Truss Bridge Design

Truss bridges are the oldest known bridges of the modern era. These bridges are constructed by connecting elements through compression, tension or both. Due to their unique structure - in which materials are used efficiently, truss bridges are considered to be quite economical from the point of construction. The triangular pattern used in construction of truss bridges offers strength to the set-up of beams in the whole structure.

What is a Truss?
It is a structure composed of triangular units connected at joints called nodes. The triangular units, that form a truss, are slender and straight in shape. Classification of a truss design/structure is primarily done into planar and space frame. All components of a space frame truss together attain a 3-dimensional form. Planar truss on the other hand has a 2-dimensional design. Truss is a structure commonly used for constructing bridges. However, the use of truss design is not just limited to building bridges. Even roofs are built by means of a truss design. Scope of this article is limited to discussing the truss bridge design, so let's find more on this topic.

Truss Bridge Design
While designing a truss bridge, certain assumptions are taken into consideration along with the application of Newton's laws of motion. It is assumed that truss bridges are designed with the help of pin joints. The pin joints are points at which straight components of the truss structure meet. The above assumption leads us to the conclusion that only the forces of compression and tension would act on the components of a truss structure. A truss bridge design consists of vertical, lower, horizontal and diagonal members. The vertical members of a truss bridge face tensile stress while the lower horizontal ones are under a stress that results from bending, tension and also shear stress. Diagonal members which run outwards are under compression while the inner diagonals face tensile stress. There are many different types of truss bridges. Some of the types of truss bridge designs prove advantageous if wood is used in the construction; this is especially useful to protect the components or truss members that face compression.

Types of Truss Bridges
There are many different truss bridge designs being used as per specific needs of topography of a particular region. Explanation pertaining to the different truss bridge types is presented one-by-one.

Howe Truss
The Howe truss is made up of diagonal and vertical members. Diagonal members of this type of bridge slope towards the center of the bridge in an upward manner. This bridge is named after William Howe - who designed it in 1840.

Pratt Truss
The design of the Pratt truss is similar to that of Howe truss, except for the diagonals that slope downwards in the former. Caleb and Thomas Pratt invented the Pratt truss in 1844. Maximum length of the Pratt truss bridge can be 250 feet; this bridge is commonly used for supporting railways.

Bailey Bridge
This bridge is generally used by the military to cover gaps and trenches of up to 60 m. The Bailey Bridge can bear the weight of tanks and doesn't require any kind of heavy machinery or tools for construction. The specialty of construction process for bailey bridge is that pre-fabricated components are used; this also makes it possible to speed up the process of construction. The bridge (due to this advantage) is therefore, also used in the form of temporary makeshift crossings.

K Truss
In the K Truss bridges, length of members undergoing compression is reduced. This reduction in length enables components of bridge to endure the compressional force. Design of the K truss bridge is complicated and it is considered to be one of the hardest bridges to build.

Bowstring Arch Truss
The Bowstring arch truss bridge, also known as Tied arch bridge, was patented by Mr. Squire Whipple in the year 1840. The vertical load faced by thrust arches in this bridge is transferred along the path of the arc. Diagonal components of the truss help in holding other parts of the bridge intact.

King Post Truss
The King post truss is also known as the 'crown post'. King post is the main part of this structure and connects apex portion of the bridge to its base. These posts were used in the construction of Parish churches which formed an integral part of the Medieval architecture. The tithe barn structures, used for storage of farm produce also incorporated the king post in its structure.

The truss bridge information presented in this article is about different forms of truss structure utilized in the construction of bridges. The truss bridge design has thus, emerged as an important part in modern architecture and in the construction of many famous bridges.