What holds the balcony up in a cinema or theatre?
The huge overhanging theatre balconies with their hundreds of seats don’t seem to be supported by very much, but in fact they are examples of a common type of structure called a cantilever. A cantilever is really just a beam that is supported at one end only instead of two. To understand how beams and cantilevers support large weights, get a drinking straw that has pleats in the middle and balance it across the tops of two mugs or glasses. You now have a simple beam supported at two ends. Gently push down with your finger near the centre of the straw and look at what happens to the pleats. Those on the top side are squeezed together as the beam bends, while the pleats on the underside are spread apart. Engineers call these two different stresses on a beam compression and tension. Compression takes place when something is being squashed, like the top of your straw beam. Tension takes place when something is being pulled apart, like the bottom of the beam. To stop a beam from bending und
