How bridges balance forces
Forces make things move, but they also hold them still. It's far
from obvious, but when something like a skyscraper looms high above
us or a bridge stretches out beneath our feet, hidden forces are hard
at work: a bridge goes nowhere because all the forces acting on it
are perfectly in balance. Bridge designers, in short, are force
The biggest and most pervasive force in the universe, gravity, is
constantly tugging things down, which isn't such a problem for a
skyscraper, because the ground underneath pushes straight back up
again. But a bridge spanning a river, valley, sea, or road is quite
different: the huge deck (the main horizontal platform of a
bridge) has no support directly beneath it. The longer the bridge,
the more it weighs, the more it carries, and the bigger the risk
it'll collapse. Bridges certainly do fall down from time to time, and
quite spectacularly, but most stand happily still for years, decades,
or even centuries. They do it by carefully balancing two main kinds
of forces called compression (a pushing or squeezing force, acting inward) and
tension (a pulling or stretching force, acting outward), channeling the load
(the total weight of the bridge and the things it carries) onto
abutments (the supports at either side) and piers
(one or more supports in the middle). Although there are many kinds
of bridges, virtually all of them work by balancing compressive
forces in some places with tensile forces elsewhere, so there's no
overall force to cause motion and do damage.
Artwork: Balancing forces in a bridge: Different types of bridges carry loads through the forces of compression ("squeezing"—shown here by red lines) and tension ("stretching"—shown by blue lines): 1) A beam bridge has its beam partly in tension and partly in compression, with the abutments (side pillars) in compression; 2) An arch bridge supports loads through compression; 3) A suspension bridge has its piers (towers) in compression and the deck hangs from thick suspension cables by thinner cables, all of which are in tension. 4) A cable-stayed bridge is similar but the deck hangs directly from the piers from cables. The piers are in compression and the cables are in tension. 5) A truss bridge is a kind of reinforced beam bridge. Like a beam bridge, the top is in compression and the bottom in tension. The diagonal trusses are in tension and the vertical ones are in compression. 6) A cantilever bridge balances tension forces above the bridge deck with compression forces below.
If a bridge is unloaded, all it really has to do is support its
own weight (the dead load), so the tension and compression in
its structure are essentially static forces (ones that don't cause
movement), changing little from hour to hour or day to day. However,
by definition bridges have to carry changing amounts of weight (the
live load) from things like railroad trains, cars, or people,
which can increase the ordinary tensile or compressive forces quite
dramatically. Rail bridges, for example, bend and flex every time a
heavy train crosses over them and then "relax" again as soon as
the load has passed by.
Bridges also have to bear ever-changing environmental forces. Arch bridges over rivers,
for example, have to cope with water backing up behind them (their abutments often have strategically
placed openings to let high flood water drain through). Suspension
bridges that carry cars tend to bear the same loads all day long,
though, often sited in windy estuaries, they also have to endure
squalling gusts of wind, which can set up a twisting force,
called torsion, in the bridge deck.
(Modern suspension bridges tackle this problem by having decks with
aerodynamically designed cross sections, tested in
and may be reinforced with trusses underneath.) Loads that cause a bridge to
move back and forth can be particularly dangerous if they make it
vibrate wildly at its so-called natural or resonant frequency.
Resonance, as this is known, is what makes wine glasses
shatter when opera singers get a bit too close; the "singing" of
the wind can have equally catastrophic effects on a bridge.
Bridges through history
People find bridges bewitching and bewildering at the same time.
Why are there so many different types? How do engineers choose one
kind over another? Why have people tended to build different kinds of
bridges in different periods of history? There's are easy answers to
all these questions—and complex ones too.
Photo: Bridges are a product of their place and time:
place, because they have to be tailored to specific local conditions; and time, because they have always drawn on the most advanced materials and engineering technology available. This is a view of the revolutionary Brooklyn Bridge in New York City, taken from the Brooklyn side. Credit: Photographs in the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.
One simple answer is that, over thousands of years of human
civilization, engineers have gradually developed more sophisticated
bridge designs that can span ever greater distances. The oldest
bridge types, beams and arches, can only stretch so far before they
collapse under their own weight; more sophisticated versions of these
designs (truss, box girder, and cantilever bridges) can reach
further; and suspension and cable-stayed bridges can go further
still. This gradual evolution—and extension—of bridges has been
made possible partly by a deeper understanding of engineering, but
also by the development of far stronger materials. Arch bridges were
popular in the Middle Ages, for example, because they were quick and
easy to build from locally sourced materials and lasted a long time
with little or no maintenance. When Ironbridge,
the world's first cast iron (arch) bridge, was built at Coalbrookdale in Shropshire, England, in 1779, it
revolutionized bridge construction; during the 19th century, hundreds
of other bridges were built from iron and later steel, including New
York City's famous 1883 Brooklyn Bridge, with a span of 486m (1595ft). Suspension and cable-stayed bridges rely on those most
dependable of modern materials, reinforced concrete and steel. Some
of the newest bridges naturally use the very latest composite
Types of bridges
While it's easy to discuss bridges in this fairly abstract and
theoretical way, it's much more interesting to look at some specifics
by examining each major type of bridge in turn.
Photo: A beam bridge carrying a railway line over a road in Dorset, England. Note the abutment on the right-hand side that stops the bridge from collapsing down the hill toward us.
A beam is the simplest (and often cheapest) kind of bridge: a
deck, spanning a relatively short distance, held up
by a pair of abutments (the vertical supports at either end).
Stand on a plank (the deck) stretched between a couple of chairs (the abutments)
and you'll make it flex downward in the middle, so it's slightly longer underneath
and slightly shorter on top. That tells us that the bottom of a beam
is in tension (pulled longer than it would ordinarily be), while the
top is in compression (squashed shorter). The load on a bridge like
this is transmitted through the beam to the abutments at either end,
which are also compressed (squashed downward). The longer the beam,
the more likely it is to sag in the middle, which is why basic beam
bridges are usually quite short. Modern beam bridges can be much
longer, if they're built with box girders (huge hollow boxes
made from repeating sections of steel girders and/or reinforced
concrete) or braced with trusses (diagonal reinforcements)
either on the side or underneath. Beams are
explained further in our article on how
Photo: The Pulteney Bridge in Bath, England is made up of three stone arches. Completed in 1773, it's modeled on the Ponte Vecchio in Florence, Italy.
Arches are the only kinds of bridges supported entirely by forces
of compression. There is some tension underneath an arch, but it's
usually negligible unless the arch is large and shallow. That makes
sense, if you think about it, because an infinitely wide arch would
just be a horizontal beam, with its lower side in tension.
A bridge deck resting on an arch pushes down on the curve of stones
(or metal components) underneath it, squashing them tightly
together and effectively making them stronger. The load on a stone
arch bridge is transmitted through the central stone (called the keystone),
around the curve of other stones, and into the abutments,
where the solid ground on either side pushes back upward and inward. Like beam
bridges, arches are relatively simple and cheap to construct, and
don't need to block a road or river with central piers. They can
easily exceed the span of a basic beam, though their big drawback is that they
need large abutments, so they're not always an efficient way of
bridging something like a highway if a lot of clearance is needed underneath.
Viaducts and aqueducts (water-carrying bridges) are often variations on arch bridges,
with multiple arches side by side.
Examples of arch bridges include the Mostar Bridge in Bosnia Herzegovina and the Charles Bridge in Prague.
Photo: A truss bridge carrying a pedestrian walkway over a railroad line in Dorset, England.
One way to extend the reach of a basic beam bridge is to reinforce
it—and engineers have found the best way to do that is with a
system of diagonal, triangular bars on the sides, which are called
trusses. There are many ways of arranging trusses to support a
bridge, giving a variety of intricate and often attractive lattice patterns;
lenticular (curved) trusses, used in the Royal Albert Bridge in the top
photo, are one example. A typical truss bridge looks like a hollow box with open
or closed vertical sides and roof, the sides reinforced with diagonal trusses,
and the base resting on girders.
Photo: The Huey P. Long cantilevered bridge on the Mississippi River near New Orleans,
under construction in the early 1930s. Note how the apparently unsupported spans at either end are extending outwards into thin
air from the piers—the cantilever principle at work.
Picture restored from an original by Marcus Lamkin
courtesy of US Library of Congress.
Two back-to-back beams extending outward from a pier can
balance one another—just as a tightrope walker can balance
by holding both arms straight out from her body. That's the basic idea behind
the cantilever bridge. Normally, when we talk about a cantilever, we mean a beam supported at only one
end, like a diving board or see-saw only much more rigid. In a cantilever bridge,
there's usually a pair of cantilevers extending from each
pier, with a short beam bridge in between, linking them
together; alternatively, some have a cantilever extending out
from each abutment toward the middle, with a beam bridging them. Cantilever bridges are
sometimes hard to recognize because they're typically reinforced with girders
and trusses, but easier to spot if you remember that they have
multiple sections and often have at least one pier in the middle.
The world's most famous cantilever bridge, the
Forth Bridge in
Scotland, has three cantilevers (reinforced with a lattice of trusses)
with two shorter beam bridges in between them. The world's longest cantilever is the
very similar Quebec Bridge,
at just under 1km long (987m or 3239ft to be exact).
Other examples of cantilever bridges include the Queensboro Bridge in New York City and the Crescent City Connection in New Orleans.
Photo: The Tamar Bridge, completed in 1961, spans the River Tamar, the boundary between Cornwall and Devon, England, alongside Brunel's 1859 rail bridge (which you can see in the background). Notice the truss and girder reinforcements under the deck.
If you need a bridge that spans even further, a suspension bridge
of some kind is really your only option. The genius of a suspension
bridge lies in using very tall piers with huge, curving main cables
strung between them. Dozens of thinner vertical suspension cables of
varying length hang down from the main cables and support the immense
weight of the deck and the loads it carries. (And although people
always notice the cables in a suspension bridge, they often fail to
spot the girders and trusses reinforcing the deck underneath. This is
a subtle and quite important point: most bridges are actually
composites of two or more of the basic bridge types.) The biggest
bridges all use the suspension approach; the world's longest,
the Akashi Kaikyō in Japan, is 3.9km (2.4 miles) long.
Famous suspension bridges include the Humber Bridge
and the Clifton Suspension Bridge in England,
the Golden Gate Bridge in California,
and the Brooklyn Bridge in Manhattan, New York City.
Photo: The Arthur Ravenel, Jr. cable-stayed bridge in Charleston S.C.
Picture courtesy of Carol M. Highsmith's America Project in the Carol M. Highsmith Archive,
US Library of Congress.
A big drawback of suspension bridges is that they need to be
anchored to the ground on either side. That's not always possible if
there isn't room for the cables or appropriate bedrock to anchor them
into. A different kind of suspension bridge, known as a cable-stayed
bridge, does away with this by balancing two sets of suspension
cables either side of each pier, which supports the load. In a
"normal" suspension bridge, the deck hangs from cables of varying
length that are themselves supported by the immensely strong main
suspension cables. In a cable-stayed bridge, there's only one set of
cables that fan out, diagonally, from each pier to the bridge deck,
which tends to be stronger and bulkier than in a suspension bridge.
Cable-stayed bridges are significantly shorter than conventional
suspension bridges and generally don't span distances much greater
than 1km; the world's longest is currently the
Russky Bridge in
Vladivostok, Russia, at 1.1km (3622ft).
Other examples include the Vasco da Gama Bridge
the Millau Viaduct in France,
the Hangzou Bridge in China,
and the Chords Bridge in Jerusalem.
Photo: A pontoon bridge laid across the Euphrates River in Iraq. Photo by Kevin C. Quihuis, Jr. courtesy of US Marine Corps.
Boats obviously float on water, so if you need to build a temporary bridge in a hurry, floating a deck on a series of boats is one possible solution. A floating bridge like this is called a pontoon—and it's widely used by the military for improvised river crossings (such as when existing bridges have been blown up for strategic reasons). Well-organized armies have prebuilt sections of pontoon bridges that they can float into place and bolt together, wherever and whenever they need to. The main problems with pontoon bridges are basic instability and the relatively light loads they can carry. Since the deck floats very close to the waterline, a pontoon bridge automatically blocks boats from using a river, though it's usually possible to detach a section or two from the middle and float it aside to let river traffic navigate through.
Through arch and tied-arch (bowstring)
Photo: Hell Gate Bridge, a through-arch railroad bridge in New York City, pictured
around 1915–1920. Note how the bridge deck cuts horizontally through the arch (so
some of the arch is above the deck and some below) and the big abutments at either end
holding the arch in place. Photo by Detroit Publishing Co. courtesy of US Library of Congress.
Hang a beam bridge from an overhead arch and what you get is called a
through-arch bridge (if the deck cuts through the arch) or a
tied-arch bridge (if the deck ties the arch in place at its base).
Both kinds are a bit like suspension bridges, because the
deck and its load hang from the arch. However, although they look
very similar, they balance forces in different ways.
In a through-arch bridge, just like with a conventional stone or brick arch, the ends of the arches
push ("thrust") outward and need pushing back by the abutments. Bridges like this
are sometimes called thrust arches for this reason.
Examples of through-arch bridges include the Sydney Harbor Bridge
the Hell Gate Bridge in New York City
(shown here), and the Tyne Road Bridge in Newcastle, England.
Photo: Daniel Carter Beard Bridge, a tied-arch bridge over the Ohio River.
Note how the deck sits at the base of the arch (in the other words, the whole arch is
above the deck) and ties it in place; for that reason, no big abutments are needed
to hold the arch in place. Picture courtesy of the Carol M. Highsmith Archive,
US Library of Congress, Prints and Photographs Division.
In a tied-arch bridge, while the arch supports the
deck, the deck also stops the arch from pushing outward, holding
it in place, so the arch and deck balance one another.
Just as a cable-stayed bridge is more self-supporting than
a suspension bridge, because it does away with the anchoring cables,
so tied-arch bridges are more self-supporting than a conventional
arch, because they have less need for sturdy abutments. Tied-arch
bridges are sometimes called bowstrings because they resemble the
arch of a bow pulled out ready to fire an arrow, and because
the crossbar ties the bow together in a similar way.
Examples of tied-arch bridges include the Puente de la Barqueta in Spain and
the St Georges Bridge in Delaware, USA.
The Chaotianmen Bridge in China is both a
through-arch and a tied-arch bridge.
Lifting and swinging bridges
Photo: The El Ferdan Swing Bridge carries a railroad line over the Suez Canal in Egypt. Spanning 340m (1100 ft), it's the longest swing bridge in the world. Photo by Daniel Meshel courtesy of
Conventional bridges are impractical if something like a low road
has to cross a river or canal through which tall boats need to pass.
In that case, we need a mechanical bridge with a deck that can lift up or swing aside whenever necessary. Tower
Bridge in London, England is a kind of double drawbridge: it has a split deck that lifts up
in the center. There are many examples of swing bridges all over the
world. Some have two moving parts that swing to the sides, leaving a central channel clear; others
swivel on a central support to open up one or two clean channels of a waterway either side.
How do you design a bridge?
Just as bridges balance competing forces from different
directions, so engineers have to balance all kinds of considerations
when they plan a new bridge.
How far does the bridge need to stretch? That will usually
determine the type of bridge that's needed. A very short span (over a
small river, road, or rail track) could merit just a low-cost beam or
truss; suspension and cable-stayed bridges will generally be
unnecessarily complex and expensive; and arched bridges are built
much less often than they were in the Middle Ages, partly because
other types of bridges use the available space more efficiently. As
we've already seen, the type of bridge determines the materials used,
to a very large extent. Even so, there may be scope for using local
materials so a bridge blends into its environment. That was certainly
a feature of traditional arched bridges, often built from local rock
or stone. Modern bridges are usually built from steel and concrete
and have to rely on design to integrate themselves into their
surroundings instead. Box-girder bridges are often manufactured in sections,
off-site, which means they can be very rapidly erected. Unfortunately, it
also tends to mean that they look very similar and generic.
The place where a bridge is being built is also a critically
important factor. Is the ground firm enough to take large abutments
for an arch? Is there solid bedrock into which suspension cables can
be anchored (and, if not, would a cable-stayed bridge be better)? If
the bridge has to cross a river, how can piers and towers be safely
sunk into its bed so they're not scoured away by the rushing water.
The exact location of a bridge is carefully chosen to simplify
construction, reduce cost, and ensure the bridge is strong and
durable. It's not always possible for bridges to cross in a perfectly
straight line, however; bridges sometimes have to cross at an angle
(which gives what's known as a skew bridge), curve, or change direction
from one section to another. Modern box girder bridges, built from
modular deck sections, are easy to curve through even quite dramatic
Photo: The Calstock Viaduct has carried railroad trains high over the River Tamar at Calstock, Cornwall in England since its completion in 1908. Imagine the engineering challenges here. You have to carry very heavy loads (trains) at a high level over quite a wide
river valley while allowing enough clearance for sailing boats to pass beneath. A single arch so high and wide
would have been impossible to build, so the designer opted for a 12-arch viaduct instead. An ugly bridge could
easily have spoiled the beautiful landscape here, but this elegant structure serves as a perfect focal point.
Although it looks like stone, the material is actually precast concrete.
Apart from the dead and live load, what kinds of occasional,
transient forces might the bridge need to withstand? Are there
earthquakes or hurricanes and, if so, how can the bridge be designed
to survive them? Will a river bridge be able to cope with floods? And what
about the loads it will carry? If it's a road bridge, how much is
traffic likely to increase over the coming years and decades, and
will the bridge always be strong enough to handle them? What if
several of these transient forces occur at the same time? For
example, suppose a bridge simultaneously has to handle high winds,
immense pressure from rising water levels, and heavy traffic?
Engineers have to consider all kinds of other factors beside the
basic type, location, and strength of a bridge. For example, does a
bridge have to carry different types of traffic (a railroad, cars,
and pedestrians) and how will it separate them? What about safety
considerations (stopping speeding cars from plunging over the edge),
and issues like minimizing the risk of suicides (a particular problem
for some of the world's tallest bridges)? What kind of maintenance
will the bridge need, from regular concrete inspections to systematic
painting to protect against corrosion?
Bridges in harmony
Science, technology, and engineering
give us confidence we can build stone, iron, steel, or concrete
bridges that will survive for many decades. But there's much more to
a bridge than merely staying upright as humdrum loads shuttle over
it. Think of some of the world's greatest bridges—the
Stari Most arch at Mostar, the Brooklyn suspension bridge in Manhattan, the Forth Railway cantilever bridge, or the very recent,
cable-stayed Millau Viaduct in France, for example—and you'll
quickly realize that great bridges are as breathtakingly memorable
as great buildings. Sitting in a river or straddling a valley, you
could argue that a bridge disrupts the balance of nature. But
bridges connect people and communities together, and many
would contest that great bridges are true world wonders that enhance
their environment. Who, for example, can imagine San Francisco Bay
without the Golden Gate Bridge? So it's maybe just as true to argue that the genius of a great bridge lies in forging a partnership
between people and place so that engineering and nature sit happily,
side by side.