Types of Bridges. A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle. Designs of bridges vary depending on the function of the bridge, the nature of the terrain where the bridge is constructed, the material used to make it and the funds available to build it.
Types of Bridges
Generally there are several types of bridges based on their construction method:-beam bridges, cantilever bridges, arch bridges, suspension bridges
Beam bridges
They are the simplest in structural forms. They are supported by an abutment at each end of the deck. Since no moments are transferred through the support their structural type is known as simply supported. Bridges designed for modern infrastructure will usually be constructed of steel or reinforced concrete, or a combination of both. The concrete used can either be reinforced, prestressed or post-tensioned.
The simplicity of the beam bridge made it the first type of bridge ever built. It’s still the cheapest to build. All you need is a crossbeam covering the span, supported by an abutment at each end. One type of beam bridge is a girder bridge, which employs steel girders as reinforcement.
Gravity is a bigger challenge when constructing a bridge because, unlike a building, most of what’s underneath is empty space. A beam bridge might be supported only by two abutments, one at either end, to counter gravity and bear the entirety of its load.
But here’s the danger of beam bridges: The longer a bridge is and the more people, cars, and other things it carries, the heavier its total load. And the farther apart a beam bridge’s abutments are, the less stable the structure is.
By adding supports in the middle, known as piers or stanchions, and connecting sections between them, you can create a very long, stable bridge. Examples are the 3.2-mile Yolo Causeway near Sacramento, California, or the 24-mile Lake Pontchartrain Causeway in Louisiana.
In beam bridges, the force of compression pushes the load inward onto piers at the middle of the bridge. Simultaneously, the pulling or stretching force of tension pulls the load outward toward the abutments at both ends of the bridge.
Constructions could have various beams side by side (with a deck across the top of them), to a main beam either side supporting a deck between them. The main beams could be I-beams, trusses, or box girders. They could be half-through, or braced across the top to create a through bridge. Beam bridges are not limited to a single span. Some viaducts such as the interchanges under construction in Nairobi – Thika highway and Eastern by pass at North Airport road and City Cabanas have multiple simply supported spans supported by piers.
Beam bridges are often only used for really short distances because, unlike truss bridges, they have no built in supports. The only supports are provided by piers. Some of the major bridges in the country under this category include: Nyali and Mtwapa bridges both in Mombasa city. The construction of the two bridges started in 1978 and completed in 1980 and was done by Sumitomo Construction Company under the supervision of H.P. Gauff.
Cantilever bridges
They are built using cantilevers—horizontal beams supported on only one end. Most cantilever bridges use a pair of continuous spans that extend from opposite sides of the supporting piers to meet at the center of the obstacle the bridge crosses.
Some bridges are built using cantilever construction. This type uses a pillar anchored vertically into the ground to support a horizontal deck extending out from one or both sides across the span. The load often is supported from both above and below. A diving board or platform is a good example of cantilever construction.
The world’s longest cantilever span belongs to the Quebec Bridge in Canada, which was built in 1919 and extends 1,800 feet. It surpassed the length of the Forth Bridge in Scotland, which was completed in 1890.
In the U.S., the central section of Conde B. McCullough Memorial Bridge over Coos Bay in Oregon is a cantilever. So is the eastern section of the San Francisco-Oakland Bay Bridge (not to be confused with the Golden Gate). Many cantilever bridges are combined with other types to cross a single span.
Cantilever bridges are often supported with trusses. A bridge truss takes the load off the deck and transfers it to the supporting piers and abutments, helping the cantilevers withstand tension in the upper supports and compression in the lower ones.
Cantilever bridges are constructed using much the same materials & techniques as beam bridges. The difference comes in the action of the forces through the bridge. The largest cantilever bridge is the 549-metre Quebec Bridge in Quebec, Canada. In the country, the nearest cantilever bridge from Nairobi is across River Athi along Athi River – Namanga road.
Arch bridges
Arch bridges are those that use arches as the main structural component. Basic arch bridges are differentiated by the number of hinges used to allow the bridge to accommodate different loads and stresses. Some arch bridges don’t include any hinges at all. Arch bridges include those where the arch is underneath the bridge, not above it, provided that the trusses are arranged vertically and not diagonally
The Romans built more than 1,000 stone arch bridges, some of which still survive, such as the Pont-Saint-Martin bridge in Italy’s Aosta Valley (built in the first century BCE). This bridge design dates back more than 3,000 years. Concrete is now also used to build modern arch bridges.
A bridge’s load is the weight of the bridge itself (called the dead load), combined with the weight of whatever it carries (the live load). An arch bridge uses the forces of load and gravity, which otherwise might send a bridge tumbling downward, to hold it up instead.
An arch bridge works by conveying the downward pressure of gravity inward to the center of the structure — toward a central stone called the keystone — rather than straight down. This principle is called compression, and it enables the arch below to support the surface, or deck, above it.
Fixed arch bridges can be destabilized by temperature fluctuations, so the arch design is sometimes modified with hinges at each base and even the center of the span. This helps longer arch bridges adapt to the expansion or contraction of their materials when temperatures change drastically.
In Kenya most of the arch bridges are found along the Kenya- Uganda Railway line. A typical example of arch bridge is Railway over bridge across Uhuru Highway. Like beam bridges, arch bridges can be made of both steel and concrete.
Suspension bridges
In their simplest form were originally made from rope and wood. Modern suspension bridges use a box section roadway supported by high tensile strength cables. The suspension design is the other design for very long bridges. Here pylons or towers are erected and wires hold the deck of the bridge in place on the deck’s underside
A suspension bridge is perhaps the most recognizable bridge type in the United States, thanks to the iconic Golden Gate Bridge in San Francisco. Completed in 1937, it draws more than 10 million visitors each year to marvel at its twin 746-foot orange towers and sweeping Art Deco effect.
Suspension bridges are just what their name sounds like: They’re stabilized with vertical pillars or pylons connected by suspension cables. Attached to these main cables are smaller, vertical suspenders that hold up the bridge deck using tension, the main force that sustains suspension bridges.
Though the first suspension bridges were made of simple ropes supporting wooden planks, now the suspension technique supports long spans over broad channels. But because these bridges are only fixed to the earth in a couple of places (the towers or pillars), they can sway in the wind or vibrate when crossed by heavy traffic.
Wind or movement across a bridge can create vibrations. When these reach a certain frequency, it can cause a dangerous phenomenon called resonance — the same principle that causes a glass to shatter if a trained singer hits a high enough note. If vibrations are intense enough, they can disrupt bridge crossings and cause a collapse.
Suspension bridges also can be affected by torsion, a twisting force often caused by environmental factors like wind, which can create dangerous movement. If the surface of a bridge twists enough while travelers are on it, they can be thrown off.
And while torsion creates stress on a vertical plane, shear imparts a similar effect horizontally. It happens when environmental forces put pressure in opposite directions on a single, fastened part of a bridge, breaking it like a stick between two hands.
In the early nineteenth century, suspension bridges used iron chains for cables. The high tensile cables used in most modern suspension bridges were introduced in the late nineteenth century. Today, the cables are made of thousands of individual steel wires bound tightly together. Steel, which is very strong under tension, is an ideal material for cables; a single steel wire, only 0.1 inch thick, can support over half a ton without breaking.
In Kenya two common bridges of the type are found across Tana River in the North Eastern Kenya. Bura Bridge is a cable stay bridge constructed in 1985. It has three spans totaling 118 metres. The cable anchorage is external. Masalani Bridge is a self anchored suspension bridge across Tana River at Hola. It has a span of 135 metres with the centre span of 92 metres. The construction of Masalani Bridge was completed in 2007. Another suspension bridge in Kenya is Thua Bridge in Kitui.
CABLE-STAYED BRIDGE
A cable-stayed bridge is a variation on the suspension bridge that connects the crossbeam or bridge deck directly to pillars or towers. There’s no main cable, just a large number of vertical suspenders affixed to the top of the tower. These suspenders use tension to help keep the bridge deck stable and in place.
The Strömsund Bridge in Sweden is considered the first modern cable-stayed bridge. The three-span structure was completed in 1956. Its steel and concrete deck is suspended by diagonal cables from two pylons.
The glass-decked Sundial Bridge, built in 2004 across the Sacramento River in Redding, California, makes use of the cable-stay technique in conjunction with elements of cantilever and suspension. The famed Brooklyn Bridge, which opened in 1883, is a hybrid cable-stayed and suspension bridge.
TIED-ARCH BRIDGE (BOWSTRING)
A tied-arch bridge combines features of an arch bridge and a suspension bridge. It uses horizontal thrust from both sides to support an arched structure, as in a regular arch bridge. But instead of an arch supporting the structure from below, the arch rises above the road, and vertical ties descend to increase support of the decking.
These are also called bowstring bridges since they look like a bow from the side. This bow uses the tension of its vertical cables, together with the compression of the arch, to support the load and keep the bridge stable.
The wooden-decked Blackfriars Street Bridge in London, Ontario, Canada, is an example of this style. It carried vehicle traffic from the year it was built, 1875, until 2013, when it was permanently closed.
U.S. bowstring bridges include the Fort Pitt Bridge over the Monongahela River in Pittsburgh, the Lowry Avenue Bridge over the Mississippi River in Minneapolis, and the Daniel W. Hoan Memorial Bridge in Milwaukee.
TRUSS BRIDGE
A truss bridge distributes its load across a series of small sections fitted together. Formed by structural beams for smaller bridges or box girders for larger ones, bridge trusses are typically bound together by welded or riveted joints in a series of triangles.
Vertical steel or wooden supports help hold up the bridge using tension, while the diagonal truss supports add stability via compression, directing the load toward the center, similar to an arch.
The fairly inexpensive truss design has been around for a long time. Most truss bridges were built of wood in the 19th century, before a shift to iron and steel. The familiar Pratt truss design features diagonal segments that slope toward the bridge’s center. It was introduced by Thomas Willis Pratt in 1844.
The Cottonwood River Pratt Truss Bridge in Cedar Point, Kansas, is a classic example of this practical and widely used design. It was built in 1916 by the Missouri Valley Bridge Company and is on the U.S. National Register of Historic Places.
Other kinds of truss designs include:
- Baltimore truss
- Howe truss
- Long truss
- Vierendeel truss
- Warren truss
The Ikitsuki Bridge in Japan holds the world’s longest continuous truss span. Completed in 1991, it checks in at 1,300 feet. The Francis Scott Key Bridge over Baltimore Harbor is another notable example of this design.
