CRITICAL ANALYSIS OF NEW SIDINGS ROAD BRIDGE AND

FOOTBRIDGE OVER FABIAN WAY.

C P Nichols

University of Bath

Abstract: This paper provides detailed information and analysis of the Fabian Way Road and Footbridge. The

analysis will include sections on bridge aesthetics, design, loading, construction and strength.

Keywords: Fabian Way Bridge, Cable Stayed, Statement Bridge, Park and Ride, Pedestrian

Figure 1: Elevation of Fabian Way Bridge [1]

1 Introduction

The New Sidings Bridge, or Fabian Way Bridge as it

is referred to throughout this paper, is a cable stayed

bridge spanning Fabian Way (A483) in Swansea. It is

designed to link a Park & Ride scheme on the outskirts on

Swansea with an express bus link. The bridge also

incorporates a walking and cycle route which links up

with a national cycle path. The bridge has a skew span of

approximately 60m.

Figure 2: Aerial photo showing location of bridge with

red line. The M4 is to the East, and Swansea to the West.

[2]

Proceedings of Bridge Engineering 2 Conference 2008 16 April 2008, University of Bath, Bath, UK

Mr C P Nichols: cpn21@bath.ac.uk

2 Bridge Aesthetics

The analysis of the aesthetics of this bridge will be

based around the ideas of bridge engineer Fritz Leohardt.

He believed that there were ten areas of bridge aesthetics

which all needed to be considered during the design of a

bridge. These 10 areas are:-

1. Fulfilment of Function

2. Proportions of the bridge

3. Order within the structure

4. Refinement of design

5. Integration into the environment

6. Surface texture

7. Colour of components

8. Character

9. Complexity in variety

10. Incorporation of nature

Fabian Way Bridge is simple but effective in its

construction and design and satisfies the needs of the

Highways Agency in that it is unusual and dramatic and

helps to minimise the chances of motorists falling asleep.

It is obvious to all that the single mast is held up in

position by the two sets of cables, which are in turn held

down by ground anchors, and the single cables from the

mast support the deck. There are also visible expansion

joints at either end of the bridge which help to reassure the

public that the bridge will work.

The proportions of the bridge seem sensible on first

impressions. The slender mast appears to be elegant,

lightweight and in proportion with other aspects of the

bridge, as it should be for a cable stayed bridge. The

depth of the deck appears to be a suitable size for the span

required. However, the cross section does not change size

between cable supports as might be expected due to the

moments developed in the structure. This enables the

bridge to have a greater sense of flow which is enhanced

by the curvature of the bridge. This is obvious when

observed from walking over the pedestrian side.

One proportion that might at first seem unreasonable

is the large size of the central beam. This beam is used to

secure the cable ties along with providing the support to

cantilever the road and footpath of each side of the bridge.

However, this particular element is very much unnoticed

at first despite being vital for the correct functioning of

the bridge.

Although the basic structure of the bridge, its mast

and deck, are simple and effective the cables used to

support the mast are not ordered. They purposefully cross

each other to minimise the number of different cable

lengths needed. This results in an elegant appearance

from some angles but then a messy appearance from

others.

Figure 3: Crossing cables [1]

The parapets used throughout the bridge are identical

for each side of the bridge and provide consistency and

order. This is also echoed in the use of lighting of the

bridge. There are lights positioned at regular intervals

along the length of the footpath for pedestrians and

cyclists.

Figure 4: Parapet on pedestrian side of bridge [1]

The elegant mast that adds so much to the beautiful

nature of the bridge also shows the refinement of the

design. The mast tapers towards its anchored and free

ends as well as increasing in size near the cable

attachments. This is due to the greater resistance needed

at these locations. One refinement that could have been

made would be to space the cables equally over the whole

span of the bridge. This can be seen as the first cable

supporting the bridge is a greater distance away from the

mast than it is from the next supporting cable.

This area of Swansea is undergoing a great deal of

regeneration and already has one iconic cable stayed

bridge, “The Sail Bridge”. This addition of a second

cable stayed bridge into the city on one of the main

transport routes helps to set the scene for people visiting

as well as integrating the new bridge. Care has also been

taken to ensure that the bridge does not interfere with the

surrounding area. This was achieved by placing the

ground anchors and mast support onto the industrial side

of the bridge rather than the densely populated residential

side.

The texture, finish and colour of a bridge are vital,

especially for a bridge such as this due to the speed at

which pedestrians and cyclists will cross it. The structure

of the bridge is mostly steel which fits in with the heritage

of the local area. This steel, which is generally regarded

as the finish of choice in urban environments, has been

painted throughout the bridge to create a sleek finish as

well as to highlight the slender deck and supporting mast.

This high quality painted finish has been used for the steel

parapets and the lights which both help to unify the

bridge. This choice of white also helps to achieve a

feeling of lightness across the bridge which is regarded as

a vital requirement for pedestrian bridges. The overall

view of bridge is also immaculate at night time. A bridge

specific lighting solution was design by architects to

ensure the elegant nature of the bridges vital components

could be seen when travelling into Swansea at any time of

day. This solution can be seen in Figure 5.

Figure 5: Bridge Lighting Scheme at night [3]

Tarmac has been used as the surfacing on both the

road and pedestrian sides of the bridge. This finish was

chosen to provide a seamless continuation over the bridge

from the approaches.

One possible problem with the finish of the bridge

was the connection of the steel cables to the central beam.

This problem has been overcome by providing a white

metal screen which is bolted to the top of the central

beam. This screen gives the illusion the steel cables

simply disappear into the bridge and does not spoil the

otherwise excellent finish and texture.

Figure 6: Screen to obscure cable to beam connection.[1]

Although the bridge maybe lacking in vast amounts

of character at this early stage in its life it is likely it will

develop over time as it is another focal point for a rapidly

developing area of South Wales. The character the bridge

has at this time is due to its simple but effective nature.

Various important structural elements are purposefully

exposed which makes it easier for the general public to

understand and appreciated the mechanics of bridge. This

includes the cable connections to the ground anchor

which can be seen below in Figure 7.

Figure 7: Ground anchor with cable connections [1]

The bridge as a whole is visually excellent and

stimulating. Its elegant nature lends itself to be easily

understandable as previously mentioned. There are some

elements that are obviously complex to anyone that sees

the bridge close up. The most obvious of these is the

crossing cables which appear confusing from most angles.

However, it has hidden secrets such as connections, and

its construction which are not obvious and that add to the

complexity. This leads to the bridge becoming more

complex the longer it is examined.

Although the bridge does not immediately

incorporate nature it does incorporate heritage. The

bridge is regarded locally as “The Second Sail Bridge”.

This stems from “The Sail Bridge” which was a cable

stayed bridge built to connected the new SA1

development and the old Maritime Quarter. This second

bridge continues the theme of integrating the land with

the sea which was started by “The Sail Bridge”. The

bridge also respects nature in its design. The abutments

from the previous “Sidings Bridge” were used at either

end of the bridge with the foundations for ground anchors

and mast been located on the South Western edges of the

site in order to not disturb any buildings on the North

Eastern edge. There was also not the need for extra

ground based supports anywhere on the bridge which

minimises the effect the bridge has on nature and the local

environment.

3 Bridge Design and Construction

3.1 Choice of Bridge Type

As Fabian Way Bridge is a joint pedestrian and road

bridge it has to perform on two accounts. Not only must

it fulfil the needs of a pedestrian bridge in being

lightweight and elegant but it must also perform suitable

well for the road vehicles which pass underneath and

over. As a result a cable-stayed bridge is a good option.

Cable-Stayed bridges are not only chosen for their

aesthetic credentials but also for their mechanics. They

are stiffer than suspension bridges due to the stay cables.

The cables travel directly from the deck to the pylon

which is a more direct load path. It is this direct load path

which helps to increase stiffness. This is coupled with the

advantage that each cable is relatively thin in comparison

the suspension bridges and the tedious process of spinning

is not necessary. This speeds up construction and any

replacements needed later in the bridges life.

3.2 Structural Form

The bridge has a skew span of approximately 60m

with a total width of approximately 10m. A single plane

system was used to support the continuous central beam

which runs the length of the bridge between each

abutment. The deck consists of numerous cantilever,

tapered I-Sections which differ in size with respect to

whether they support the roadway or footpath. The cable

ties used to support the deck are 56mm fully locked coil

cable system and 36mm 1030 with nut and washer to link

the mast with the ground anchors. The cables are spaced

over the length of the bridge at approximately 6m centres.

An asymmetric, single plane cable system was chosen

for its aesthetic quality. The use of a single plane system

for the cables means the overall system of the bridge

relies heavily on the torsional stiffness of the decks. The

reliance can be reduced by having the cables more closely

spaced than would first seem necessary. This leads to the

deck only being necessary to resist the bending induced.

For Fabian Way the problem of torsional stiffness is

even greater as only one side of the bridge carries

relatively heavy traffic loading. This explains why, on

inspection, the cantilever beams are much larger for the

bus carrying lane than the pedestrian and cyclist’s lane.

Due to aforementioned problems a compromise has

been necessary. The spans between the cables have been

reduced slightly from what would originally seem

acceptable. This helps to reduce the bending generated in

the deck. However, the section size of the deck is still

larger than expected in order to deal with the large

difference in loading between each side.

It is common for the mast of cable stayed pedestrian

bridges to be inclined in an attempt to reduce bending as

much as possible. This technique has been continued on

Fabian Way Bridge. However, the mast is not inclined in

the direction that would first be thought appropriate.

3.3 Preliminary Design Procedure

For the design and analysis of cable-stayed bridges

the loading is applied with the deck modelled as a

continuous section over each of the cable supports. The

support reactions provide the force in each of the cable

ties and, therefore, the necessary cross sectional area. The

bending moments in the deck and the longitudinal forces

induced in the cables lead to the sizing of the deck section

required. The size of the pylon is determined by the

amount of compression it carries along with the amount of

bending generated under live load applied at the worst

case positions for traffic along the bridge.

3.4 Construction

A common way of constructing cable-stayed bridges

is to use suspended cantilever construction. This form of

construction is used to reduce the hogging moments

which occur over the piers of bridges. Fabian Way does

not have any intermediate piers but this form of

construction could have worked by attaching the ties to

the ground anchors as the ties were secured to the bridge.

The construction of Fabian Way was, however,

slightly different. Temporary formwork was erected at

the half way point on the central reservation of Fabian

Way itself as well as near each abutment. This enabled

the central steel beam to be transported to site as two

separate beams. The South Western beam was lifted into

place first and rested on the temporary formwork.

Figure 8: First Part of Central Beam [3]

Next, the mast, which was transported as one

complete element, was lifted into place by 3 separate

cranes. As the second part of the central beam was lifted

into place on its temporary supports the mast was secured

to the 2 ground anchors with 1 cable to each. The

remaining mast to anchor ties were attached to the mast

for future requirements but left unattached to the ground

anchors. This is shown in Figure 9.

Once the complete central beam was in place the

addition of the cantilever steel beams took place with the

cable ties being secured to the deck and ground anchors

as required. This method is a derivation of suspended

cantilever construction. The cable ties are attached to the

deck and ground anchors when required in order to

stabilise the system as more of the cantilever beams and

associated structure is added.

Once all the cantilever beams were added to each

side of the main central beam steel plates were added to

the beams to brace the structure and provide a surface

suitable for laying tarmac as well as securing the various

important items of bridge furniture.

Figure 9: Mast and Ties during construction [3]

4 Bridge Loading

This cable stayed bridge across Fabian Way has been

designed to British Standard BS 5400 (BS 5400) [4]. BS

5400 is a limit state design code which involves a check

to prevent collapse at the Ultimate Limit State (ULS)

along with a check on the serviceability of the bridge,

such as deflections, at the Serviceability Limit State

(SLS). The bridge is subjected to five main types of

loading: dead load, super-imposed dead load, live load,

wind load and temperature effects. In order to design the

bridge effectively a combination of the above loads will

need to be checked at both limit states.

Fabian Way Bridge will have a relatively small dead

load in comparison to many other bridges as it is not a

standard highway bridge. This is compounded by the fact

that the deck is mainly constructed from steel which is

light in comparison to pre-cast concrete decks. The two

halves of the bridge are subjected to slightly different

dead loads due to the different sizes of steel cantilever

sections used. However I have assumed that the

difference in weight is negligible and have used the dead

load for the road carriageway for both carriageways.

The super-imposed dead load (SDL) is also relatively

small over the whole bridge. The components of SDL in

this case are tarmac, parapets, various lighting appliances

and the bridges own drainage.

The live loading acting on the bridge is traffic

loading on the South Eastern side with pedestrian loading

on the North Western side.

Wind loading on the bridge was also checked but due

to its urban location and the stiffness provided by the

deck structure it is anticipated that normal wind loading

will not be the determining factor in the loading on the

bridge. However it is possible that wind uplift could have

a large impact on the loading of the bridge.

The effects of temperature on the bridge are also

expected to be minimal due to the inclusion of expansion

joints in the bridge. However, it is still necessary to

check this area for overall shrinkage or expansion of the

bridge along with differential movements that might be

experienced.

One load case that would be considered if this was a

concrete bridge would be creep. This is, however, not

required for Fabian Way as its main construction material

is steel which does not suffer from creep.

Although it will not be considered in this paper in

reality it would be necessary to check the steel cables for

stress relaxation. This is to ensure they do not loose any

of their strength due to being heavily loaded for a

significant period of time.

4.1 Assumed Dimensions

Table 1: Assumed Dimensions

Variable

Length (m) 60

Road Width (m) 4.5

Footpath Width (m) 4.5

Bridge Width (m) 10

Min Depth of Deck (m) 0.5

Max Depth of Deck (m) 1

Height of Mast (m) 60

4.2 Load Factors

Partial load factors are applied to all the load

condition but the value varies bet ULS and SLS. These

factored values are then combined in order to calculate

the worst possible load combinations for the bridge.

There are two factors used throughout bridge engineering:

γfl and γf3.

The factors used throughout the analysis for dead

load and SDL are shown below in Table 2 and Table 3.

The values are taken from BS 5400, Part 2.

Table 2: Dead Load Safety Factors

Steel Factors of Safety

γfl ULS = 1.05

SLS = 1.00

γf3 ULS = 1.10

SLS = 1.00

Table 3: SDL Safety Factors

Steel Factors of Safety

γfl ULS = 1.75

SLS =1.20

4.3 Dead, Super-Imposed Dead and Live Loads

4.3.1 Dead Load and Super-Imposed Dead Load

As I do not have access to technical drawings I have

had to estimate values for the dead load of the bridge.

These estimations are based on a unit weight of steel of

78.5kN/m3 and dimensions of elements gauged from site

pictures. I have assumed that g = 10N/Kg in all my

estimations.

Table 4: Dead and SDL for bridge

Material Load (kN/m)

Parapets 1.25

Surfacing 0.92

Steel Plate Road Surface 3.53

Steel Beams 6.64

Steel Edge Beams 3.50

Cables 2.23

Total SDL 2.17

Total Dead Load 15.9

The bridge cross section varies between each side of

the bridge but is constant along the length of the bridge.

4.3.3 Live Loading

For pedestrian bridges the standard loading applied is

5kN/m2

if under 36m. Fabian Way Bridge is

approximately 60m long and is therefore reduced by a

factor k which is calculated as follows:-

k = (nominal HA UDL for length x 10)/(L + 270)

nominal HA UDL for length = 151.(1/L)0.475

nominal HA UDL for length = 21.6kN/m

nominal HA UDL for area = 7.2kN/m2

k = (7.2 x 10) / (60 + 270)

k = 0.218

Loading = 5 x 0.218

Loading = 1.09kN/m2

For the traffic loading it is necessary to first

determine the number of notional lanes. I have assumed

throughout these calculations that the roadway is 3m wide

and this culminates in the roadway having only 1 notional

lane. This is used to correct the nominal HA UDL value

calculated above to accommodate for the carriageway

width being less than 4.6m and, therefore, less than two

notional lanes.

The notional load calculated above, 7.2kN/m2, is

subsequently multiplied by γfl and γf3 to obtain design

loadings. The calculated values are shown below in Table

5.

Table 5: Design Loadings for Roadway

Factor

Load

(kN/m2)

Design Load

(kN/m2)

ULS 1.50 7.20 10.8 γfl

SLS 1.20 7.20 8.64

ULS 1.10 7.20 7.92 γf3

SLS 1.00 7.20 7.20

Although there are bollards to prevent vehicular

access to from the South Western edge on the footpath

side of the bridge these are not repeated on the North

Eastern edge. As a result it will be necessary to check

this side for vehicular loading to prevent against damage

that could be caused by vandalism. However, it would

not be possible to accommodate HB vehicles on either

side of the bridge and therefore it is not necessary to

design the bridge for these loadings.

The parapets of the bridge have to be designed to

withstand 1.4kN/m over the complete length of the

bridge. This amounts to 84kN total.

The abutments that have been used for the bridge are

the original abutments from the previous Sidings Bridge.

They are less than 4.5m from the edge of the road and,

therefore, would need to be checked for accidental

vehicle loading in accordance with clause 6.8 from

BS5400-2. Even though the abutments should withstand

the forces applied it is necessary to complete the check as

a precaution.

4.4 Temperature Effects

4.4.1 Effective Temperature

If a bridge is not design to withstand the effects of

temperature large stresses can be induced in the structure.

These large stresses can cause expansion within the

bridge and could, if large enough, provoke a failure. This

bridge has been designed to withstand these increases

though and, as a result, appears to have expansion joints

at both ends. This enables any expansion that does occur

to be managed. It is possible to calculate the movement

that the expansion joints need to be designed for by using

the following equations.

δ = εl (1)

ε = ∆T α (2)

Table 6: Temperature Variables

Variable Value

∆T 23oC

α Co/1012 6−×

E 200,000 N/mm2

The average temperature of Swansea was calculated

to be approximately 11oC. This temperature, along with

the maximum of 33oC and minimum of -12

oC (taken from

BS5400-2) were used to calculate ∆T. The end value was

calculated to be 23oC. The calculations for δ are shown

below:-

( )

( ) ( )mm56.16

106010276

276

101223

36

6

=

×××=

=

××=

−

−

δ

δ

µεε

ε

As the bridge is restrained at both ends it feels a

compressive stress that is calculated below.

E⋅= εσ (3)

( )2

6

/2.55

20000010276

mmN=

××= −

σ

σ

This is a considerable stress that is being induced in

the structure and expansion joints are obviously necessary.

However, the bridge must be designed to deal with these

increases in stresses in the case of a blockage in the

expansion joint which could prevent the bridge from

being able to expand where it was designed.

4.4.2 Temperature Difference

Assuming that Fabian Way is a Group 2 bridge deck

(Figure 9 BS5400-2) there will be a variation in the

temperature of the deck from ambient temperature at the

bottom of the deck profile and 24oC above ambient

temperature at the top of the deck profile. This increase in

temperature leads to the bridge experiencing a stress of

57.6N/mm2.

The difference between the temperatures at the top

and bottom of the bridge deck induces extra axial forces

and bending moments within the bridge.

The extra axial force is calculated to be 97.2MN and

the moment induced by the temperature being equal to

7211kNm.

4.5 Wind Loading

The bridge over Fabian Way is difficult to analyse for

wind loading. This is mainly due to the lack of advanced

knowledge of how these types of structures behave under

wind loading. One vital requirement of the bridge is that

the cable ties used to support the bridge are pre-stressed to

a sufficient level so that, in the case of wind uplift, they

do not slacken off.

A rough analysis can be carried out using various

equations from BS5400-2.

4.5.1 Maximum Wind Speed

The maximum wind speed that occurs at the site of

the bridge is calculated using the following equation.

211 SSvKVc = (4)

The variables for use with the above equation are

taken from figures and tables in BS5400-2. The height of

the site is assumed to be 10m and the values are taken

from this. As it is a footbridge it is possible to modify the

final value by a factor m. The variables are show below in

Table 7.

Table 7: Wind Gust Calculations Variables

Variable Value

V (m/s) 30

K1 1.49

S1 1

S2 1

m 0.8

The maximum wind gust can be estimated as:-

smV

V

SSvKV

c

c

c

/79.35

8.01149.130

211

=

××××=

=

4.5.2 Horizontal Wind Load

The horizontal wind load acts at the centroid of the

part of the bridge under consideration. For this

calculation I have decided to consider the deck. The

equation is:-

Dt CqAP 1= (5)

In the above equation q is the dynamic pressure head.

2

613.0 cVq = ` (6)

2

2

/2.785

79.35613.0

mNq

q

=

×=

A1 is the solid horizontal projected area in m2 and CD

is the drag coefficient. CD is derived from the b/d ratio

and then a value is taken from the relevant table in

BS5400-2. The two options below are for if a gust blows

with and without a bus crossing the bridge

101

10

1

10

==

==

==

d

b

depthd

widthb

5.24

10

4

10

==

==

==

d

b

depthd

widthb

These, in theory, would lead to having a CD

coefficient of 1.05 or 1.40. However, as part of this

bridge is a footbridge it is necessary to raise the value to

the minimum coefficient for foot/cycle track bridges. The

value used is, therefore, 2.

kNP

NP

P

CqAP

t

t

t

Dt

2.94

94224

2602.785

1

=

=

××=

=

4.5.3 Longitudinal Wind Loads

The longitudinal wind load on both the structure and

parapets has also been calculated. The loads on the

structure are shown below.

DLS CqAP 125.0= (7)

kNP

NP

P

LS

LS

LS

6.23

23556

2602.78525.0

=

=

×××=

The wind loads acting on the parapets of the structure

are shown below.

tL PP 8.0= (8)

kNP

P

L

L

36.75

2.948.0

=

×=

This shows that the parapet of the bridge should be

designed in order to withstand 75.36 applied as a point

load.

4.5.4 Uplift and Downward force

It is necessary to calculate this as the final value gives

an indication as to the amount of pre-stress that is needed

in the cable ties. This is to ensure they do not slacken of

if the bridge deck is forced into an uplift position.

Lv CqAP 3= (9)

In this case q remains the dynamic pressure head

calculated earlier. A3 is the plan area of the deck and CL is

another coefficient which is dependant on the b/d ratio.

kNP

NP

P

v

v

v

146

2.146047

31.06002.785

=

=

××=

However, the CL value for the cross section is

inaccurate as the cross section of Fabian Way is not a

standard cross-section. Therefore as a conservative

estimate CL is increased to 0.75. This could be reduced in

practise by wind tunnel testing. This rise in CL increases

Pv to 353.5kN.

4.5.5 Wind Loading Combinations

There are various wind loads which need to be

considered throughout the design of bridges. The

combinations are their final values are shown below in

Table 8.

Table 8: Wind Loading Combinations

Combination Value (kN) Value (kN/m)

Pt 94.2 1.57

Pt + Pv 447.7 7.46

Pt – Pv -259.3 -4.32

PL 75.36 1.26

0.5Pt + PL + 0.5Pv 299.11 4.99

0.5Pt + PL – 0.5Pv -54.19 -0.90

This table shows that the worst case for wind loading

for this bridge is Pt + Pv = 240.2kN. As a result this is the

value that should be used for all the subsequent load

combinations.

4.6 Vibrations

As part of Fabian Way Bridge is a footbridge it is

necessary to pay close attention to the vibrations of the

bridge. In order to analyse these effectively we initially

calculated the fundamental natural frequency of the

bridge, fo, using Euler’s differential equation. To

calculate the second moment of area for the section it has

been necessary to use the same estimated sizes as used

earlier when calculating the dead load of the bridge. For

ease of calculations I have assumed the beams are of a

constant height equal to the average over the whole

length. Another estimation I had to make was that second

moment of area was an average of the second moment of

area of the deck alone and the second moment of area of

the beams in elevation. This leads to the second moment

of area being equal to 0.158m4.

It was necessary to make other assumptions before

calculating the fundamental frequency. The mode of

vibration was based on a clamped-clamped beam. In

reality it would be more accurate to model the bridge as

pinned-pinned but this is not possible with this method of

calculation. For this analysis the contribution of stiffness

provided by the single plane of cable ties has been

ignored. Therefore the value calculated for the

fundamental frequency will be lower than value

experienced by the bridge.

( ) 4

2

...

lmIElnn βω = (10)

( )( )

Hz7.27

60.159010158.10200

37.22

1

4

39

1

=

××=−

ω

ω

This value of 27.7Hz is safely within the permissible

range for the frequency of vibration. In reality the bridge

would have a larger fundamental frequency due to the

additional stiffness provided to structure by the cable ties.

The result of the fundamental frequency leads onto the

maximum permissible acceleration:-

ofa 5.0= (11)

2/63.2

7.275.0

sma

a

=

×=

4.7 Load Combinations

There are 5 combinations of loads which need to be

checked for Fabian Way Bridge. These combinations

need to be checked at both the ULS and SLS. However

only the ULS calculations are shown as these are the

worst case scenario’s in each case.

4.7.1 Combination 1

Combination 1 consists of all permanent loads plus

primary live loads.

Comb 1 = Dead Load + SDL + Live Load (12)

All the loads in the above equations are multiplied by

their specific γfl and γf3 values. The calculations for the

Ultimate and Serviceability limit states are shown below.

ULS:

( )mkN /42.34

2.75.117.275.19.1505.110.1

=

×+×+××

4.7.2 Combination 2

Combination 2 consists of all the loads from

combination loads plus the worst case wind load which is

shown in Table 7.

ULS: ( )

mkN /5.41

46.710.12.725.117.275.19.1505.110.1

=

×+×+×+××

4.7.3 Combination 3

Combination 3 consists of the loading from

combination 1 plus the temperature loading induced in the

bridge.

ULS: ( )

mkN /4.37

5.330.12.725.117.275.19.1505.110.1

=

×+×+×+××

4.7.4 Combination 4

Combination 4 consists of all permanent loads plus

secondary live loads such as skidding, longitudinal and

collision loads. All these secondary live loads must be

coupled with the associated primary live loads.

For this particular bridge though it is not necessary to

check the parapet collision as no HB vehicles are able to

travel over the bridge. Neither is it necessary to calculate

the centrifugal loading as the bridge is not curve in plan.

The calculation for combination 4 has, however, been

omitted from this report as it is not the most significant

for the bridge.

4.7.5 Combination 5

Combination 5 includes all permanent loads plus the

loads due to friction at the supports. Once again this

calculation has been omitted as it is not the worst case

scenario for the bridge.

4.7.6 Conclusion on Load Combinations

As can be seen from the calculations above load

combination 2 is the most severe for this particular bridge.

As a result the value of 574kN/m will be used throughout

the analysis of the bridge.

5 Structural Analysis

The bridge carries the majority of its load through its

cable ties and steel mast. The cables are in tension and

the mast is in compression. The abutments will also be in

compression.

5.1 Bending Moments

A basic bending moment diagram is created by

assuming the loading acting on the road way acts over all

the bridge. The loads used are the worst case values from

Load Combination 2. The max moments are stated below.

Mmax Hogging = 160kNm

Mmax Sagging = 115kNm

The max sagging moment occurs in the span furthest

from the central point of the bridge and the maximum

hogging moment occurs at the cable tie position that is

adjacent to the maximum sagging moment position, but

closer to the centre of the bridge.

5.2 Cable Strength

These cable strength calculations are taken from

reference [5]

Cable Area = Max Force / Stress (13)

Stress = Ultimate Stress / Factor of Safety (14)

= 1570 / 5

= 314kN

Max Force = 160kN

( )

2

3

510

31410160

mmArea

Area

=

×=

Cable Area = 2.rπ

mmdiameter

mmradius

radius

48.25

74.12

510

=

=

=π

The bridge uses 56mm cables for the mast to deck

ties and from my basic calculations you can see that these

cables are not oversized.

5.2 Deflections

The maximum deflection will occur at the same

location as the maximum sagging moment. This equation

is slightly inaccurate as it assumes the part of the bridge

being considered is acting as a simply supported beam.

As can be seen the deflection is well within the allowable

range.

EI

wL

384

5 4

max =δ (15)

( ) ( )mm42.0

1058.110200384

75003235

max

113

4

max

=

××××

××=

δ

δ

360lim

span=δ (16)

mm21360

7500lim ==δ

6 Foundations

The foundations of bridges are very important and

can often amount to around 50% of the total cost for a

bridge. This was not, however, such a problem for Fabian

Way Bridge as it is on the site on an old Sidings Railway

Bridge. As a result the abutments on each side will have

been designed to cope with railway freight loading and,

therefore, are perfectly acceptable for a small footpath

and park and ride bridge. However, some work was

undertaken in order to modify the existing abutments to

accommodate the alter bridge cross section.

6.1 Ground Anchor

The only major work that was needed for the

foundations were the ground anchor’s for the cables on

the South Western side of the bridge along with the

foundations for the mast itself.

Although exact information is not available on the

foundations used it can be seen in Figure 7 that the ground

anchor is concrete. This leads me to think that the

foundations below could either be a continuation of the

concrete or, more likely, be friction piles. These would

have been used to resist the tension force exerted on the

anchor by the cable ties.

6.2 Approach Foundations

It is likely that at either end of the bridge a jockey

slab has been provided in order to ensure a smooth

transition on and off the bridge for the park and ride

buses.

7 Susceptibility to Intentional Damage

As this is a dual purpose bridge there is the possibility

of it being damaged by vehicles. The bridge has been

designed to resist parapet loading from cars. However,

the road is not wide enough to allow large vehicles such

as Heavy Goods Vehicles across.

However, I think the possible problem of intentional

damage by vehicles was in the process of being removed

when I visited the site in February 2008. Whilst I was

visiting a team of contractors were installing an electronic

reader system along with a barrier similar to the one

shown below in Figure 10.

Figure 10: Road Blocker [6]

The bridge has no intermediate piers but the

abutments present would have been checked against

vehicle collisions in accordance with BS5400.

The ground anchors have been placed behind

standard crash barriers which help to minimise any

possible damage from collision. However, the concrete

base to the ground anchors and mast foundations look of a

sufficient size to resist the collision loading that would

occur if the crash barriers present did not reduce the

energy of an accident by the prescribed 80%.

Although vandalism, such as graffiti, has not been a

problem in its short life it could be a problem in the

future. The areas most susceptible to graffiti are the

foundations for the ground anchors and the central beam.

If this was to occur it could be easily removed by simply

re-painting over the area.

8 Possible Future Changes

The bridge is very new and was officially opened in

November 2007. There is no room to accommodate a

wider bridge although, if necessary, the current footpath

across the bridge could be replaced with a roadway if it

was ever desired.

On the whole it is regarded as a ‘Statement Bridge’

and the first sign and welcome to a newly revived

Swansea.

It main purpose was also to reduce the number of

cars travelling in and out of Swansea and to cut the

journey time. This has been achieved as expected by

linking the bridge into the express bus route.

This linking has continued by including the bridge

into a National Cycle Network which is visible on all new

Ordnance Survey Maps.

There is, finally, the option to significantly increase

the bus flow over the bridge. On my visit the frequency

seemed to be around one bus every 10 minutes. As the

traffic only travels in one direction this could be increased

with no adverse affects on travel time.

9 Conclusion

The bridge over Fabian Way in Swansea is very well

designed. It provides a dramatic welcome to Swansea for

visitors along with being functional. It is not only a great

engineering solution but also a striking piece of

architecture.

10 Acknowledgements

The guideline notes for this paper were provided by

Professor Tim Ibell. Along with his lecture series ‘Bridge

Engineering 1’ it proved essential in gaining all the

information needed. I would like to thank him for

making these easily available.

11 References

[1] Pictures taken by Mr. C P Nichols, 7th

Feb 2008.

[2] http://maps.google.co.uk

[3] News Articles, Press Releases. Various contributors.

http://www.swansea.gov.uk

[4] BS 5400-2:2006. Steel, Concrete and Composite

Bridges – Part 2: Specification for Loads. BSI

[5] Seward, D. 2003. Cable Sizing Equation,

Understanding Structures Analysis Materials and

Design, 3rd

Edition

[6] Avon Barrier Company, Technical Details

http://www.avon-

barrier.co.uk/rb680roadblocker.html