COMPREHENSION CHECK

CANTILEVER BRIDGES

Cantilever bridgesconsist of two independent beams called cantilevers thatextend from opposite banks of a waterway. The two cantilevers are joined together above the middle of the waterway by a beam, girder, or truss. Cantilever bridges may have spans as long as about 1,800 feet (549 meters).

Each cantilever has two sections, an anchor arm and a cantilever arm. The anchor arm extends between an abutment and a pier. One end of the cantilever arm is supported by the pier, and the other end extends freely over the waterway. The free ends of the two cantilevers are joined together by a suspended span.

Most cantilever bridges have two anchor spans and one center span. Each anchor span consists of an anchor arm. The suspended span and the two cantilever arms make up the center span.

Many cantilever bridges have truss frameworks. Most bridges of the cantilever type are made of steel or prestressed concrete.

Suspension bridges

Suspension bridgesare perhaps the most impressive type of bridge because of their long main span and especially attractive appearance. These bridges have a roadway that hangs from steel cables that are supported by two high towers. Suspension bridges are used to span great distances. Most suspension bridges have a main span more than 1,000 feet (300 meters) long. Some have a main span longer than 4,000 feet (1,200 meters). Suspension bridges also are used to cross deep water or steep canyons, and in other places where the construction of piers is especially difficult and expensive. These bridges require only two piers, each of which supports a tower.

The main span of a suspension bridge stretches between the two towers. Each of two side spans extends between a tower and an anchorage. Most anchorages are huge blocks of concrete set at the ends of the bridge.

The cables that are supported by the towers are called the main cables. A suspension bridge has at least two main cables. Each of these cables extends from one end of the bridge to the other and is secured at each end by an anchorage. The main cables are connected to the top end of vertical suspender cables. The bottom end of each suspender cable attaches to the roadway of the bridge.

A suspension bridge may sway in a strong wind. To minimize such movement, most suspension bridges have a thick structure that supports the roadway. This type of structure helps stiffen the bridge and is called a stiffening girder or stiffening truss.

CABLE-STAYED BRIDGES

Moveable bridges

Moveable bridges have a roadway that is moved entirely or partially to provide enough clearance for large ships to pass. There are three types of moveable bridges, bascule bridges, vertical lift bridges, and swing bridges. A bascule bridge tilts upward to open. Some bascule bridges open at one end, and others the middle. A vertical lift bridge has a roadway that extends between two towers. The roadway rises between the towers, and ships pass underneath. A swing bridge is mounted on a central pier. The bridge swings sideways to enable ships to pass.

Bascule bridge

Vertical lift bridge

VIADUCT. AQUEDUCT.

Viaductis like a bridge, except that it crosses over dry land instead of water. Some viaducts do cross water, but they also cross dry land instead of merely extending from one bank to the other as a bridge does.

Most viaducts consist of a series of supports under beam-and-slab or arch construction. Viaducts carry railroad tracks over valleys and gorges. Some viaducts are built higher than the general level of the land to carry railroads over highways or to make a safe crossing for highways over railroads. The ancient Romans built the first viaducts. The aqueducts they built to carry water to cities often also served as roadways. One of the longest viaducts ever built was a portion of the 110-mile (177-kilometer) Key West extension of the Florida East Coast Railway. Viaducts that extended over the open sea formed 30 miles (48 kilometers) of this extension. Parts were destroyed by a hurricane in 1935, but these were rebuilt as a highway a few years later. The main part of the pier viaduct over the mouth of the River Tay in Scotland has 84 steel spans and is over 2 miles (3.2 kilometers) long. The Tunkhannock viaduct on the Lackawanna Railway is one of the largest steel and concrete viaducts in the world. The Tunkhannock viaduct is 2,375 feet (724 meters) long and includes 10 spans of 180 feet (55 meters) each. Another famous viaduct is the 3-mile (5.6-kilometer) Pulaski Skyway between Newark and Jersey City. Other well-known viaducts are the Pecos River viaduct in Texas and the Landwasser viaduct across Albula Pass, in the canton of Craubunden, Switzerland.

Historically, many agricultural societies have constructed aqueducts to irrigate crops. Archimedes invented the water screw to raise water for use in irrigation of croplands, Another widespread use for aqueducts is to supply large cities with clean drinking water Some of the famed Roman aqueducts still supply water to Rome today. In California, USA, three large aqueducts supply water over hundreds of miles to the Los Angeles area. Two are from the Owens River area and a third is from the Colorado River. In more recent times, aqueducts were used for transportation purposes to allow canal barges to cross ravines or valleys. During the Industrial Revolution of the 18th century, many aqueducts were constructed as pan of the general boom in canal-building. In modern civil engineering projects, detailed study and analysis of open channel how is commonly required to support flood control, irrigation systems, and large water supply systems when an aqueduct rather than a pipeline is the preferred solution. The aqueduct is a simple way to get water to other ends of a field. In the past, aqueducts often had channels made of dirt or other porous materials. Significant amounts of water arc lost through such unlined aqueducts. As water gets increasingly scarce, these canals are being lined with concrete, polymers or impermeable soil. In some cases, a new aqueduct is built alongside the old one because it cannot be shut down during construction. The Central Arizona Project Aqueduct, the largest and most expensive aqueduct system ever constructed in the United States.