Category: Engineering—Structural Engineering
Project Idea by: Rosemary Kalonaros and Megan Ganzenmuller
In designing a bridge, an engineer must consider the types of forces that will be applied to the bridge. A force is a push or pull on an object. Two of the forces that must be considered when designing a bridge are gravity forces and lateral forces. Weight is the measure of gravity, which on Earth is a measure of the force pulling objects toward Earth's center. The weight or gravity of an object increases as the mass of the object increases.
Weight causes some structures to bend downward. When this happens, compression and tension are produced. Compression is a force that pushes materials together, and tension is a force that pulls materials apart. In the diagram, the weight of the bridge as well as the weight of the car causes the beam to bend. The top edge of the beam has shortened because the compression forces, indicated by the arrows pointing toward each other in the illustration, squeeze the materials together. The bottom edge of the beam has lengthened due to tension forces, indicated by the arrows pointing in opposite directions, stretching the material.
Lateral forces are those directed at the side of the bridge or other structure. These forces include those generated by things such as the wind, earthquakes, and explosions. As a system (different parts working together as one unit), a structure must be designed so that it can resist all forces to which it is subjected, including gravitational and lateral forces.
The most basic bridge design is a beam bridge. A log across a creek is one of the simplest beam bridges. Since beam bridges cannot span great distances, other types of bridges have been designed such as truss bridges. A truss bridge is a bridge with trusses (a simple skeletal structure made up of straight beams forming triangular shapes). A triangle is a geometric shape that has a small amount of flexibility. By using many combinations of triangles in a truss, the unit can be designed to carry a great deal of weight and span great distances. A project question might be, "What effect does triangle size have on the strength of a truss bridge?"
Clues for Your Investigation
Build small truss bridges with different-sized trusses and test their strength. The bridges can be built with simple materials such as craft sticks, toothpicks, and straws. The bridges must be of comparable length, width, and height, with trusses containing different-sized triangles. Design a way to test the strength of the bridges such as supporting the bridge ends and adding a measured amount of weight until each bridge breaks.
Independent Variable: Size of triangles
Dependent Variable: Strength of truss bridge
Controlled Variables: Type of building materials, type of strength-testing device, size of bridges
Control: Truss with the median size of triangles
Other Questions to Explore
- How do the vertical beams affect the strength of a truss bridge?
- How does the strength of different truss designs, such as the Howe, the Pratt, and the Warren, compare?
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