Platforming Article #5
Legs, Legs, Legs! (Betty Grable, Eat Your Heart Out!)
Once again, welcome to the Techies Corner. In the last few articles we have talked about platforms, the types of platforms, special platforms and how to brace them. This month we will look at legs, legs of many types and how to make them and what their pros and cons are.
Legs fall into two major types, those that support by friction pressure and/or the shear strength of fasteners, called the standard leg, and those that support by direct, in-line compression, called compression legs. I am sure that there will be several types of legs that I will miss or forget. Please contact me directly and I will make space for those and include them in next months article.The basic difference between the two types of legs is manner in which they support their loads. A friction/shear leg relies on the tightness of the fasteners, the sideways friction generated between the leg and the platform and the sheer strength of the chosen fastener. The compression leg relies on the direct in line compression of the leg material.
The standard leg uses friction as a major element of support. If you have ever seen a magician or box-juggler perform a juggling act with three or more cigar boxes (or similar size/shape cardboard boxes) tossing them into the air and catching them between other boxes, you have seen the friction part of the puzzle. If you have ever used a C clamp to fasten a temporary leg to a platform or to support a piece of work in the shop you have used this principle. The method of fastening the leg to the platform can add to the friction element. Bolts with nuts add the greatest amount of friction force, screws or lag screws a moderate amount and nails or pneumatic staples, the least.
The other half of the equation in the standard leg is the shear strength of the various fasteners. When the leg is fastened to the platform frame, there is a shearing action formed, much like the shearing action performed by common sewing scissors
The shear strength of the fastener added to the friction force is the total strength of the standard type of leg. It should be obvious by now that fasteners which produce a compressive force will be stronger than those that do not. All fasteners produce some degree of compressive force, smooth shank nails the least, bolts and nuts the most, with screws, staples and ring shank nails falling in the middle.
What may not be obvious is that more fasteners do not necessarily mean a stronger joint. If wood is the construction material and a leg is between 2 ½ and 3 ½ wide and a platform frame is between 3 ½ and 5 ½ wide, there are a limited number of fasteners that can be inserted in any joint. At some point the grain of the wood will split or there are so many holes made for bolts that there is no wood left. What is the right number of fasteners, is there a specific pattern that is stronger? With nails, screws or staples, the maximum number is 8 if both leg and frame are nominal 6 wide. With bolts, the maximum is 5. If either member is less than 6 nominal wide the maximum number drops.
Any additional fasteners than shown above will either add so little extra strength as to be useless or they will actually start to weaken the joint.
The most common standard leg is the ordinary 2x4. It is cheap, strong and easy. Other standard legs are single piece legs made of 2x6, or 4x4. Manufactured legs, those made of more than a single piece of lumber, are almost always made in an L section. Other variations are the box tube and the T section. L section legs are the most structurally sound in terms of weight versus strength. The box leg is used where the bulk look is desired but weight is a factor. The
T section leg is used when light weight is needed and for design
reasons the T shape is either visible and more desirable or the
bracing is easier to attach.
While wood is not the only material used for standard legs, it is the most common. Steel is also used as a standard leg. Square or rectangular steel tube is the most common form of steel leg. Other forms are angle iron, slotted angle iron, Tele-spar, Uni-strut and pipe.
These are all fastened to the inside of the platform frame just as other standard legs. While through bolts are the most common form of fastening steel legs to platforms, screws and nails through predrilled holes are another method. One last method of fastening steel legs to platforms is by pneumatic T nails. These are specially hardened nails that can penetrate 16-gauge steel tube. Personally I only recommend through bolts to attach steel legs.
The second type of leg is the compression leg. It is called that because the actual support of the leg is in direct compression under the frame. If 100% of the force was straight down and there was absolutely no sideways force, a compression leg would support the load with out any fasteners or bracing actually connecting it to the platform. Of course we all know that the entropy of the entire universe would have to come to a halt for that degree of stillness to exist for more than a very brief moment. Once the leg is braced and secured to other legs, gravity can become the only actual attachment between the legs and the platform. The parallel platform is an example of this as are several commercially built platform systems.
The most common compression leg is the one shown above, a 2 x 4 leg with a ¾ ply cleat attached. For strength in attachment to the platform, the cleat needs to be only 10 ½ long, or about 3 times the width of the platform frame and the same width as the leg. However the best practice is to continue the cleat flush to the bottom of the leg. This allows bracing and stiffeners on the inside of the legs to all follow the same plane. In addition the full-length cleat adds a great deal of stiffness to the leg, especially over 30 long. One variation of this leg is to use 2 x 6 as the leg but to keep the cleat only 3 ½ wide and flush to one side. This allows the leg to be placed at the corner of the platform and have the leg actually support both frame pieces at that corner.
Compression legs can also be used with platforms whose frames are one by stock or 5/4 stock, the leg material is simply the same thickness as the frame.
A very good variation on the compression leg is a combination of the L section leg and the common compression leg. The leg is made with by six stock for the leg, thickness to match the platform frames. The cleat section is also L shaped. These legs provide a very stiff, strong leg that needs little or no cross bracing at 3 tall or less.
All of the metal leg types can also be made in to compression legs. To make compression legs of metal requires welding corner brackets to hold the legs and support the platform frames. There many different methods of making the legs, some with a universal bracket that can be fitted with different lengths of legs to suit the show and others that have a permanent length of leg welded to each bracket. The brackets are all similar in that they have a flange that supports the frame and a face with predrilled holes for bolts or screws. The main difference between different types is whether the socket for the leg (or the leg itself) is welded on under the bracket or inside the predrilled faces.
The socket type of bracket can also be welded under the corner of the flange but then, if the bracket is left on the platform, it is permanently legged to the height dictated by the length of the socket.
The standard and compression legs cover a large number of the legging situations you will encounter, but by no means will they suit every situation.
The main advantages of the standard leg are:
It is the least expensive legging system for material costs on a one-time basis
when using a simple leg such as a 2x4.
They are the quickest method unless you already have a large supply of legs of
the desired height in stock, especially if you are using a simple leg such
as a 2x4. This can also translate into additional $$ savings if your
carpenters are paid by the hour.
Leg length is determined by the thickness of the platform lid, which tends to be
very consistent compared to the width of framing members.
Requires the least amount of skill on the part of the carpenters.
The disadvantages are:
They tend to shorten the life of stock platforms by chewing up the corners with
numerous bolt/screw holes.
They are not as strong as compression legs and are rarely suitable for heavy
and/or prolonged dancing especially of the clog, tap or step styles.
The main advantages of the compression legs are:
They are far stronger and stiffer when properly braced.
They do not tear up the corners of stock platforms.
If they are built well, they can be used over and over again, more times than
standard legs, which can result in a materials saving over time.
The disadvantages are:
They require greater skill and more time to construct.
The length of the leg is dependent on the width of the platform frame members
which can vary from one lumber lot to the next even when purchased from
the same lumber company. Not every stock platform is the same.
They require more storage space than standard legs.
As can be readily seen, there are many way of legging platforms and I have just touched on some of the major ways to do it. What is right for you depends on the skills of your workers and the tools you have available. It also depends on your budget both in terms of time and money. It can also depend on your available storage area, if you cant store it, do you need to build it to last 20 years?
Next Month we will look at a method of supporting platforms that is very strong and stable but isnt really legs. It is also very good for supporting raked or sloped stage sections. It is the Stud Wall method.
Until then, keep the green side up, dont sweat the small stuff, .. and remember . Its all small stuff!