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Mimimizing weight of a rolling platform

Paul Pfeiffer | Posted in General Questions on

Not sure if this is too off-topic to be against the rules, but I thought the people here might have some good input.

How would you design a rolling platform about 32″ tall x 36″ wide x 60″ long to be (1) safe as a stage prop to be danced upon but (2) lightweight (3) easy to build?

I built one a few years ago that you could call platform-framing-inspired. It is excessively heavy.  Some of that is also due to it’s modular design–I wanted it to be easily disassembled for storage, but that feature has never been exercised.  Anyway, the first thing I would do differently is probably use thinner plywood and a little less framing, but perhaps there is a better way to go about it?

Would it be smarter to build cross-sectionally?  That is, make rectangular cross-sections out of 2/4s then join them with plywood, essentially making one big box?  Maybe use 3/8″ plywood/OSB on all surfaces except the top where it would be 1/2″ or 5/8″.  Or even just use cross braces at all surfaces except the top.  I’m guessing this case would call for at least 3 structural cross-sections, which would then call for 6 wheels instead of 4.  Plus I may want some minimal framing (2x2s?) between cross-sections at least on the top in order to screw down the perimeter of the plywood.

Or another idea  is to frame one big cube (or more properly, right rectangular prism) and surface/cross-brace it.

Is there some other way to go about this that I’m missing?  Other materials? Recommendations for appropriate plywood/OSB thickness?  A subfloor material lighter than plywood?

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Replies

  1. Walter Ahlgrim | | #1

    Consider using one of these plastic folding tables for the top. They seem very strong for how little they weigh. At this moment they are priced at only 29$.

    I have not tried dancing on them but if you replaced the legs with a rolling base seems like you would be set.

    https://www.menards.com/main/grocery-home/furniture/folding-tables-chairs/pdg-reg-6-folding-banquet-table/804/p-1444450879064-c-7726.htm?tid=-1225203754813964435&ipos=7

  2. Expert Member
    Zephyr7 | | #2

    Waaay back I used to work with this stuff (I started out working in a recording studio and doing theater work, which sometimes included set construction). It's been a while, but it was fun. You don't have same kind of longevity issues with set pieces since they tend to be torn apart after a few months, but you still need them to be safe.

    When you say "danced on", is that one dancer? Three? It would be a tight fit for multiple people on there, but I don't know what your plans are. I would likely build this with a 2x4 framed top, with runners on the long sides and probably two cross pieces between the ends (additional cross pieces on the end). This would mean each supported area of plywood would be 36" x 20". I would use 3/4" plywood for the top. You could probably use 1/2" safely, but it would be more boomy while being danced on, and you probably don't want to build a drum. You don't need anything heavier than a 2x4 for the beams on the long axis, since the plywood sides will act to stiffen the framing on top. The same goes for the cross pieces on the ends, which could be made with 2x3 or even 2x2, but it has to be fastened securely, and ideally glued to the end panel plywood if you go with a 2x2.

    I would use light plywood for the sides, probably 3/8", maybe 1/4". The plywood doesn't need to be super hefty on the sides since it's not carrying much load, but it is helping with shear strength, so you need to fasten it well to the top framing. I would use 2x2s vertically in each of the four corners of the box.

    The lower framing is interesting, since it carries loads in unusual ways. I'm assuming, since you said "rolling platform", that you'll have some casters on the bottom. I'd use all four corners for these, and I'd make them all swivel casters so that your stage crew can move the set piece in any direction. This will make things easier when it comes time to stuff it in the wings at scene change time. You want the casters recessed up into the box so that the plywood sides create a skirt to hide the caster, but you need that skirt to stop above the stage a bit, so allow about a 1/4" to 1/2" between the floor and the bottom edge of the side panels of the box. The casters take load, so you need a solid mounting block. Cut pieces of 2x6 or whatever fits the caster's mount plate. Fasten them to make small, flat, "platforms" connected in the lower corners but up above the bottom so that the caster remains mostly hidden. For the corner, the ideal support is to cut down the edge of a 2x4 to fit the corner diagonally, then notch it to support the lower edge of the top framing. The 2x4 will then stick out diagonally from the corner along the top of the caster mounting plate, which will help to resist that plate bending upwards under load. Fasten the 2x4 to the side panels well.

    I would probably put a 2x2 along the lower edge of the side panels so that they don't bow in and out, which they will when made with thin plywood. I might add a single 2x2 cross piece in the middle to help stiffen things up a bit.

    Is this to building code? Nope, not even close! But this isn't a project subject to code, and you have different requirements. You can use clever contruction to keep things safe while minimizing weight. I'm going to sketch a few drawings here to post since this is probably a little confusing to follow.

    BE SURE to test everything before you have cast members on top. The tradition is for the set construction crew guys to jump on everything and make sure it's solid. My quick sketch isn't perfect for perspective, but should be close enough to understand clearly. Note that I didn't show fastners to the ends and side edge of the caster plate, which should all be securely fastened to the side and end panels. The sides and ends are being used to keep the structure together, so the facade AND the framing carry load here. This is how you keep weight down -- make ALL the parts of the piece to "work", and only put additional material where it's needed to carry load (top framing, and vertical supports). Keep lower framing minimized. Use the plywood sides to stiffen the top piece and keep the assembly together.

    You need a bigger piece of wood for the caster plates because those areas take localized loads (point loads on the casters, not distributed loads like the rest of the assembly), so it has to be solid so that the casters don't cause it to deflect. The vertical corner pieces are positioned as they are to help keep the caster plate horizontal, which is why they are angled in and aren't just aligned with one or the other face of the box. Everything is laid out with an eye towards the forces involved so that material is only placed where it's needed, and not wasted in other places where it will only add weight. Glue is helpful at all the framing joints, but especially in the corners and where the two plywood end panels attach, since those will have the highest forces, along with the caster plates.

    The top piece is a tradeoff between heavier plywood or more cross pieces. More cross pieces allows the use of thinner plywood for the same stiffness. The tradeoff is what is heavier to get the level of stiffness (and reduction of boominess from the dancer) you need with a minimum of weight. Keep in mind the cross pieces should attach with at least three fasteners (16d nails or similar size structural screws). The connections all need to be solid, so you need to use enough fasteners.

    You need to test this for safety after building it. I would if it were my crew, but remember that you and your set construction crew are responsible for the safety of all of your cast members, so it is your responsibility to test everything you build and ensure it's all safe. Pay particular attention to all connections (places that are nailed, screwed, and/or glued), because those areas may gradually seperate over time and not stay safely together. Dancing is dynamic loading, similar to pounding on something with a hammer, so it's subjecting additional stresses to the assembly beyond what would be seen with more static load like someone sitting on it.

    Be careful, and always think SAFETY FIRST.

    Bill

    1. Paul Pfeiffer | | #3

      Wow, thanks! The diagonal pillars is a clever idea. Does your design have any plywood on the lower framing? I guess I can see that it should not be subject to much racking force.

      As I thought about this after posting the question I was starting to come up with some truss-like designs for the two long sides, but it would probably be more useful to have sheathed sides. I'll have to think about that--the sides will be covered up with decorative stuff anyway.

      This is for my wife's dance studio, so it'll be high school age girls dancing on it (and one at a time doing any sort of dancing--there could be two or more remaining nearly stationary). It's also something that will be kept and used year-after-year. The performances are usually at a local high school so I got inspired by seeing how much lighter some of their props were (if I recall correctly, they had something of similar size made almost entirely out of 1/2" plywood and metal brackets for connectors. I'm not sure I'm willing to go that lightweight, but it's a nice reference point for a lower bound on structural integrity).

      1. Expert Member
        Zephyr7 | | #4

        If they're permanent, I would absolutely glue the connections and also screw them. Glued and screwed is a pretty solid connection. You could cut down on weight some more by cutting out some of the side panels in less stressed areas. An idea of how to estimate where those areas of less stress are is to look up some pics of "lattic trusses". Imagine the perimeter and some "X's" and you're headed the right direction. Leave the "connections" between parts oversized to act like gusset plates. If you do this, you need to use 1/2" plywood for more strength, but you'd make up for the weight by removing the unneeded areas. Trusses in aircraft are made this way to save on weight, for example.

        I wouldn't use plywood on the lower framing. At most I would make an X brace between the four corners with a gusset plate in the center of the X, possible including the middle brack to make it look like an asterisk (*). I would build that out of 2x2. I might just put guesset plates at the corners. All you need to do is keep the bottom sides square to each other, there aren't many other forces at work down there aside from the vertical load carried by the corners. Keep in mind that diagonal braces like this are acting mostly in tension, so you don't need a lot of thickness in the wood since sag/deflection don't really matter. It's all about thinking about where forces are, and forces are like arrows point in the direction of the forces. Visualize those arrows, how they all point down to the ground for the dancer on top, and how they push horizontally when you push the platform around. Racking is diagonal forces. In the engineering world, this is called "statics", because all those forces need to zero out -- if they don't zero out, you have dynamics, and dynamics means things are moving. In this situation, "moving" means things are moving apart from each other, which is the engineering way to say "AH! It's FALLING DOWN!", obviously something you want to avoid :-)

        Plywood is surprisingly strong when used as the web of a beam. Think about a very big I joist -- the web might only be 3/8" OSB between the 2x4 chords on the top and bottom of the truss. The reason for this is the load is carried primarily by the chords of the truss, with the web maintaining the spacing between them. Flanges of a steel I beam are pretty much the same as the chords of an I joist. You put the material where the forces are, and you don't waste material where they aren't. If you look closely at a steel I beam, you'll notice the flanges are thicker than the web, and this is why.

        Engineering is just applied physics. Think your problem through and you can accomplish whatever you need to.

        Bill

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