Tank Overturning Stability Criteria

[Muhammad@ZEL]

Hi All,
I hope this message finds you well.
I am currently reviewing the anchorage and overturning stability requirements for vertical aboveground storage tanks designed in accordance with API 650, particularly under the combined influence of wind and seismic loads.

Based on my understanding so far, the need for anchorage arises primarily from two major load scenarios:

Wind Case

1. For unanchored tanks, API 650 outlines the following uplift stability criteria under wind loads and internal pressure:​

Unanchored tanks shall satisfy all of the following conditions:​

1.  0.6Mw + MPi < MDL / 1.5 + MDLR
2.  Mw + Fp(MPi) < (MDL + MF)/2 + MDLR
3.  Mws + Fp(MPi) < MDL / 1.5 + MDLR

For unanchored tanks with supported cone roofs, satisfying the following condition applies:
  Mws + Fp(MPi) < MDL / 1.5 + MDLR

It is clear that:

• The tank's ability to resist wind uplift strongly depends on the actual liquid level at the time of the wind event.
• An empty or near-empty tank becomes particularly vulnerable to uplift.

I would appreciate further clarification on how to treat uplift stability in scenarios where the tank may be only partially filled (e.g., dead stock level only) — which is a likely operational scenario in industry. In cases where a tank operates at minimum dead stock level (e.g., 5%–10% fill), how is the resisting moment treated in wind uplift checks? Is the liquid weight ignored entirely?
And Also

• if I run calculations using full liquid height (up to shell height), the tank passes easily.
• However, if I use the actual design liquid level (DLL) instead of shell height — uplift may fail, requiring anchors

So, what’s the proper industry practice here? Should wind uplift checks be done using:

• Design Liquid Level (DLL)?
• Full shell height?
• Or even dead stock level (worst case)? (do we separately consider it ?)

Seismic Case
Also, in API's seismic Examples, the product height is taken up to full shell height — not the design liquid level.

1. Is it acceptable to use design liquid level (DLL) instead of full shell height when calculating seismic overturning moment and anchor bolt sizing?
2. While testing different design scenarios, I observed that:

When I use the full shell height as the product height in seismic overturning calculations, the required anchor bolt size increases noticeably. However, if I reduce the product height to the design liquid level the required bolt size decreases.

Freeboard
API 650 Annex E (Seismic Design) considers sloshing and requires freeboard per Table E.7 for SUG II & III, but it's optional for SUG I.
"Sloshing of the liquid within the tank or vessel shall be considered in determining the freeboard required above the top capacity liquid level."
“Purchaser shall specify whether freeboard is desired for SUG I tanks.”
So does the freeboard affect anchor design or overturning checks, or is it only for sloshing margin?


Tank Design Data (For Reference):
Location: Pakistan
Seismic Zone: 2A
Peak Ground Acceleration (Sp): 0.16g
Seismic Use Group (SUG): I
Site Class: D
Tank Diameter: 8.0 m
Shell Height: 13.0 m
Product Specific Gravity: 0.76
Roof Type: Self-supporting Cone
Bottom Type: Cone Down
Shell Thickness (1st course, corroded): 6 mm
Bottom Plate Thickness: 8 mm
Wind Speed: 160 km/h (3-sec gust)

Best regards,