At 06:49 PM 11/2/2006, John Barrett wrote:
>I'm looking to design a freestanding tower that can support up to
>500lbs of load at 50 feet with a wind loading of approx 40 sqft and
>100 mph survivability. Guy wires are not an option for the location
>we must use. I have a few problems to solve and am not finding much
>in the way of references online to work out all the details.
That's a pretty burly tower.. Not something you want to engineer
casually.. but, if you want to ballpark it, to find out how big a job
you're getting into...
100 mi/hr is 25lbs/sq ft or thereabouts, so you're looking at a wind
load of 1000 lbs, not counting the probably significant wind load of
the tower itself (it's gonna be bigger than a foot in diameter, so
you've got another 50 square feet of area, just for the tower).
so let's say you've got a bending moment of, say, 1000lb*50 ft (50000
lb ft) + 1250lb*25 ft( 31250 lb ft).. call it 100,000 lb ft so you
pick up some margin.
A piece of 12" pipe, 1/2" wall 50 ft long would weigh about 3000 lbs,
and would have a bending load failure of about 4000 lbs at the top,
so it's in the ballpark. (I assumed mild steel with a yield of 50 ksi)
Don't forget you also need to design for seismic loads in many
places. 1/3 g on a 4000 lb structure is 1300 lbs.
>1> what kind of pipe/tubing to use (diameter/thickness) -- assuming
>that the "mast" will be a single run of pipe (welded and reinforced
>with smaller tube inside at the joints if needed).. This has to be
>able to support the 500lbs dead weight load in the tilt-down
>position if a tiltover is recommended. Stepping the pipe size down
>as we get higher is ok (unless a carraige setup is recommended)
Since the tube has to support a 1000 lb wind load when vertical, it
will support a 500 lb load at the end when horizontal.
>2> footing recomendations -- how wide a hole and how deep ?
enormous (by ham tower standards).. You've got a pretty big bending
moment trying to overturn that block of concrete. Might want to
consider something like a vertical pier. Drill a hole 3 or 4 ft in
diameter many ft deep, fill it with rebar and concrete. A lot
depends on your soil properties.
>3> should I look at a tilt and crank-up design to reduce the
>stresses when raising/lowering the tower ?? or perhaps a "carraige"
>design where the load is pulled up the tower after it is erect ??
>
>This is obviousy overkill for an antenna tower :) and while there
>will be antennas on it., it will mostly be used to support a wind
>turbine. (that is until I build the 2nd one to support a stacked beam :)
>
>now I just need the references or some advice to work out the rest :)
This is a pretty cookbook thing in the wind turbine business. I
would imagine that they have standard designs, just like they do for
antenna towers. Have you checked sources like Home Power magazine?
It's not going to be cheap, but at least, in most states you can get
a tax credit or rebate to help pay for it.
You could also look into commercial poles for supporting traffic
signals and things like lights over freeway overpasses. 50 ft is
fairly short in that business, and, again, they'll have stock designs
and design recommendations for the footing. (The stoplight down at
the corner near my house is probably 40 ft tall, but has a 40-50 ft
canteliever sticking out. It's also held down by a dozen or so 1" or
larger bolts into a concrete caisson that was probably 30 ft deep
with a BIG rebar cage. OTOH, they were able to install with only
jackhammering out one little segment of sidewalk.
If you could possibly get away from the "monopole" type support, life
would be much easier. There's a reason why farm windmills are made
on lattice style A-frame structures with a wide base.
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