What is Steel Tank Design used for in structural engineering?

Steel Tank Design provides values required for steel design calculations including capacity checks, connection design, and detailing. It is referenced in structural design codes and used by practicing engineers during the design of buildings, bridges, and industrial structures.

Can I use these reference values directly in my design?

Yes, provided the values match your governing code edition and project specifications. Always cross-reference against the primary source (code document, manufacturer data, or published standard). The information on this page is for educational use — final verification by a licensed engineer is required.

How often are these reference values updated?

Reference content is maintained to align with current code editions. When a new edition of AISC 360, AS 4100, EN 1993, or CSA S16 is published, values are reviewed and updated. Check the last-modified date at the bottom of the page and verify against the current edition for your jurisdiction.

Steel Storage Tank Design Guide -- API 650

The API 650 standard (Welded Steel Tanks for Oil Storage, 13th Edition, 2021) is the worldwide benchmark for aboveground steel storage tanks. Originally developed by the American Petroleum Institute for the oil and gas industry, API 650 is now used across industries -- water, chemicals, agriculture, mining -- wherever large volumes of liquid need to be stored in flat-bottomed, cylindrical, welded steel tanks. Its empirical design rules, refined over nearly a century of operating experience, produce safe, economical designs without the complexity of finite element analysis.

Shell Thickness Determination

The tank shell resists hydrostatic pressure from the stored liquid. Because the pressure increases linearly with depth, the required shell thickness also increases with depth -- a tank shell is fabricated from multiple cylindrical courses (rings), each a constant thickness suited to the pressure at its base.

One-foot method (API 650 Section 5.6.3): This simplified method calculates the required thickness at a point 1 foot above the bottom of each course. The shell is considered as a series of independent rings, each designed for the hydrostatic pressure at the point 1 foot above its bottom:

Design condition: t_d = 2.6 x D x (H - 1) x G / Sd Hydrostatic test condition: t_t = 2.6 x D x (H - 1) / St

Where: D = tank diameter (ft), H = distance from top of shell to bottom of course (ft), G = design specific gravity of stored product, Sd = allowable design stress (psi) from API 650 Table 5-2a (23,200 psi for A36, 28,000 psi for A572 Gr. 50/ASTM A516 Gr. 70), St = allowable test stress (30,000 psi for A36, 36,000 psi for A572 Gr. 50).

Variable-design-point method (API 650 Section 5.6.4): A more refined approach that accounts for the actual stress distribution -- the lower, thicker courses restrain the upper, thinner courses through shell bending continuity, permitting thinner upper courses. The ratio t_actual / t_one-foot increases with height, typically saving 10-15% of shell plate weight for tall tanks (H/D > 1.0).

Worked example -- 100-ft dia x 48-ft tall oil tank: Product: crude oil, G = 0.85. Steel: A36 (Sd = 23,200 psi, St = 30,000 psi). Six 8-ft courses.

Course 1 (bottom, H = 48 ft): t_d = 2.6 x 100 x (48 - 1) x 0.85 / 23,200 = 2.6 x 100 x 47 x 0.85 / 23,200 = 10,374 / 23,200 = 0.447 in. t_t = 2.6 x 100 x 47 / 30,000 = 12,220 / 30,000 = 0.407 in. Design controls: 0.447 in. Add corrosion allowance: 1/16 in internal + 1/16 in external = 0.125 in. Total: 0.572 in. Use 5/8 in (0.625 in) plate.

Course 2 (H = 40 ft): t_d = 2.6 x 100 x (40 - 1) x 0.85 / 23,200 = 2.6 x 100 x 39 x 0.85 / 23,200 = 8,619 / 23,200 = 0.371 in. Add CA: 0.125 in. Total: 0.496 in. Use 1/2 in plate.

Course 3 (H = 32 ft): t_d = 2.6 x 100 x 31 x 0.85 / 23,200 = 6,851 / 23,200 = 0.295 in. Add CA: 0.420 in. Use 7/16 in plate.

Course 4 (H = 24 ft): t_d = 2.6 x 100 x 23 x 0.85 / 23,200 = 5,083 / 23,200 = 0.219 in. Minimum t plus CA per API 650 Section 5.6.1.1: minimum shell thickness = 3/16 in = 0.1875 in for D <= 120 ft. With CA: 0.1875 + 0.125 = 0.3125 in. Use 3/8 in plate.

Courses 5 and 6 follow the same pattern and will be governed by the 3/8 in minimum.

The step-thickness design results in a shell whose thickness decreases from bottom to top -- an efficient use of material that follows the hydrostatic pressure gradient. The butt-weld joints between different-thickness courses must be prepared with a 3:1 taper (API 650 Figure 5-2) to avoid stress concentrations at the thickness transition.

Wind Girder Design

Open-top tanks (common for water storage and some chemical applications) require a primary wind girder near the top to maintain shell roundness under wind. Without the girder, wind creates suction on the leeward side that can buckle the unrestrained top edge inward. Fixed-roof tanks do not require a separate wind girder because the roof structure provides the necessary circumferential stiffness.

Primary wind girder (API 650 Section 5.9.4): Required section modulus Z_req = 0.01 x D^2 x H1 (US customary units: D in ft, H1 in ft, Z in in^3). H1 = vertical distance from the wind girder to the top angle or the next wind girder above. For a 100-ft dia open-top tank with the wind girder placed at 45 ft from the base (H1 = 3 ft from girder to top): Z_req = 0.01 x 100^2 x 3 = 0.01 x 10,000 x 3 = 300 in^3.

The section modulus provided by the girder must include the contribution of a portion of the shell: 16 x t of shell above and below the girder. For t = 0.25 in: shell width contributing = 2 x 16 x 0.25 = 8 in. The shell contribution to Z is typically small (Z_shell = width x t^2 / 4 = 8 x 0.25^2 / 4 = 0.125 in^3), so the girder itself must provide effectively the full required modulus. A C15x33.9 channel has Zx = 43.6 in^3 (not enough), requiring a built-up section or a larger rolled section.

Intermediate wind girders (API 650 Section 5.9.5): The maximum permissible unstiffened shell height between wind girders: H1_max = 6 x (100 x t) x sqrt((100 x t) / D)^3. For t = 0.25 in, D = 100 ft: H1_max = 6 x 25 x sqrt(25/100)^3 = 150 x sqrt(0.25^3) = 150 x 0.125 = 18.75 ft. If the total shell height (48 ft) exceeds the sum of the unstiffened heights for all wind girders, additional intermediate wind girders are required at vertical intervals that satisfy H1 <= H1_max.

Anchor Bolt Design for Uplift

Flat-bottom tanks are inherently stable against overturning under gravity loads, but wind and seismic forces can create net uplift at the shell-to-foundation interface. API 650 Section 5.12 establishes the anchorage requirements.

Wind uplift: The wind overturning moment M_w = V_w x H/2 (approximate, assuming uniform wind pressure over height). V_w = total wind force = q_z x C_f x D x H per ASCE 7. For the 100-ft x 48-ft tank with 115 mph wind: V_w = 30.6 x 0.6 x 100 x 48 = 88.1 kips. M_w = 88.1 x 48/2 = 2,115 kip-ft. Resisting moment from self-weight: W_shell = 235 kips (calculated from plate weights), W_roof = 65 kips (fixed cone roof, 3/16 in plate), W_bottom = 85 kips (1/4 in bottom plate). Total W = 385 kips. M_R = 385 x 100/2 = 19,250 kip-ft > M_w. No uplift -- anchor bolts not required by wind.

Seismic uplift (API 650 Annex E): For seismic design, the impulsive and convective (sloshing) masses of the stored liquid create base shear and overturning moment. The seismic design is more complex than wind because the liquid mass does not move in unison with the tank -- the lower portion (impulsive) moves with the tank shell, while the upper portion (convective/sloshing) moves in long-period sloshing waves. Annex E provides detailed procedures that typically control for tanks in Seismic Design Categories C through F. When seismic overturning exceeds the resisting moment, anchor bolts are sized for the tension demand: F_bolt = 4 x M_seis / (n x D_bc) - W/n, where n = number of bolts and D_bc = bolt circle diameter.

Anchor bolt design per ACI 318-19 Chapter 17: The anchor bolt chair (stiffened bracket welded to the shell) distributes bolt tension into the shell. The chair top plate must resist bending: t_top >= sqrt(4 x F_bolt x e / (phi x Fy x b_eff)). Bolts must be checked for steel tensile strength, concrete breakout, pullout, and side-face blowout. Embedment depth: minimum 12 bolt diameters for headed anchors. For a 1-1/2-inch diameter F1554 Grade 55 bolt (f_uta = 75 ksi), the steel tensile capacity is N_sa = 0.75 x 75 x 1.41 = 79.3 kips per bolt.

Roof Types and Design

Fixed cone roof: The most common configuration, either self-supporting (no interior columns, for diameters up to approximately 60 ft) or column-supported (interior structural frame with rafters, columns, and girders). Roof slope: minimum 3/4:12 (9.5 degrees) for drainage. Roof plate minimum thickness per API 650 Section 5.10.2.2: t = D / 400 x sin(theta) >= 3/16 in. Live load: 20 psf minimum, or ground snow load if higher.

Floating roof: An open-top tank with a roof that floats on the liquid surface, eliminating the vapor space and reducing evaporative emissions by 95+ percent. Required by EPA regulations (40 CFR Part 60 Subpart Kb) for many petroleum products. Two types: external (exposed to weather, with roof drains and rolling ladder) and internal (inside a fixed-roof tank, protected from rain and wind).

Aluminum dome roof: A clear-span geodesic aluminum frame covered with aluminum panels, supported at the shell top angle. Light weight (2-3 psf self-weight), corrosion-resistant, and economical for large-diameter tanks (100-300 ft). Wind uplift must be checked; dome roofs develop significant net uplift that must be resisted by the shell-to-roof junction detail.

Try it now: Check beam capacity for tank support platforms

Related Tools and References

Disclaimer

This page is for educational and reference use only. Tank design must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) for the specific product stored, site conditions (wind, seismic, snow), local building codes, and environmental regulations (including spill containment per API 650 Appendix I and EPA 40 CFR 112).