A look at the different types of bridges

Staff Writer
Mount Shasta Herald

The Interstate 35 bridge that collapsed in Minneapolis was a truss bridge, one of six main bridge types. Here's a quick look at the different types:


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.


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 1,500 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. Suspended from two high locations over a river or canyon, simple suspension bridges follow a shallow downward arc and are not suited for modern roads and railroads. Advances in materials and design led to the development of the suspended-deck suspension bridge, a modern bridge capable of carrying vehicles and light rail. Instead of the deck following the downward arc of the main load-bearing cables (or chains), these cables are suspended between towers, and vertical suspender cables carry the weight of the deck below, upon which traffic crosses. This arrangement allows the deck to be level or to arc slightly upward for additional clearance.


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.

Source: Wikipedia.org