We will begin by looking at the three categories of bridge design that solve the problem of carrying a load across a gap. Theoretical principles and suitable building materials will be discussed.
Within each category, we will see how the basic type has been developed or 'hybridised' over the centuries to give diversity in style and function. Finally, we will look at current trends in bridge design and try to identify some likely directions that bridge building could take in the future.
The beam bridge
This is the earliest and simplest kind of bridge: a fallen tree lying over a stream, perhaps, or a log lodged across a narrow chasm? We still come across these when walking in the hills, and there is no way of dating the first time some hominid straightened or stabilised such a 'beam' and thereby became the world's first bridge engineer.
The design is as simple as a single rigid 'beam', resting on supports at either end and unsupported in the middle. The weight of the beam, and of any traffic on it, is carried directly to the ground by the supports, often called 'piers' in the trade.
© structurae.de (Jacques Mossot)
The beam need not be of any particular shape and there are no other elements besides the piers to help dissipate the load. Hence the piers take the full weight of the load and are said to be in 'compression'. This means that they are being squashed by the forces at the top and bottom, and must be built from materials that can resist such forces without crumpling.
The fact that the load tends to stress the beam itself - the top surface of the beam being shortened slightly and the bottom surface stretched - shows that the top is in ‘compression’ while the bottom is in 'tension'.
Suitable materials for a beam
© structurae.de (Martin Kelly)
The degree to which bridge components can withstand these opposing forces depends largely upon the material from which that bridge is made. Early beam bridges across streams were often made from stone 'clapper' slabs laid across the narrow gaps between piles of stones in the river bed itself.
Stone is very strong under compression and is thus ideal as a material for the piers, but it is not strong under tension, cracking easily if given a sudden blow. Therefore, it is not ideal for use as a beam despite its apparent strength. In addition, stone slabs are very heavy and therefore difficult to manoeuvre into position.
By contrast, wooden tree trunks or cut planks form a very strong beam when laid across a stream, despite their comparative lightness. Wood is able to carry both compression and tension equally well – wood is organic, after all, and heavy horizontal branches often carry similar loads to bridges themselves. Indeed, the longitudinal fibres within wood are designed to spread stresses within a tree without breaking.
Of course, wood is not as hard wearing as stone and needs replacing relatively often.
© Simon Dakeyne
Most simple beam bridges have been made from wooden planks, laid on stone pier supports. Relatively recently, materials have been developed that are stronger, weight for weight, than wood, and we will be looking at some of these later in this module. These days, bridges carrying the huge weights of trains across small roads are almost always beam bridges made from steel. They are very robust and stable, and often do not require any intermediate pier.
© structurae.de (Michael Gonzalez )
For a wider crossing, however, a series of intermediate piers may be needed, effectively forming several 'mini' beam bridges. Such a structure can form a hugely long bridge over, say, a valley floor, a fairly shallow lake or an estuary.
© Simon Dakeyne
This latter solution does produce a significant new obstacle to shipping, however. Several major bridges of this type have been catastrophically struck by ships, often in foggy conditions.
The world's longest bridge (but with no exceptional individual span) crosses Lake Pontchartrain in the United States. In total, the bridge is 38 kilometres long between the banks.
Less spectacularly, many of the bridges across motorways, railways and valleys are simply concrete beams – with a variety of pier designs that lend a little interest to the eye.
© structurae.de (Klaus Föhl)
The Golden Gate bridge, San Fransisco.
Nantua viaduct from the Autoroute A40, France.
Stone slab or ‘clapper’ bridge.
A simple rail beam bridge.
The 1906 King Edward rail bridge, Newcastle-upon-Tyne, UK.
Redheugh Bridge, Newcastle. Beam bridges rarely span more than 80 metres.