So today, I headed down to one of the places I frequently freelance for as they asked me to come down and weld for them... (they really ought to hire a staffer that can weld... I cost them almost as much every year as if they were to get another full time position) Anyway, the project du jour was to weld the frames for some stress skin platforms... For those who may not know what a stress skin platform/door/wall is, let me explain... In order for something to be a stress skin, it requires a few things... It needs the two outside surfaces to be flat, uninterupted sheets of some material... The material only matters in terms of what you are doing with the unit... You would use plywood for a deck/platform... You might use paper, or wood laminate if its a door/wall... The next component of the stress skin is the core... The core needs to be able to be glued extremely well to the outer skins, as well as have a degree of non-compressability... You can use styrofoam, or wood slats, or in my case today, square steel tubeing... In reality, the strength of the unit does not come from the core material, so you can actually make platforms with styrofoam cores that support lots of weight...
If you're still haveing a problem wrapping you head around this think of corrugated cardboard... That's basically a stress skin situation... Now, what happens it you slice through one surface of the cardboard?... You can bend the sheet at that point in one direction easily right?... Ok... Hang on to that thought...
The technical director (the person in charge of a scene shop) tells me what we're doing, and how the units are going to be skined... At this point, my brain starts screaming what the hell?... The intention is to simply glue sheets of plywood to the steel tube core... Ok so far, but the deck is 11'6" x 6'... Plywood generally comes at 4' x 8'... (yes, I know you can get it bigger, but I've never seen 6'x12' plywood, and the price would be insane anyhow...) Not a problem in my mind... instead of useing 3/4" plywood on the top, you use two layers of 3/8" plywood, and offset the seams, and use a ton of glue... Basically you do the same on the bottom with two layers of 1/4" ply... (the bottom can be thiner since nobody is walking on that part) That, however, is not what the intention was... They simply intend to use 3/4" ply on the top, and 3/8" ply on the bottom, with noting overlapping the seams...
Effectively, they have decided to build a deck with two skins, however, it is not a stress skin method, and will not behave in the same manor... All the strength from this unit is going to have to come from the steel frames I made...
Yes, I brought this to their attention... They did decide to go with the double layer on the top skin, but not the bottom skin... I suppose its better than nothing, but it still isn't going to behave like a stress skin...
Stress skins work because the two skins resist stretching and compressing, which means they don't bend... The core simply keeps the skins apart... If the skin isn't one continuous material it doesn't work... Particularly on platforms, the bottom is where the most strain is...
Now, this isn't a new technology... I-beams work in basically the same way... If you decided to use this method, and you didn't know all about it, don't you think you might research it a tad... Or hell at the very least, ask someone that has built and used these units before, before you jump in to a situation that is of structural importance... People have to stand on this thing, elevated off the ground... There are times I wish that TD's had to get certified like they do in most European countries... ('course, then I think about haveing to go back to school to get that certification, and decide that perhaps I'll just keep muddleing through life, fixing other people problems for them...)
Posted by Backstage at November 19, 2003 06:28 PM