BridgeBuilder

Inspiration

Physics is an essential part of early education, and a hands-on experience in middle or high school builds a foundation that lasts a lifetime. Yet structural mechanics is usually taught through flat textbook diagrams where concepts like tension, compression, and buckling feel disconnected from reality. In physics, interactivity matters. We created BridgeBuilder MR to turn any bedroom or classroom into a mixed-reality engineering lab—letting students step up to a gap in their actual room, sketch a 3D bridge in mid-air, and immediately discover why bare spans collapse while triangles, piers, and road decks hold strong.

What it does

BridgeBuilder MR blends realistic structural engineering with your physical environment on the Meta Quest 2, allowing anyone to build, test, and iterate on 3D bridges in real space:

Anchor to Your Room: Players select a design challenge (factoring in car weight, wind speed, tire grip, and material budget) and tap two spots in their physical room with the controllers to place support pads across a "valley."

Draw in 3D Space: Holding the trigger lets players sketch structural beams directly in mid-air. Drawn strokes automatically smooth out, straighten into taut members, divide into equal segments, and snap to existing joints or floor-grid piers. Drawing two parallel lines from pad to pad automatically generates a drivable road deck.

Live Dead-Load Preview: While building, the bridge dynamically colors itself to show how hard each piece works just to support its own weight, flagging unanchored floating pieces in purple.

Stress Test & Progressive Collapse: Pressing A sends a car driving across the span. Under load, beams shift from green to yellow to pulsing red accompanied by rising audio creaks. Overloaded members snap and redistribute forces in real time—triggering piece-by-piece progressive collapse while a plain-English panel explains what broke and why before resetting your design so you can fix it.

How we built it

We built the front-end Mixed Reality experience in Unity 6 for the Meta Quest 2 (using Quest Passthrough, XR controller tracking, and procedural mesh generation) backed by an engine-free C# physics core (BridgeBuilder.Core) running on a background thread. The solver scales room builds up 20x and uses an implicit Newmark integrator with Reverse Cuthill-McKee matrix ordering to simulate 3D rigid-jointed frame mechanics (tension, compression, Euler buckling, and bending), two-way suspension car coupling, and wind loads—all verified by 150+ xUnit tests running in GitHub Actions.

Challenges we ran into

Getting up to speed with a brand-new XR platform came with steep Unity-to-Quest setup and hardware compatibility hurdles. We also had to figure out how to make shaky freehand 3D air-drawing snap cleanly into valid geometry, keep matrix physics running smoothly on mobile VR hardware, and coordinate parallel code collaboration without Unity scene merge conflicts.

Accomplishments that we're proud of

We are proud of building a genuine 3D structural physics solver from scratch rather than faking collapses with basic game joints, translating complex stress math into intuitive color and audio cues, and shipping a working Mixed Reality prototype on a new platform backed by 150+ automated unit tests.

What we learned

We learned how to develop for Meta Quest Mixed Reality, how decoupling core C# physics from Unity drastically speeds up testing, and how prototyping multiple versions in parallel—guided by structured GitHub Issues—lets a team combine the best features without stepping on each other's toes.

What's next for BridgeBuilder

Next up is on-headset tuning for material strengths and budgets, rendering visual member bowing before buckling, and replacing the placeholder car box with a detailed 3D model. Long-term, we plan to add multiple materials and beam thicknesses, a dedicated tension/compression color view, sandbox free play, and Quest room-scanning so bridges can anchor directly onto real furniture.