A steel wind tower is built from several tubular sections, and each joint between them is a bolted ring flange. These tower flange connections — together with the foundation and yaw joints — carry the entire bending load of the turbine, and the ring of bolts around each flange is one of the most heavily fatigue-loaded elements in the structure.
§ 01 What a tower flange connection is
Where two tower sections meet, each section end carries a thick steel ring (the flange) welded to the shell. The two flanges are bolted face-to-face by a ring of high-strength bolts — typically class 10.9 flange bolts from M36 to M64, with anywhere from ~80 to 150 bolts per ring. The same arrangement joins the tower base to the foundation and the tower top to the yaw/nacelle.
§ 02 L-flange vs T-flange
Two geometries dominate:
- L-flange — the flange ring turns inward at the bottom of the section like an "L", and the bolts pass vertically through it. It is the established, lower-cost standard for most onshore towers.
- T-flange — the ring is symmetric about the shell wall (a "T" cross-section), placing bolts on both sides of the shell. This carries higher loads and reduces the prying effect, so it appears on very large and offshore towers where loads are highest.
| Attribute | L-flange | T-flange |
|---|---|---|
| Bolt position | One side of shell | Both sides of shell |
| Prying effect | Higher | Lower |
| Load capacity | Standard | Higher |
| Cost | Lower | Higher |
| Typical use | Onshore towers | Large / offshore |
§ 03 How the bolts are loaded
Wind thrust bends the tower, so on the windward side of each flange the joint is pulled apart while on the leeward side it is pushed together — and this pattern rotates around the ring as the wind direction changes. Each bolt therefore sees a fluctuating tensile load on top of its preload. In an L-flange the offset between bolt line and shell wall adds a prying amplification that increases the cyclic stress — which is precisely why preload and flange stiffness matter so much.
§ 04 Preload and fatigue
The flange joint is designed so that the bolt preload keeps the faces clamped across the full wind load spectrum for the turbine's life. That means the preload target (typically ~70% of proof load) and the tightening accuracy are central design parameters, not site details — covered with indicative figures in preload values for M36/M42 tower bolts. Flange flatness and fit-up also matter: gaps force bolts to bend during tightening and reduce achievable clamp force.
§ 05 Bolting practice
- Multi-pass tightening in a defined cross / star sequence so the ring pulls down evenly.
- Accurate preload — large rings are often hydraulically tensioned rather than torqued; see tensioning vs torquing.
- Re-torque / re-check at the intervals in the maintenance plan, because new joints settle.
- Correct coating & documentation — class 10.9 with a coating matched to the environment and a 3.1 certificate.