A Scheme-Stage Aspect-Ratio Criterion for Determining When Pedestals Require Anchor Reinforcement in Elevated Tank Towers

Abstract Drained elevated steel tanks under skew wind can place windward stanchion bases in tension, and that tension must pass through headed bolts into concrete pedestals whose plan size limits the concrete breakout resistance. Current design practice discovers whether anchor reinforcement is needed only after the tower has been analysed and the pedestals sized. This paper derives a criterion that makes the decision at scheme stage. Rigid-group equilibrium with EN 1990 and EN 1991-1-4 actions shows that the design uplift equals the factored stabilising load per stanchion multiplied by (η − 1), where η is a dimensionless demand ratio. The EN 1992-4 narrow-member rule caps the breakout resistance of any symmetric bolt group in a square pedestal of side a at (√6/9)k₁√f ck a 1.5 /γ Mc , independent of bolt diameter and grade. Combining the two gives the base aspect ratio at which concrete-only anchorage ends, (H/B) 2 = (H/B) 1 (1 + ρ a ), where (H/B) 1 is the onset of uplift and ρ a is the ratio of total pedestal resistance to factored stabilising load. The criterion was checked against a three-dimensional frame model over 108 configurations and classified 297 of 300 randomly generated towers in the same region as frame analysis combined with EN 1992-4; an 8% demand allowance for multi-leg towers with corner-supported tanks removed the single unconservative case. At a basic wind velocity of 40 m/s, (H/B) 2 was 1.37 to 2.17 for a 15 m tower with a 100 m³ tank on 700 mm C30/37 pedestals, well below the aspect ratios of built towers, so anchor reinforcement should be planned from the outset.