Benjamin Baker’s Human Cantilever Model of the Forth Bridge

The Forth Bridge is a cantilever railway bridge across the Firth of Forth in the east of Scotland, 9 miles west of central Edinburgh. Completed in 1890, it is considered as a symbol of Scotland (having been voted Scotland’s greatest man-made wonder in 2016), and is a UNESCO World Heritage Site. It was designed by English engineers Sir John Fowler and Sir Benjamin Baker.

After the collapse of an earlier Tay Bridge in 1879 (with the loss of an estimated 75 lives), Benjamin Baker and Sir John Fowler submitted a design to the Forth Bridge Company on the cantilever and central girder principle. Fowler and Baker were well-established engineers whose long list of achievements included a substantial role in constructing the London underground rail network.

For many years an exponent of the use of cantilevers as the most effective means of constructing long-span bridges, Baker devised the human cantilever to explain the principle at a lecture to the Royal Institution in London in 1887.

To demonstrate the stability of the cantilever, Benjamin Baker designed a demonstration with three men, two chairs, two piles of bricks, and four broomsticks. With their outstretched arms, the people on the left and right serve to transfer the load of the suspended person (center) to the anchors (pile of bricks on left and right). As he explained, “when a load is put on the central girder by a person sitting on it, the men’s arms and the anchorage ropes come into tension, and the men’s bodies from the shoulders downwards and the sticks come into compression.”

Hanging by a Thread

Forth Bridge’s engineers created one of the first living models of the bridge in 1885. A closer look at the photograph makes a question if the model was successful, both in explaining the cantilever and transmitting confidence.

‘Forth Bridge Construction’, possibly Evelyn George Carey, 1885.

The building in the background is not set on the ground; it is supported by construction materials, creating a feeling of uncertainty and improvisation. The composition is off-centre, and the window and the construction site are distracting. The wrinkled drawing on the wall looks makeshift. There are construction materials scattered around, making it difficult to see what is holding the wooden sticks. Wooden boards under the left chair try to compensate the unevenness of the ground, aggravating the sense of precariousness.

The feeling of instability is even more pronounced if we look at the young man sitting in the middle. His face looks worried, and his awkward body language – a stiff pose, hands rigidly placed on his lap – suggests uneasiness. Looking even closer, we notice that his seat is tilted and secured by sloppy, thin strings. They seem to be on the verge of breaking. He is, literally, hanging by a thread. One cannot help but wonder if he fell off the model in the end.

Stronger Ropes, Stronger Message

Two years later, at a talk held before the Royal Institution, Benjamin Baker showed a photograph depicting a notably different setting.

‘Living Model Illustrating Principle of the Forth Bridge’, published in ‘The Forth Bridge’, W. Westhofen, 1890.

The perfectly centered building is sturdier and firmly set on an even, cleaner ground than in the first version. There are no distracting elements. The drawing on the wall looks more polished. The chairs are levelled and the brick structure holding the model on each side is clear. The two men sitting on the chairs are the same as in the previous image. Their faces show the same expression. But their bodies are now much more confident and upright, their limbs straight and symmetrical.

Even the ropes are thicker, enhancing the strength of the structure. The ropes holding the seat are shorter and were put closer to the ends of the sticks, positioning the middle seat higher. The analogy with the cantilever principle applied to the bridge became more pronounced.

Looking Confident to Create Confidence

The most striking difference is, however, the man sitting in the middle. The insecure looking, puny boy has disappeared. In his place we find a man with a knowledgeable-looking countenance, dressed in better clothes, his clock chain partially showing. The man’s hands are firmly placed on the wooden board. His legs are crossed, revealing how relaxed and assured he is. There is even another image of this same model where he crosses his arms with confidence, looking almost defiantly at the camera. He is telling us that he will not fall.

‘Demonstration of the Cantilever Principle of the Forth Bridge’, 1887.

The 1887 model is clearly much more effective in showing that the cantilever principle applied to bridges is stable and safe. More than explaining an engineering principle, it has a reassuring agency of its own. It shows how a model can be both objectively accurate and subjectively persuasive.

The well-dressed man of this second model is Kaichi Watanabe, a Japanese engineer who studied in Glasgow and who worked at the Forth Bridge construction.

The Forth Bridge under construction.

The Forth Bridge under construction.

(via V&A blog)