For a cylindrical tank shell (e.g., storage vessel), how does the required course thickness vary from bottom to top, assuming the uppermost course already exceeds the minimum code thickness for the diameter?

Introduction / Context:
Atmospheric and low-pressure cylindrical storage tanks are commonly fabricated in “courses” (rings) of plate. Hydrostatic pressure from the liquid contents is greatest at the bottom, so required shell thickness varies with height to meet stress limits while minimizing material cost.

Given Data / Assumptions:

• Vertical, cylindrical shell storing liquid.
• Uppermost course already meets or exceeds minimum thickness required by code for the diameter.
• Primary load considered is hydrostatic head; wind and seismic are secondary for this conceptual question.

Concept / Approach:
Hoop stress in a thin cylinder is σ_θ = p * r / t. Since hydrostatic pressure p increases linearly with depth, the highest stress demand occurs near the bottom course. To keep stress within allowable limits, bottom courses are made thicker. Moving upward, the hydrostatic head decreases, permitting thinner courses while still satisfying code requirements.

Step-by-Step Reasoning:

Verification / Alternative check:
API/IS tank design practices provide annular plates and thicker first courses to handle bottom stresses, with successively thinner upper courses as allowed by design calculations and minimum-thickness rules.

Why Other Options Are Wrong:

• Increasing upwards contradicts hydrostatics.
• Uniform thickness is uneconomical for large diameters/heights.
• Vacuum may require stiffening but does not reverse the general hydrostatic trend for liquid-filled service.

Common Pitfalls:
Ignoring corrosion allowance and external loads; using too-thin top courses that violate minimum-code plate thickness or fabrication constraints.

Final Answer:
Decreases upwards (thicker at the bottom due to hydrostatic head)