My parents have a pond on their land with a small island in the middle, and I want to build a real rope bridge so my kids can cross over to it. Rather than eyeballing rope diameters and hoping for the best, I figured I'd properly work out the sag, tension, and required rope rating first — and since I was doing the maths anyway, it turned into this tool.

The problem: given a rope strung between two points — a handline, a monkey bridge, an improvised crossing — how much does it actually sag under a person's weight, and is the rope even rated for the tension that puts on it? Those two questions (sag/tension geometry, and rope selection) are usually answered by looking up tables or reaching for a spreadsheet.

My approach was to build the underlying math as a small, standalone, fully unit-tested package (@jsfc/bridge-physics) rather than baking it into a component: an exact catenary solve for when the rope's own weight dominates, a closed-form parabolic approximation for a loaded deck, and a rope-sizing step that applies a 10:1 safety factor — the life-safety rigging convention for rope that directly supports a person, rather than the lower factors used for general lifting gear. That package has no UI dependencies at all, so it's just as usable from a script or a test as it is from this tool.

The outcome is this interactive designer: adjust the span, sag, crossing load, and safety factor, flip between the catenary and parabolic models, and see the resulting tension, the required minimum breaking strength, and a recommended rope diameter update live. It's shipped as its own federated micro-frontend — a separately deployable app that this page loads at runtime — proving out the same Module Federation setup the rest of this site's portfolio pieces can plug into.