Bridges

There are six main types of bridges: beam bridges, cantilever bridges, arch bridges, suspension bridges, cable-stayed bridges and truss bridges.

A beam bridge is a rigid, horizontal structure that rests on two end supports, and carries traffic loads by acting structurally as a beam. It is a direct descendant of the log bridge, now more normally made from shallow steel 'I' beams, box girders, reinforced concrete, or post-tensioned concrete. It is frequently used in pedestrian bridges and for highway overpasses and flyovers. As is its ancestor, this bridge is in structural terms the simplest of the many bridge types.

A cantilever bridge is a bridge built using cantilevers: structures that project horizontally into space, supported on only one end. For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, or box girders built from prestressed concrete. The steel truss cantilever bridge was a major engineering breakthrough when first put into practice, as it can span distances of over 1500 feet, and can be more easily constructed at difficult crossings by virtue of using little or no falsework.

An arch bridge is a bridge with abutments at each end shaped as a curved arch. Arch bridges work by transferring the weight of the bridge and its loads partially into a horizontal thrust restrained by the abutments at either side. A viaduct (a long bridge) may be made from a series of arches, although other more economical structures are typically used today.

A suspension bridge is a type of bridge that has been created since ancient times as early as 100 AD. Simple suspension bridges, for use by pedestrians and livestock, are still constructed, based upon the ancient Inca rope bridge. One of the oldest of engineering forms, suspension bridges were constructed by primitive peoples using vines for cables and mounting the roadway directly on the cables.

A cable-stayed bridge is a bridge that consists of one or more columns (normally referred to as towers or pylons), with cables supporting the bridge deck. There are two major classes of cable-stayed bridges, differentiated by how the cables are connected to the tower(s). In a harp design, the cables are made nearly parallel by attaching cables to various points on the tower so that the height of attachment of each on the tower is similar to the distance from the tower along the roadway to its lower attachment. In a fan design, the cables all connect to or pass over the top of the tower. The cable-stay design is the optimum bridge for a span length between that of cantilever bridges and suspension bridges. Within this range of span lengths a suspension bridge would require a great deal more cable, while a full cantilever bridge would require considerably more material and be substantially heavier.

A truss bridge is a bridge composed of connected elements (typically straight) which may be stressed from tension, compression, or sometimes both in response to dynamic loads. Truss bridges are one of the oldest types of modern bridges. This type of bridge structure has a fairly simple design and is particularly cheap to construct owing to its efficient use of materials. For purposes of analysis most truss bridges may be considered to be pin jointed where the straight components meet. A more complex analysis may be required where rigid joints impose significant bending loads upon the elements.

At 6:01:38 pm, CDT on August 1, 2007, with rush hour bridge traffic moving slowly through the limited number of lanes, the southern end of the span suddenly gave way and the middle and north parts of the span immediately collapsed into the river and onto the riverbanks below, toppling 50 feet eastward in the process. Within the next ten seconds, the northern end of the span followed but fell straight down, sending a total of more than 50 vehicles, their occupants and several construction workers into the river or onto its banks. Northern sections fell onto three unoccupied freight train cars sitting in the trainyard in that sector.
Sequential images of the collapse were taken by an outdoor security camera located at the parking lot entrance of the control facility for the Lower Saint Anthony Falls Lock and Dam. The sequence shows that the collapse started at the southern end of the span.

For more information and histoty on the I-35 Mississippi River bridge Click Here!

More information:I-35W Bridge Close Up!

A truss arch bridge combines elements of the truss bridge and the arch bridge. The actual resolution of forces will depend upon the design. If no horizontal thrusting forces are generated this becomes an arch shaped truss, essentially a bent beam. If horizontal thrust is generated but the apex of the arch is a pin joint, this is termed a three-hinged arch. If no hinge exists at the apex, it will normally be a two-hinged arch. In the Iron Bridge, the structure of each frame emulates the kind of structure that previously had been made of wood. Such a wood structure uses closely fitted beams pinned together, so the members within the frames are not free to move relative to one another, as they are in a pin-jointed truss structure that allows rotation at the pin joint. Such rigid structures (which impose bending stresses upon the elements) were further developed in the 20th century as the Vierendeel truss.

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Much of the information used here has been researched from Wikipedia, the free encyclopedia.

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