HNC/HND Engineering
Engineering Design
SOMETHING ABOUT
BRIDGES
EXTRACT TAKEN FROM
“STRUCTURES OR WHY THINGS
DON’T FALL DOWN”
BY J.E. GORDON
HNC/HND Engineering
Engineering Design
ARCH BRIDGES
Arch bridges have always been popular and, in various forms, they are
still very much in fashion. A simple masonry arch can quite safely be built
with a span of well over 60 metres. For most sites, if there are objections they
are likely to be associated with the cost or with the rise of the arch and with
the load on the abutments or foundations.
If we are concerned with the plain, semi-circular masonry arch which
was widely used in roman and medieval times (fig. 1), then one of the facts of
life is that the rise of the arch must be about half its span. Thus a 100 foot
span will call for a rise of at least 50 feet – in practice rather more. This is all
very well if the bridge is spanning a ravine which is more than 50 feet deep,
because the arch can then be sunk so that its crown is level with the roadway
on either side. However, if the bridge is to be built on flat ground, then we
have alternatives, on the one hand, of a hump-backed bridge, which is
inconvenient and dangerous, or, on the other, of having to build long and
expensive sloping approaches.
Fig. 1 – traditional masonry arch
The problem became particularly important with the coming of railways,
because trains don’t like hump-backed bridges – or indeed gradients of any
kind – and the expense of earth moving to make embankments for a flat
approach is a serious matter. One way of getting around the difficulty, at least
to a certain extent, is to build a rather flat arch with considerably less rise. In
1837, faced with the problem of getting the Great Western Railway across the
Thames at Maidenhead, Isambard Kingdom Brunel built a bridge of two brick
arches, each with a span of 128 feet and a rise of only 24 feet. (Fig. 2).
Both the public and experts were horrified, and the papers were full of
letters prophesying that the bridge would never stand. To keep the
correspondence and the publicity going, and perhaps to gratify his sense of
humour, Brunel delayed removing the wooden centering or false-work on
which the arches had been erected. Naturally, it was said that he was
frightened to do so. When, after about a year, the centering was destroyed in
a storm, the arches stood perfectly well. Brunel then revealed the centering
had, in fact, been eased to a clearance of a few inches soon after the
brickwork was in place and had been doing nothing at all for many months.
The bridge is still there today, carrying trains about ten times as heavy as
Brunel ever intended.
HNC/HND Engineering
Engineering Design
Fig. 2 – Brunel's railway bridge in Maidenhead
When we flatten the shape of an arch, so as to reduce the rise in
proportion to the span, the compressive thrust between the voussoirs (A
voussoir is a wedge-shaped element in an arch) of the arch-ring is
considerably increased, as we should expect. However, the compressive
stresses are still, as a rule, well below the crushing strength of the masonry,
and the voussoirs of the arch are seldom in any danger of being broken,
although the deflections which occur when the arch settles after the centering
is removed may be quite large and often amount to several inches.
Any real damage to a ‘flat’ arch, however, is most likely to be a
consequence of the greater thrust which must come upon the abutments. If
the foundations are of a solid material, such as rock, all will be well, but if they
are upon soft ground there may be serious trouble if they yield too much.
Unfortunately, the need for a long, flat arch is most likely to occur when we
are bridging rivers which flow across a level and boggy country.
It is for these reasons that bridges are often built with many small
arches; in fact nearly all long medieval bridges are multi-arch bridges. The
objection to this way of doing things is that the cost of building the supporting
piers – usually under water and often on soft ground – is high, and,
furthermore, the numerous piers and narrow arches obstruct the channel and
may cause floods and danger to navigation.
HNC/HND Engineering
Engineering Design
CAST-IRON BRIDGES
Some of the objections to arched bridges can be got over by making
them from less traditional materials. By the 1770s people like John Wilkinson
(1728-1808) – who had greatly cheapened the manufacture of cast iron by
improvements to blast-furnaces – began to cast voussoirs from iron. Cast
iron is a totally different type of material from wrought iron and steel because,
unlike these substances, it is very brittle. It resembles stone in being strong in
compression but weak in tension, and so, in building construction, it has to be
treated rather like masonry.
An advantage of cast iron is that it is possible to cast architectural
members, such as voussoirs, in the form of an open, trellis-like framework, so
that there can be an enormous reduction in weight, as compared with
traditional masonry. Furthermore, it is generally cheaper to cast iron than to
carve stone, and, before taste degenerated around the time of the first Reform
Bill, these iron castings were often of very attractive shape.
The benefit of cast iron to bridge-building was two fold. In the first
place, there was a saving in labour and transport costs; but more significantly
the reduction in the weight of arches diminished the magnitude of the thrust
upon the abutments and thus enabled engineers to build flatter arches with
cheaper foundations.
Curiously, one of the first people to take advantage of this technique
was the American Thomas Paine (1737-1809), who is famous in the history
books as the author of “The Rights of Man”. Paine planned to build a great
cast-iron bridge, which he had designed himself, across the Schuylkill River,
near Philadelphia. He came to England to order the castings, and while they
were being made he decided, as a supporter of the French revolution, to pay
a visit to his Jacobin friends in Paris. These gentlemen put him in prison and
very nearly guillotined him. He was just saved by the fall of Robespierre.
As a result of the delay, Paine’s finances collapsed and the castings
were sold to build a bridge over the Wear at Sunderland. The arch, which
was finished in 1796, had a clear span of 236 feet with a rise of only 34 feet.
The reason why Brunel did not use cast iron for the Maidenhead Bridge forty
years later was probably that he was afraid that the vibrations of the trains
would crack the brittle cast iron. In any case, his brick arches worked very
well.
During the nineteenth century a great many cast-iron arch bridges were
built. Although nearly all of them were successful, the method is scarcely
ever used today, chiefly because there are now cheaper ways of doing the
same job. Unfortunately, a very flat cast-iron arch looks, superficially, rather
like a beam. Structurally the two are quite different, since the arch is, or
should be, entirely in compression, whereas the underside of a beam is in
tension. If the material can be relied upon to carry tensile stresses, then a
beam is often lighter and cheaper than an arch, for a comparable service.
Some of the early engineers, notably Robert Stevenson (1803-59),
were tempted by this prospect of economy to venture into using cast-iron
beams. Because of Robert Stephenson’s outstanding professional reputation
the railway companies were persuaded to build several hundred cast-iron
beam bridges. However, as we have said, cast iron is weak and treacherous
HNC/HND Engineering
Engineering Design
in tension, and these bridges turned out to be very dangerous indeed. In the
end, every one of them had to be replaced and the expense to the companies
was naturally severe.
HNC/HND Engineering
Engineering Design
ARCH BRIDGE WITH SUSPENDED ROADWAY
A modern tendency in building large arch bridges is to use a suspended
roadway. If we split the arch-ring into two parallel elements, which are made
from steel or reinforced concrete, then we can hang the roadway from the
arches, at any level we like, in much the same way as is done in suspension
bridges (Fig. 3). There is then, of course, no restriction on the rise of the arch.
Fig.3 some examples of arch bridges with suspended roadways
The Hell Gate Bridge in New York (1915), which is at least 300 metres
span, and the Sydney Harbour Bridge (1930) which has a span of 500 metres,
are steel bridges of this type. In such bridges the main loads are carried
entirely in compression in the arches, and the hanging roadway is free from
longitudinal stresses. In big bridges the thrust upon the abutments is
therefore considerable, and very reliable foundations are needed. Both the
Hell Gate and the Sydney Harbour bridges are founded on solid rock.
HNC/HND Engineering
Engineering Design
SUSPENSION BRIDGES
Masonry arches have a number of advantages. As we have seen in the last
chapter, they are comparatively easy to design, since one can generally scale
up from previous experience quite safely. In fact, as professor Heyman
remarks, it is very difficult to design an arch which will actually fall down. This
feat was, in fact, achieved by a certain William Edwards a Pontypridd in 1751,
but I do not think there is any record of it happening since. Again, arches are
not unduly sensitive to a reasonable amount of movement in the foundations.
However, foundations of some sort there must be; and on soft ground they
have a way of being both troublesome and expensive.
Furthermore, although the maintenance cost of masonry is usually low,
the first cost has always been high, and this is particularly the case with large
bridges, which need elaborate centering during erection. For these reasons
there has always been a demand for something cheap and cheerful in the
bridge line. In primitive countries suspension bridges of various sorts were
fairly common; these were made from rope or other kinds of vegetable fibre.
Rope suspension bridges were also used by military engineers for temporary
bridging, notably by Wellington’s sappers during the peninsular war.
However, although rope is a strong and reliable material for carrying
tension when it is new, ropes made from plant fibres deteriorate fairly quickly
in the open and become undependable-as the more interesting personalities
in the neighbourhood of the bridge of San Luis Rey discovered. For a
permanent suspension bridge, cables of iron or steel are necessary. Cast
iron was far too brittle and steel was not commercially available until relatively
recently, but wrought iron is fairly strong and very tough; also it is
exceptionally resistant to corrosion.
Although a footbridge 20 metres long, made with iron chains, was
erected over the Tees in 1741, wrought iron was generally too expensive to
be used at all widely in bridge-building until the puddling process was
introduced about 1790.
Fig. 4 – Telford's Menai suspension bridge
After this, wrought-iron chains became comparatively cheap. In the Tees
Bridge the flooring was attached directly to the chains in the primitive manner,
so that the bridge was impassable to vehicles and must have been both steep
and alarming for pedestrians. The modern system of supporting cables from
high towers and hanging the roadway below the cables was invented by
HNC/HND Engineering
Engineering Design
James Finlay, of Pennsylvania, who began to build bridges of this kind around
1796.
The combination of a suspended, level roadway with the availability of
wrought-iron chains at a reasonable price made the suspension bridge an
attractive proposition for carrying wheeled traffic over wide rivers. For many
situations these bridges were much cheaper and more practical then large
masonry bridges. The idea was taken up very actively in many countries, and
especially by Thomas Telford, whose bridge across the Menai Straits (see fig.
4), was finished in 1825; it has a centre span of 166 metres, by far the longest
in existence.
Telford’s chains, like all the suspension chains used in bridges at that
time, were made from flat plates or links, joined by bolts or pins, very much
like the links of a modern bicycle chain. The concentration of stress at the pin
joints calls for a tough and ductile material, such as wrought iron, and indeed
chains of this type have been very successful and have seldom given any
trouble. Although wrought iron is reliable in tension, it is not especially strong,
and Telford wisely kept the highest nominal stress in his chains down to about
55 MPa, which is less than a third of the breaking stress (i.e. it had a safety
factor of greater than three).
In these circumstances a great deal of the strength of the chains was
devoted to supporting their own weight, and Telford was of the opinion that
the Menai bridge represented about the maximum safe span for a suspension
bridge, using the materials of the day. Although Brunel eventually showed
that Telford was being rather cautious – Brunel's Clifton Bridge has a span of
190 metres – yet for many years the span of the Menai Bridge remained a
record; and, in any case, the limitations of wrought-iron chains were clearly
within sight.
The recent fashion for road suspension bridges of great length is made
possible by the availability of high tensile steel wire. This material is very
much stronger than wrought iron or mild steel and can therefore support a
much greater length of its own weight. High tensile steel is more brittle than
wrought iron, but this can be accepted, since the cable is continuous and
does not have to have links with pinned joints, which are particularly
vulnerable to cracking. Again, instead of having only three or four plate links
in parallel with each other in each element of chain cable, the wire cables are
woven from many hundred separate wires, so that the failure of any individual
wire is not likely to be dangerous (See Fig. 5)
Fig. 5 – The Severn suspension bridge. High tensile steel cables with ten
times the tensile strength of wrought iron enable bridges nearly ten times as
long as Telford’s Menai bridge to be constructed
HNC/HND Engineering
Engineering Design
As an example of the sort of thing one can do nowadays, the new
Humber motorway bridge has a clear span of 1388 metres, which is over eight
times the length that Telford thought practicable. This is made possible by the
fact that the suspension wires operate, quite safely, at a working stress of 580
MPa, which is more than ten times the stress in Telford’s wrought-iron chains.