Model Description
This demonstration shows the simplicity of a truss. The equipment consists of a single triangle made of two rulers and string – forming a truss. The truss is held on two desks by students and given a load by another student. This shows the innate strength of the simple triangular shape used in trusses. This demonstration should take 5-10 minutes.
Engineering Principle
The basic equations required for analysis are the equations of equilibrium and trigonometry. However, no real numerical analysis is required beyond measuring the load applied if scales are placed under each leg of the Ruler Truss (not shown). The model is used primarily for developing a feel for the importance of triangles as a structure, especially when considering truss stability.
What You Need
| Item | Quantity | Description/Clarification |
|---|---|---|
| Yardstick | 2 | The yardsticks will be attached at one end with a bolt and a nut so that they can rotate freely. They will be attached at the other end by a string that is about 30 inches long so that the three parts form a triangle. $5 each; 15 minutes |
| Bolt | 1 | The bolt (2 inch x ¼ inch) will be used to connect one end of the yardsticks. $3 |
| Nut | 1 | The nut (1/4 diameter) will be used to secure the two yardsticks in place forming a generally frictionless pin. $1 |
| String | 30 inches | The string will be attached to the far end of the yardsticks to create a triangular truss. $5 |
How It’s Done
Before Class: Set up the ruler truss with the 2 rulers, bolt, nut, and string.
In Class:
Set up the rulers and string on two desks and assign two students to hold the bottoms of the rulers using only their thumb and index finger (ruler weight actually resting on the desks) without the string being taut. The students are only trying to keep the structure vertical.
Ask another student to apply a force to the top of the rulers. The students holding the bottom using only their fingers should not be able to resist the outward movement of the rulers as the student presses down. Based on the loading applied, ask the students what type of load is in each leg of their simple truss? Pull from the students that the rulers are in compression while the string is in tension.
Additional Application: A discussion can ensue concerning the definition of a truss: a structure composed of slender members joined together at end points by frictionless pins; loads only applied at joints; all real truss members are 2-force members; and member weight is negligible. Slender members and frictionless pins are obvious, but the fact that truss members are 2-force members jumps out with this simple model. Additionally, vertical stability becomes an issue that the two students holding the bottom can address by how hard it is to keep the structure vertical if the other student does not load vertically. How do we keep a truss from falling over? Can we ensure loads are only vertical? Etc.?