The gin pole design
Prairie Turbines uses is very common in smaller towers and
allows for service of the generator without climbing the tower. With this design
the tower is mounted on a pivot, and the gin pole at a 90 degree angle with
guide wires. Most towers of this design are raised and lowered by cables running
through a block and tackle, and ultimately attached to a pick-up truck or
tractor. I have trees that are almost 90' tall in the vicinity of the only
location I could erect my tower, and my wife would not approve (nor was I
excited about) removing a bunch of very nicely matured oak trees. I performed a
good site assessment and determined I needed 130' of tower to clear the trees by
the recommended amount for wind turbines (30' from the bottom of blades to top
of any trees within 300').
I am a pilot and aware of FAA issues with towers, so I did some research with both my local governing body and the FAA/FCC regarding tower zoning, registration and lighting requirements. The first good news was that the local governing body had no restrictions (or reporting requirements) for wind generators. As long as I met the federal standards, they were fine. I own 30 acres of wooded property and the generator was going to be in the middle of this parcel so neighbors seeing it (or hearing it) would not be an issue. The FAA's policy for towers is advisory (recommendation) only, unless they are in the immediate vicinity of an airport. The FCC is another story. They adopt the FAA "recommendations" as regulatory. The good news, there is no registration or marking/lighting requirements for towers under 200'.
The first "on site" project was the tower pedestal and the 4 anchor point pods. The 5 pods were all pretty much the same design other than the actual anchors coming out of the concrete. The concrete pads/anchors consist of a 7' x 7' x 18" base with a 4' x 4' x 5' tall column on top. The bases have a re-rod grid that is extended up into the second pour, the column, with 8 "S" shaped pieces of re-rod. The columns are also reinforced, from the 8 re-rod ties to the upper metal work coming out of the concrete. In the interest of keeping cost down (for both form material and concrete) I used my portable 5-6 cubic foot mixer (gas powered) for all the cement work. Each pour was about 1 1/2 to 1 3/4 yards, so only took about 2 hours to complete, and the lower and upper forms were reused for all 5 anchor pods.
Several pictures showing the construction of the tower footings -- Click on pictures for full size
The "gin pole" cement pod was unique in that it was all above ground due to the elevation change of my property at this location. The elevation drop worked to my advantage for the gin pole, as a substantial truss was needed that required space. It was a disadvantage for the pod though, as this pod was to see the most stress of the 5 pods. Being the anchor point for the
gin pole and winch assembly, this pod would see upwards of 20,000 lbs. of lift during the initial pick of the tower. With 3 1/2 yards of concrete coming in at 14,000 lbs., without ballast this pod would come off the ground before lifting the tower. Fortunately, I had a 200-300 ton rock pile from a past "soccer field" project that was begging to be used somewhere. I
placed about 30 ton of rock on this pod. I also placed 10-15 ton of rock on the other pods before the final top dirt for extra ballast as well.
Note: More information from OtherPower on another gin pole style tower...
October 20, 2008