Geometry of Bridge Construction
The four kinds of bridges and some combinations
A. The beam or truss bridge is, in effect, a pair of girders supporting a deck spanning the gap between two piers. Such a beam has to withstand both compression in its upper parts and tension in its lower parts. Where it passes over supports, other forces come into play. A beam may be a hollow box girder or an open frame or truss.
B. An arch bridge can be designed so that no part of it has to withstand tension. Concrete is well suited to arched bridge design. When reinforced concrete is used, a more elegant and sometimes less costly arch can be designed and most concrete arch bridges are reinforced.
C. A suspension bridge consists, basically, of a deck suspended from cables slung between high towers. The cables of high tensile steel wire can support an immense weight. The towers are in compression and the deck, often consisting of a long slender truss (used as a hollow beam), is supported at frequent intervals along its length.
D. A cantilever bridge is generally carried by two beams, each supported at one end. Unlike a simple beam supported at both ends, the cantilever must resist tension in its upper half and compression in its lower. A fifth type arrived on the scene in 1952 the first modern cable-stayed bridges were built in Germany and Sweden. There are also many other composite forms of bridges. The bridle-chord bridge is a combination of a long beam (usually a trussed girder) partially supported by steel wires from a tower at one end, or from towers at each end. Most cantilever bridges are designed so that a gap remains between two cantilevered arms that reach out from their abutments: the gap is bridged by a simple beam.
Some Examples
History Of Bridges
Bridge study has revealed that people have been carrying out bridge construction since humans first assembled into groups. The initial bridge design was basically felled trees that were utilized for moving over the ditches and rivers, and concrete bridges were rare. With the advance of civilization, techniques were discovered to use rocks, stones, mortar, and other materials for the creation of stronger and extended bridges. Subsequently, as the engineers and physicists advanced in the design, materials, and construction technology, modern materials like steel and aluminum were introduced for bridges.
Bridge Construction during 20th Century
The bridge construction skills progressed rapidly during the 20th century. At the end of the century, new techniques were developed that improved the design, strength, and durability of the bridges. Steel bridges were strongly riveted instead of the previous practice of using bolts. Concrete bridges were being cast at the desired place, instead of being precast. Huge bridge elements made from bars and small sections were used, and not rolled as one part. Before the 1980s, the majority of bridge designs included expansion joints for decks, including expansion and fixed support bearings. This technique was used to permit structural expansion and contraction. However, the expansion joints are likely to be filled with debris, and bearings often weaken over time. Thus, the structure is hardened, and maintenance requirements are increased. The bridge engineers explored methods to reduce this trouble, and finally the expansion joints and bearings were eliminated to develop a joint-less bridge. This type of bridge is constructed on a flexible foundation that may expand or contract with negligible trouble.
Modern Bridge Co 
nstruction Techniques
New technologies are expected to meet the challenging and varying requirements, and also offer options that will guide to innovative engineering and bridge construction standards. With the beginning of the new century, bridge construction is being revolutionized. Modern construction methods and the latest advanced materials are being evolved. Construction technologies like post tensioning, reinforced ground walls, and soil freezing are being developed. Modern surveying techniques are being used that have facilitated the soil selection, and other design parameters, through the use of optical and infrared technology. Progress in the deck technology is creating lighter and stronger decks. Bearings, joints, and seismic elements have become more effective since advanced testing facilities have been introduced. Consistent, economical, fast, and programmed inspection systems will emerge.