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Learning Activity #1:

Build a Model of a Truss Bridge
Overview of the Activity
In this learning activity, you will build a model truss bridge that has already been designed for you. When construction is complete, you will load the bridge to determine if it performs as its designer intended. With the load in place, you will be able to observe how thestructure works—how the various structural members work together to carry the load safely and efficiently. And at the end of the project, you will save the model as evidence of your bridge-building skill. Don’t break it! We will be using it again in subsequent learning activities.

Design is the essence of engineering. The only way to truly appreciate the challenges and rewards of engineeringis to actively engage in the creative process of design. So why, in this learning activity, will we devote considerable effort to building a bridge that has already been designed by someone else? It is true that building an existing design will not allow you to exercise a lot of creativity; nonetheless, this activity will provide you with valuable preparation for learning how to design a structure.Building an existing design will allow you to:
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Learn many key concepts about trusses and structural behavior that you’ll use when you design your own bridge in Learning Activity #5. Familiarize with the engineering characteristics of a rather unique building material—cardboard from a manila file folder. Learn some special construction techniques appropriate for this material. Workwith confidence, knowing that your bridge will carry the prescribed loading successfully, as long as you build the structure with care. Learn about the challenges faced by real-world construction contractors, who are often required to build structures that have been designed by someone else.


Learning Objectives
As a result of this learning activity, you will be able to do the following:n n n n n n n

Explain what a truss is. Identify the major components of a truss bridge. Identify the types of truss bridges. Explain the following fundamental structural engineering concepts: force, load, reaction, equilibrium, tension, compression, and strength. Explain how a truss bridge works—how each individual component contributes to the ability of the entire structure to carry a load.Explain the roles of the four key players in the design-construction process—the Owner, the Design Professional, the Constructor, and the Project Manager. Explain how construction quality affects the performance of a structure.

1. Component Parts of a Truss Bridge
What is a Truss?
A truss is a structure composed of members connected together to form a rigid framework. Members arethe load-carrying components of a structure. In most trusses, members are arranged in interconnected triangles, as shown below. Because of this configuration, truss members carry load primarily in tension and compression. (We’ll discuss these terms in Section 3 below.) Because trusses are very strong for their weight, they are often used to span long distances. They have been used extensively inbridges since the early 19th century; however, truss bridges have become somewhat less common in recent years. Today trusses are often used in the roofs of buildings and stadiums, in towers, construction cranes, and many similar structures and machines.

A typical truss bridge. Note that the structure is composed entirely of interconnected triangles.

Trusses, like all structures, are designedby civil engineers with special expertise in structural analysis and design. These men and women are called structural engineers.

Component Parts

The major components of a typical truss bridge are illustrated in the two diagrams below. The elevation view shows the bridge from the side. The isometric view is a three-dimensional representation of the structure. Note that certain members are...