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MAKING THE MODERN WORLD
Stories about the lives we've made

module:Bridges

page:Glossary

A
abutments  
The side support at either end of an arch bridge, necessary to withstand the horizontal forces generated by the arch's shape.
anchorages  
The points at which the main cables on a suspension bridge are 'anchored' to the ground at either end of the bridge.
aqueduct  
A channel carrying water from its source (a lake or spring) to where it is needed. The gradient of an aqueduct has to be extraordinarily constant (typically to within 10cm per km of aqueduct) for the flow to be reliable.
aramid  
An artificially-made organic material that forms very long and stiff fibres.
arch  
A span that consists of an upwardly curved beam. The forces from the centre are distributed outwards. There are many types of arch, including Corbelled, Roman, Gothic, Arabic, Elliptic etc.
B
beam  
A rigid horizontal element that is used to carry a load. A beam bridge often consists of a road deck reinforced with girders. A simple example of a beam is an ordinary table top.
bending moment  
The internal force that prevents an object from bending freely under the action of the external forces. Sagging and hogging are visible signs of the Bending Moment within the beam.
bridge(s)  
A link or connection between two objects - usually places either side of an obstacle, such as a river, chasm, or estuary.
buttress(es)  
A structure, usually brick or stone, built against a wall for support or reinforcement to resist the sideways pressure of the weight of the building.
C
cable stay  
A bridge design in which the road deck is supported by a series of cables attached to the top of one or more towers.
cables  
Extremely strong wires, formed from thousands of single steel threads bound together.
cantilever  
A beam that is supported at one end only, and free at the other. A simple example is a diving board. Many bridges are constructed from a pier outwards, the two 'arms' balancing each other, but unsupported. This is called the 'cantilever' method of construction.
caissons  
A cofferdam is a temporary watertight enclosure constructed on the spot where a pier is to be built. A cofferdam usually consists of sheets of steel driven into the ground to create a walled chamber. The cofferdam is then pumped dry to expose the riverbed. Soil can be excavated to bedrock, or piles can be driven to create the pier foundation. The cofferdam is removed after the foundation and pier are constructed.
cofferdams  
A caisson is a large cylinder or box chamber that is sunk into the riverbed. The excavation and foundation work takes place within the submerged caisson. Some caissons are removed after construction, while others are left in place, filled with concrete, and used as part of a permanent foundation.
composites  
Materials whose properties derive from the mixture of their component materials. Individual glass fibres held together by a plastic resin forms the easily moulded, yet strong, 'fibreglass' material used for canoes, car bodies etc. Recently, carbon or aramid fibres have been similarly used to create very light, stiff materials for beams.
compression  
Any force that acts in order to shorten, or push together the ends of, a structural element. When pushing your hands together, your arms will be under compression. The towers of a suspension bridge, and the piers of an arch bridge, are under compression.
compressive stresses  
Any force that acts in order to shorten, or push together the ends of, a structural element. When pushing your hands together, your arms will be under compression. The towers of a suspension bridge, and the piers of an arch bridge, are under compression.
corbelled  
An arch constructed by successive layers of brick or stone projecting further towards each other from either side of the arch, until the gap is spanned.
D
deck  
The part of a bridge that carries the roadway. Usually horizontal, and often suspended from cables or resting on an arch.
E
elliptical arch  
The exact shape of an arch may vary hugely. The simplest is the semi-circular, but a shallow, flattened semi-circle becomes an ellipse, and many mediaeval stone bridges were built with elliptical arches spanning rivers.
estuaries  
The mouth of a river where it empties into the sea. Due to erosion, estuaries are often extremely wide. This width, coupled with the tidal currents up and down the river, require great expertise to span with a bridge.
F
Finite Particle Analysis (FPA)  
A heavily computerised approach to the analysis of stress and the behaviour of the components within a structure. This approach puts together the behaviour of thousands of separate sub-sections, all interacting with each other. The millions of calculations needed made this approach impossible until very recently.
footers  
The stones at the base of a bridge structure that take the loads onto the foundations.
G
girders  
A beam, usually made from concrete or steel, that is designed to strengthen another structural element. Concrete beam bridges often have steel girders beneath the road deck in order to help bear the tensile forces.
H
hangers  
The vertical rods or cables that are directly attached to the road deck of a suspension bridge, and 'hang' from the two main suspension cables that pass over the towers.
hogging  
The upwards bending in a beam, usually over its piers, that is the counterbalance to sagging in other sections of the beam.
I
J
K
keystone  
The 'keystone' is the central stone in an arch, and begins the distribution of the vertical 'load' forces down and around the arch. The keystone is the last voussoir to be placed in position, and allows each half of the arch to support the other.
L
load  
A force that is to be carried by a structure. Examples include the weight of traffic on a bridge and the wind on the side of a tent.
M
midpoint  
The central point of a structure. This is often where the structure is at its weakest, and the load at its greatest.
modelling  
A simplification of the real-life object and situation that preserves their essential nature, and allows a solution using mathematics. A good model produces a solution that reflects the main features of the original situation.
N
O
P
pier(s)  
The part of a bridge that supports the horizontal element, and carries the load to the ground, especially the intermediate support in a multi-arched bridge.
Q
R
reinforcing  
Where a structure such as a girder or a concrete slab has been strengthened or stiffened by extra material or by the addition of trusses.
resonance  
The build-up of oscillatory, or wave-like, motion in an object such as a plucked guitar string or the deck of a suspension bridge in a wind. The control of such motion requires the removal, or absorption, of the energy of the wave, preventing its build-up.
S
sagging  
The tendency of a beam to bend downwards, when acted on by a load, including its own weight.
social cohesion  
The degree to which a society is effective in holding together its individual members by its shared laws, customs and public behaviour. A high level of shared resources (such as cities, roads, bridges and cinemas) demands shared responsibility and ownership - and hence social cohesion.
span  
The unsupported length of a bridge, between its towers or piers.
steel  
The ubiquitous construction material consisting of iron alloyed to a carefully regulated proportion of carbon. Steel is less brittle than iron, and can take both tensile and compressive stresses, which makes it ideal for girders and cables. The super-long decks of most suspension bridges are made from steel.
stress  
The physical demands laid upon an object or material by the forces acting on it. Stresses can often lead to splitting, cracking, stretching and so on.
struts  
A part of a framework that is carrying compressive forces, or keeping two elements of a structure apart.
suspended deck  
A roadway or railway that hangs from cables slung over towers or from the underside of an arch.
T
tensile stresses  
Any force that acts in order to lengthen, or pull apart the ends of, a structural element. When hanging from a rope, both the rope and your arms, will be under Tension.
tension  
Any force that acts in order to lengthen, or pull apart the ends of, a structural element. When hanging from a rope, both the rope and your arms, will be under Tension.
ties  
A part of a framework that is carrying tensile forces, or keeping two elements of a structure together.
trusses  
A framework of connected members, usually made from steel, which together bear the loads on a bridge. At any particular moment, some of the members will be under tension and others under compression.
U
V
viaduct  
A road or rail bridge of considerable length, usually carrying vehicles at an elevated height across a valley or round a mountain side, as opposed to simply across a specific obstacle.
voussoirs  
The trapezoidal stones that are often used to form a rounded arch. The central voussoir is known as the 'keystone' and begins the distribution of the vertical 'load' forces down and around the arch. The keystone is the last voussoir to be placed in position, and allows each half of the arch to support the other.
W
wind tunnels  
A tunnel used by designers to test the effects of high winds on a planned structure. This may be part of a full-size structure, or a small-scale model of the whole structure. Huge fans drive air down the tunnel to simulate the effect of the wind in real-life.
X
Y
Z

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