OneTaskBy Tim King · Updated
Bridge Builder Game
Build a Truss Bridge, Send a Truck Across and Watch It Hold or Snap
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How the Bridge Builder works
This bridge builder game tests whether your bridge holds. A deck of planks spans the gap between two piers, braced by a Warren truss of beams. Press Send the truck and a loaded truck rolls on from the left. Every beam is coloured by how hard it is working, green through to red, and any beam stretched past the snap limit breaks and drops out. Click above the deck to add a joint, drag between joints or plank ends to add a beam, and tune stiffness, snap strain and truck load to see what gets across.
Bridge builder game: what it does and why
Bridge-building games are popular because they teach a real idea with no equations: a structure stands or falls on how it routes force to the ground. This one runs a 2D physics engine in your browser, with the deck made of hinged planks and the bracing made of spring-like beams that report how far they have stretched.
Why a truss. A flat row of planks on its own sags into a curve and the hinges between them carry all the load, which is the worst possible shape. Adding a row of joints above the deck and connecting them in a zig-zag turns the whole thing into a truss. Now a load on the deck pushes the top chord into compression and pulls the diagonals and the deck into tension, and those straight-line forces are far easier for a beam to carry than bending. Watch the colours as the truck crosses: the diagonals nearest the wheels light up first, and the top chord over the middle of the span carries the most as the truck reaches midspan.
Strain and snapping. Each beam has the length it was built at. Strain is how far it is currently stretched or squashed, expressed as a percentage of one plank length. The snap strain slider sets the limit; past it the beam is removed and the load it was carrying jumps to its neighbours, which is how real collapses cascade. A stiffer beam stretches less for the same force, so raising stiffness lets the same bridge carry a heavier truck — up to a point, since a very stiff structure transfers shocks from the wheels straight into the joints.
Building your own. The generated truss is a starting point, not the answer. Adding a second row of joints, doubling the diagonals under the middle planks, or tying the top chord down to the piers all change how load spreads. A joint added with a click floats until you connect it, so drag a beam to it from a plank end or another joint. Beams are massless, joints are light, and the piers are immovable; the deck planks and the truck are the only things with real weight. Everything runs locally; Pause, Fullscreen and Save PNG work as usual, and Rebuild the bridge starts over with a fresh truss.
FAQ
Why does the bridge collapse in the middle?
That is where the bending moment is greatest. Add a joint above the middle of the span and tie it to the plank ends with diagonals so the load goes into the truss rather than the deck hinges.
What do the colours mean?
Strain. Green is a beam near its built length, red is one close to the snap limit, whether stretched or squashed. Watch which beams go red first as the truck crosses; those need help.
Does the truck weight change the truss?
No, only the load. Raise the truck load to test a design; if a beam snaps, either add a beam to share its load or raise stiffness so it stretches less.
Can I delete a beam or joint?
Not individually. Rebuild the bridge to start from the generated truss, or let a bad beam snap and carry on. A snapped beam is gone for good, so the fastest way to test a change is to rebuild, add your beams and run the truck again.
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