Make every guy anchor strong enough to hold twice the maximum cable tension, measured along the line of the cable, and set the base on a concrete pad with anchor bolts. For a 12 m tower with a 3 m rotor in a 25 m/s storm the top guy carries 4,965 N, so its anchor must hold 9,930 N, or 2,233 lb, which exceeds the low end of a deadman’s range of 2,000 to 5,000 lb; moving the anchors out from 65% to 80% of the tower height lowers that to 1,948 lb.
Part of Advanced Wind Turbine
The foundation carries the tower’s weight into the ground; the guy anchors resist the cables’ pull so the tower cannot tip over.
The Pad Is Small And The Anchors Carry The Rules
The carrier asks for a concrete pad 60 × 60 × 60 cm at the tower base with embedded anchor bolts, concrete blocks or screw anchors buried at each guy point, and, in rocky soil, anchor bolts drilled and epoxied into bedrock. Guy Wire Anchors adds the rules: anchors at 60 to 80% of the tower height from the base, every anchor able to hold at least twice the maximum expected load, a deadman rated 2,000 to 5,000 lb at 4 feet depth, a screw anchor 1,000 to 3,000 lb (poor in sand and gravel), and a come-along test on screw anchors by pulling with your full body weight.
The Load Is Along The Cable, Not Just Sideways
The 12 m tower in Guyed Tower (Steel Pipe), in a 25 m/s storm, has a top guy tension of 4,965 N when its anchor stands at 65% of the height (7.8 m out), where the cable is 57° above horizontal. Twice that is the anchor’s required capacity: 2 × 4,965 = 9,930 N, which is 9,930 ÷ 4.448 = 2,233 lb. It splits into an uplift of 9,930 × sin 57° = 8,327 N and a horizontal pull of 9,930 × cos 57° = 5,412 N. Moving the anchor out lowers the tension:
| Anchor distance | Cable angle | Cable tension | Anchor rating (2×) |
|---|---|---|---|
| 40% of height | 68.2° | 7,286 N | 14,572 N (3,276 lb) |
| 65% | 57.0° | 4,965 N | 9,930 N (2,233 lb) |
| 80% | 51.3° | 4,333 N | 8,666 N (1,948 lb) |
| 100% | 45.0° | 3,827 N | 7,654 N (1,721 lb) |
(Thrust 2,706 N from the tower page; tension = thrust ÷ cos of the angle.) The Guy Wire Anchors page warns that anchors closer than 60% of the height need much stronger anchors; the table shows the cost, a 47% rise from 65% to 40%. A deadman rated at the low end of 2,000 lb falls short of the 65% case, and a 1,000 lb screw anchor is at less than half the need. Use a deadman at the upper part of its range, two screw anchors side by side, or a rock anchor, and set the anchors farther out where there is room.
A Weight Check On A Deadman
A weight-only estimate gives a floor. A concrete block 0.6 × 0.6 × 0.3 m weighs 0.108 m³ × 2,400 kg/m³ = 259 kg, which is 2.5 kN (the corpus’s 600 lb for 2 × 2 × 1 ft implies the same density). The column of soil above it, 0.6 × 0.6 × 0.9 m at 1,600 kg/m³ (a recalled soil density), is 518 kg or 5.1 kN. Together 7.6 kN, short of the 8.3 kN of uplift required. A real deadman does better than its weight, since the soil around it resists being pulled out as a wedge and the horizontal pull bears on the soil face. That is why the corpus’s tested range (8.9 to 22 kN) is the better guide. Bury the rod or cable so it pulls in line with the guy, and tamp the backfill every 6 inches, as the Guy Wire Anchors page directs.
The Base Carries Weight Only
A tower on a hinge at its base cannot resist tipping; the guys do that. The pad carries compression. Assume the mast and turbine at 1.1 kN and the guy vertical parts at 4.2 kN, a total of 5.3 kN, plus the pad’s own weight: 0.6 × 0.6 × 0.6 m = 0.216 m³ × 2,400 = 518 kg = 5.1 kN. The bearing pressure is (5.3 + 5.1) ÷ 0.36 m² = 28.9 kPa. Compare that with the strength of your soil; the corpus gives no bearing figure, and for a larger load use Foundation Work with your own loads. Set the anchor bolts in the wet concrete with a template, since drilled-in bolts in cured concrete are weaker.
Method: Place And Prove The Anchors
Step 1: mark the base and the anchor points on circles round it, 120° apart, at the chosen distance, with the side anchors on the hinge axis (Tilt-Up Design for Maintenance). Step 2: dig, set and backfill each anchor, with the rod or cable leaving the soil at the guy’s angle. Step 3: attach cables with thimbles and shackles, as in the Guy Wire Anchors page. Step 4: proof-test each anchor before raising the tower, pulling it along the cable line with a come-along and a scale or a known weight. Pull to the working tension (4,965 N here) first, and watch for any movement. I suggest going to 1.5 times it if the soil is stable; the corpus’s own test is the come-along pull of a person’s weight. Any anchor that moves is not finished. Step 5: re-check the anchors after the first heavy rain and each year.
Common Mistakes
- Sizing the anchor for the sideways pull only — the load is the cable tension along its line.
- Mixed anchor types in one set — they give at different loads.
- Backfill left loose — the anchor creeps.
Safety
Excavations and a failing anchor
Do not stand in a deep anchor pit unshored, and never test an anchor with your body in line with the cable: a cable or shackle that fails snaps back. Keep everyone out of the tower’s fall zone during the proof test, and test anchors before the mast goes up, not after.
What’s Next
- Learn next: Electrical System for the wiring that comes down the tower
- Combine with: Tower & Mounting