Cylindrical storage tanks: roof column (internal support) requirements are primarily driven by span. For which tank size category is roof column support typically required?

Introduction / Context:
Fixed-roof storage tanks (e.g., cone roof) must support roof dead load, live load (snow, maintenance), and wind uplift. Whether internal columns are needed depends largely on roof span and framing efficiency.

Given Data / Assumptions:

• Conventional atmospheric cylindrical steel tanks.
• Standard roof types (cone or dome) with typical structural framing.
• Design per common industrial standards for snow/wind loads.

Concept / Approach:
As tank diameter increases, roof span and bending moments in roof framing increase rapidly. Beyond a certain diameter, practical roof plate thickness and rafter sizes become excessive without intermediate support. Central columns (or multiple columns) reduce span and member sizes, providing an economical and safe solution for large-diameter tanks.

Step-by-Step Solution:
Relate roof load effects to span (diameter); larger diameter → larger bending moments.Recognize that small tanks can use self-supporting roofs, but large ones need internal columns.Choose “large diameter tanks.”

Verification / Alternative check:
Fabricator guidelines specify self-supporting cone roofs up to limited diameters; above these, column-supported roofs are standard.

Why Other Options Are Wrong:
Small diameter: Span is short; self-supporting roofs are common.Small diameter tall tanks: Height does not govern roof span capacity.All tanks: Overly broad; many small tanks do not require columns.

Common Pitfalls:
Ignoring corrosion allowance on columns in corrosive service; overlooking interference with mixers or internal floating roofs.

Final Answer:
large diameter tanks.