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blecrone 09-23-2005 10:43 AM

Framing a curved platform/stage
We are adding on to the platform/stage in our church auditorium and are planning to make the new part of the stage curved. The stage is about 14-inches high off the floor and we plan to use 2x12's supported on 2x "sleepers" to get the correct height. We will then apply 3/4" plywood subflooring and then finish off with some type flooring (haven't decided yet).I have put together some drawings of how I think it would be best to frame the curve, but I'm not sure if this is best or not. The drawings are PDF files here:

Specifically, do we need the blocking along the face of the curve to support the front edge of the plywood? I would prefer not to do it because it is a lot of extra work, and it certainly isn't necessary for the joists to be stable at such a short length. If we do need it, what would be the best way to attach it to the joints. End-nailing would not be possible since the blocking is all in line with each other. I was thinking we would need to use a metal framing angle, the kind where you can adjust the angle by bending it.

I would appreciate any advice. Thanks!

Teetorbilt 09-23-2005 04:27 PM

You gain a lot of unseen strength due to the fact that it is an arc. I'd leave out the blocking and make the arc structural. If I were doing it, I'd laminate formboard until it reached about 1"+ thickness. In an arc and that thickness, you could have tap dancing elephants for entertainment.

KenTheHandyMan 09-24-2005 12:30 AM

One thing that I WILL say here, things like this look fine on paper, when it comes down to the DOING, that's another story. The only proper way to do this is with a laminated box-end, either formboard like Teetor says, or bendable plywood (aka Italian Bending Plywood). Your drawings don't show supporting framing, but you will find that this isn't quite as simple as the drawing suggests. Depending on how you are finishing this, the way the curve is framed can be crucial. It will not look right unless it is an actual curved piece. Cutting the end of the joists will be a trick in this situation.

If I were doing this, I would layout my arc on the floor, using 3/4" plywood doubled up as my top and bottom plates, (which I would cut out with a jigsaw), then frame the supporting wall that the joists will rest on. Then you could layout the joists and scribe the angle onto the joists, and subtract whatever thickness your builtup box end is and make the cut.

In other words, I would stress every aspect of the framing as an arc or true curve, and not an 'envelope' which is using several small straight lines to draw the illusion of a curve. When you try to carpet this or whatever you're going to have done, that illusion will be found less than adequate.

blecrone 09-26-2005 06:29 AM

Thanks for the thoughts. Here is how it worked out. We used 1/2" pvc conduit with string stretched between the ends (sort of like a big "bow") to create the arc, whcih we then traced out on the floor. We rough cut all the joists then laid them half of them in position, and marked them to length and the correct angle. We then used a sliding t-bevel to transfer the angle to a table saw. We then cut the joists to final length and angle in pairs, since they are a mirror image of each other on each side of the centerline of the arc. We fastened one end of the joists to the existing stage framing using joist hangers, and the other end toe nailed/screwed to the blocks, which were screwed to the floor. Then we ripped the 1/2" plywood to width, and bent it along the face of the joists, fastening it with 3" screws and construction adhesive. Finally we laid the subfloor down on top, marked the curve on it from below and cut it with a jigsaw. We'll then go back and trim the edge of the subfloor with a flush trim router bit to get it exact. I was pleased with the way it turned out, the curve is pretty smooth and uniform. Thanks again for your input. I'll try and post a pic or two if I get a chance.

KenTheHandyMan 09-26-2005 08:46 AM

Excellent! Sounds like you did a wonderful job. If you are carpeting, be sure to bullnose the top corner. Give it a bit of roundover, otherwise it will cut into the carpet over time.

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