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

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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)

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

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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.

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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

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

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Stress distribution over a stiffening ring

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

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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

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Code provisions

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Code provisions

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