Hello JLB,
I went back and read your original post, and have a few comments.
You said that the Exmork 3 kw wind turbine, which you are considering using, generates 6500 pounds of rotor thrust in a 110 mph wind. I don't know where you live, but the building code where I live only requires structures to be designed for a maximum wind speed of 80 mph. 110 mph sounds pretty stringent. If your house and outbuildings were designed to the 80 mph windspeed criterion, and a 90 mph wind blows them away, it won't matter to you whether your tower is still standing or not. You got bigger problems. My suggestion is to design the tower to the same wind speed criterion as your house, no more.
Secondly, the calculation of rotor thrust (which you stated was 6500 lbs.) is wrong. Rotor thrust can be calculated as follows:
RT = 2 * pi * (R**2) * rho * (V**2) * a * (1-a)
where:
RT = rotor thrust in pounds
pi = 3.1417
R = radius of the rotor in feet
rho = mass density if air = 0.002378 slugs per cubic foot at sea level
V = velocity of the wind in feet per second
a = interference factor. RT is at a maximum when "a" is equal to 1/2
* means multiply
** means squared
For a side-furling wind turbine (such as the DanB 10 footer) that is set up to furl at 25 mph (37 feet per second), that is the correct windspeed to use in the above formula when calculating rotor thrust, not 110 mph or 80 mph, or whatever the structural windspeed design criterion is. After furling is complete, assuming that the rotor is sideways to the wind, the force on the tower is strictly dependent on drag on the tower and turbine, not rotor thrust. If the wind blows hard enough, the tower is coming down even with no wind turbine on it, so the drag of the wind turbine is essentially irrelevant.
However, you are considering the Exmork 3 kw wind turbine, and it is not a side furling machine because the tail boom does not fold up when it furls. It does yaw, however. Even though I own three Exmork wind turbines, I haven't flown any of them yet, so I don't know the extent to which the rotor turns sideways into the wind, but I'd bet dollars to doughnuts that it doesn't go very far. So I calculated the worst case scenario, a 110 mph wind with the Exmork rotor facing directly into the wind. Using those assumptions, the rotor thrust comes out to 4236 lbs, not 6500 lbs as you stated.
You said that your lamp posts have a 1 1/4 inch thick base of 16 inches diameter, but to analyze the strength of the tower, an engineer would need to know the bolt circle diameter. Let's assume that the bolt circle diameter is 12 inches. You said that you plan to add ten feet to the lamp posts. That would make the tower 45 feet high. A force of 4236 lbs acting at a distance of 45 feet generates a moment of 191,000 foot lbs. (force times moment arm). To keep the tower from tipping over, the foundation bolts have to generate an equal moment in the opposite direction. You didn't say how many foundation bolts there are, but 12 bolts is not unusual. The most effective bolt is the one farthest from the downwind edge of the base plate, since that edge would be the axis of rotation (giving that bolt the largest moment arm).
Assume for a moment that that bolt is the only bolt, because the nuts were removed from the other eleven bolts. That bolt is 14 inches (1.17 feet) from the axis of rotation, and it would be subjected to a tension force of 163,000 lbs. Assume that steel has a tensile strength of 50,000 psi; it would then require 3.26 square inches of bolt cross-section to keep the tower up. A two inch diameter bolt would do the job. Anchor bolts are usually 1 1/4 inch diameter, 3 of those bolts would do the job.
So, looks like your lamp post base would be okay under these assumptions. That 3/16 inch wall thickness looks a little shaky though. I'd want lots of long gussets.
If it were me, I'd add the ten feet tower extension to the bottom of the tower, and use 20 inch pipe with a 24 inch bolt circle at the base, and at least 12 bolts, just to add some factor of safety. My Chinese hydraulic tower has a 24 inch bolt circle.
Those so-called professional engineers you consult aren't afraid of liability--if they do their job right there isn't any liability. They're probably incompetent. I have met lots of them who were downright stupid. As an example, I once asked one of them to draw me a freebody diagram of a block and tackle, and he had some of the ropes in compression. Ever try pushing on a string?
poco