![[Diagram]](platform.gif)
One solution to this that I have used is steel. 1"x3"x14 gauge rectangular tubing makes a platform that is essentially indestructible as compared to any wood material. It is however somewhat heavier. I have used this material in two configurations that are common to 2x4 wood construction. These are with one or with two interior supports in a 4x8 platform. The 3/4 plywood tops are attached with 1 1/2" Phillips wafer head Tek screws. For those new to Tek screws, they drill a hole in the steel and tap threads into it as they are installed. The wafer heads have a large enough diameter head that pulling through the plywood is not an issue. The rectangular tubing is stiff enough that it can be supported at the 4 corners for most applications, going to 2 additional center legs when a very rigid platform is desired or the loading is higher that usual (audience platforms especially). We have never bent one of these frames and am still using the same 12 we built 10 years ago. Flat head slotted machine screws rather than carriage bolts are used for attaching legs to these platforms. The platforms are fitted with coffin locks, welded to the frames, which are sometimes useful for large areas if the crew remembers to align them properly. We often just C-clamp platforms together mixing steel and wood frames as convenient.
UMBC has produced a traveling production called "Shakespeare On Wheels" for the last nine years. Steel construction is especially attractive for this application, that being an outdoor set on a flatbed trailer that is set up in a different location every two days, for its strength and its imperviousness to water and heat. The few wood framed elements we have tried to use have disintegrated in short order and had to be rebuilt. To be fair, we have had repairs to do with the steel construction, principally an occasional cracked weld on the stage floor side frames which travel vertically forming the sides of the trailer. This is principally travel damage from trailer flex as the weight bounces. The only complete unit replacement was a 1" tubing bi-fold flat that was cartwheeled across a large parking by 80 MPH winds. In this case, it was easier to rebuild than straighten.
The floor framing for the sides of the "Shakespeare on Wheels" stage is the same 1x3 tubing described above. It is constructed in 8x8 sections to be manageable in size and weight; and because the first two or three years we rented the trailer and had to completely dismantle and store the stage every year. These 8x8 sections are attached to the trailer with three 12" strap hinges and supported at the opposite end with two telescoping legs of Unistrut TelSpar. This is square tubing in sizes that fit inside one another and in this case have holes every inch. We set up almost anywhere and a flat surface in never available. This trailer has set up with a differential of as much as 8 feet in distance from stage deck to ground at diagonal corners of the trailer. This is admittedly not an everyday variation, a foot or so is more normal so that two three foot pieces of TelSpar can adapt sufficiently to get within an inch of level with plywood shims under the leg used to get within 1/4" of level. The plywood also keeps the leg from sinking into the ground or pavement, which can happen on hot days. But this doesn't have much to do with platforms so back to the subject.
The "Shakespeare on Wheels" project has always used the 1x3 steel for large platforms or one piece of 1 1/2" square tubing for smaller areas where deflection is not an issue. The height of thesecond level of the set has always been 7' 0", a height selected because, with some hinged fold and stack provisions of the early design, this height allowed us to stay below the legal height for travelling without an escort. Last year, with a new designer the second level was placed at 7' 6". This was not a construction problem but did create a problem in that it became too high for the average crew member to lift into place from the deck. In fact most of them had to stand on a step to reach it. The spans were long which meant that the units would be very heavy. I decided this was unacceptable for logistical and safety reasons and began to look for a solution.
As it turned out, I already had used the solution a number of years earlier to build very low, large turntables. The problem was to reduce the weight and still be able to span the distances required. OK, who's ahead of me? The answer is to replace mass with structure in the form of a truss. I was able to reduce the weight of the frame by one-third without increasing the size at all. I also reduced the weight further by replacing the 3/4 plywood that would normally be used with 1/2 plywood, eliminating a third of the wood weight. This overall reduced the weight by a third! The weight of the usual platform of 1x3 x 14 ga tubing with 3/4 plywood is 150#. The weight of the truss platform with 1/2" plywood is 100#. I was a little surprised to get that much reduction. Half the weight reduction is in the frame and half is in the reduction of plywood thickness.
The way a truss works is that if you have two members that are kept parallel, bending requires that either one gets longer or the other gets shorter. This is of course very difficult so they try to get closer together instead. So maintaining the spacing is the key. More distance apart increases strength. More connection points, i.e., less distance between connection points, to maintain spacing increases strength. Diagonal bracing between spacing/ connection points also increases strength. A common example is the bar joists often used to support roofs of buildings and, of course, bridges often use trusses to span large distances.
For this application, I used two pieces of 1" x 18 gauge (.049 inch) square tubing one inch apart. One inch spacing allowed used the same tubing in the same orientation for spacers. I used three inch pieces, spaced approximate 12 inches apart, and welded one half inch at the four corners of the three inch piece. Six inch pieces were used at the corners placed such that the corners were interlocked by the center piece. Internal cross supports were constructed in the same way with the ends extending to interlock with the sides. Cross supports were placed on two foot centers. A three inch piece of TelSpar was welded into the corners to accept the support legs. An added advantage of the interlock in the corner is that the hex bolt that secures the leg can be recessed into the frame without any effort. Another advantage is the slots between the spacers make the platform much easier to handle. The single problem with these platforms was that an actor managed to fracture the plywood in the center of span between supports where there was a void in the plywood. I will reinforce this by adding a single tubing piece down the center line of the platform. This will effectively reduce the spans to two feet square.