Bridges

When people in early days moved from one place to another, they had to cross forests, plains and mountains. When it comes to small or big rivers they used to wade through or swim. In mountains there are deep gorges which were difficult to negotiate. Human mind found ways by using wood, ropes to make bridges over the gap, over which they can cross.

A wooden log spanning a gap is probably the first bridge and then came ropeways by tying to two trees on the banks. As an advance rudimentary engineering two wooden trusses with planks in between was a much better choice to move men and material. Ropeway also went through the same process by two of them with planks in between.

When quarrying of stone came into existence, a slab of stone was used as a bridge. This is pre history. By experimenting it was found that an arch can transfer much better load, and the arch bridges were made out of precut stone blocks jutting against each other, which are still in existence in Europe and India. When lime came into the field it was used as a cementing material between stone blocks. The same was used for brick arches.

Cement replaced lime and concrete a mixture of broken stone, sand and cement was found to be a convenient alternative for foundations. When steel rods are used by pouring concrete around it, otherwise known as rcc, it was possible to make beams of large spans in constructing bridges, whereas piers were of stone brick or concrete. Later foundations and piers were also done by rcc.

For small bridges open foundation with concrete or rcc was adopted. In large bridges across big rivers, where scour is a consideration, wells of brick on a wooden curb were adopted in railway and some roads in late 19^th century onwards. This is still in vogue with steel curbs and rcc staining. The annular space is dug out and a portion of well sinks further casting is done, till it reaches the founding depth. This varies from 30 ft to 150ft depending on the soil strata and loading. In some cases piling of rcc, pre cast or cast in situ are found to be economical. Steel pipes also can be used as piles.

Piers are either of brick or stone masonry, cement concrete or rcc and sometimes prestressed concrete. The superstructure is be rcc beam with slab at top, rcc double cantilever arch, prestressed beam, with slab. Most of the old road bridges and present railway bridges are by steel trusses, with steel deck. Then there are suspension bridges, cable stayed bridges, cantilever steel bridges. There are road cum rail bridges also.

The design and method of construction of each one is different, based on the length of the bridge, soil type for foundation, traffic load, dead and moving load. Consideration of wind load and earthquake is also mandatory.