Petroski on Engineering: Design Begets Design

New designs have a variety of origins. Some are prompted by necessity, some by desire, and some by just plain playfulness.

An interesting historical example that incorporates all three forms of prompting is the Forth Bridge, which was designed and constructed in the 1880s and which still carries railroad trains high above the River Forth near Edinburgh, Scotland.

The Forth Bridge and another across the River Tay about 35 miles north at Dundee had their origins in the need for fixed crossings over the respective rivers so that the North British Railway could do away with the use of ferries to transfer rolling stock wherever the rails came to a river bank. By bridging the two rivers, the railway company could reduce travel time between Edinburgh and Dundee, and so better compete with the inland Caledonian Railway.

The commission to design the two bridges was given to veteran engineer Thomas Bouch. Both river crossings were to be made at their wide estuaries, which the Scots call "firths," and so long bridges were required. The Firth of Tay is shallow, however, and so Bouch designed a bridge with many piers connected by relatively short girders, except for the so-called "high girders" over the navigation channels. When the Tay Bridge opened in 1878, it was the longest in length of any bridge in the world, reaching across almost two miles of water.

As ordinary a design as it was, the Tay Bridge lasted less than two years. During a storm in December 1879, its high-girder trusses collapsed while a train was passing over them, taking the lives of 75 people. The accident naturally led to an investigation, and a court of inquiry found that the structure had been "badly designed, badly constructed, and badly maintained," and engineer Bouch, who had been knighted when the bridge had been completed, was discredited and the commission to design a span over the Forth was taken from him.

A new Tay Bridge, designed by railway engineer William Henry Barlow, was built right beside the failed one, with the stubs of the piers remaining exposed to this day as a reminder of the tragedy. The commission to design a bridge across the Forth was given to the engineering firm of Sir William Fowler and his young partner, Benjamin Baker. There was no confidence in the suspension bridge that Bouch had designed for the Forth, and the railway desired that Fowler and Baker come up with a novel design that would in no way remind potential rail passengers of either of Bouch's designs.

Baker was the lead designer for the new bridge across the Forth, and he came up with a structure based on the cantilever principle. Like a cantilever beam that is supported at one end only, Baker's bridge reached out from its towers without any need for additional support from below. This was an ideal choice for the site, since the Firth of Forth is deep and so erecting a supporting structure in it would be time consuming, dangerous, and expensive. Only three sets of deep foundations, over 1,700 feet apart and supporting 300-foot-high piers, had to be constructed in the water. The daring configuration would give the Forth Bridge the longest spans in the world, a record that would not be surpassed for almost three decades.