The Human Cantilever Model of The Forth Bridge

LEARNING OBJECTIVES This is a kinesthetic activity in which students interact with a physical model to understand the behavior of a , based on a ‘human cantilever’ demonstration of the Forth Bridge devised by bridge engineer Benjamin Baker.

After this activity, students should be able to: 1. Demonstrate how a cantilever bridge relies on a balance of forces 2. Describe the role of the different structural elements in a cantile- Kaichi Watanabe (center) demonstrating the cantilever principle. ver bridge and identify those that source: public domain are under compression or under tension and four broomsticks. With their out- sides feel on their legs? Quantitatively 3. Reason about the stability of the stretched arms, the people on the left estimate how much force the rope cantilever and relate changes in and right serve to transfer the load of exerts on their feet. Predict how this one part of (e.g. greater the suspended person (center) to the force change if no one was sitting in the load) to changes in other parts of anchors (pile of bricks on left and right). center? What if there were two people the bridge sitting in the center? In this activity, we recreate this classic Materials List demonstration, through which students Which of the structural elements are in - 4 planks of wood of size 30” x 4” x 1.5” can experience and understand how compression and which are in tension? - 1 plank of wood of size 24” x 6” x 1.5” forces flow through a cantilever bridge, - A drill and gain an understanding of the role Through this activity, students - Nylon rope (under 0.5” diameter) of balance in this type of structure. should recognize that when a load is suspended on the central section Procedure of the bridge, the upper beams of Background Drill 0.5” diameter holes in wood planks the cantilever arms are stretched in Cantilever bridges can be constructed and thread rope through the holes as tension, while the lower beams are without the need for a large amount shown in the figure. Ensure that the squeezed in compression. Furthermore, of falsework and are therefore useful knots are carefully tied so that they will they should also recognize that each of in spanning difficult crossings. The not come undone under heavy loads. the anchors need to support one half of Forth Bridge is a cantilever railway Smoothen the edges of the 30” long the suspended weight. bridge spanning 8,296 feet over the cantilever arms so they can be easily of Forth in East , and at gripped. Contributors the time of its construction in 1890 This model was assembled by Serguei was the longest-spanning cantilever Students can demonstrate Bagrianski and Matthew Streeter. bridge in the world. This was a bold understanding of this exercise by Additional teaching materials were application of the cantilever principle answering questions as follows: developed by Maria Garlock, Negar on such a large scale, at a time when Khorasani, Aatish Bhatia. this technique had only been used for What do the people standing on the major bridges for a few decades.

To demonstrate the stability of the cantilever, Benjamin Baker designed the following demonstration with three men, two chairs, two piles of bricks,

This material is based upon work supported by the National Science Foundation under Grant No. NSF 14-32426, 14-31717, and 14-31609. Any opinions, findings, conclusions or recommendations expressed in the materials provided are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. The Creative Art of Structural and Civil Engineering casce.princeton.edu