Relative plate thicknesses in vertical storage tanks: how do the bottom and roof plate thicknesses typically compare with the cylindrical shell thickness?

Introduction / Context:
Atmospheric and low-pressure vertical storage tanks are designed so that the cylindrical shell sustains hydrostatic pressure and wind/seismic loads, while the bottom and roof primarily provide containment and closure. Recognizing the typical thickness hierarchy helps in preliminary design and cost estimation.

Given Data / Assumptions:

• Vertical, cylindrical storage vessels for liquids at near-ambient conditions.
• Bottom plates resting on a foundation or annular ring; roof plates forming a fixed roof (e.g., cone or dome).
• No special vacuum or pressure design beyond normal venting.

Concept / Approach:
The shell experiences the highest membrane stresses from hydrostatic head and environmental loads, so it generally requires greater thickness. The roof carries comparatively small loads (self-weight, occasional snow/wind), and the bottom is supported by the foundation. Therefore, roof and bottom plates are typically thinner, while shell courses often get thicker toward the base where liquid head is greatest.

Step-by-Step Solution:
Identify major load paths: shell → hydrostatic and lateral loads.Bottom → supported; thickness driven by corrosion and local support details.Roof → light loads; thickness driven by stiffness and corrosion allowance.Conclusion: bottom and roof plates are usually thinner than shell courses.

Verification / Alternative check:
Common standards (e.g., API 650 practice) show graduated shell thickness with typically lighter roof/bottom plates subject to minimum practical and corrosion allowances.

Why Other Options Are Wrong:
(a) and (e) contradict common practice; (c) ignores graduated shell thickness; (d) overstates variability—while materials and temperature matter, the usual pattern remains thinner roof/bottom.

Common Pitfalls:
Over-thickening the roof and bottom without corrosion rationale; ignoring bottom annular plate requirements near shell for settlement and corrosion.

Final Answer:
Both are usually thinner than the shell