Bridge Design part 4 By Alan Pennington, materials taken from and adapted West Point Bridge Design Note: have pdf file of bridge designs ready for structural analysis

Learning Objectives Calculate the structure of various bridges. Determine the load strength of every member in a truss. Evaluate a truss, to determine if it can carry a given load safely.

Using vectors we can make diagrams of the different types of bridge designs and calculate how the forces will be applied to different parts of the bridge

Create the Structural Model To model a Bridge, we must define (1)the geometry of the structure, (2) the loads, (3) the supports and reactions. We begin by drawing the three-dimensional bridge structure as a pair of two-dimensional trusses. Since these two trusses are identical, we only need to analyze one of them. The geometry of the truss is shown below. The dimensions indicate the locations of the centerlines of the members.

Determining Internal Member Forces Decide on the total amount of weight that the bridge will carry. This is W. Note that the total load applied to each truss is only half of the total weight (0.5W), because the bridge is made of two main trusses, one on each side Next to each member on the diagram, you will see a decimal number. For each member, multiply the decimal number by the value of W you determined. This product is the internal member force, expressed in the same units you used for W. If the decimal value is positive, the member is in tension. If it is negative, the member is in compression.

For example, let’s assume that this truss has a total load W=10 newtons, divide by two because of two sides for 5 newtons.divide by 3 for the three members where the load is applied. Then the bottom chord member on the left-hand side of the truss has an internal force of

Use the supp_civil_strcanal.pdf file to make worksheets and have students calculate different parts of the different bridge designs