Bridge Design part 1 By Alan Pennington, materials taken from and adapted West Point Bridge Design

Learning Objectives Understand what a truss is and identify the major components of a truss bridge. Explain the following fundamental structural engineering concepts: force, load, reaction, equilibrium, tension, compression, and strength. Explain how a truss bridge works

What is a Truss? A truss is a bridge composed of members connected together to form a rigid framework. Members are the load-carrying components of a structure. In most trusses, members are arranged in interconnected triangles, Because of this design, truss members carry load primarily in tension and compression. Because trusses are very strong for their weight, they are often used to span long distances.

How does a truss bridge carries load We will need to introduce (or perhaps review) some basic concepts from physics. Forces A force is simply a push or a pull applied to an object. A force always has both magnitude(size) and direction. Mathematically, we represent a force as a vector. A vector is a quantity that has both magnitude and direction. To show a force on a picture or diagram, we normally represent it as an arrow (which shows the direction) and a magnitude (in units of force, such as pounds or newtons)

Internal Member Forces When you apply external loads to a structure, external reactions occur at the supports. But internal forces are also developed within each structural member. In a truss, these internal member forces will always be either tension or compression. A member in tension is being stretched. Tension force tends to make a member longer. A member in compression is being squashed. Compression force makes a member shorter.

What causes the string to break? The string breaks when its internal member force becomes larger than its strength. This observation leads us to two closely related definitions: (1) The strength of a structural component is the largest internal force the component can experience before it fails. (2) Failure occurs when the internal force in a structural component becomes larger than its strength.