Cedar osprey pole...like a mast?

In a marsh along the Bronx River we want to install an osprey pole and platform. The experts have told us that we will need 26 feet (6 feet in ground) of length. Due to the area we cannot get heavy equipment and we can't carry a full length pole to the site. I want to assemble on site a 24-26 foot pole using sistered 12 foot cedar 2x4s. Any suggestions on best method to make a pole that is stable enough to hold up to 100 lbs on a 3 foot by 3 foot platform at the top?

 

So you need 20ft air height ?

 

use heavy galvanized screws or stainless screws and cordless drills to assemble it. I would use a length of 6x6 pressure treated for the part you bury in the ground (it will hold up better than the cedar), the cedar should be okay for above ground. There is a fairly complicated analysis to optimize the design of a built-up member for lateral wind loads in combination with the end load. You have axial compression with lateral bending of a cantilevered column. I have software for doing solid sawn or metal columns, but a built-up member requires a lot of hand calculations (the software is not smart enough to do it). For a solid sawn column of Western Cedar you would need about a 10x10 (net 9.5x9.5 inches) column. So if you intend to use 2x3 you will be looking at a truss type tower with cross bracing.

something like this:

 

A 6" square section, tapering to 4" at the head will easily support this platform. This is a solid (laminated) 26' spar. If it's solid cedar (depends on species) it'll be in the 100 pounds range, when assembled. It'll sway around a bit in high winds, but will stand. If it was a stayed pole, you could reduce the dimensions by 45% or so (3 5/8" x 2.5"). If hollow and stayed 3 3/4"x2 5/8", a huge weight reduction (70 pounds lighter w/ 20% wall) could be realized, which is easy enough to carry on your shoulder and drop in a hole. Pressure treated seems the only reasonable choice for in the ground stuff.

 

Use (to make the section) a pair of 2"x6"s and a pair of 2x3s so you have a box section that's 5 1/2" x 5 1/2". Spruce would be fine and a lot cheaper than cedar. The lower part can be pressure treated if need be. Stagger joints every two feet and glue with liquid nails or equal.
2 x 3 pressure treated doesn't exist so rip them from a 2 x 8 (2 x 6s are one saw kerf too narrow but could be fudged).
For fasteners, use 2-3 # of 3" ceramic coated or epoxy coated screws every foot or so.

 

What is a stayed pole. Lighter sounds good.
They do not want using pressure treated in the area.

 

I cannot use pressure treated , but then it does not have to last forever, just a few breeding seasons. How long might spruce last?

 

Yes, to get it above a berm.

 

Do not use spruce, it has almost no rot resistance and it will rot out at the base in less than a season.

If you can not use pressure treated than cedar is your best bet. It should hold up for a number of years. Redwood and cypress also have good rot resistance but are much more costly. If possible you might consider steel legs you drive into the ground and than bolt the ceder to it.

PAR, your size is too small. This would not be high quality boat building lumber I would suspect either. 6" square is way too small for an unbraced column because it is below the minimum slenderness ratio , meaning it risks a buckling failure. The minimum slenderness ratio requires a min of 9.5x9.5 column if it is going to be 20 ft high, even with no lateral wind loads.

I was assuming wind loads of 85 mph with high exposure, plus the combined load of 100 lbs on top, and I was assuming the platform itself adds another 75 lbs. You have combination loading of both axial compression and bending, compounding the buckling problem.

Also, you should know that most places in the United states, if it is over 6' high, you will need a permit because it is considered a hazard. With a building permit it means you will need to meet the building code and will require an analysis by a licensed engineer. Unless you have some sort of exemption (not likely) or you intend to "boot leg" it in, you will have to pay for a designed structure.

I do this for a living in Washington state, I am very familiar with codes and standards. I can not help you there since I am not licensed in your state.

If you want to use 12 ft 2x4s to build it, the most cost effective way (and easiest to transport) is to build it as a four legged cross braced tower as I have shown above.

 

In the ground, not too long. I can appreciate they don't want you to use pressure treated (Our kids can be around it, just not animals). So use cedar.
To be honest, a solid cedar tree would be ideal. Ten to twenty years maybe, longer if in muck. You really should consider a one piece 8" tree (4" at the top). Three-four guys could shoulder it. And if you have to make it out of two or three pieces, a slash cut (scarf) about 4 ft long with some big (1/2") galvanized bolts would make a good joint. Bear in mind, some shrinkage will loosen the bolts if the tree is unseasoned, so use a few more than would be needed at first, say six bolts, as much drifts as bolts.
I agree that spruce shouldn't be buried.
I don't agree that PAR's sizing (or mine) is small. Imagine what it would take to blow down or "dismast" a cedar tree of that size buried 6 ft deep. This requires a building permit? It's a temporary "structure" but I'd believe anything nowadays so I'd have to say I just don't know, especially in the Bronx.
Regarding the beefiness issue. This isn't a mast. It's going to sway like a real tree would. Later, the woodpeckers will have their turn when the core rots.

 

I admit not running numbers, but just some in head math and a 6" (5.5" is probably fine too) base, with a continuous taper to 4" at the head will do. It will not be rigid, but this probably isn't want you want anyway. The in ground portion could be painted with a single part polyurethane, which will improve it's durability considerably if untreated wood. I like Allan's idea of a box section, which will be considerably lighter than a solid tree or laminate.