Short answer: no. Many storage tanks are designed to stand on their foundation without anchor bolts. Whether a tank needs anchors depends on an engineering check of wind, seismic loads, and internal pressure, including the case where the tank is empty. Size and weight alone do not decide it. When anchors are required, they have to be designed together with the tank shell and the foundation, before concrete is poured.
At MMI Tank we design, fabricate, and erect welded and bolted steel tanks and build the foundations they sit on, so we see this decision from both sides of the anchor bolt. This guide covers what drives the decision, what an anchorage system includes, and what project teams should check before construction.
What Anchor Bolts Do on a Storage Tank
Anchor bolts tie the bottom of the tank shell to the foundation. Their job is to resist uplift: forces that try to lift one side of the tank, slide it, or tip it over. A tank that stays put under every design load without them is called self-anchored (or unanchored). A tank that needs them is mechanically anchored.
A full tank is heavy and very stable. The problem cases are usually the opposite: a tall, light, empty tank in high wind, or a tank whose contents slosh and rock it during an earthquake.
The Four Loads That Decide It
1. Wind
Wind pushes on the shell and roof and creates an overturning moment at the base. The resisting force is the weight of the shell, roof, and attached structure. That is why the check is made with the tank empty: during first fill, cleaning, inspection, or repair there is no liquid weight helping hold it down. Tall, narrow tanks in open, high-wind sites are the most likely to need anchors for wind.
2. Seismic
In an earthquake, the stored liquid moves. Part of it moves with the shell and part of it sloshes, and both push the tank sideways and try to rock it off its base. For welded tanks, API 650 Annex E uses an anchorage ratio to decide whether a self-anchored tank is stable or needs mechanical anchors. Seismic is often the controlling case in the western US, including parts of Arizona, California, Nevada, and Utah.
3. Internal pressure
A tank that operates under internal pressure, even a small amount above atmospheric, pushes up on the roof. If that uplift exceeds the weight of the roof and shell, the tank must be anchored to keep the bottom from lifting at the edge. Vent sizing and pressure settings feed directly into this check.
4. Tank proportions
Height-to-diameter ratio matters as much as capacity. Two tanks with the same gallons can land on different sides of the line: a short, wide tank resists overturning far better than a tall, narrow one. If a site forces a tall, slender tank, plan on anchorage being part of the conversation.
Which Standard Applies
The tank standard sets the design method, so the answer is tied to which code your tank is built to:
- API 650 for welded tanks in petroleum, chemical, and process service. Covers wind overturning stability, seismic design (Annex E), and anchorage for internal pressure.
- AWWA D100 for welded steel water tanks, and AWWA D103 for factory-coated bolted water tanks. Both include wind and seismic stability and anchorage requirements.
- NFPA 22 for fire-protection water tanks, which references the AWWA standards for the tank structure.
Not sure which one your project falls under? See API 650 vs AWWA D100: When to Use Which Standard.
What an Anchorage System Includes
If the design calls for anchors, it is more than a set of bolts cast into concrete. A complete anchorage system covers:
- Anchor bolt size and quantity, sized for the design uplift plus corrosion allowance.
- Spacing around the tank circumference, evenly distributed so the shell is loaded uniformly.
- Anchor chairs or attachments welded or bolted to the shell that transfer load from the shell into the bolt without overstressing the shell plate.
- Embedment into the foundation: depth, edge distance, and reinforcement so the concrete does not break out before the bolt yields.
- Foundation design heavy enough, or wide enough, to resist the uplift the anchors pass into it. A ringwall or slab that works for a self-anchored tank may not work once anchors are added.
- Installation tolerances for bolt location and projection, so the tank actually lines up when it is set.
Where Projects Go Wrong
The most common anchor problem is not engineering. It is coordination. The tank designer, the foundation designer, and the concrete contractor are often different firms working from different drawings. Typical results:
- Bolt circle diameter on the foundation drawing does not match the tank drawing.
- Bolt count, size, or projection differs between the two sets.
- Anchor chairs land on a shell seam, a nozzle, or a manway.
- The foundation was designed for a self-anchored tank, then the tank design changed.
Each of these is found when the erection crew arrives, after the concrete has cured. The fix is coring and epoxy anchors, a modified foundation, or redesigned chairs, and all of them cost schedule.
Checklist Before Concrete Is Poured
- Confirm the tank design calculations state whether the tank is self-anchored or mechanically anchored, and for which load case.
- Compare tank and foundation drawings side by side: bolt circle, quantity, size, material, projection, and orientation relative to nozzles.
- Confirm the foundation was designed for the anchor uplift loads in the tank calculations.
- Check that the site wind speed, seismic data, and design pressure used by both designers match.
- Use an anchor bolt template during the pour and survey bolt locations before the tank is set.
The simplest way to avoid the mismatch is to have one team responsible for both. MMI designs the tank and builds the tank foundation under the same project, so the anchor layout is set once and checked against both.
Frequently Asked Questions
Do small storage tanks need anchor bolts?
Sometimes. Small tanks are light, which works against them in wind, especially when empty. A small, tall tank on an exposed site can need anchors where a much larger, wider tank does not.
Do bolted steel tanks need anchors?
Bolted tanks follow the same logic as welded tanks: wind, seismic, and pressure decide. Because bolted tanks are often lighter for the same capacity, anchorage is common. Compare the two construction types in Welded vs Bolted Steel Tanks.
Why is the empty tank the worst case?
The liquid’s weight is what holds a full tank down. Empty, only the steel resists wind overturning. Tanks are empty during first fill, cleaning, inspection, and repair, so the design has to cover it.
Can anchors be added to an existing tank?
Yes, but it usually means adding shell attachments and installing post-installed anchors in an existing foundation, which must be checked for the new loads. It is often part of a repair or re-rating project. See Tank Repair and Modification (API 653).
Who decides whether a tank is anchored?
The tank designer, using the governing standard and the project’s site data. The foundation engineer then designs for the anchor loads. Both need the same inputs and the same final drawings.
Plan the Tank and Foundation Together
MMI Tank & Industrial Services engineers, fabricates, and erects field-erected welded tanks and bolted steel tanks, and builds the foundations under them, so anchorage is coordinated from the first drawing. Request a quote and send your site location, capacity, and service; we will tell you early whether anchors are in play.
This article expands on a post from MMI’s LinkedIn page. Follow MMI on LinkedIn for new tank engineering tips every Monday and Friday.
