Why are deeper trusses stronger?
How truss depth lowers the forces in the top and bottom chords, why it makes bridges stiffer, and what limits how deep you can go.
Updated · 1 min read
Short answer: the top and bottom chords act like a pair that resists bending. The farther apart they are, the less force each has to carry to resist the same load. A deeper truss means smaller forces in the chords.
The idea
A load in the middle of a span tries to bend the bridge. The top chord resists by being pushed, and the bottom chord by being pulled. Those two forces, a distance apart, make a "couple" that balances the bend. For the same bend, doubling the distance halves the force in each chord.
You can see it in the worked example: raising the apex from 100 mm to 200 mm cut the bottom chord force from about 0.75 P to about 0.38 P.
Why not make it very deep?
- Rules. Contests set a maximum height. See reading your rulebook.
- Long diagonals. Deeper means longer diagonals, and long pushed sticks buckle. See preventing buckling.
- Stability. A tall narrow truss can tip over sideways unless braced. See twisting and leaning.
- More weight. Taller verticals and diagonals add mass.
A good habit
Use as much depth as the rules and your bracing allow, then check the longest pushed members. See truss basics.