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HonestAbe5424 12-19-2012 12:18 PM

Structural input need for 20' high free standing deck
Hello everyone,

I am a simple enthusiast that usually makes up for lack of formal structural knowledge with a little extra elbow grease. Well, for this project, I am reaching out to the community here for some expert advice.

I am building a luxurious elevated hunting cabin. The interior of the walled structure needs to be at least 8x8 - hopefully 10x10. This cabin will sit atop a 16x16 deck which will be elevated 15ft in the air by 4 utility poles.

Right now here is my plan.

Sink 6' holes for the 4 utility poles and use about 2 bags of quikrete in each. I will space these 8x8. And then simply build a 16x16 deck at the 15' mark of the poles.

I want each pole to extend up another 7-8 feet past the deck floor to become the corner posts for the future walls....

Is there anything I'm really overlooking?

nealtw 12-19-2012 12:57 PM

Hay; welcome to the site.
I see two problems right off the top. A 4x4 fence post 30" deep can take 2 bags of ready mix so you would be looking at a lot more for your posts.
It sounds like you want to cantilever the floor or deck in two directions, that would be tricky and a cantilever should not be more than 1/3 the total length of the joists.

HonestAbe5424 12-19-2012 01:31 PM

1 Attachment(s)
You're right about the amount of ready mix, I honestly hadn't given it much though - but they will be anchored well.

I was unsure about the deck design - specifically what you are mentioning with the 1/3 rule. Now did you mean it would be tricky do to the 4ft overhang on both left and right... or that it would be tricky to do the overhang on the left/right AND the top/bottom?

The way I thought it in my head was - each 4ft overhang is either 25% or 33% overhang.

I attached a diagram - please let me know!

By the way, Thank you so much for replying.

nealtw 12-19-2012 01:42 PM

I can't access you drawing. If you're thinking of a 10x 10 building with a 42" wrap around deck all sides, you could make that work.
You lost me on the top and bottom thing.

HonestAbe5424 12-19-2012 01:51 PM

10x10 with a wrap around deck on all sides is precisely what I want... Can I still accomplish this using the four utility poles ?

nealtw 12-19-2012 02:19 PM

So you would place timbers or beams front and back sticking out both sides and place your floor joists across those, sticking out front and back. The problem that comes to mind is that the cantilever depends on something holding down the other end. On each side you will have some joists with nothing above them. So you would have to use some type of ties to hold them down when you load one corner.So then the problems become.
Proper size of the beams to be used.
How best to tie the beams to the posts.
How best to tie floor joists to the beams
How best to cross brace the posts as they will move around.

HonestAbe5424 12-19-2012 02:21 PM

I see what you're saying. I got some thinking and some work to do!

nealtw 12-19-2012 02:44 PM

Heres a good article to read and after you have the deck plan you still need a plan for keeping your shed floor dry and warm.

majstor 01-20-2013 11:09 AM

I find great tutorial how to build a roof on the porch

Wuzzat? 01-20-2013 02:57 PM

If you will be using a rifle with a scope in moderately windy conditions you might need a lot of rigidity. Use bracing or gusset plates on your columns; IIRC a triangle is the only geometric figure that cannot change shape without changing the length of its sides.

I recommend trying out existing decks or other structures with similar cross sections in windy conditions. An 8' H x 10' W plate makes quite a sail and there are web formulas for the force on this. I know fence posts have to be able to withstand some bending torque.

12.00 >enter hole dia.
6.00 >enter sq. post side dim
113.10 =calc'd hole area
36.00 =calc'd post area
77.10 =calc'd (hole area - post area)
72.00 >enter hole depth
3.21 = calc'd cu ft concrete volume

For a 7" dia. hole and a 4" sq. post it comes to about 1 cu ft of concrete.

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