Thursday, June 20, 2013

Floating Tower Base

I had a couple of hours last evening, just enough time to build and install the floating base for the opportunistically-guyed Site-C tower. It is floating in the sense that it is entirely above the frost line. The conventional base for a Golden Nugget tower is to secure the base plate with 3 x 1-meter steel studs that are driven into the ground. The floating design -- which is entirely my own -- is worth a few words.

My requirements for the base include:
  • Do not interfere with the septic system tile bed under the tower.
  • Support the dead weight of the tower and antennas (and ice), plus the vertical force due to guy tension from preload and wind load.
  • Secure the base from lateral forces due to wind and from accidental or deliberate interference.
  • Easy to remove and restore the lawn when the tower is dismantled.
  • Survivability of at least 5 years.
The base is quite simple. The only material is two 8'-long 4x4 pressure-treated lumber rated for burial, plus 8 galvanized steel spikes. These can be purchased at almost any large building supply store. Galvanized lag bolts and washers will be used to secure the tower base plate to the base.

Each piece of lumber is cut into 3 sections: 3' and 2 x 2.5'. The longer sections form the bottom of the base, and the 4 shorter ones are secured across those. The depth of the base is about 6", leaving approximately 1" above grade (2 x 3.5" = 7"). There is just enough space between the upper sections to allow water to drain, so that is does not pool for long between the base plate and base.

The tricky part of the installation is getting everything level while not leaving any air gaps beneath any part of the base. The picture shows the base centred, levelled and ready for final hammering and packing of soil. The unrestored tower base plate is included in the picture for perspective.

The base gets its strength by coupling to the soil, both downward and laterally. This is fine for wire antennas. If you plan to install a yagi or similar rotatable antenna it is important to add diagonal strength to the base so that it better resists twisting. Unlike a concrete base there is not enough mass or overburden to resist much uplift force so I'll be out of luck if a tornado strikes. These are happily rare in the Ottawa valley. Frost heave during spring thaw is modest in my experience here (~1 cm), which I judge to be negligible in this application.

Once installed and the sod replaced it makes for a tidy installation. Even the local waterfowl seem unperturbed by its presence. The trees that will serve as guy anchors bracket the picture. Notice that the base and lawn are not on the same plane. The base is level, the lawn is not. Never assume that the ground is level. Measure!

The last test is to ensure that after all that surveying, calculating and construction work that the base is actually in the correct position. I did this by placing a pad of paper on the base, marking the point where the base plate will be centred, and then sighting along a straight edge to the 3 anchor points. With lines drawn it is a simple matter to use a protractor to measure the angles between guys.

The result is quite good considering that I intended to not adjust for minor asymmetries. About 1° of the 3° error is because I hadn't taken into account that the southwest tree bends a bit towards the south and the anchor point is above head height. At a distance of ~9 meters from the base a 1° lateral offset is equal to ~16 cm (6").

With all the ground prepared I now have some finishing work to do on the tower, after which it will be ready to be raised.

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