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Bridges Require Beams to Span the Gaps between Piers
Bridges are essentially meant to cross gaps caused by sudden changes in elevation that come in the way of a road, rail, or pipeline alignment because of rivers, streams, road and railway crossings, or valleys. The main structural components of a bridge are the piers or foundations and the structure that spans between the piers. This structure can be a supporting arrangement of beams or trusses and a deck which will allow the vehicles or materials to be transported over the gap being bridged.
Beam bridges are mainly used for smaller spans, and their spans will depend on the materials being used for the bridge construction. A number of beams may be used over adjacent piers and the top of the beams will be connected by the deck element. Such construction is referred to as multi-beam bridge construction. Materials used for the multi-beams may be steel, reinforced concrete, or pre-stressed concrete. Quite often the depth of the beam becomes a limiting factor, especially in cases where headroom for areas below the bridge becomes a limiting factor. For larger spans the use of trusses or cables becomes more economical. Such truss or cable stayed bridges are also more aesthetically pleasing.
Image Source : Wikimedia: Multi-beamed bridge
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Requirements of Multi Beam Bridges
Multi-beam bridges have a number of beams covering the gaps between the piers of a bridge. The beams are required to be able to take the loads that are expected to be taken over the bridge. Such loads are dispersed over the various beams through the slab or deck element and can consist of the static loads of the vehicles and or materials, plus the dynamic loads caused by the moving loads. Beams of bridges also need to be designed for wind forces and vibrations that can be caused by the moving loads and wind. Vibrations are known to be very potent forces and have been known to have caused bridge collapses due to the harmonics set up by vibrations. All the beams across a single span may be connected laterally with proper structural elements, and these can help to reduce vibration and lateral forces and add to the structural stability of the entire structure.
Beams of bridges are made of I beams made of steel or concrete or even box girders made of the same materials. Box girders are said to be more resistant to twisting and therefore can resist vibrations and the effect of moving loads far more easily. When a load is transmitted from the deck to the beam, the top layers of the beam get compressed, but the lower edges of the beam are under tension. This tension is not suitable for concrete unless it has been properly reinforced. Pre-stressing is used for bridge beams and enables the top layer of the beam to be under tension and the bottom layer to be under compression. So when the loads are taken by the beams when placed in position, they find it easier to resist the forces imposed on them.
Image Source: Wikimedia : Bridge
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Construction of Multi Beam Bridges
Bridge building techniques have evolved vastly to allow the prefabrication of beams, whether of steel or concrete, in controlled conditions like a workshop or yard and then transporting the beams to the site by various methods. Beams can be launched from the piers or in case of river or sea crossings can be floated to site and raised onto the piers. Engineers have been quite often known to have used tidal forces to lift large spans on to piers without the use of cranes. It is also quite normal for very large spans being made up of short segments that are then joined together at site and techniques of pre stressing used to hold the segments together.
Stress and strain gauges are nowadays made part of beam and multi-beam bridge structures, and the information from these gauges is transmitted electronically to a monitoring site not necessarily at the bridge location. This allows engineers to monitor the health of bridges so that corrective action can be taken whenever needed.
Image Source: Wikimedia: Autostrada