Comparative Study of Bridges
Student Internship Research Project by Gurubaran Magesh
Grade V Student, Student Intern, Adaptive Instruction
Gurubaran Magesh is a 5th grader living in Bengaluru. He has strong interests in Geography (mainly political), Maps and Railways and Urban planning.
Comparative Study of Bridges
What are the various types of bridges? What are the key factors to decide a type of bridge for a place?
Research Guide: L.N.Venkataraman
Bridges are a structure built to span a physical obstacle usually water but it can also be a valley, road, rail or swamp. The earliest bridges were built in the Neolithic period when they were built from fallen trees, stepping stones, boardwalks and logs and until the early 1900s they were made of timber, stone and masonry. Modern bridges are almost exclusively made of reinforced concrete, prestressed concrete and stainless steel. The many Types of bridges will dedicate more of their capacity to better handle specific types of forces. Small bridges can be built in only 2 to 4 days while for bigger bridges like suspension and cable stayed bridges it can take between 1 and 1½ years. Most bridges have a life span of around 50-70 years.
There a few types of bridges that we’re going to discuss today. They are beam, truss, cantilever, arch, tied arch, suspension and cable stayed bridges. And the main parts of bridges are abutment, girder, bearing, pile and pile cap, pier and pier cap and deck.
Beam bridges are the simplest of all bridges. They are supported by a substructure unit at each end which can be simply supported when the beam only connects a single span or continuous when connected across two or more spans. When there two or more spans the intermediate supports are called piers. In modern times, beam bridges are usually made from steel or reinforced concrete or a combination of both. Beam bridges are often used only for short distances because they have no support in them and the only supports are provided by piers. The longest beam bridge in the world is the Lake Ponchartrain Causeway in USA which is roughly 25 kilometers long.
Beam bridges are built when,
- The distance to cover is short.
- The banks of river or valley has hard Rock or soil which can take load.
- If more piers are needed, then the river bed should not have loose soil.
Truss bridges are descendants of beam bridges and have gained quite a lot of popularity over the years. The bridge has a load bearing superstructure which is composed of a truss and a structure of connected elements that usually form triangular units. The connected elements are tensioned or compressed or sometimes even both for stressing. The truss is usually made of metal or steel but in the 19th century (1800’s) iron was adopted for lifting heavier loads. Most bridges are used for short to medium distances and not very long unless continuous. The longest truss bridge in the world is the Ikitsuki Bridge in Kyushu, Japan which is 960 meters or 0.96 kilometres long. The Godavari bridge in Rajahmundry, Andhra Pradesh is also a suspension bridge.
Truss bridges are used when:
- There is a need to carry heavier loads than beam bridges.
- Banks and piers are not that strong to handle loads required for beam bridges.
A cantilever bridge is a bridge built using cantilevers. Cantilevers are structures that project into space horizontally and are supported on only one end. For small and short bridges, the cantilevers could be as simple as beams; however, large cantilever bridges made to handle road or rail traffic use steel trusses or box girder made from prestressed concrete. This type of bridge is usually built for medium distance span in certain sites, especially for great loading. A famous example of a cantilever bridge is the Forth Bridge in Scotland, near Edinburgh and the Howrah Bridge in Kolkata. This bridge also held the title of longest span in the world for nearly 30 years until it was surpassed by the Quebec bridge.
- Cantilever bridges are used when
- One side of the bank is not sturdy enough to hold a pier
- The places where piers can be placed in the river are few and hence one cannot build many piers.
- Span between two piers are of reasonable length.
An arch bridge is a bridge with abutments on each end shaped as a curved arch. Arch bridges transfer the bridge’s weight and loads partially to a horizontal thrust restrained by the abutment at either side. Many modern arch bridges are made from reinforced concrete or steel. Many of these bridges often bear some of their load by tension within their structure. Arch bridges are usually short but they are often set end to-end-to form a larger total length. The most famous example of an arch bridge is the Pont du Gard aqueduct in Nîmes, France.
Arch bridges are used when:
- Build bridge across deep valleys in rocky mountains
- The site does not require access to complex materials such as steel or metal as arch bridges can be built with just bricks.
Suspension bridges are straight and diagonal. A suspension bridge carries vertical loads through the curved cables are in tension. These loads are transferred both to the towers which carry them by vertical compression to the ground and to the anchorages which must resist the inward and sometimes vertical pull of the cables. Suspension bridges are either made of steel or concrete but early ones were made of stone. Steel rope is used for the cables as well as steel wire strand cables. Arguably the most famous bridge in the world, the Golden Gate Bridge is also a suspension bridge as well as the Howrah Bridge in Kolkata which is also a cantilever bridge.
Suspension bridges are used when one need to build bridge across long rivers, but the river bed is not strong enough to build piers in it.
Cable Stayed Bridge
A cable stayed bridge is a bridge form in which the weight of the deck is supported by a number of nearly straight diagonal cables in tension running directly to one of more vertical towers. The towers transfer the cable forces to the foundation through vertical compression. Like suspension bridges, cable stayed bridges are made from steel or concrete. The Vidyasagar Setu also in Kolkata and also in Howrah and the Russky Bridge in Vladivostok, Russia are examples of cable stayed bridges.
Cable stayed bridges are used in places where spans need to be longer than cantilever bridges but short enough so a suspension bridge is not economically practical.
As you see, there many types to build bridges and these are just the main ones. So next time you say bridge, remember to specify which type.