A human model where you can see and feel the forces

The gigantic 2,200 meter (1.6 mile) long Firth of Forth Railway Bridge in Scotland is nine miles west of central Edinburgh. Those three cantilever steel structures are 100.6 meters (330 feet) tall, and at high tide the railroad tracks are 46 meters (151 feet) above the water. That bridge opened on March 4, 1890. You can read a brief history here. 

How do those cantilever structures work? Benjamin Baker, one of the designers, gave a lecture at the Royal Institution in London on May 20, 1887 in which he showed a lantern slide with a human model that very cleverly let his audience both see and feel just what was going on:

“Two men sitting on chairs extended their arms and supported the same by grasping sticks butting against the chairs. This represented the two double cantilevers. The central girder was represented by a short stick slung from one arm of each man and the anchorages by ropes extending from the other arms to a couple of piles of brick. When stresses are brought on this system by a load on the central girder, the men’s arms and the anchorage ropes come into tension and the sticks and chair legs into compression.

In the Forth Bridge you have to imagine the chairs placed a third of a mile apart and the men’s heads to be 300 ft. above the ground. Their arms are represented by huge steel lattice members, and the sticks or props by steel tubes 12 ft. in diameter and 1-1⁄4 in. thick.”

The March-April issue of American Scientist magazine contains an article by Henry Petroski titled An Anthropomorphic Model that describes the history of that famous visual aid. It turns out Kaichi Watanabe, a young engineer who was a constuction foreman and Mr. Baker’s assistant, was seated on the platform in the middle of the image. 

An image of the bridge came from Wikimedia Commons.