Critical Load

This lesson explores the concepts of structural engineering and how to measure the critical load or the maximum weight a structure can bear. Students design and build a structure designed to hold increasingly greater weight, while determining the structure’s critical load.

  • Learn about civil engineering and the testing of building structure.
  • Learn about efficiency ratings and critical load.
  • Learn about teamwork and engineering problem solving.

Age Levels: 8 – 14

Lesson Plan Overview

Build Materials (For each team)

Required Materials

  • 12 index cards
  • 1 roll of tape

Alternative Materials

  • Game cards (i.e. Uno), playing cards, pieces of cardstock

Testing Materials

  • Plastic container to put the weight in
  • Weight to test structure: coins, marbles, Legos, sand, etc.

Alternative Testing Materials

  • Flat surface to stack weight (i.e. folder, cardboard, notebook, etc.)
  • Weight to test structure: books, stacks of magazines, cans of food, etc.


  • Plastic container to put the weight in
  • Weight to test structure: coins, marbles, Legos, sand, etc.
    • Separate weight into the containers in equal amounts adding up to a minimum total weight of 4 pounds

Alternative Testing Materials

  • Flat surface to stack weight (i.e. folder, cardboard, notebook, etc.)
  • Weight to test structure: books, stacks of magazines, cans of food, etc.


  • Each team should first predict the “critical load” of their structure and document it.
  • Test each team’s structure by first placing the structure on the floor, on a table or on a desk.
  • Begin carefully placing the containers of weight (one by one) on top of each structure.
  • Try to stop placing the weight just before each structure meets the critical load (amount just prior to failure). Document the critical load of each structure.
    • If the structure does collapse, document the last amount of weight it was able to withstand.
  • Compare each team’s predicted critical load to the testing results. As a class, discuss the strengths and weaknesses of the designs.
An inside look at how 3 Civil Engineers are changing the world. (Video 6:22)

Source: ASCE YouTube Channel

Finding the strongest strength. Learn how different shapes relate to the strength of a structure. (Video 3:34)

Source: PBS Learning Media, Zoom

Earthquake-proof buildings? Explore ideas for earthquake proofing buildings with hands-on design challenges. (Video 5:57)

Source: QuadSquad YouTube Channel

Design Challenge

 You are a team of engineers all working together, using the engineering design process, to design a structure using 12 index cards and tape to hold a minimum of 4 pounds of weight without collapsing. Before testing your structure, make a prediction of the critical load of your structure (the weight at which you think your structure will fail) and write it down.


  • Structure must hold a minimum 4 pounds of weight


  • Work in teams of 2-3
  • Use 12 cards or fewer
  • Cards may not be cut or torn


  1. Break class into teams of 2-3.
  2. Hand out the Measuring Critical Load worksheet, as well as some sheets of paper for sketching designs.
  3. Discuss the topics in the Background Concepts Section.
  4. Review the Engineering Design Process, Design Challenge, Criteria, Constraints and Materials. If time allows, review “Real World Applications” prior to conducting the design challenge.
  5. Before instructing students to start brainstorming and sketching their designs, ask them to consider the following:
    – What shapes are strong?
    – Will the weights stack well or do you need a container to hold them?
    – How big is the container to hold the weight and will it fit on top of their design?
  6. Provide each team with their materials. Ask each team to predict the “critical load” of their structure and write it down.
  7. Announce the amount of time they have to design, build, and test their structure (1 hour recommended).
  8. Use a timer or an on-line stopwatch (count down feature) to ensure you keep on time. ( Give students regular “time checks” so they stay on task. If they are struggling, ask questions that will lead them to a solution quicker.
  9. After the teams have completed their structures, test the designs.
  10. Test each team’s structure by adding measurable weight (minimum of 4 pounds) to determine at what weight the team’s structure will collapse. This is each structure’s “critical load“ or amount just prior to failure.
  11. Teams then compare their predictions to their testing results and discuss their designs with the class.
    After determining the “critical load” of each team’s design, it’s time to have some fun! Keep stacking weight on the designs until they are as flat as a pancake.
  12. As a class, discuss the student reflection questions.
  13. For more content on the topic, see the “Digging Deeper” and “Real World Applications” sections.

Student Reflection (engineering notebook)

  1. What was your structure’s “critical load?”
  2. How close were you to your prediction?
  3. What aspects of your design do you think helped its ability to hold more weight?
  4. What aspects of your design do you think hindered its ability to hold more weight?
  5. What was the highest critical load in your classroom?
  6. What was the difference in the winner’s design and yours? Or…if your team had the winning structure, what do you think set your structure apart from the rest?
  7. If you could do your design all over….what would you change, and why?
  8. What human factors do you think a civil/structural engineer needs to take into consideration when planning an office building? (examples are the weight of people and furnishings, need for water, fresh air, escape routes)

Time Modification

The lesson can be done in as little as 1 class period for older students. However, to help students from feeling rushed and to ensure student success (especially for younger students), split the lesson into two periods giving students more time to brainstorm, test ideas and finalize their design. Conduct the testing and debrief in the next class period.

Divide into teams
Review the challenge and criteria constraints
Brainstorm possible solutions (sketch while you brainstorm!)
Choose best solution and build a prototype
Test then redesign until solution is optimized
Reflect as a team and debrief as a class

What Civil Engineers Do

Civil engineers are problem solvers, meeting the challenges of pollution, traffic congestion, drinking water and energy needs, urban redevelopment, and community planning. This activity focuses on the work of structural engineers who face the challenge of designing structures that support their own weight and the loads they carry, and that resist wind, temperature, earthquake, and many other forces.

Famous Building Failures

The John Hancock Tower in Boston, Massachusetts (right) is said to have been “known more for its early engineering flaws than for its architectural achievement.” Wind-induced swaying was so large, it was said to cause motion sickness for people on the upper-floors. This problem was solved by adding a pair of 300-ton dampers on the 58th floor. Another unrelated but serious problem was that 65 of its

10,344 floor-to-ceiling plate-glass window panes fell out of the building to the ground during construction — luckily no injuries resulted to either workers or passersby!

Efficiency Ratings and Critical Load

The efficiency rating measures the weight that will cause a structure to fail divided by the weight of the structure itself. The most efficient structures are strong and lightweight – a difficult combination to achieve. For example, roofers in areas which experience heavy snows must factor in the weight of a massive snowstorm into designing the strength of the roof. The weight at which a building or structure fails is called the “critical load.”

  • Civil Engineers: Problem solvers, meeting the challenges of pollution, traffic congestion, drinking water and energy needs, urban redevelopment, and community planning.
  • Constraints: Limitations with material, time, size of team, etc.
  • Criteria: Conditions that the design must satisfy like its overall size, etc.
  • Critical Load: The weight at which a building or structure fails.
  • Efficiency Rating: Measures the weight that will cause a structure to fail divided by the weight of the structure itself.
  • Engineers: Inventors and problem-solvers of the world. Twenty-five major specialties are recognized in engineering (see infographic).
  • Engineering Design Process: Process engineers use to solve problems.
  • Iteration: Test & redesign is one iteration. Repeat (multiple iterations).
  • Prototype: A working model of the solution to be tested.

Internet Connections

Recommended Reading

  • Stacking the Deck: Secrets of the World’s Master Card Architect by Bryan Berg
  • Why Buildings Stand Up: The Strength of Architecture by Mario Salvadori
  • Why Buildings Fall Down: How Structures Fail Architecture by Mario Salvadori

Writing Activity

Write an essay or a paragraph describing a recognizable building in your town. Include the history, interesting challenges to the building’s engineering, and challenges that the engineers faced in design and construction.

Global Math Standard Aligned Lesson Concepts
Number and Operations Basic Calculations, Formulas
Geometry (Shape and Space)
Measurement Charting and presentation
Data Handling and Analysis Evaluation of Data Results, comparison with other student results
Statistical Investigation
Probability and Estimation Predicting outcomes
Global Science Standard Aligned Lesson Concepts
 Materials and Properties Conductors and Insulators, Testing
Forces and Motion
 Electricity/Magnetism Electric Circuitry
Community and Health
Natural Resources
Earth Systems
 Biology/Bioengineering Wearables and Sensors
Physics (light, sound, heat)
Global Technology Standard Aligned Lesson Concepts
Information Technology Coding
Systems Integration of parts to a whole
Innovation Wearables and Sensors
Data Organization
Data Representation Chart Development and Presentation
Overarching Skills Aligned Lesson Concepts
Communication Presentation and Audience Feedback
Critical Thinking
Creativity Design and Implementation of Design
Collaboration Teamwork
Problem Solving Project Planning
Organization Parts Management
Scientific Thinking/Research
Careers and Degrees

Build a Chair Lift

Research and Planning

You are part of a team of engineers who have been given the challenge of building a chair lift to carry a ping pong ball up the mountain (from the floor of your classroom to the top of a desk or chair) using materials provided to you.  Your lift must both carry the ball up the mountain and also back down without the ball dropping out. How you design your chairlift and the chair that will carry the ball, and what materials you use are up to you!

Design Phase

You have been provided with many materials from which to design and build your own chairlift and chair.  Consider which materials you would like to use, and list them in the box below.  On a separate piece of paper, draw a diagram of the system you intend to build.


Parts Required:














Build it!  Test it!
Next build your chairlift and test it.  You may share unused building materials with other teams — and trade materials too.  Be sure to watch what other teams are doing and consider the aspects of different designs that might be an improvement on your team’s plan.

You may decide to completely change your design when in the manufacturing phase — and you may ask for additional materials, or try different solutions as you build. 


 Build a Racquet


Complete the reflection questions below:


1) How similar was your original design to the actual chair lift your team built?






2) If you found you needed to make changes during the construction phase, describe why your team decided to make revisions.






3) Was your chairlift able to carry the ping pong ball up and down the mountain without it falling out of the chair you designed?






4) Which chairlift system that another team developed was the most effective or interesting to you? Why?






5) Do you think that this activity was more rewarding to do as a team, or would you have preferred to work alone on it? Why?






6) If you could have used one additional material (tape, glue, wood sticks, foil — as examples) which would you choose and why?