What it shows
Railway and road bridges use trusses because a triangle is rigid and each member carries either tension or compression. The project builds a Warren truss, loads it step by step until it fails, and compares the failure load with a simple member-force calculation.
The science
In a Warren truss, the diagonals form equilateral triangles so loads on the joints become axial forces. The top chord is mostly in compression and the bottom chord in tension. For a simple span with a central load W, each support carries W/2, and the force in a member can be found by resolving forces at the joints. Gluing joints are the weak point, so the real failure load is lower than the member strength.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- About 150 flat wooden ice-cream sticks
- Wood glue (white PVA)
- Two tables or two stacks of books 40 cm apart
- A bucket and a measured amount of sand or water in bottles to load
- A weighing scale
- A ruler and protractor
- Pins and cling film for the template
Safety
For this project: Wear eye protection when loading to failure, because sticks can snap. Keep feet and hands away from the falling load. An adult must supervise.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
10 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Draw a truss 40 cm long and 10 cm tall made of equilateral triangles on a board, and cover it with cling film.
Glue sticks along the lines in two layers, using pins to hold them in place.
Make two identical truss sides and join them with cross-braces at the top and bottom to form a 3D bridge.
Let the glue dry for at least 24 hours and check weight of the finished bridge.
Place the bridge across the gap between supports with 2 cm resting on each.
Hang a bucket at the centre and add water or sand in small steps of 200 g, measuring the sag with a ruler after each step.
Continue until the bridge fails and note which member or joint broke first.
Calculate the force in the mid-span chord member for the failure load using joint analysis and compare with the strength of the sticks.
Plot load against deflection to see the linear and failure regions.
Calculate the strength to weight ratio of the bridge and write the report with photos and sketches.
What you should see
A well-made truss carries about 10 to 25 kg before failure from a self-weight of 150 g. The load against deflection graph is linear first and then flattens, and failure usually occurs at a glued joint or at compression buckling.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why are triangles used in trusses?
A triangle cannot change shape without changing the length of a side, so it is rigid.
2Which members are in tension?
The bottom chord and some diagonals carry tension, while the top chord is in compression.
3What does buckling mean?
A long thin member in compression bends sideways and fails at a load lower than its crushing strength.
4What is strength to weight ratio?
Failure load divided by the bridge weight, a measure of design efficiency.
Take it further
Build a Pratt truss of the same weight and compare the failure load with the Warren truss.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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Mini and Major Project Guide
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