SOURCE Lesson Plans Detail

Engineering the Perfect Bridge

Topic Bridge design and construction
Program Brown Science Prep
Developed by Manu Venkat, Colin Gould
Developer Type High school students

Overview / Purpose / Essential Questions

Why do architects and engineers build bridges, buildings and other structures the way they do? What are some of the challenges that they face and how do they meet these challenges?

Performance / Lesson Objective(s)

Learn about some of the types of forces that act on structures. Learn how engineers and architects build structures to resist these forces.

Lesson Materials



Lesson Motivation

Examples of bridge designs are all around us, and it is important for students to be able to identify the forces at play in keeping these bridges sturdy.

Lesson Activities

Bridge Building Competition


The most common kinds of forces that act on structures are compressive forces, tensile forces, and torsion forces forces.

Compressive forces:

-Pushing forces that compress a part of the structure
-The type of force a rod experiences when you push on both ends
-Shorter things are generally more resistant to compressive forces
-When you push on both ends of a long, thin rod (like a meter stick), it will tend to bend to relieve the force

-If you add too much force, it will eventually break

Tensile forces:

-Pulling forces that stretch a part of the structure
-The type of force a rod experiences when you pull on both ends
-Unlike compressive forces, the resistance of a material to tensile forces does not usually depend on length

Torsion Forces:
-Twisting forces that torque parts of the structure
-The type of force a rod experiences when you grab both ends and twist in opposite directions
-The way a material responds to torsion has to do with the properties of the material, though long segments of materials are often more susceptible to torsion that short ones

Building Materials:
- Certain materials are better or worse at dealing with each of the three forces.
- Demonstrations (if materials are available. if not, just talk about the materials):

- Paper: bad at resisting compression and torsion (just bends), good at resisting tension (takes relatively large amount of effort to pull ends of a paper until it rips.

- Roll 4 sheets of paper into cylinders, tape the cylinders to keep them from unravelling, stand them up, and place books on top of them until they fall.  In this form, paper is resistant to compression.  Shows that shape of material has impact on how the material deals with a force.

- Metal: good at resisting tension, bad at resisting compression (will bend)

- Plastic: same as metal

When engineers build structures, they take these forces into account and design structures to counteract them.

What do engineers do in situations where two types of forces act on the same object?  A concrete beam would break because of the tension at the bottom.  A steel beam would break because of the compressive forces on the top.  Answer: use two materials together! Reinforced concrete is a good example.

The strongest type of geometric shape is the triangle. Engineers and architects often incorporate triangles into the structures they design in order to relieve tensile and compressive forces.

Another important structure that engineers and architects use in their designs is the I-beam. The I-beam (sometimes called H-beam), is named for its resemblance to a capital letter I. These beams are extremely resistant to compressive forces that act on them vertically.


Introduce the rules of the bridge building contest to the kids.

Discuss with them where the three forces might act on their bridge and how they could counter these forces and the material constraints with some of the engineering strategies you have discussed.

ACTIVITY: Bridge Building

-The bridges will span a gap of 12 inches
-The max length for the bridges is 14 inches


50 straws
3 rubber bands
15 large paper clips
30 standard paper clips
1 foot 6 inches of tape

The bridges will be judged by a ratio of how much mass they can hold to their mass-- the highest ratio wins.

Wrap up / Conclusion

Have the kids discuss how their team’s bridge performed, where the structure broke (due to what type of forces?), and how the design could be improved.

Follow up

During the next lesson, announce the winner of the contest. 

Supporting Web Information

Pre Assessment Plan

List three types of forces that a building or structure may experience.

What is the strongest geometric shape and why?

Post Assessment Plan

Repeat the pre-assessment questions to see whether students have understood the basic concepts from the lesson.

List three types of forces that a building or structure may experience.

What is the strongest geometric shape and why?

Supplies List


Alignment Info

Audience(s) High school students
STEM Area(s) Engineering
Physical Sciences (RI GSE) PS3.9-11.8a
Students demonstrate an understanding of forces and motion by… predicting and/or graphing the path of an object in different reference planes and explain how and why (forces) it occurs.
Activity Type(s) Hands-on
Grade Level(s) High School
Version 1
Created 11/11/2012 10:51 AM
Updated 12/20/2018 11:41 AM