Wind Girder and Roof Structure Design for Large Steel Water Storage Tanks

The two structural elements most often under-specified in modular steel tank projects are the wind girder and the roof structure. Both are resisting loads that are hard to visualise. Lateral wind pressure on a thin-walled cylinder does not look dangerous from the outside, but it can buckle the wall panels at wind speeds well below the design maximum if the girder is undersized or misplaced. Seismic sloshing forces on a roof spanning 10 to 20 metres above a 5ML body of water are similarly invisible until the event occurs. Getting these two elements right is where structural engineering adds real value in a tank project.

What Wind Girders Do

A wind girder is a ring beam that runs around the circumference of the tank wall at a specified height. Its function is to prevent the thin-walled cylindrical panels from buckling inward under lateral wind pressure. Without the girder, the wall acts as a thin shell supported only at the base ring and at the top ring, and the critical buckling mode is an oval distortion of the cylinder cross-section that begins at mid-height.

The lateral wind pressure on a cylindrical tank is not uniform around the circumference. The windward face receives positive pressure, the leeward face receives negative pressure (suction), and the side faces receive a more complex pressure distribution that depends on the Reynolds number and tank aspect ratio. The peak differential pressure across the cylinder at design wind speed for a tank in an open exposure category is typically 0.8 to 1.2 kPa, which on a 20m-diameter tank translates to a total lateral force of 100 to 150 kN per metre of wall height.

The wind girder reduces the effective unsupported length of the wall, which dramatically increases the buckling resistance. A wall panel that would buckle under 0.6 kPa without a girder can resist 1.2 kPa with a girder at mid-height, because the critical buckling length has been halved.

Wind Girder Sizing for Different Tank Diameters

Wind girder sizing is governed by the required flexural stiffness of the ring, which depends on the wind pressure, the tank diameter, and the panel wall stiffness. Smaller-diameter tanks have an inherent geometric advantage because the curvature of the cylinder wall provides in-plane compression resistance that larger-diameter tanks lack. A 10m-diameter tank wall is stiffer against lateral buckling than a 20m-diameter wall made of the same panel thickness, simply because the arc length for a given chord deflection is shorter.

For tanks up to 12m in diameter with wall heights up to 6m, a single top-of-wall wind girder using a 150x90x10 cold-formed channel section is typically adequate for wind speeds up to 50 m/s. For tanks above 15m in diameter, or wall heights above 7m, the girder section increases and an intermediate girder at mid-height becomes necessary for the design wind speed of most Australian sites.

The connection between the wind girder and the wall panels is critical. The girder must be bolted or welded to the top course of panels at close enough spacing to transfer the lateral load from the panels to the girder without local buckling at the connection points. PBE specifies the connection detail as part of the wind girder design, including the bolt size, spacing, and edge distance from the panel top.

Roof Structure Design for Large Tanks

The roof structure for a bolted steel panel tank spans from the top ring of the cylindrical wall to a central support grid. For small tanks up to 10m in diameter, a simple pitch-hip roof with a king post at the centre is common. For tanks above 15m in diameter, a truss grid with two or more internal columns is needed to keep the truss span manageable.

For a typical 5ML tank with a 20m diameter, the roof structure uses seven trusses arranged radially from the centre column to the perimeter top ring. The trusses span approximately 10m from the central column ring to the perimeter. Purlins span between adjacent trusses at approximately 3.0 to 3.5m spacing, which suits standard roof sheeting spans and keeps the purlin section weight to 75x50x4 cold-formed channel or similar.

Truss depth affects the span-to-depth ratio, which in turn affects the deflection under full snow and wind load. For a 10m span truss carrying a 1.0 kPa roof live load and a 0.75 kPa wind uplift load, a truss depth of 1.0m gives a span-to-depth ratio of 10:1, which is acceptable for a simply supported cold-formed steel truss. Deeper trusses reduce the chord forces and simplify the connection design but increase the head room required above the top water level.

Seismic Sloshing and Freeboard

Seismic sloshing is the most frequently overlooked load case in large tank design. When the ground accelerates horizontally during an earthquake, the fluid surface does not remain level. It oscillates as a standing wave, with the wave height determined by the seismic spectral acceleration at the sloshing period, the tank diameter, and the depth of water.

The sloshing period for a large tank is much longer than the period of the tank structure itself. For a 20m-diameter tank with 16m of water depth, the fundamental sloshing period is approximately 8 to 10 seconds. This means the sloshing response is governed by the long-period part of the seismic spectrum, where the spectral accelerations are lower than at the short periods that govern the structural inertia loads. The sloshing wave height is therefore not necessarily higher in a high-seismic region than in a low-seismic region, but it is significant in all seismic zones and must be calculated for each project.

The freeboard between the operating water level and the underside of the roof structure must be at least equal to the calculated sloshing wave height. PBE calculates the sloshing wave height to AS 1170.4 for each project and specifies the freeboard as a dimension on the structural drawings. For tanks above 3ML, the minimum freeboard is 750mm in seismic zone 1 and typically 900mm in seismic zone 2.

Anchor Bolt Design for Overturning

The anchor bolts at the base of a bolted steel panel tank resist two separate overturning actions. Wind overturning produces a moment about the tank base that puts the windward anchor bolts in tension. Seismic overturning produces a similar moment from the horizontal inertia of the tank structure and the convective (sloshing) mass of the stored water.

For a 3ML tank on a level site in wind region B with an open exposure category, the wind overturning moment at full design wind speed is approximately 8,000 to 12,000 kN-m, depending on tank height and diameter. This translates to anchor bolt tension forces of 60 to 100 kN per bolt for a typical bolt spacing of 600mm around the base ring. High-strength anchors with 200 to 250mm embedded length in a 300mm-thick slab are the typical specification for this load range.

The seismic overturning case for the same tank in seismic zone 1 adds a horizontal seismic force of approximately 0.08g on the total mass of the water plus the tank structure. For a 3ML tank (3,000 tonne water mass), the seismic base shear is approximately 2,400 kN and the overturning moment is approximately 6,000 to 9,000 kN-m depending on the effective height of the water centroid. The governing anchor bolt load is typically the seismic case combined with the empty tank case, where there is no stabilising water weight on the leeward side.

PBE designs anchor bolts for both wind and seismic overturning, checking the critical combination of load cases (full, half-full, and empty tank conditions). The anchor bolt layout, diameter, and embedment depth are specified on the structural drawings and form part of the structural certificate.

Cold-Formed Panel Stiffness and Bolt Pattern Specification

The horizontal bolt spacing between adjacent panels affects both the connection capacity and the ease of field assembly. A tighter bolt spacing of 100mm provides higher shear capacity at the connection but requires more bolts per panel and more time to install. A looser spacing of 120mm reduces the bolt count but may be inadequate for the lower courses of a deep tank where the hydrostatic shear force per connection is highest.

G350 grade panels with 4mm thickness are the standard specification for the lower courses of tanks above 4m in wall height. G250 grade panels at 3mm thickness are adequate for the upper courses where the hydrostatic pressure is low. Using G350 consistently throughout the wall is conservative and adds cost but simplifies panel procurement and field assembly, as the same panel grade can be used in every position.

The engineer’s role in bolt pattern specification is to confirm that the supplied bolt pattern provides adequate connection capacity at the critical load combination for each course, including the combined effect of hydrostatic pressure and seismic shear. This is not a standard check in the tank supplier’s design tables, which are typically based on static hydrostatic loads alone.

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Frequently Asked Questions

Does a single wind girder at the top of the wall always provide enough lateral restraint?

For most tanks below 8m in wall height, a single top-of-wall wind girder is adequate. For taller walls or larger-diameter tanks in high-wind regions, an intermediate girder at mid-height is typically required. The critical check is the buckling resistance of the panel wall between the base ring and the girder, which depends on the panel thickness, the wall height, the tank diameter, and the design wind pressure.

What roof structure configuration is typical for a 5ML tank?

A 5ML tank with a 20m diameter typically uses seven radial trusses from a central column ring to the perimeter top ring, with purlins spanning between trusses at approximately 3.0 to 3.5m spacing. The truss span is approximately 10m and the truss depth is typically 900mm to 1,200mm depending on the roof live load and wind uplift design requirements.

How is the sloshing wave height calculated for a large steel tank?

The sloshing wave height is calculated to AS 1170.4 using the convective mass fraction of the stored water, the sloshing period (which depends on tank diameter and water depth), and the spectral acceleration at the sloshing period for the site’s seismic hazard. For a 20m-diameter tank with 16m water depth in seismic zone 1, the sloshing wave height is typically 500 to 750mm.

What bolt size is typically used for base ring anchor bolts on a 3ML tank?

For a 3ML tank in wind region B, M24 or M30 high-strength anchor bolts at 500 to 600mm centres around the base ring are a typical specification. The exact size and spacing depend on the anchor bolt tension force from the governing overturning case and the concrete slab edge distance and embedment length available.

Can the structural engineer provide the slab design for the tank base?

The concrete slab is typically a separate scope. PBE provides the anchor bolt loads and base ring reaction forces in the structural documentation for use by the civil engineer responsible for the slab. For projects where PBE is engaged for the full scope, slab design can be included as an additional item. The slab design requires geotechnical information, including bearing capacity and groundwater data, which is typically available from a separate geotechnical report.