Bridge, structure that spans horizontally between supports, whose function is to carry vertical loads. The prototypical bridge is quite simple—two supports holding up a beam—yet the engineering problems that must be overcome even in this simple form are inherent in every bridge: the supports must be strong enough to hold the structure up, and the span between supports must be strong enough to carry the loads. Spans are generally made as short as possible; long spans are justified where good foundations are limited—for example, over estuaries with deep water.

**Seto Great Bridge**The multiple-span Seto Great Bridge over the Inland Sea, linking Kojima, Honshu, with Sakaide, Shikoku, Japan.

The northern terminus of the Akashi Strait Bridge in Terumi ward, southern Kobe, Hyogo prefecture, west-central Japan. The bridge spans the Akashi Strait and links Awaji Island to Honshu.

All major bridges are built with the public’s money. Therefore, bridge design that best serves the public interest has a threefold goal: to be as efficient, as economical, and as elegant as is safely possible. Efficiency is a scientific principle that puts a value on reducing materials while increasing performance. Economy is a social principle that puts value on reducing the costs of construction and maintenance while retaining efficiency. Finally, elegance is a symbolic or visual principle that puts value on the personal expression of the designer without compromising performance or economy. There is little disagreement over what constitutes efficiency and economy, but the definition of elegance has always been controversial.

Modern designers have written about elegance or aesthetics since the early 19th century, beginning with the Scottish engineer Thomas Telford. Bridges ultimately belong to the general public, which is the final arbiter of this issue, but in general there are three positions taken by professionals. The first principle holds that the structure of a bridge is the province of the engineer and that beauty is fully achieved only by the addition of architecture. The second idea, arguing from the standpoint of pure engineering, insists that bridges making the most efficient possible use of materials are by definition beautiful. The third case holds that architecture is not needed but that engineers must think about how to make the structure beautiful. This last principle recognizes the fact that engineers have many possible choices of roughly equal efficiency and economy and can therefore express their own aesthetic ideas without adding significantly to materials or cost.

Thomas Telford's suspension bridge across the River Conwy, leading to Conwy Castle, Conwy county borough, Wales. — Thomas Telford’s suspension bridge across the River Conwy, leading to Conwy Castle, Conwy county borough, Wales.

Generally speaking, bridges can be divided into two categories: standard overpass bridges or unique-design bridges over rivers, chasms, or estuaries. This article describes features common to both types, but it concentrates on the unique bridges because of their greater technical, economic, and aesthetic interest.

The elements of bridge design

Basic forms

There are six basic bridge forms: the beam, the truss, the arch, the suspension, the cantilever, and the cable-stay.

Beam

The beam bridge is the most common bridge form. A beam carries vertical loads by bending. As the beam bridge bends, it undergoes horizontal compression on the top. At the same time, the bottom of the beam is subjected to horizontal tension. The supports carry the loads from the beam by compression vertically to the foundations.

When a bridge is made up of beams spanning between only two supports, it is called a simply supported beam bridge. If two or more beams are joined rigidly together over supports, the bridge becomes continuous.

Arch bridges

Arches are normally fabricated on-site. After the building of abutments (and piers, if the bridge is multiarch), a falsework is constructed. For a concrete arch, metal or wooden falsework and forms hold the cast concrete and are later removed. For steel arches, a cantilevering method is standard. Each side of an arch is built out toward the other, supported by temporary cables above or by falsework below until the ends meet. At this point the arch becomes self-supporting, and the cables or falsework are removed.

Suspension bridges

When bridges requiring piers are built over a body of water, foundations are made by sinking caissons into the riverbed and filling them with concrete. Caissons are large boxes or cylinders that have been made from wood, metal, or concrete. In the case of suspension bridges, towers are built atop the caissons. The first suspension-bridge towers were stone, but now they are either steel or concrete. Next, the anchorages are built on both ends, usually of reinforced concrete with embedded steel eyebars to which the cables will be fastened. An eyebar is a length of metal with a hole (or “eye”) at the ends. Cables for the first suspension bridges were made of linked wrought-iron eyebars; now, however, cables are generally made of thousands of steel wires spun together at the construction site. Spinning is done by rope pulleys that carry each wire across the top of the towers to the opposite anchorage and back. The wires are then bundled and covered to prevent corrosion. When the cables are complete, suspenders are hung, and finally the deck is erected—usually by floating deck sections out on ships, hoisting them with cranes, and securing them to the suspenders.

Cantilever

A beam is said to be cantilevered when it projects outward, supported only at one end. A cantilever bridge is generally made with three spans, of which the outer spans are both anchored down at the shore and cantilever out over the channel to be crossed. The central span rests on the cantilevered arms extending from the outer spans; it carries vertical loads like a simply supported beam or a truss—that is, by tension forces in the lower chords and compression in the upper chords. The cantilevers carry their loads by tension in the upper chords and compression in the lower ones. Inner towers carry those forces by compression to the foundation, and outer towers carry the forces by tension to the far foundations.

Cable-stay

Cable-stayed bridges carry the vertical main-span loads by nearly straight diagonal cables in tension. The towers transfer the cable forces to the foundations through vertical compression. The tensile forces in the cables also put the deck into horizontal compression.

Materials

The four primary materials used for bridges have been wood, stone, iron, and concrete. Of these, iron has had the greatest effect on modern bridges. From iron, steel is made, and steel is used to make reinforced and prestressed concrete. Modern bridges are almost exclusively built with steel, reinforced concrete, and prestressed concrete.

Wood and stone

Wood is relatively weak in both compression and tension, but it has almost always been widely available and inexpensive. Wood has been used effectively for small bridges that carry light loads, such as footbridges. Engineers now incorporate laminated wooden beams and arches into some modern bridges.

Stone is strong in compression but weak in tension. Its primary application has been in arches, piers, and abutments.

Iron and steel

The first iron used during the Industrial Revolution was cast iron, which is strong in compression but weak in tension. Wrought iron, on the other hand, is as strong in compression as cast iron, but it also has much greater tensile strength. Steel is an even further refinement of iron and is yet stronger, superior to any iron in both tension and compression. Steel can be made to varying strengths, some alloys being five times stronger than others. The engineer refers to these as high-strength steels.

Concrete

Concrete is an artificial stone made from a mixture of water, sand, gravel, and a binder such as cement. Like stone, it is strong in compression and weak in tension. Concrete with steel bars embedded in it is called reinforced concrete. Reinforcement allows for less concrete to be used because the steel carries all the tension; also, the concrete protects the steel from corrosion and fire.

Prestressed concrete is an important variation of reinforced concrete. A typical process, called post-tensioned prestressing, involves casting concrete beams with longitudinal holes for steel tendons—cables or bars—like reinforced concrete, but the holes for the tendons are curved upward from end to end, and the tendons, once fitted inside, are stretched and then anchored at the ends. The tendons, now under high tension, pull the two anchored ends together, putting the beam into compression. In addition, the curved tendons exert an upward force, and the designer can make this upward force counteract much of the downward load expected to be carried by the beam. Prestressed concrete reduces the amount of steel and concrete needed in a structure, leading to lighter designs that are often less expensive than designs of reinforced concrete.

Construction

Beam bridges

All bridges need to be secure at the foundations and abutments. In the case of a typical overpass beam bridge with one support in the middle, construction begins with the casting of concrete footings for the pier and abutments. Where the soil is especially weak, wooden or steel piles are driven to support the footings. After the concrete piers and abutments have hardened sufficiently, the erection of a concrete or steel superstructure begins. Steel beams are generally made in a factory, shipped to the site, and set in place by cranes. For short spans, steel beams are usually formed as a single unit. At the site, they are placed parallel to each other, with temporary forms between them so that a concrete deck can be cast on top. The beams usually have metal pieces welded on their top flanges, around which the concrete is poured. These pieces provide a connection between beam and slab, thus producing a composite structure.

For longer spans, steel beams are made in the form of plate girders. A plate girder is an I beam consisting of separate top and bottom flanges welded or bolted to a vertical web. While beams for short spans are usually of a constant depth, beams for longer spans are often haunched—that is, deeper at the supports and shallower at mid-span. Haunching stiffens the beam at the supports, thereby reducing bending at mid-span.

Arch bridges

Suspension bridges

Cantilever bridges

Like suspension bridges, steel cantilever bridges generally carry heavy loads over water, so their construction begins with the sinking of caissons and the erection of towers and anchorages. For steel cantilever bridges, the steel frame is built out from the towers toward the centre and the abutments. When a shorter central span is required, it is usually floated out and raised into place. The deck is added last.

**Forth Railway Bridge**

Construction of the Forth Railway Bridge, designed by Sir John Fowler and Sir Benjamin Baker on the cantilever principle, crossing the Firth of Forth in Scotland, c. 1860s.

The cantilever method for erecting prestressed concrete bridges consists of building a concrete cantilever in short segments, prestressing each succeeding segment onto the earlier ones. Each new segment is supported by the previous segment while it is being cast, thus avoiding the need for falsework.

Cable-stayed bridges

Construction of cable-stayed bridges usually follows the cantilever method. After the tower is built, one cable and a section of the deck are constructed in each direction. Each section of the deck is prestressed before continuing. The process is repeated until the deck sections meet in the middle, where they are connected. The ends are anchored at the abutments.

**cable-stayed bridge construction**

Five steps in the cantilever-method construction of a cable-stayed bridge.

Performance in service

**See the testing of new bridge design which is capable of resisting earthquakes**

Testing new bridge designs capable of resisting significant earthquake damage.

Displayed by permission of The Regents of the University of California. All rights reserved. (A Britannica Publishing Partner)See all videos for this article

Bridges are designed, first, to carry their own permanent weight, or dead load; second, to carry traffic, or live loads; and, finally, to resist natural forces such as winds or earthquakes.

The primary function of a bridge is to carry traffic loads: heavy trucks, cars, and trains. Engineers must estimate the traffic loading. On short spans, it is possible that the maximum conceivable load will be achieved—that is to say, on spans of less than 30 metres (100 feet), four heavy trucks may cross at the same time, two in each direction. On longer spans of a thousand metres or more, the maximum conceivable load is such a remote possibility (imagine the Golden Gate Bridge with only heavy trucks crossing bumper-to-bumper in each direction at the same time) that the cost of designing for it is unreasonable. Therefore, engineers use probable loads as a basis for design.

In order to carry traffic, the structure must have some weight, and on short spans this dead load weight is usually less than the live loads. On longer spans, however, the dead load is greater than live loads, and, as spans get longer, it becomes more important to design forms that minimize dead load. In general, shorter spans are built with beams, hollow boxes, trusses, arches, and continuous versions of the same, while longer spans use cantilever, cable-stay, and suspension forms. As spans get longer, questions of shape, materials, and form become increasingly important. New forms have evolved to provide longer spans with more strength from less material.

Forces of nature

Dead and live weight are essentially vertical loads, whereas forces from nature may be either vertical or horizontal. Wind causes two important loads, one called static and the other dynamic. Static wind load is the horizontal pressure that tries to push a bridge sideways. Dynamic wind load gives rise to vertical motion, creating oscillations in any direction. Like the breaking of an overused violin string, oscillations are vibrations that can cause a bridge to fail. If a deck is thin and not properly shaped and supported, it may experience dangerous vertical or torsional (twisting) movements.

The expansion and contraction of bridge materials by heat and cold have been minimized by the use of expansion joints in the deck along with bearings at the abutments and at the tops of piers. Bearings allow the bridge to react to varying temperatures without causing detrimental stress to the material. In arches, engineers sometimes design hinges to reduce stresses caused by temperature movement.

Modern bridges must also withstand natural disasters such as tropical cyclones and earthquakes. In general, earthquakes are best withstood by structures that carry as light a dead weight as possible, because the horizontal forces that arise from ground accelerations are proportional to the weight of the structure. (This phenomenon is explained by the fundamental Newtonian law of force equals mass times acceleration.) For cyclones, it is generally best that the bridge be aerodynamically designed to have little solid material facing the winds, so that they may pass through or around the bridge without setting up dangerous oscillations.

Source: britannica

Total Views: 211 ,

3 thoughts on “Bridge Design, Types And Parts”

Engr Oyejide Akinboro says:

at 9:02 am

Nice

0

0

sende isaac terkimbir says:

at 9:04 pm

Good tutoring

0

0

Secret Aesthetics says:

at 11:29 am

Botox (Botulinum toxin) type A is a purified neurotoxin derived from the bacterium Clostridium botulinum. It works by blocking the impulses to muscles thereby restricting their movement. Clinical grade cosmetic Botox ® is a prescription only medication extracted and purified from the original paralytic agent. Once injected, it temporarily weakens the contraction of the targeted muscle, resulting in the softening and relaxing of the lines and wrinkles caused by the underlying muscle, creating a dramatically smoother, younger-looking appearance. Although there is no reversal agent to date, it is naturally broken down by the body over months, allowing movement to gradually return. The idea is that by reducing the movement of muscles in these overactive areas, the overlying skin is given time to rest and recover, allowing the skin creases and wrinkles to soften and eventually fade away. Depending on each individual’s wishes, the dose and treatment regime can be adjusted to allow for a more natural softening of expressions and lines to a complete smoothening of skin texture. For the first few years, more frequent treatments are required to ‘re-educate’ the muscles.

0

0