Parabolic Arch Bridge (IDEA 210)

IDEA 210: Mechanics and Materials.

Project 2: the bridge

The brief was a 3 ft clear span, a deck no wider than 7 inches, and a design judged on structural response, material efficiency (load carried divided by self-weight), and aesthetics.

My bridge passes load from the deck to a parabolic arch through post-tensioned cables. Each cable is three braided fishing lines, braided together again and fixed into 3D printed ends, and MDF spacers let the system be fine-tensioned. Each arch is a sandwich of two 1/8 in MDF sub-arches, glued and fixtured. Because the laser cutters were too small, each sub-arch was cut in three pieces. The braced joints are offset between the two layers so they don't line up into a weak point, yet every brace ties into both layers.

Results

• The bridge tied the university record at 175 lb. There was no more weight available to test to failure.
• Self-weight was 6.6 lb, an efficiency of 26.52.
• Maximum deflection, measured with an ultrasonic distance sensor, was 0.12 in.

The same course's first project was a load path model of the Miho Museum Bridge.