Foundation Load Report for API 650 Tanks: The Complete Set of Loads Civil Engineers Actually Need

The Four Load Cases: Operating, Empty, Hydrotest, and Seismic

Every API 650 tank foundation report must cover four distinct load cases. Civil engineers designing a ring beam, raft slab, or pile foundation need all four — because each one governs a different failure mode and a different structural check. Providing only the operating load, as many tank engineers do, is insufficient and can result in an under-designed foundation.

The four load cases are:

1. Operating case: The tank at maximum liquid level with the design specific gravity, all equipment weight (shell, roof, bottom, nozzles, insulation, internals) included. This is the normal service condition and governs the bearing pressure check under sustained loading.
2. Empty case: The tank empty — no product, no hydrotest water. Wind or seismic loading applies. This case governs when checking for potential foundation uplift or overturning when there is no liquid ballast. It is also the governing case for verifying that the tank does not slide or tip before the product is first introduced.
3. Hydrotest case: The tank filled with water at SG = 1.0 to the hydrotest fill height, per API 650 §7.3. No wind or seismic load is applied concurrently — the hydrotest is a controlled, static operation. This is almost always the heaviest vertical load on the foundation, and it must be provided separately.
4. Seismic case: The tank at operating weight plus seismic inertia loads from the Appendix E analysis. This case includes the horizontal seismic base shear and an overturning moment that can exceed the wind case significantly in high-seismic zones.

Each of these cases produces a different combination of vertical load, horizontal shear, and overturning moment. The civil engineer must envelope all four cases when sizing the foundation elements.

Weight Breakdown: Shell, Roof, Bottom, Fittings, and Product

The vertical load in the operating case is the sum of all component weights. Getting this breakdown right matters — not just for the total foundation load, but because the civil engineer may need individual component weights to calculate the centre of gravity and the moment arm for wind overturning.

The weight components are:

• Shell weight: Sum of all courses — plate width × plate thickness × circumference × steel density (7,850 kg/m³). Each course is calculated individually, then summed.
• Roof weight: Roof plate plus structural framing (rafters, columns, girder ring). For a self-supporting cone or dome roof, the plate weight dominates. For a supported cone roof, the structural frame can add 15–30% on top of the plate weight.
• Bottom plate weight: Annular plate ring plus sketch plates (interior bottom). The sketch plate area is approximately π/4 × D² minus the annular ring area. Both are at the same nominal thickness (typically 6–8 mm for sketch plates, slightly thicker for annular plates).
• Nozzle and fitting weights: Individual nozzles, manways, and fittings can be calculated from the nozzle schedule, but for preliminary foundation loads they are commonly estimated at 3–5% of the shell weight. TankCode 650 calculates nozzle weights individually from the nozzle schedule and adds them explicitly.
• Product weight: Wᵐ = (π/4) × D² × Hₗ × SG × 9.81 kN/m³, where Hₗ is the maximum design liquid height and D is the nominal diameter. For SG < 1.0, the hydrotest will always exceed this.

The total operating vertical load is the sum of all steel components plus the product weight. For insulated tanks, add insulation and cladding weight. For tanks with heating coils or internal structures, add those weights separately with a note on the application point.

Overturning Moment from Wind and Seismic

The overturning moment is provided at the shell-to-bottom junction (the base of the tank). This is the correct application point for the civil engineer — it is the moment that must be resisted by the foundation, not the moment at the centre of the projected wind area or at the centre of mass.

For wind overturning, the moment at the base comes from two components: the wind pressure on the projected cylindrical area of the shell, and the wind pressure on the projected area of the roof. The resultant horizontal force acts at a height above the base, and the moment is that force multiplied by its height. API 650 §5.11 provides the wind pressure and load combination rules.

The civil engineer needs not just the moment but also the derived uplift and compression forces at the tank base perimeter. For a simplified circular footprint, the edge reaction from overturning can be estimated as:

• Net uplift at tension side = Mₒ- / (π/4 × D) — this is the maximum uplift per unit length of circumference
• Vertical reaction distribution follows an approximately linear variation across the diameter

For the wind case, also provide the total horizontal shear force at the base — this is the total wind force on the projected area of the shell and roof combined. The civil engineer needs this to check the sliding resistance of the foundation and to design any mechanical shear transfer devices (anchor bolts, shear keys) at the base slab interface.

Note that for API 650, wind load is defined at a 3-second gust speed and a specific exposure category. Always state the design wind speed, exposure category, and whether the wind map used is ASCE 7 or a locally specified value.

Seismic Base Shear and Its Load Path

The seismic base shear from API 650 Appendix E is the total horizontal seismic force at the tank base, combining the impulsive and convective components by the square root of sum of squares (SRSS) method:

V = √(Vᵢ² + Vᴄ²)

Where Vᵢ is the impulsive base shear (the tank structure and the portion of liquid moving with the tank wall) and Vᴄ is the convective base shear (the sloshing liquid mass). These are computed at different natural periods and combined by SRSS rather than simple addition.

This seismic base shear is a lateral load that must be transferred from the tank bottom plate through the foundation interface. The load path is:

1. Seismic inertia acts on the tank structure and liquid mass.
2. The resultant lateral force is transferred to the bottom plate via the shell-to-bottom connection.
3. The bottom plate transfers the force to the foundation through friction (for self-anchored tanks) or through the anchor bolt shear resistance and shear keys (for mechanically anchored tanks).

A common error is to assume that because a tank passes the J-ratio anchorage check (meaning no tensile uplift bolts are required), no shear transfer mechanism is needed either. This is incorrect — the lateral base shear must still be transferred regardless of the uplift anchorage status. The civil engineer needs the base shear value to verify that the friction coefficient between the bottom plate and foundation or the bearing pad is sufficient, and to design shear lugs or keys where required.

The seismic overturning moment at the base is separately computed from the effective moment arm of the impulsive and convective masses. For the impulsive component, the centroid of the pressure distribution is approximately at 0.375 × H above the base for a filled tank (where H is the liquid height). This is the moment arm used in API 650 Appendix E §E.7.2. The convective component acts at a higher elevation — typically at 0.5 to 0.7 × H depending on the tank slenderness ratio.

What the Civil Engineer Actually Needs (and What Gets Missed)

There are three things that are consistently missing from foundation load reports delivered by tank engineers to civil teams:

1. Loads at the correct elevation. The overturning moment must be reported at the shell-to-bottom junction — not at the mid-height of the shell, not at the centre of mass. The civil engineer will add this moment to the foundation's own weight effects and compute bearing pressures at the underside of the ring beam or raft. If the moment is provided at the wrong elevation, the civil engineer must back-calculate the height and re-derive the base moment, introducing unnecessary opportunity for error.

2. The hydrotest load case. This is the single most commonly missing load case. Many tank engineers provide operating and seismic loads only. For tanks designed for light products (SG < 0.85), the hydrotest vertical load can be 20–35% higher than the operating vertical load. A ring beam designed only for operating loads will be under-sized for the hydrotest fill. Since the hydrotest occurs before commissioning — often before the foundation has fully cured or settled — this matters for both structural capacity and for interpreting settlement readings.

3. Differential settlement tolerance. API 650 Appendix B provides guidance on the maximum allowable differential settlement for a tank. The civil engineer designing the foundation needs to know the tank's settlement tolerance — both the allowable overall tilt and the allowable short-wavelength (edge settlement) differential — so they can specify an appropriate foundation design. This information must come from the tank engineer; it cannot be derived by the civil engineer from the structural drawings alone.

Hydrotest is almost always the heaviest vertical load on the foundation — and it is often the case that was never provided to the civil engineer. A tank designed for SG = 0.75 gasoline carries 33% more weight during the hydrotest than at operating conditions.

Practical Format for the Foundation Load Report

The most effective foundation load report is a single table with one row per load case and one column per load component. This format prevents the majority of civil/tank miscommunications because both parties can see the complete load set in one view, verify that all four cases are present, and confirm the application point without cross-referencing multiple calculation sheets.

The recommended six-column format is shown below:

In addition to the load table, the foundation load report should include the following supporting information:

• Tank geometry summary: Nominal diameter (inside), shell height, design liquid height, hydrotest fill height, and number of shell courses. The civil engineer needs these to verify load calculations independently.
• Design parameters: Design specific gravity, operating SG, wind speed and exposure category, seismic spectral accelerations (S₁ and S₂), site class, and importance factor. These define the load basis and must be traceable to the project specification.
• Weight schedule: A breakdown of the total vertical load by component — shell, roof, bottom, nozzles and fittings, and product (or hydrotest water). This allows the civil engineer to check the total against their own estimates and to locate each weight component at its correct elevation for moment calculations.
• Differential settlement note: State the maximum allowable differential settlement at the tank base, referencing API 650 Appendix B. This is the tolerance the foundation must achieve under the applied loads.
• Application point confirmation: Explicitly state that all moments are provided at the shell-to-bottom junction elevation. If the foundation introduces an eccentricity (e.g., a ring beam whose centroid is below the tank base plate), the civil engineer must apply an additional moment correction — but only if they know the reference elevation.

Finally, if the tank is anchored, also provide the anchor bolt load — the tensile uplift force per bolt — as a separate line item. This is not a foundation load in the traditional sense, but the civil engineer needs it to design the anchor bolt embedment into the ring beam or raft.

Related reading: Continue with Shell Course Thickness Design, Seismic Design of API 650 Tanks, and Anchorage Design Guide to keep the full API 650 design workflow connected.