TRUSSES

A truss is a structure comprising five or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes.

 External forces and reactions to those forces are considered to act only at the nodes and result in forces in the members which are either tensile or compressive forces.

Moments (torques) are explicitly excluded because, and only because, all the joints in a truss are treated as revolute.

Types of Trusses:

Pratt truss:

A Pratt Truss Bridge is a bridge with both vertical and diagonal members arranged in triangular shapes, between the upper and lower chords. The diagonals may be stressed from tension or compression, to support dynamic loads. They are economic to construct as they don't use a lot of material.

  

Scissor truss:

A scissor truss is a type of truss in which the lateral beams form a kind of symmetrical shape, like the two shears of a pair of scissors. In general, the term "truss" refers to a structure that holds a roof in place. Residential roof trusses are commonly made of wood with steel accessories, where some truss types are made of steel. Trusses provide the framework for a roof.

Royal Albert Bridge:

The Royal Albert Bridge is a railway bridge that spans the River Tamar in England, United Kingdom between Plymouth, on the Devon bank, and Saltash on the Cornish bank. Its unique design consists of two 455 feet (138.7 m) lenticular iron trusses 100 feet (30.5 m) above the water, with conventional plate-girder approach spans. 

Lattice truss:

The lattice design fastened simple, diagonally set planks with treenails, or wooden pins, into crisscrossing truss system secured by top and bottom chords. Thus, Town's truss eliminated the need for large and expensive timbers, used in the Burr truss' series of arches, and streamlined the intricate, time-intensive labor of fastening mortise-and-ten on joints into the simple slotting and wedging of treenails. Town's innovative truss design is visible today in two of Connecticut's three remaining covered bridges, Bull's Bridge in Kent and West Cornwall Bridge in Cornwall and Sharon.

  

Allan Truss:

The Allan truss bridge is named after Percy Allan, a famous Australian architect and engineer who designed the bridge.

Allan Truss :

His design consists of vertical and diagonal arrangements of trusses which are were originally composed of steel.

Today many of the bridges based on Percy Allan’s design are composed of iron bark or even timber to reduce the cost.

The Hampden Bridge in New South Wales Australia was the first bridge to be built on Percy Allan’s design.

 

Howe Truss:

Howe truss bridge is a basic design of truss bridges based on which many engineers gave modified designs for many other famous truss bridges of the world.

It has a simple design which consists of diagonal and vertical trusses. The diagonal trusses move upwards and slope towards the center of the bridge. The vertical members of the truss are in tension while the members of the diagonal truss are in compression.

This bridge was first designed by William Howe in 1840. Jay Bridge in New York and Sandy creek covered bridge in Jefferson County, Missouri are to bridges based on Howe’s design.

Pratt Truss Bridge:

The design of the Pratt truss is bridge is the opposite of the Howe Truss Bridge. Although they too like the Howe truss have vertical and diagonal members, the major difference between the two is that in the Pratt Truss Bridge the Diagonal slope down towards the center.

This bridge was invented by Thomas and Caleb Pratt in1844. Originally this bridge was built with iron but not wood is also used in its making. This bridge can be extended up to 250 feet therefore it is commonly used as a railway bridge.

Bowstring Arch Truss:

The bowstring Arch truss or the Tied Arch Bridge was designed in 1840 by Squire Whipple. The bridge is composed of Thrust Arches which transform their vertical load along the arc of the arch.

At the ends of the arc, the thrust arches are resolved into two components a vertical and a horizontal thrust.

The vertical thrust is usually equal to the weight and load of the bridge section while the horizontal typically is taken down to the ground. If the arch is bowstring the horizontal thrust is taken to the opposite side of the arch by a chord.

The load which is faced vertically by the thrust arches are usually transferred along the path of the arc.

King Post Truss:

The king post truss which is also known as the ‘crown post’ truss bridge is probably the simplest type of truss bridges.

It simply consists of a single vertical support on which two angled trusses lean to.

It is also the oldest or most probably the first kind of truss bridges ever made. They were common during the medieval times.

 Lattice Truss:

Lattice truss bridges are another type of simple truss bridges which are comparatively easier to construct than the other types of bridges.

They are usually built of iron, steel or wood and are common in small towns and cities.