An Introduction to Stressed Skin Platforming
Welcome to this months Techies Corner. The last two articles have discussed types of stock platforming that have been around for several centuries, the parallel, and for almost a century, the plywood covered 4x8 unit. This months article will be about a type of stock platform first developed about 1990 at the Yale School of Drama, called a Triscuit.
The Triscuit is four foot by four foot, stressskin unit 2 3/8 thick. To truly describe a triscuit, we will first have to explain just what a stress skin unit is. For those of you who already know, skip the next few paragraphs.Stress skin almost defines itself, i.e. a unit with a skin under stress. But what does that really mean? Actually it means a unit with two skins which oppose each other in the direction that they handle stress. Between the two skins is a core to which the skins are completely bonded. The core may be continuous, such as foam core mounting board, or open like the honeycomb construction of a hollow core door. In order for a stress skin unit to flex or bend, one skin has to stretch and the other has to compress see Illustration #1
Now lets look at the same units with a load applied.
All of us confront stress skin units everyday; the most common is the hollow core door. Other examples are plywood, steel I-beams in construction and many of the tabletops in folding business tables. If you do artwork or photo mounting you may have come across a product called foam core. All of these are examples of stress skin construction.
The hollow core door has a very thin skin about 1/16 to 1/8 thick on each side. The outside frame between the skins is about 2wide on the sides with 4 extra in the doorknob position. The top and bottom frames are usually about 2 wide. The interior of the door is usually filled with CARDBOARD! The cardboard is about 1/32 thick and formed into a honeycomb with the cardboard on edge and each cell of the honeycomb is about 1 to 1 ½ across. When the door is assembled, every edge of the honeycomb is covered with an adhesive; every edge and face of each component that comes in contact with another is coated with an adhesive. When the door is finished it is very stiff. Many of you have made drafting tables, shelves and benches with old doors. You may have noticed that even though the door can support a great deal of weight, it is lightweight and easy to handle. If you have ever lifted a solid core door, you know they are very heavy and awkward to handle.
To make a comparison that will be readily understood, a common stock platform, using 2x4 for the framing members and ¾ plywood for the lid, will sag noticeably with a 150 lb. weight in the center if it is supported only at the ends. A 4x8 stress skin unit with a 2x4 frame with 8 members on 16 centers and with a 5/8 lid and a 3/8 bottom will support almost 2,000 lbs. evenly distributed which is about 60 lbs. psf. or over 500 lbs. in the center before it deflects more than ¼.
That is the essence of stress skin construction. A very high strength to weight ratio. In the construction industry, reduced weight usually means less materiel and less materiel means reduced manufacturing costs. In theatre the reduced weight means easier handling which can result in time savings or labor savings.
The Triscuit platform takes advantage of the stress skin principle to make a unit that is small enough and light enough for one person to handle yet is very strong. Because of its size and thickness, triscuits can be stored in smaller spaces. For comparison, four 4x8 stock platforms with a 2x4 frame, require a space 4 x 8 and 17 deep. Eight triscuits (equal in floor area to the four 4x8 units) can be stored in a space 4 x 4 x 19 deep, just an inch more than half the space.
Strong, light, store in less space, why dont we all use triscuits? There are some drawbacks. First of all they take longer to build. Second they must be built with more care and skill than a plain stock unit. Third they rely on the studwall system or similar system of legging and other legging systems will not work.
Are triscuits right for you? I cant answer that because every theatre has different needs, space and personnel. Right or wrong depends on your mix of these things. Now heres how to build a triscuit.
All framing is 5/4 stock @ 2 true wide. Depending on the lumber dealers in your area, 5/4 nominal stock may be anywhere from 1 to 1 3/16 True thickness. Assembly of the frame is with one 4 drywall screw, predrilled at each joint. Note that the screws at the corners must be off set to miss the bolt down hole. All joints are glued and the end grain pieces are coated with glue. After a 3 to 5 minute wait, they are coated again and then assembled.
The skin on both sides is 5/8 plywood. OSB would also work but add about 10% to the weight of the unit. The frame is completely brush coated with glue on the surface and the ply is then screwed down with 1 5/8 screws on 6 centers. The original design called for nails but I strongly advise screws. Great care must be taken at this time to insure that the unit is very flat and not skewed from corner to corner. Flip the unit over and repeat the skin attachment on the other side. When the glue has set, drill and counter bore the corners and your triscuit is done.
The Texas Triscuit
Due to problems with lumber supply of 5/4 stock in his region, Tim Francis, technical director at Trinity University developed what he calls a Texas Triscuit. Although the Texas Triscuit is not a stress skin unit, I include it here because of the close similarity of the units. In fact it should be obvious that one could very easily turn it into a stress skin by merely adding the second skin on the bottom.
The basic unit shares the same advantages of the triscuit. It is light, easily handled and stores in very little space. It has a few advantages over the original triscuit. It is actually lighter at about 56-lbs. total weight. It uses less material, thus cheaper. The frame is steel, thus virtually indestructible, the lid can be easily replaced when it is damaged or worn out. As might be expected there are also some disadvantages. The first is that the frame is made of steel and must be welded. If welding is beyond your means or skills, then you can not build a Texas Triscuit in your own shop. Although it will support as much weight, it is ever so slightly more springy under an active load. I suspect that adding a lower skin, even if it were only ¼ or 3/8 instead of the 5/8, and both skins were attached with a continuous bead of construction adhesive, then the unit would actually be stiffer than the original triscuit. Now to the point, here is the Texas Triscuit.
The frame is 1 ½, 16 ga. square steel tube. The first step is to cut the steel to length and drill the four corner holes. Next all joints are welded with a 1/8 bead all around. The most common problem in the construction is failure to maintain a square and flat structure during the welding process. The best way to achieve this is to build a jig of 2x4 flat on top of a worktable covered with hardboard. The jig should leave all the weld joints exposed. Next a series of tack welds will help to stabilize the unit. Tack the corners, top and bottom of each joint. Next complete all fillet welds on the inside seams, welding from the center of the joint out in both directions. The reason for this is to balance the stress and expansion/contraction that welding creates in all joints. Next weld the outside of the four corners, followed by the top surface. Carefully lift the frame from the jig, reverse it and finish the welds on the other side. Note that you can not be too accurate in building the jig. Every minute you spend on it will save you time in construction and help to insure accuracy. All the welds on the top, bottom and outside are then ground flush. Be sure to file or grind the insides of the exposed open tube at the corners to prevent finger catchers.
If you intend one lid to last the life of the unit the top may be fastened on with T nails and a construction adhesive. If you think you will replace the wood lid before the frame wears out, then drill point tek-screws are the answer. In either case, the fasteners should be on 6 centers all around. The construction adhesive method will result in a slightly stiffer unit, but it is very difficult and tedious to remove if you want to change the lid.
As I mentioned earlier, both of these platform types are designed to be legged with a studwall system of supports. Later in this series I will be covering legging systems of many types.
As with all articles, if you are reading this in the archives, the illustrations will be missing. Drop me an e-mail and I will be glad to send them to you. Until next month, keep the green side up, dont sweat the small stuff and remember .. Its all small stuff!