A Warren truss: predicting tension and compression by the method of joints

The idea

Each joint of a pin-jointed truss is in equilibrium, so resolving forces at the joints gives every member's tension or compression, and the compression members are the ones most likely to buckle.

Safety card

What you need

• Pop sticks (about 114 mm long) and PVA or low-temperature hot glue
• Template of a Warren truss: bottom joints A, B, C at 0, 100 and 200 mm; top joints D and E above the middles of AB and BC, making equilateral triangles of side 100 mm
• Two supports under A and C; a hanger at B; slotted masses in 500 g steps; a foam-filled box under the truss
• Kitchen scale; safety glasses

How to do it

1. Build two identical trusses from the template and glue them into a pair joined by cross pieces.
2. Before loading, draw a free-body diagram of each joint for a 20 N load at B and find every member force by the method of joints.
3. Mark each member T (tension) or C (compression) on the model.
4. Load the pair at B in 500 g steps and watch which members bow first. Record the failure load and the first member to fail.
5. Weigh the truss and work out its strength-to-mass ratio.
6. Redesign by doubling only the compression members, rebuild, retest and compare ratios.

What you should see

For a 20 N load at B the supports each carry 10.0 N. The bottom members AB and BC carry 5.77 N tension, the inner diagonals DB and BE 11.55 N tension, the end diagonals AD and EC 11.55 N compression and the top member DE 11.55 N compression (20 ÷ √3 = 11.55 N; half of that is 5.77 N). A slender pop stick bows (buckles) under a far smaller force than it can carry in tension, so the compression members are expected to fail first unless a glued joint lets go earlier; doubling them targets that failure. The learner knows it worked when their hand calculation matches these member forces, they record whether the first failure was a member marked C or a glued joint, and they compare the strength-to-mass ratios of the two designs.

What changes

What you change

which members are doubled

What you measure

failure load (N) and strength-to-mass ratio

What you keep the same

• the same template and sticks
• the same glue and drying time
• load at joint B
• 500 g steps

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

• All members of a bridge are squashed by the load (some are stretched).
• The member nearest the load carries the most force (forces depend on geometry, not only closeness).
• Stronger glue alone makes a stronger truss (slender compression members buckle whatever the glue).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

• Industrial Technology 7–10 Syllabus (2019), NESA. Current; its Engineering courses are not available after December 2028. Code read from the official syllabus document (DOCX) on 2026-09-22.IND5-4IND5-8
• Engineering Technology 7–10 Syllabus (2024), NESA. Implementation from 2027, so this code describes the future syllabus. Code read from the outcomes page on 2026-09-22.EGT5-MEA-01EGT5-EVL-01
• Australian Curriculum v9AC9TDE10K03AC9TDE10K06AC9TDE10P02

Sources

The pages the author read to write this activity.

1. curriculum.nsw.edu.au/learning-areas/tas/engineering-technology-7-10-2024/outcomes
2. www.nsw.gov.au/education-and-training/nesa/curriculum/tas/industrial-technology-7-10-2019
3. www.teachengineering.org/lessons/view/ind-2472-analysis-forces-truss-bridge-lesson
4. www.teachengineering.org/activities/view/cub_trusses_lesson01_activity1