Learning Activity #5:
Design and Build
a Model Truss Bridge
In this learning activity, we will design, build, and test a model truss bridge. We will analyze the Owner’s
needs, then formulate specific design requirements. We will develop a truss configuration, analyze the structure, design each individual member and connection, then develop plans andspecifications. Finally, we will
build the bridge and test it to verify that it can carry load safely.
Overview of the Activity
In Learning Activity #1, we played the role of the Constructor and built a model bridge that had been
designed by someone else. In Learning Activity #5, we will assume the role of the Design Professional and
design a new bridge with the same span length butwith a different loading and a very different geometric
configuration. In doing so, we will learn a process that can be used to design a bridge with practically any span
length, loading, or configuration.
This project provides an opportunity to apply everything we have seen in the previous four learning activities. We will see how the various elements of the engineering design process fittogether—how scientific
principles, mathematic tools, engineering concepts, experimental data, and practical considerations contribute
to the final product. We’ll see how the truss configuration is tailored to the Owner’s needs; how the structural
model is derived from the truss configuration; how structural analysis results and experimental data contribute
to the design of structural members; how thesize and shape of connections are determined; how constructability considerations affect the final design; and how engineering computations are translated into the drawings
and schedules required for construction. Finally, we will build the bridge we designed—a great way to check
the validity of the design and the accuracy of the plans and specifications.
As aresult of this learning activity, you will be able to do the following:
Explain how design-build project delivery differs from design-bid-build project delivery.
Explain how the factor of safety is used in design.
Explain how scientific principles, mathematic tools, engineering concepts, experimental data, and practical
considerations contribute to the engineering designprocess.
Design a model truss bridge to meet a set of design requirements.
Build a model truss bridge, consistent with a set of plans and specifications.
To successfully complete this learning activity, you must understand the following key terms and concepts
from previous learning activities:
factor of safety
plans & specifications
If you need to refresh your memory on any of these terms, see theGlossary in Appendix D.
Using the Factor of Safety in Design
When we analyzed a structure in Learning Activity #3, we used the following definition for the factor of
To use this equation, we first determined the internal force in each member (by doing a structural analysis)
and the strength of each member (by using our experimental data from Learning Activity #2). Then weused
these numbers to calculate a unique factor of safety for every member in the structure. In short, we used
known values of internal force and strength to calculate unknown factors of safety.
When we design a structure, we need to select members that are strong enough to carry load safely. Thus,
in design, the unknown quantity in the equation above is the strength. The known quantities...