# Supported Cantilever



## Chuwwy (Nov 16, 2016)

Here is what is wanted:
A 8' x 12' platform with a 3'x12' section cantilevered but supported with bracing to 3 posts at 10' roughly off the ground. Check out the images for a better look (It does not have cross bracing on legs). Due to load in doors, the structure would have to be (2) 6x8. Now I know that it appears sketchy and more of a design for a bookshelf, not something that will support actors. Has anyone had a similar scenario with a supported cantilever? Perhaps thoughts on faking the look? I'd love some ideas before I opening up the structural design book to do free body diagrams and calculations.


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## Van (Nov 16, 2016)

This looks very stable, and sturdy to me. That is, it passes the smell test. I'd want to look at the loads on it. How many people. what kind of movement. The diagonal support underneath buys you a lot os safety margin, especially at that angle. Remember, however, the load from that support will be transferred to a lateral load on the 'pile' supporting it. If the thought is to make that column out of 3/4 ply then I would want to see it made from good quality materials, I would want an internal gusset, or center member running the entire length with interior blocking probably 24"oc. All the seams of all the faces and the internal structure need to be well over lapped. I'd look at shouldering the 'piles' or columns or whatever, so the base is a bit wider than the top section. By reinforcing the framing that surrounds that hole those columns whill be able to carry a lot more of the direct load. 
Oh I should say, I'm picturing this as being constructed from Stressed skin platforms yes? I think i's imminently doable in that way it's designed by do the math, check the loads.


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## Chuwwy (Nov 17, 2016)

Van said:


> This looks very stable, and sturdy to me. That is, it passes the smell test. I'd want to look at the loads on it. How many people. what kind of movement. The diagonal support underneath buys you a lot os safety margin, especially at that angle. Remember, however, the load from that support will be transferred to a lateral load on the 'pile' supporting it. If the thought is to make that column out of 3/4 ply then I would want to see it made from good quality materials, I would want an internal gusset, or center member running the entire length with interior blocking probably 24"oc. All the seams of all the faces and the internal structure need to be well over lapped. I'd look at shouldering the 'piles' or columns or whatever, so the base is a bit wider than the top section. By reinforcing the framing that surrounds that hole those columns whill be able to carry a lot more of the direct load.
> Oh I should say, I'm picturing this as being constructed from Stressed skin platforms yes? I think i's imminently doable in that way it's designed by do the math, check the loads.



(2) 6x8 stressed skin platforms. I was not thinking stressed skin but that would definitely be stronger. 

I've been reading the structural design book-not an easy read. So wouldn't the stressed skin platform as a whole be considered the beam rather than say a 2x4 stile of a regular 2x4 framed platform?

I did get more information. The live load would include (4) actors so 800lbs. Also the 'second floor' would actually support 3-400lbs of lighting instruments on pipes. 

While the original support was closer to 45 degrees, wouldn't a larger (even though a steeper angle) support or 'triangle' strengthen the front. I was thinking some walls (that would need inner bracing) and try to keep the center open as a walkway. 

Can't say I've done a stressed skin larger than 4x4. I've seen them supported every 2' & 4'. Don't know about spanning 6' with only two walls though.


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## BillConnerFASTC (Nov 17, 2016)

Make sure back is anchored real well if columns are free standing. Thanks


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## kicknargel (Nov 19, 2016)

The 6' span between the gusset braces requires more than 2x4 framing along that leading edge. Either stressed skin as discussed, or thicker framing.


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## Kfred (Nov 21, 2016)

I do that similar load in all the time.
I would use 2x6 at least for frame.
Your 45 degree braces will transfer alot of load at 10 feet. so i would make my legs 2x6 as well... or make a "hog trough" leg .and put your braces against the 2x edge rather than on the 6 flat edge.


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## porkchop (Nov 21, 2016)

If it was me I'd want to see it made out of welded steel, but your design seems to have a reasonable beginning.


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## Chuwwy (Dec 7, 2016)

kicknargel said:


> The 6' span between the gusset braces requires more than 2x4 framing along that leading edge. Either stressed skin as discussed, or thicker framing.



Appreciate all the comments. 

Can someone clarify supports for stressed skins? Perhaps I'm thinking of the stressed skin incorrectly. If going the stressed skin route, should the stingers go in the 6' direction and not the 8' direction if the knee walls are running parallel with 8' section? If I did go with stringers in the 8' direction, wouldn't diagonal braces (gold members) transfer the load back to the larger gussets?


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## BillConnerFASTC (Dec 7, 2016)

I feel like this is getting over complicated and I'm not sure what the final appearance is to be, especially of the angled braces in front of (?) the big columns. And at 10' off the ground, I would look more carefully at those diagonals. Concentrated loads on a 2 x 4 at 10' long cause some concern. It seems like you need a beam in the 12' dimension at the top of the big columns - what is the fulcrum of the cantilever. Looks like a 3' cantilever - probably could be done easier with larger framing, like maybe 2 x 6 s (or steel) in the 8' dimension. Stressed skin decks have some weird issues across a fulcrum that frankly I'd have to open a few books or look at grad school notes but it's the way the forces in the skins change from compression to tension.

If the trapezoidal frames - the columns with the diagonal - were a rigid unit - like all 2 x 4 with ply skins on each side - then a stressed skin deck could span the 12' dimension - even without the center frame and column if you wanted.

No matter what methods or design you end up with, I still worry about it all tipping over. Anchoring the back edge is never easy, the pull out forces in a stage floor are hard to achieve. A couple of barrels of water of some jersey barriers would be nice to anchor to.


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## Ech725 (Dec 8, 2016)

Check out https://law.resource.org/pub/us/code/bsc.ca.gov/sibr/org.apawood.U813.pdf for detailed stressed skin fabrication. It is the Design and Fabrication of Stressed Skin Plywood Panels.


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## Chuwwy (Feb 20, 2017)

Just wanted to share the final product. Ended up being (2) 6'x6' stressed-skin platforms. The ship ladder and fireman pole helped anchor it, but (2) pipe legs were added for safety due to dancing and additional weight of top columns with lighting instruments. While there are stud walls underneath, it was secured to a permanent structure in the space with pipe (swivel flanges & cheeseborough) That really took away any movement. So it turned out well for not being steel.


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## BillConnerFASTC (Feb 20, 2017)

Very nice. Congratulations.


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## GreyWyvern (Feb 21, 2017)

Chuwwy said:


> Just wanted to share the final product. Ended up being (2) 6'x6' stressed-skin platforms. The ship ladder and fireman pole helped anchor it, but (2) pipe legs were added for safety due to dancing and additional weight of top columns with lighting instruments. While there are stud walls underneath, it was secured to a permanent structure in the space with pipe (swivel flanges & cheeseborough) That really took away any movement. So it turned out well for not being steel.
> View attachment 14370View attachment 14371View attachment 14372View attachment 14373


Thanks for returning to let us know how it looked and worked. That is much appreciated.


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