Transcript Document
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.