1. Design of stiffening rings Prof. Ch. Baniotopoulos I. Lavassas, G. Nikolaidis, P.Zervas Institute of Steel Structures Aristotle Univ. of Thessaloniki, Greece

2. Typical stiffening ring arrangement When used, stiffening rings are placed over the height of the cylinder, separating it to regions Distances between rings are a question of buckling limit state design (LS3)

3. Effects of ring stiffeners on Von mises stress Left (stiffened): max σ vm =266 Mpa Right (unstiffened) max σ vm =300 Mpa

4. Effects of ring stiffening rings on meridional stress Left (stiffened): max σ z =219 Mpa Right (unstiffened) max σ z =240 Mpa The existence of rings should have no effect to the meridional stress. The difference appears around door position.

5. Effects of stiffening rings on circumferencial shell stresses Left (stiffened): max σ θ =63 Mpa Circumferencial stress is distributed over the rings Stiffeners are forcing the cylinder sections to remain circular Right (unstiffened) max σ θ =109 Mpa Circumferencial stress is concentrated to the flange positions where the ovalling deformed shape of the cylinder is forced to circular due to the stiffness of the flange

6. Stresses on stiffening rings & flanges Left (stiffened): max σ vm =115 Mpa Right (unstiffened) max σ vm =180 Mpa

7. Stress distribution over a stiffening ring

8. Effect of stiffeners to the shell buckling behaviour Buckling mode shapes on a stiffened shell remain within one stiffened region

9. Use of stiffening rings Advantage: - Use of stiffening rings may reduce shell thicknesses mainly to the upper parts of the tower where local buckling is the main design issue Disadvantage: - Although the cost of the materials is less in a stiffened tower, the cost of the work is more The total cost of the tower may be bigger for the stiffened tower

10. Code provisions‏

12. Code provisions