Atmospheric storage: a cylindrical tank may use a self-supported conical roof under what size condition?

Introduction / Context:
Fixed roofs for atmospheric storage tanks include self-supported cones, raftersupported cones, and domes. The choice depends on diameter, loadings, and construction economy. Self-supported conical roofs avoid rafters but are limited by structural spanning capability.

Given Data / Assumptions:

• Standard above-ground cylindrical tanks for hydrocarbons or chemicals.
• Normal environmental loads (wind, rain) and no extraordinary roof appurtenance loads.

Concept / Approach:
Self-supported cones are practical only up to moderate diameters, because plate thickness and required cone angle become prohibitive for large spans. A typical rule places the upper limit near 15–20 m, above which rafters or column-supported roofs are used to limit plate thickness and deflection.

Step-by-Step Solution:

Verification / Alternative check:
Tank standards and vendor practices show transitions from self-supported to supported roofs as diameters increase beyond ~15–20 m for economy and strength.

Why Other Options Are Wrong:

• > 50 m: Typically requires rafters or column support.
• Thickness criterion alone (c) is not the selection basis; spanning capability governs.
• “Irrespective of diameter” ignores structural span limitations.

Common Pitfalls:
Ignoring corrosion allowance and live loads; not considering roof appurtenances (nozzles, platforms) that further constrain self-support feasibility.

Final Answer:
If its diameter is less than about 15–20 metres.