Multi-Beam Bridge Construction

A bridge is essentially a construction that is built to cover a road, valley, water body, or other natural obstacles to provide a route over the barrier. Several bridge designs are used that depend upon their function and the soil conditions of the site for bridge construction. A bridge is described normally by its form of construction, like beam, truss, arch, etc. A bridge may also be characterized by the construction materials used, like concrete, stone, and metal. A bridge may have different types of spans that include simple, cantilever, continuous etc.

Multi-beam bridges are a very common form of construction for roads and other bridges that are required to span across distances less than one hundred meters. The beams are connected by an horizontal element that allows transport of men, materials, and vehicles.

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 prestressed 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.

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.

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.

Beam bridges can be simple and made of wood beams. Heavy beams are carried by a bridge crane using a beam clamp to hold the beams. Bridge load rating is executed to establish the loads that can be safely carried by the beam bridges. It is essential to calculate the bending moments in a beam to establish a safe design of the beam bridges.

Types of beam bridge
Beam bridges basically consist of beam that is laid across the piers or supports. The beam should possess the strength to bear the loads that are expected to be placed on it. These loads are borne by the bridge piers. The loads cause the beam top edge to be compressed, while the lower edge is being stretched and is under tension.

Existing beam bridges are formed by girders, normally box girders, trusses or I-beams, that are supported on strong piers.
  1. Box girders are stretched, box shaped elements that are more suitable to bear the twisting loads.
  2. Trusses consist of one or more triangular units connected at joints or nodes.
  3. I-beams are economical and simple to fabricate. They are simply beams with an I-shaped or H-shaped cross-section. The horizontal elements of the "I" design are flanges and the vertical is the web of the construction.
Other beam bridges may be fabricated from concrete beams that are pre-stressed. These materials possess the steel characteristics to endure loads in tension, and concrete strength to bear the compression loads.

The beam bridge's strength is largely influenced by the distance between the piers. Therefore, the beam bridges are normally not suitable for longer length, unless several such bridges are connected with each other.

The beam bridge's span is dependent upon the beam weight and the materials strength. As the bridge material thickens, its capacity to hold the loads increases. Therefore, the span could also be increased. However, a sturdy beam may become too heavy, and sag. The beam bridges can be supported by the utilization of trusses.

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