Hello again, and welcome to the Techies 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. 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.Before we describe specific legging systems we need to define just what a legging system does and how it does it. What is legging? Legging is: A system of raising a platform to a desired height, spaced to prevent sagging and with sufficient cross bracing to prevent lateral movement. The function of any legging system is to transfer the load on the platform to the structure it rests on, whether that is the permanent stage floor or other temporary stage platforming. To tell you just how legs and bracing perform that task will require some drawings, a brief explanation of the geometry of a triangles and the ability to multiply by 50.
First lets 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 its 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 1x4 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 6tall platform with 2x4 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.
Now it seems like I have contradicted myself. I said that you could build a platform with a leg 50 times taller than it is thick and then I tell you that it will collapse. The 50:1 leg will support the platform and the load, but only if the weight is completely still and there is no sideways motion caused by walking actors, dancers etc. Our next task is to brace the platform to stop any swaying or sideways motion and this is where the triangle comes into play. The triangle is the only geometric form that can not change its shape without breaking either a side or separating a joint.
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.