA

loading of BS 153: Part 3A, it was found that vehicle

loading for a 4 lane bridge was found to be around 20-

160% higher than the original design loadings.

Observations carried out on the bridge in the

1970s found substantial fatigue cracks occurring in the

welded joints at various points on the bridge. Also

external cracking of road surfaces around the Beachley

viaduct and Wye bridge boxes had caused ingression of

corrosion pitting of the deck plate. Also fractures of the

outer wires in some hangers, due to increased

frequency in the vicinity of the lower sockets, had

occurred. The primary cause would be due to the

passage of individual heavy goods vehicles along the

bridge.

After a further investigation was commissioned,

by the Department of Transport, the appraisal was

extended to cover other areas showing that the towers,

hangers, and deck were currently designed to too low a

safety margin.

Single lane operation was thus introduced in the

early weekday hours between 4am- 8am when heavy

goods vehicles where >25% proportion of the traffic

load travelling over the bridge. This was until the

bridge was strengthened up to the required standards.

In July 1982 it was that the full strengthening work

would cost £33 million, including entire bridge

resurfacing, estimated to take 5 years to complete.

The strengthening scheme was as below:

Reinforcement of the Severn Bridge towers,

by installing new tubular columns inside the

tower boxes, and grouting up the voids

beneath the saddle troughs, and jacking the

columns up against the saddles.

Replace all hangers with larger more type

specific ones

Larger welds implementation especially on

the slow lane which large HGV vehicles

occupy

New rocker bearings to be suspended from the

ends of the deck boxes from the towers to

allow rotation to cope with the deck

movement.

Additional stiffening inside and out of deck

box. Ref [8]

However even with these strengthening

alterations the capacity of the bridge was estimated

to have been exceeded by the mid 1990s.

Strengthening work still commenced despite this

fact due to the time period needed to develop a

new crossing as it was deemed more cost effective.

10 Future of the Severn Bridge

Since the opening of the Second Severn

Crossing in 1996 the future of the First Severn

Crossing has been uncertain. At its peak the bridge was

capable of carrying 50,000 vehicles a day, however

today it only carries 25% of the estuary crossing traffic

surmounting to 15,000 vehicles a day, this coupled

with growing maintaince costs and increased closures

due to high winds and ice forming on the hangers, has

brought the efforts of maintain the crossing into

question. References [1] WILLIAM Sir A.The Severn Bridge superstructure.

Yarm : Studio G, 1966

[2] ROBERTS Sir G. Severn Bridge: design and contract

arrangements. Proc. Instn Civ. Engrs, 1968, 41, Sept, 1-48

[3] GROWRING G. Severn Bridge: Foundations+

substructure. Proc. Instn Civ. Engrs, 1968, 41, Sept, 1-48

[4] BS 5400-2:2006. British Standards: Steel, Concrete

and Composite Bridges Part 2. BSI

[5] IBELL, T. 2008. University of Bath, Bridge

Engineering 1 Lecture Series

[6]COLGARO,J.TSCUMI,M.GULVANESSIAN,H. 2002

Designers’ guide to Eurocode 1 Action on Bridges. Thomas

Telford

[7] COBB, F. 2004. Structural Engineer’s Pocket

Book.Elsevier Butterworth-Heinemann

[8]CUNINGHAME J.R and BEALE C. Strengthening and

refurbishment of Severn Crossing. Part 4: TRRL research on

Severn Crossing. Proc.Instn.Civ.Engrs.Structs&Bldgs,

1992,94,Feb,37-49