Hello again, and welcome to the Techiest Corner. In the last three articles we have looked at a number of different types of platforms. Not every type, just some of the most used and the most well-known. Now we are going to look at how to elevate or leg these platforms to the height needed for our show. There are more ways of legging a platform than there are of building the platform itself. What makes a legging system right for you depends on your particular situation.<span> </span>Do you have storage space for stock leg pieces? Do you have skilled carpenters or do you rely on a group whose skills vary from pro to rank beginner? Do you build your units on stage or in a shop off site? Look at the costs, time, skills, available tools, etc. and decide which system is best for you and your theatre.
First let’s look at the leg itself and that number 50 that I mentioned. Any leg will be very stiff in short lengths, but every leg will become flexible when it gets long enough. A piece of ¾” x ¾” pine will support 1,000 pounds when it is only one inch long.
A 12″ steel I-beam will barely support it’s own weight at 100″ tall, a stiff wind will bend and collapse it. How tall or long can a leg be and be safe? A standard formula that works for theatrical use is a 50:1 ratio. That is 50 times the narrowest cross section of the leg is the maximum height that leg can be without some form of bracing to prevent buckling or bending.
This formula will work for all materials used in general stagecraft, wood, steel tube, pipe etc. For example a simple 2″ x 4″ leg can be 45 tall before it needs bracing to prevent buckling or bending. The calculation is simple, the nominal 2″ thickness which equals 1 ½” times 50 equals 75″ or 6″-3″. If you nail two 1×4 boards together to form an “L” or “T” section, the narrowest dimension is 3 ½”. Fifty times 3 ½” equals 175″ or 14″-7″.
Now this DOES NOT mean you can build a 6″tall platform with 2×4 legs or a 14″ tall platform with “L” legs and no cross bracing. The leg is actually fastened to the platform in a very small area and the length provides a very effective lever when the platform sways the least bit. Twenty pounds of sideways push will cause the 6″ platform to fold sideways and the legs to split or tear out the fasteners. It would only take about 5 pounds of sideways push to topple the 14″ unit.
Triangular bracing is what makes tall buildings, bridges, the Eiffel tower and other construction possible. Look at a bridge or a building under construction before the bricks and facing are added. What you see is a series of triangles in some form or after.