What are Truss Bridges?
A truss bridge consists of a supporting structure with a network of straight beams placed in triangular sections. This type of construction helps this bridge in supporting huge loads. Different truss bridges were built using wood in the earlier days, as wood was found in abundance, but the modern ones are usually built with concrete, iron, or steel. They are based on the theory of Newton's laws of motion, and work on the principle of tension and compression, and the ability to distribute the forces in different ways. Tension is a force which pulls the ends away from one another, thereby making the member longer. Compression is a force that pushes the ends towards each other, thereby making the ends shorter. The terms deck, pony, and through are used to signify the placement of travel surface in the bridge, and they help categorize types of truss bridge.
Truss is a structure composed of triangular units, which consists of straight beams connected at the joints that are called nodes. The application of this principle and its improvisation further led to the invention and design of various types of truss bridges around the world. These are some of the types with examples:
- Howe Truss Bridge:
This type is named after its inventor, William Howe, and was designed for the use of timber as the compressionive elements and iron as the tensile members. Howe truss was later improvised to use steel for its construction and became a forerunner of iron bridges. This type is popular as railroad bridges, and a well-preserved example is the Comstock Bridge over Salmon river, Colchester.
- Allan Truss Bridge:
This was designed by Percy Allan, and hence the name. Hampden Bridge in Wagga Wagga, New South Wales, Australia, is one of the most famous examples of this type of bridge. It is the first of this type, and constructed with wood and ironbark for strength. This is the simplest among the other truss bridges, economical due to the use of less material, and easier to repair.
- Truss Arch Bridge:
This type of combines the design of truss and arch bridges, in which the trusses are fitted within the arch. A famous example of this type is the Iron Bridge across the river Severn, Shropshire, England.
- Bollman Truss Bridge:
Named after its inventor Wendell Bollman, this type of is built only using metals, mostly wrought iron and cast iron. Most of the railroad bridges around the world are built by adopting this design due to its ease of assembly and durability. Though common after its invention, only one bridge of this type is available today. The oldest and most historic, the Bollman Truss Rail Road Bridge in Savage, Maryland, is an example of revolutionary design in engineering history.
- Pratt Truss Bridge:
It is exactly the opposite of Howe truss bridge in structure. Here, the diagonals are in tension and the vertical elements are under compression, both sloping towards the center in a V-shape. Earlier, Pratt truss bridges were made of timber and iron, but later they were made of iron only. This type has many variations due to the modifications made on this design to make it lighter, but was originally designed by Thomas and Celeb Pratt. An example of these bridge types is the Schell Bridge in Northfield, Massachusetts.
- Bowstring Arch Truss Bridge:
The father of tied arch bridge is considered to be Squire Whipple. This involves the most complicated engineering among the various truss bridge types, where the tension of the top chord is supported by the bottom chord, rather than being supported by the ground foundation. Due to this quality, tied arch bridges are usually built in areas of unstable soil. An example of this type is the Torikai Big Bridge over the Yado river, Osaka, Japan.
- Cantilever Bridge:
Cantilever bridges are named after its use of cantilever beams, and involve one of the most complex designs among different truss bridges. For supporting heavy loads, cantilever bridges either use steel trusses or concrete box girders. For long bridges, steel truss cantilevers are used, which gives it strength and can be easily constructed. The Quebec Bridge in Quebec, Canada, is not only listed as one of the most famous bridges of the world, but is also the longest cantilever bridge around.
- Bailey Bridge:
This type was originally designed by Donald Bailey for use by military engineering units. These are portable bridges that are small enough for easy transportation, handling, installation, and re-use. They are modular bridges, and unlike previous portable designs used by the military, these do not require complicated equipment while assembling and are very cost-effective. Modern-day example of a Bailey bridge is the Bnot Ya'akov Bridge built across the Jordan river in Israel.