Introduction

Truss bridges are often studied in engineering courses, because they are easy to design and build. A truss bridge is composed of several straight members connected in a triangular configuration. When a load is applied to a truss bridge, the structural members experience tension or compression forces, but no bending. Tension forces are pulling forces, while compression forces are pushing forces. Truss bridges have a high strength-to-weight ratio, which allows them to span long distances and support a large amount of weight. In contrast, beam bridges cannot be used to span long distances, because the beam gets weaker as the supports get further apart. Trying to stiffen a beam by making it larger will only cause the beam to sag in the middle from its own weight. Instead, triangular components are often used to stiffen and support long beam spans.

Truss bridges are statically determinate. Their support forces are equal to the applied load, and the moments about any point are zero. The internal forces in a truss bridge can be determined using the method of joints or the method of sections, however the internal forces can never exceed the strength of individual members. In this activity, you will test the tensile and compressive strengths of structural components using a Vernier Dual Range Force Sensor. Strength will be defined as the maximum force a member can carry before breaking. Compressive strength depends on the length, shape, material properties, and cross-sectional area of the structural component. Tensile strength is not affected by the length or shape. In the last part of the activity, you will predict the locations of maximum compression and tension forces in a model truss bridge (the structural members most likely to break first), and test your predictions using a Vernier Force Plate.

*This activity is intended to complement the West Point Bridge Design Contest (http://bridgecontest.usma.edu/manual.htm). The test stand described in Appendix C of the West Point manual has been modified to incorporate a Vernier Dual Range Force Sensor.

Objective

In this experiment, you will:

  • Build a truss bridge that spans 24 inches, following the directions in the West Point Bridge Design Contest manual.
  • Measure the tensile and compressive forces in individual components.
  • Measure the total applied load to the bridge.

This lab uses the Vernier SensorDAQ device and the SensorDAQ driver,