Civil & Environmental
Bridge Builder Lab
This is where statics becomes a yes-or-no answer. Put a load on the bridge deck and the truss is solved with the method of joints (the same solver behind the Truss Lab), then every member is rated against its strength capacity. Safe members glow by tension (green) or compression (amber); the moment any member's force exceeds its capacity it turns red and the bridge collapses. The lesson engineers live by is built in: a structure is only as strong as its single most-stressed member — its weakest link — not its average. Push the load up, or weaken the members, and watch exactly which one gives first.
71%
Peak utilisation
40 kN
Load
60 kN
Member strength
The method, with your numbers
Method-of-joints load rating
- 1
Find the support reactions
ΣM_pin = 0 → R_roller = P·a / L ΣF_y = 0 → R_pin = P − R_roller
R_roller = 40·2 / 6 = 13.33 kN R_pin = 40 − 13.33 = 26.67 kN
The load splits between the two supports in proportion to how close it sits — the nearer support carries more.
- 2
Solve every member (method of joints)
at every joint: ΣFₓ = 0, ΣF_y = 0 → 12 equations, 12 unknowns
9 member forces solved — most-loaded: left end diagonal, F = -42.69 kN (compression)
On paper you hop joint to joint, starting where only two unknowns meet; the lab assembles all 12 equilibrium equations and solves them at once (Gaussian elimination).
- 3
Rate the critical member
u = |F| / capacity
u = 42.69 / 60 = 71%
Every member gets this check; the biggest ratio marks the weakest link — the first to break as the load climbs.
- 4
The verdict
holds ⇔ every |Fᵢ| ≤ capacity (max u ≤ 100%)
max u = 71% ≤ 100% → HOLDS
Member forces scale linearly with the load, so this bridge would first fail near 56 kN. Drag the load slider there and watch.
Drag any control above — every number here recalculates. Want this method for any problem? Step Sheets →
Load position
The deck carries the load to the supports through the truss; the solver finds the force in every member, then each is checked against its strength. The bridge holds only while every member stays inside its capacity — push the load up (or weaken the members) and the most-stressed member hits 100% first and fails, turning red. That weakest link, not the average, decides whether your bridge stands.
How to use this simulation
This is where statics becomes a yes-or-no answer. Put a load on the bridge deck and the truss is solved with the method of joints (the same solver behind the Truss Lab), then every member is rated against its strength capacity. Safe members glow by tension (green) or compression (amber); the moment any member's force exceeds its capacity it turns red and the bridge collapses. The lesson engineers live by is built in: a structure is only as strong as its single most-stressed member — its weakest link — not its average. Push the load up, or weaken the members, and watch exactly which one gives first.
Everything runs in your browser — no sign-up, no download. Change a value and the result updates instantly, so you can build a feel for how each input shapes the outcome. It pairs with Crameleon's practice exams and step sheets when you want to go from intuition to working the problems.