How do the Forth Bridge's three giant cantilevers hold it up, and what did three men on chairs show?

The Forth Bridge stays up because it is built as three giant cantilever structures, each standing on its own granite piers and reaching out on both sides, with a central truss span carried between the tips of neighbouring arms. In a cantilever, the upper parts are stretched in tension and the lower parts are squeezed in compression, and the far ends are weighted down so the whole thing cannot tip. It is a cantilever railway bridge across the Firth of Forth in Scotland, designed by the English engineers Sir John Fowler and Sir Benjamin Baker.

The engineers knew the idea was hard to picture, so in 1887 they staged a famous demonstration. The Japanese engineer Kaichi Watanabe sat between Fowler and Baker, who sat in chairs. Their outstretched arms stood for the parts in tension, the sticks they pushed against stood for the parts under compression, and bricks on the far ends stood for the end piers, weighted with cast iron. Three men and a few sticks held up a person, just as the bridge holds up a train.

Why build it this way? A few years earlier the Tay Bridge had collapsed, and confidence in Thomas Bouch, who had designed an earlier plan for the Forth, dried up and the work stopped. The inquiry found that he had not properly allowed for wind, and the Board of Trade required every bridge to be designed for a lateral wind loading of 56 pounds per square foot. The new bridge was also the first major structure in Britain to be made of steel.

Today each main span has two cantilever arms of 680 feet carrying a central truss span of 350 feet, and the Forth Bridge is still the world's second-longest single cantilever span. You may also know the saying "painting the Forth Bridge", for a task that never ends. That rests on a mistaken belief: there was a permanent maintenance crew, but painters never simply started again the moment they finished.