Building the Forth Bridge on Stage

Cantilever bridge human model

For my first time on stage at Science Showoff back in November 2011, I decided to recreate the famous public demonstration conducted by engineer Benjamin Baker to reassure the public that his planned Forth Rail Bridge would stand up. For me, this demonstration captures both the engineering daring-do and the showmanship of the period.

In Baker’s experiment, two stout volunteers sitting several metres apart represent the enormous pylons of the Forth Bridge, their arms out-stretched to represent the top chords of the structure, broom sticks stretching from hand to foot representing the bottom chords of the structure. On a seat suspended between the human pylons a slighter fellow sits representing the weight of a train passing from one structure to the next. What stops the two human pylons from see-sawing in towards the middle under the weight of the central load are the brick counterweights attached to their outer arms. These counterweights represent the massive weight of the approach gateways on either side of the bridge, and show how these gateways play an integral to the stability of the bridge.

The demonstration is beguilingly simple; recreating it on stage was not. Given the restricted performance space, I had to align the human bridge on the diagonal. Whereas the original experiment was conducted against a wall, mine was done mid-stage, without the benefit of the lateral stability that a wall would have offered. In placed of the broom sticks I created four wooden armatures to represent the bottom booms of the truss so that I could make the necessary connection with the pub chairs – these wooden arms were less sturdy than I had hoped. Finally, as I had arrived at the venue by bicycle, I needed on-site counterweights. The pub were unhappy about me using beer kegs, so one end of the structure was tied down to the underside of the stage, while the other was attached to a hefty base amp.

The rules of Science Showoff are clear: 9 minutes only on stage. Without the benefit of any rehearsal time, I took to the stage. Three volunteers were selected; all were given fake moustaches for authenticity. Everything was in position, but it all looked very shaky. With a few seconds left, the volunteer in the middle riding the bridge nervously lifted her feet from the floor. Without any wall to lean against, the whole structure began to wobble out of plane, but for a few seconds at least the span was achieved.

Sadly no photos were taken, but it is a moment I won’t easily forget. I would love to repeat this experiment, but next time I’d build more sturdy armatures designed to actually fit the seats at the venue, I’d do it on a wider stage…and I’d do it against a wall.

I didn’t know the Science Showoff team at the time, but they have since told me they were scared. Daring do indeed.