Technological Studies
Exemplar Project Enquiry
Crossing the Forth
(Advanced Higher)
8537
.
Spring 2001
HIGHER STILL
Technological
Studies
Exemplar Project Enquiry
Crossing the Forth
Advanced Higher
Support Materials
CONTENTS
Page
Table of Contents
1
List of Illustrations
2
Introduction
3
Analysis and Description
6
Conclusion
10
References
12
Technological Studies: Exemplar Project Enquiry: Crossing the Forth (AH)
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LIST OF ILLUSTRATIONS
Page
Figure 1
The Arch Bridge
3
Figure 2
The Forth Rail Bridge
4
Figure 3
The Forth Road Bridge
4
Figure 4
Suspension Cable (Parabola)
8
Figure 5
Suspension Cable (Catenary)
9
Technological Studies: Exemplar Project Enquiry: Crossing the Forth (AH)
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INTRODUCTION
The River Forth has presented a major obstacle to communication between the northeast and the south of Scotland. The shape of Scotland’s coastline is such that distances
between centres of population are out of proportion to the size of the country, and
Bridges are vital to the economy. Yet there was no road crossing east of Stirling as
recently as 1936, and vehicles, passengers and livestock had to make the time
consuming passage by ferryboat at Queensferry or Kincardine.
Up until the 18th century most bridges were arch bridges, and their design was the
province of builders, or architects. An arch bridge transmits the vertical thrust through
the stonework to its abutments, and is a very stable structure. (Figure 1) This means
that the height of the bridge is comparable to the span. Brunel’s, daring flat arches of
the Maidenhead Bridge in 1837 spanned 39 metres for a rise of only 7.5m, but this
increased the loading on the abutments, although since the thrust was compressive the
masonry was able to cope, and the bridge is still there, carrying trains ten times
heavier than those it was built for. However to cross wider rivers it was the custom to
build a series of connected arches. The railway viaducts across valleys stand today as
a tribute to the Victorian Engineers.
Figure 1 The Arch Bridge
The rapid growth of Railways in the mid nineteenth century highlighted the need for
a crossing of the Forth, although plans to construct a road crossing were drawn up as
early as 1750, before the development of railways. Travel by road was in any case
slow and difficult, and the ferries could easily cope with the demands of horse drawn
carriages, foot passengers and even livestock.
The topography of the Forth river bed made an Arch Bridge impracticable. A
Suspension Bridge was considered, but as will be seen later, such bridges were
considered to be at the limit of their span with the materials available, and were too
flexible to accept the varying loads of trains.
Technological Studies: Exemplar Project Enquiry: Crossing the Forth (AH)
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Figure 2 The Forth Rail Bridge
After many plans had been considered, the design of Sir John Fowler and Sir
Benjamin Baker was adopted and work began in 1883, and was completed in 1890.
The principle of the Forth Rail Bridge is the Cantilever. The development of road
transport and the steady improvement in the road system in the first half of the
twentieth century brought increasing demands for a road crossing of the Forth
downriver from Stirling, as the demands on the Ferries was becoming too great, and
they were time consuming, which was very frustrating as average road speeds steadily
increased. The alternative was an equally time-consuming detour. An additional
railway crossing had been built at Alloa, but it was not until 1936 that a road crossing
was built to replace the ferry at Kincardine. This was at a narrow part of the river,
and it incorporated a swing section to allow shipping to pass upstream to Alloa and
Stirling, once busy ports. Significantly the Swing Bridge has now been locked
following the decline in shipping trade at these ports.
Figure 3 The Forth Road Bridge
Technological Studies: Exemplar Project Enquiry: Crossing the Forth (AH)
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A growing public demand led to a road crossing of the Forth a short distance upstream
from the Rail Bridge. This time the Suspension Bridge was considered, because of the
large span needed, and the requirement for only two support piers which utilised a
strategically placed rock on the north side, and a pier constructed using a caisson on
the deeper south side. The high deck of this bridge as with that of the cantilevers of
the Rail Bridge allowed unrestricted access to shipping.
Rope suspension bridges have been in use for thousands of years. To construct a
strong bridge with a large span required the development of wrought iron for the
support chains, a cheap strong material, and Telford made use of this in 1825 to cross
the Menai Straits. The span of 166 m was reckoned to be approaching the limit for
this material. He worked with a highest working stress of 55 MN/m2 with a Factor of
Safety of 3. Brunel crossed the Avon Gorge at Clifton with a span of 190 m. The use
of high tensile steel wires made much larger spans possible, as the Engineer could
work with stresses of 580 MN/m2 , ten times that employed by Telford.
Work started on the Forth Road Bridge in 1958, and it was completed in 1963.
Technological Studies: Exemplar Project Enquiry: Crossing the Forth (AH)
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ANALYSIS AND DESCRIPTION
The Forth Rail Bridge consists of three double cantilevers supported by masonry
piers, the Fife Pier and the central Inchgarvie Pier built on whinstone rock, the South
Queensferry Pier on boulder clay. The central pier makes use of Inchgarvie Island,
well placed near the centre of the Firth. The ends of the bridge are not fixed to the
approach viaduct, but carry a counterbalance of 1000 tonnes to balance half the
weight of the central span each. The deck is thus in perfect balance for dead loading,
but trains entering the bridge upset this, which accounts for the wider base for the
Inchgarvie cantilevers.
The three foundations were constructed under water by sinking caissons. These 24 m
diameter cylinders were settled on the river bed, open at the base, and all the water
pumped out and kept out by compressed air. This allowed workmen working inside
the caissons to construct the masonry foundations. Once completed the remaining
space in the caissons was filled with concrete. Each foundation block was topped with
four circular granite faced piers. The steel structure was then anchored to the masonry
by 48 steel bolts each 64 mm diameter and 8 meters long. Work on each of the
cantilevers was carried out simultaneously. The main “vertical” columns are 3.7 m in
diameter and incline inwards at 80 to the horizontal. Each tower is 104 m high, and
with a clear height of 46 m above high water, allows free passage to shipping of all
types.
The central joints between the cantilevers allow for 25 mm expansion, and at each
shore end there is provision for a further allowance of 12.5 mm. An internal viaduct
constructed from two latticed girders, 5 m between centres and with cross girder every
3.4 m, carries the two rail tracks. The viaduct is supported from the cantilevers by
cross girders and trestles. It is interesting to note that, in contrast to the Road Bridge,
the design load of the Rail Bridge has gone down, since modern trains are much
lighter than the steam trains of old.
The cost of the bridge was £2 500 000, and the materials used were:
Steel
Masonry:
 Granite
 Ordinary stone
 Concrete
Rivets
55,000 tonnes
21,000 cubic metres
37.000 cubic metres
49,000 cubic metres
8 million.
4,500 workmen were employed in the construction of the Bridge.
Sadly 57 workmen lost their lives in the course of construction.
A painting schedule was drawn up to combat corrosion. The bridge to be painted
entirely every three years. 29 Painters were engaged permanently, using 17 tonnes of
paint to cover the 59 hectares of steel.
Technological Studies: Exemplar Project Enquiry: Crossing the Forth (AH)
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The campaign to build a road bridge near the rail bridge began in earnest in 1923 at a
Press conference at the Hawes Inn, South Queensferry, attended by all the towns in
the locality and motoring associations and motor traders. Discussions went on,
interrupted by the Second World War, until 1956, when it was agreed that a
suspension bridge should be built utilising the MackIntosh Rock west of the Rail
Bridge. The main span would be 1006 m long. The Government and the local
Authorities contributed part of the cost, the remainder to be by loans, repayable out of
toll revenue from the Bridge.
After clearing the sites of the north and south approaches, the first work on the bridge
was the building of a causeway 400 m out to the MackIntosh rock. This whinstone
reef is about 6 m below low water. The top of the rock was blown off to make a base
for the north tower. The south pier was built using caissons, working through boulder
clay onto sandstone at a depth of 27 m below low water.
The main towers were prefabricated in Glasgow, and three welded high tensile steel
boxes in each leg. The steel plate was up to 32 mm thick and each section weighed 35
tonnes. When assembled the towers were quite a spectacle at 157 m high. On July 5th
1961 the first cable was erected across the Forth. In all 22 cables 25 mm diameter and
1834 m long were put in place to support temporary catwalks. The two catwalks were
linked by cross bridges to assist in spinning the main cables.
The system of spinning was invented by J A Roebling in the United States in 1845 on
the Pittsburgh Canal. This was to be the first use of the system in Europe. As
mentioned earlier, a special school was set up in South Queensferry to teach the
techniques of cable-spinning.
Four tunnels had been prepared, two at either end, to form anchorage for the cables.
An idea of their size can be gathered from the fact that when completed each tunnel
was filled with 10,500 tonnes of concrete.
Two wires were passed from the shore-based drums over a travelling pulley block and
anchored in the south tunnel. The pulley block was then hauled across the river and
the wires anchored in the corresponding north tunnel. The pulley was then returned to
the south side for the next pair. Nearly 12,000 of the 5 mm diameter wires were taken
across for each main cable. The wires were laid parallel in groups, and were then
compacted into circular form by a moving mechanism of a circle of rams. The final
diameter was 600 mm. After treatment to prevent corrosion the circular cable was
bound with soft galvanised wire, an operation which required 48,000 kilometres for
the two cables. Cast steel cable bands spaced at 18 m centres were clamped round the
cables to hold the hangers for the decking. Each hanger consists of two loops of 50
mm diameter wire.
The deck was assembled outwards from the towers. The steelwork was raised by
electric derricks and assembled in front of the derrick which then moved on to the
new part and raised the next. The final piece fitted to join the two sides was accurate
to within 25 mm.
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At the peak of activity, 450 men were employed in the construction of the road bridge,
which cost in excess of £11 500 000. 29,500 tonnes of steel were used to construct the
towers, decking and cables.
The question of tolls for what many regard as part of the national road network has
long engaged public attention. The toll was set initially at two shillings and sixpence
(12½ pence). At this rate of collection with the traffic at the time the initial cost was
not being repaid. Indeed the interest on the loan was not being raised, and unpaid
interest was added to the capital. The situation was resolved mainly by the inflation of
the seventies, which in effect reduced the value of the loan, together with increased
traffic and a toll of 40p. Automatic collection of tolls was tried in an attempt to reduce
queuing, but was found to be a failure. A scheme to collect double tolls only on the
North lane is now in force.
T
P

Q
H
Vertical chains
y
O
x
Figure 4 Suspension Cable (Parabola)
Figure 4 shows part of the Suspension cable holding up the horizontal deck. If the
weight of the cable is neglected, the weight of the bridge is evenly distributed.
Consider portion OP of the cable. Forces acting are:
1. Tension at O (horizontal) H
2. Tension at P, T acting at  degrees to horizontal
3. Tensions of vertical chains (vertical).
Resolve horizontally:
H – T cos  = O
Resolve vertically:
=O
sin  - W
Where W is sum of all tensions in the support chains between O and P
Technological Studies: Exemplar Project Enquiry: Crossing the Forth (AH)
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If the weight of the portion of the bridge between O and P is w per unit length,
W = wx
Giving
and
T sin  = wx
T cos  = H
Dividing these gives
Thus
tan 
dy/dx
Integrating
y
= wx/H
= wx/H
= ½wx²/H (+C)
This is the equation of a parabola, which is the shape taken by the suspension cables.
B
A
Figure 5 Suspension Cable (Catenary)
Figure 5 shows the part of the Suspension on completion before being attached to the
deck. The weight of the cable is now significant, and is not evenly distributed in the
horizontal plane. This is shown by inspection, as there is clearly more cable in
horizontal distance B compared with the same horizontal distance A, due to the
increasing angle of the cable away from the centre. It can be shown that the shape
taken up by the freely suspended cable is a Catenary. This has a different profile to
the parabola, and though the difference is not great, it becomes significant for cables 1
kilometre in length. The bridge builders have careful calculations to make to ensure
that the length of the cables is correct so that they can change profile when connected
to the deck, and that the deck support hanger cables are of the correct length.
Technological Studies: Exemplar Project Enquiry: Crossing the Forth (AH)
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CONCLUSION
Both the Forth Rail Bridge and the Forth Road Bridge fulfil their function very well.
Each structure was built in response to a need for improved communications at a
particular time in history, and each was constructed with regard to the type of
transport involved, the topography of the area, the materials and the techniques
available at the time of construction.
Fowler and Baker, aware of the spectre of failure in the wake of the Tay Bridge
disaster, built a structure far stronger than was really necessary. The material used was
steel, comparatively cheap plentiful and easily worked, but has left a legacy of
corrosion which requires continuous maintenance. On the other hand the maximum
loading which the bridge has to withstand has gone down with time, as the diesel
trains of today are far lighter than the steam trains of old, and when the line is
electrified the loading will be even less. This is in marked contrast to the Road Bridge,
where traffic loading has increased steadily since it was built, and the bridge has
undergone strengthening once or twice already.
The suspension bridge is in theory an ideal solution for a road crossing over a wide
deep estuary, since road traffic, being in discrete packets rather than a train, can
tolerate some road movement. However the stability of a suspension bridge has been
achieved over time by trial and error. The Tacoma Narrows Bridge in America
suffered a spectacular failure in 1940 because the deck was not stiff enough, and a
wind of only 42 miles per hour which blew steadily for a few days caused the swaying
to increase until the bridge destroyed itself. Future bridges employed stiffening in the
form of box girders, but this brought new problems, as the tendency to buckle had not
been fully appreciated, and the Forth Bridge had further stiffening work on its girders
not long after it was built. In recent times the use of jacks in the towers together with
strengthening inside the towers has raised the load carrying capacity of the bridge.
The technique of spinning high tensile steel cables has been described, and the fact
that a school was set up to teach this method of construction is indicative of the
careful preparation that characterised the whole enterprise.
The employment of up to 4500 men on the Rail Bridge brought some wealth to the
area over seven years, and permanent employment for maintenance staff. Fife and the
North East enjoyed improved communications with the capital and the South. The
loss of 57 lives shows that health and safety matters were not a major consideration.
In contrast the workmen on the Road Bridge wore hard hats, safety belts, safety boots
and special clothing. Safety nets, walkways and handrails were provided as well as a
safety boat. Even so 3 workmen lost their lives during the construction. High winds
were a constant hazard, and in the worst of these, winds over 30 miles per hour, the
workmen were not expected to operate. The 74 staff of the ferries, displaced by the
construction of the bridge, were given employment in the operation of the bridge.
Technological Studies: Exemplar Project Enquiry: Crossing the Forth (AH)
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Both bridges stand as monuments to the achievements of man. Each owes its success
to the pioneering work, ingenuity, inventiveness (and the failures) of many
generations of Engineers. Each bridge is distinctively different in solving basically the
same problem, from the solid ruggedness of the Victorian ironwork to the elegant
sophistication of the nineteen sixties.
Whilst praising and admiring what we see however, we must acknowledge that also,
as with most great works, the hand of man is not flawless.
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REFERENCES
Mackie A
A Tale of Two Bridges, Commercial Enterprises, Edinburgh
Gordon J E
Structures, Penguin Books Ltd, Harmsworth
Norman E et al
Advanced Design & Technology, Longman Group Ltd
Jeans J H
Theoretical Mechanics, Ginn & Company
CD Rom
Microsoft Encarta
CD Rom
Europress Family Encyclopedia 1999
Internet Sites
www.google.com
www.hydratight.com/morlift/forth_bridge.htm
www.structurae.de/DataEnglish/str00300.html
www.geo.ed.ac.uk/scotgaz/features/featuresfirst101.html
www.webviews.co.uk
www.scotlandvacations.com/forthbridge.htm
www.forth-bridge.co.uk
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