As construction progressed, the all-steel structure drew increasing attention and visitors from both the engineering community and the general public. Because of the legacy of the Tay Bridge and the oddness of the Forth's form, Baker was prompted to prepare a public lecture in which he could explain that the new bridge was not only distinct from the Tay but also much better able to withstand the force of the wind. In the course of preparing his lecture on the unusual design, he found that he needed to solve another design problem: how to develop a visual model of the great cantilever that could be easily grasped by a general audience. It was here that playfulness reigned.
In preparing his lecture, Baker found himself looking for a way to make even non-specialists in the audience appreciate the "true nature and direction of the stresses on the Forth Bridge." After consulting with colleagues on the construction site, a "living model" of the cantilever structure was conceived as an illustration for the lecture. The tableau has also been described as a "human model," because in place of some of the structural components of the bridge were people who could experience directly the forces involved.
Fortunately, one of the assistant engineers of the project, who was also its official photographer, was present as the design of the model evolved. He captured not only the final version but also some early, less effective versions of the model. The series of photographs thus shows how the model progressed from a concept to a polished work of anthropomorphic structure.
The essential structure of the bridge consists of distinct steel towers from which cantilevers reach out toward each other and support a central spanning girder. To counterbalance the weight of these cantilevers plus the suspended central span between them -- and the trains that run along the whole assembly -- opposing cantilevers are anchored to end piers of massive masonry construction.
The human model that mimicked the actual structure consisted of a pair of chairs on each of which a person sat. These represented the steel towers. Each person held a wooden strut, one end of which was wedged between the person's thigh and the chair seat and the other end was grasped by the person's outstretched arm. Together, this arrangement represented a cantilever. The end of the counterbalancing cantilever was attached by a rope to a pile of bricks that sat on the ground. From the ends of the inside cantilevers was suspended a swing seat, representing the suspended central girder. When a person sat in this elevated swing seat, he represented a train on the bridge and so completed the model.
The seated persons felt a pull in their arms and also felt the strut pushing into their thigh, thereby experiencing both the tension in the top chord of the cantilever truss and the compression in the bottom one. The seated persons also felt their torso being pushed down onto the seat of the chair, thereby experiencing the great compression that the piers felt under the weight of the structure and its loads. The audience viewing a lantern slide of the model demonstration could feel the same forces. The model was as beautifully designed as the bridge itself.
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