Not Just Structure: The String Section

by Don Friedman on April 13, 2016

We occasionally deal with steel cable in our repair designs, and we deal with steel wire as slab reinforcing in various archaic floor systems. This is neither of those:

That’s what a grid looks like in a  working theater. Some more-or-less obscure terminology: a flyloft is the volume of space above a theater stage (and hidden from the audience by the wall above the proscenium arch), scenery flats “fly” into the flyloft by having ropes pull them upwards, and a grid is the steel-slat-floored space above a flyloft. Very old-fashioned theaters used sandbags as counterweights, hemp rope, and backstage muscle to lift flats; modern theaters use iron counterweights, wire ropes, and motors. The slat floor – visible in the photo – allows ropes to come up from the scenery, turn 90 degrees over a pulley, and run out to the motors and counterweights at the sides of the flyloft. The dozens of wire ropes in the photo indicate dozens of pieces of scenery below, since typically each flat has two ropes.

A critical tool in structural analysis is the use of force vectors, which we draw as little arrows pointing in the direction the force applies and of varying lengths depending on the size of the force. A theater grid, for a structural engineer, is a vector diagram come to life, with the cables showing us exactly where forces are applied to the grid slats and the beams that support them.

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