Bridges

a. Types of bridges.

According to the webpage http://www.brantacan.co.uk/bridgedefs.htm, the main types of bridges are arches, beam bridges, cable-stayed bridges, cantilever bridges and suspension bridges. Encarta says that there are There are basically eight common bridge designs: beam, cantilever, arch, truss, suspension, cable-stayed, movable, and floating bridges. Combination bridges may incorporate two or more of these designs into a bridge. Beam bridges typically span the shortest distances, while suspension and cable-stayed bridges span the greatest distances.
Movable bridges are the ones that have a portion which moves up or swings out to provide additional clearance beneath the bridge. They are usually found over heavily traveled waterways and there are three types of movable bridges: the bascule (drawbridge), vertical-lift, and swing bridges. "Modern bascule bridges usually have two movable spans that rise upward, opening in the middle. A vertical-lift bridge consists of a rigid deck frame held between two tall towers. The bridge opens by hoisting the entire bridge roadway upward between the towers in an elevator-like fashion. Swing bridges are mounted on a central pier and open by swinging to one side, allowing ships to pass." (encarta)
Encarta says that bascule bridges are used for short spans, vertical-lift bridges are useful for longer spans (but they must be built so they can be lifted high enough for tall ships to pass underneath) and swing bridges have the advantage of not limiting the height of passing vessels (but they restrict the horizontal clearance, or width, of passing ships).
I found this picture of a floating bridge in encarta.com. This is the description of the picture: "The simple pontoon floating bridge over the Kabul River, Pakistan, is supported by flat-bottomed boats rather than fixed piers. Pontoon bridges may also be supported by other types of floats or metal cylinders."
The bridge on the Chesapeake Bay that crosses between Norfolk and Cape Charles disappears into the water midway. It is a combination structure, the Chesapeake Bay Bridge-Tunnel combines two bridges with two tunnels that pass under major shipping channels."

This is picture is of the construction of the secound longest suspenion bridge in the world in 1998. It connects the Danish islands of Fyn and Sjælland.

"The concrete arch of the Paul Sauer Bridge spans the Storms River in South Africa, on the road between Cape Town and Port Elizabeth. An arch bridge transfers the weight of the bridge down along the arch to the abutments where, in this example, the arch meets the canyon walls." This bridge reminds me of the "acueducto de la Guaira" in Venezuela.

"The city of Seville, Spain, built six new bridges in preparation for Expo '92. Spanish architect and engineer Santiago Calatrava designed the Alamillo Bridge (1992), shown here. Lightweight and sculptural in appearance it calls to mind a harp or bird, according to Calatrava. The bridge is 200 m (656 ft) long and the single pylon that supports the roadway rises 140 m (459 ft) above road level."

b. The technology involved in bridge construction.

"Engineers must consider several factors when designing a bridge. They consider the distance to be crossed and the feature, such as a river, bay, or canyon, to be crossed. Engineers must anticipate the type of traffic and the amount of load the bridge will have to carry and the minimum span and height required for traffic traveling across and under the bridge. Temperature, environmental conditions, and the physical nature of the building site (such as the geometry of the approaches, the strength of the ground, and the depth to firm bedrock) also determine the best bridge design for a particular situation.Once engineers have the data they need in order to design a bridge, they create a work plan for constructing it. Factors to be considered include availability of materials, equipment, and trained labor; availability of workshop facilities; and local transportation to the site. These factors, in combination with the funding and time available for bridge design and construction, are the major requirements and constraints on design decisions for a particular site."
"There are four basic categories of design decisions: the type of bridge, the materials of which it will be made, the type of foundations that will support the structure, and the construction method to be used. Typically, several feasible choices exist in each category, and each option is evaluated in terms of convenience, appearance, endurance, and cost. Bridges must be convenient to build, use, and maintain. Appearance is important in gaining public approval, which is particularly critical for taxpayer-funded projects. Bridges must be designed to endure, as most structures can be expected to provide service for at least 50 to 100 years. Durability of materials and maintenance requirements are important considerations, as the true cost of a bridge is not simply the initial construction expense but the total cost of constructing and maintaining the structure throughout its service life. Good designs minimize total cost." (encarta)

c. My opinion about bridges.

They have alot of esthetic potencial that should be taken advatege by eingeniers. I think that bridges can be very beautiful like many of Calatrava's bridges. These bridges become works of art and important landmarks.