Contents
About this tool
The Tank Anchorage calculator checks the mechanical anchorage of a new tank to API 650 5.12. It works out the net uplift for each Table 5.20 load case that applies (design pressure, test pressure, wind, seismic, the two pressure combinations and frangibility), shares it between the anchors, and checks the anchor bolts or straps against the allowable stress, their number and spacing, the bolt size and eccentricity, the embedment and seismic attachment loads, the anchor chair (local shell stress, top plate, side plates and welds), and the counterbalancing weight of the ringwall.
Scope
This is an indicative check, not a certified design. It follows API 650, 14th edition (August 2025), 5.12 and Tables 5.20a/b. The anchor chair (5.12.7) is checked by the procedure 5.12.7 names, AISI Steel Plate Engineering Data Vol. 2 Part V (2011), with its ksi limits converted. A continuous top ring (AISI Figure 5-3) is checked as a ring under equal radial loads; a 5.12.8 evaluation (ASME VIII-2 Part 5) is checked from the stress results you enter, not computed. Wind and seismic loads come from the Wind Design and Seismic tools; foundation overturning and bearing are evaluated per Annex B.
Quick start
Press Load illustrative sample: a 15 m × 12 m Annex F tank (design pressure 3.5 kPa, test 4.375 kPa) with wind (PWR 1.2 kPa, PWS 860 Pa) and seismic (Mrw 2,500 kN·m, Av 0.10), held by 16 M30 ASTM F1554 Grade 55 bolts in chairs with 3 mm corrosion allowance, on a 0.6 × 1.2 m ringwall. All six cases give uplift; design pressure + seismic governs at 620.5 kN. Nothing fails.
- 1Identification: Tank ID, record type and units, which choose Table 5.20a (SI) or 5.20b (US customary).
- 2Section 1: diameter, height, bottom course thickness and its material, which gives Fty.
- 3Section 2: nominal shell, roof and attachment weights with their corrosion allowances; the tool builds W1, W2 and W3, or you enter them.
- 4Section 3: the loads that apply. A case is checked only when its loads are entered.
- 5Section 4: bolts in chairs or straps, their number, material, size and corrosion allowance, the eccentricity and thermal growth.
- 6Section 5: the chair dimensions (or leave them blank). Section 6: the ringwall, then the record items, Calculate & Save and Print / PDF report.
Units and rule set
- Units choose the rule set (1.1.3). SI uses Table 5.20a: pressures in kPa, the constant 785, spacing 3 m, bolt Fy not over 380 MPa, em = 54 + d/2 + 500 Et D T. US customary uses Table 5.20b: pressures in inches of water, 4.08, 10 ft, 55,000 lbf/in², em = 2.125 + d/2 + 6 Et D T.
- The two tables are close but not exact conversions: 4.08 lbf per in. of water per ft² is 784.3 in SI units, against the 785 printed in 5.20a. A pressure case reads about 0.1 % differently between them.
- Bolt lists follow the rule set: ISO metric coarse (M24 – M80) for SI, UNC / 8-UN (1 – 3 in.) for US. Saved records are stored in SI and reopen in the rule set they were saved in.
Resisting weights (section 2)
| Weight | Table 5.20 definition | Sample |
|---|---|---|
| Shell, corroded | Nominal less π D H × CA × 77 kN/m³ | 300 − π × 15 × 12 × 1.5 mm × 77 = 300 − 65.31 = 234.69 kN |
| Roof plates, corroded | Nominal × (t − CA) / t | 70 × 4 / 5 = 56 kN |
| W1 | Corroded roof plates + corroded shell + corroded attachments | 234.69 + 56 + 15 = 305.69 kN |
| W2 | Corroded shell + attachments + the roof plates and framing carried by the shell | 234.69 + 15 + 56 = 305.69 kN |
| W3 | Nominal roof plates + shell + attachments | 300 + 70 + 15 = 385 kN |
- The roof plates and framing carried by the shell (for W2) are entered already corroded: for a column-supported roof this is only the part at the shell.
- Entering W1, W2 or W3 directly overrides the built-up value; the results table marks it “entered”.
- The Shell Design and Roof Design tools give the shell and roof weights.
Uplift load cases (section 3)
| Case | Net uplift U, N (Table 5.20a) | Sample U, kN | Stress / allowable, MPa |
|---|---|---|---|
| Design pressure | [Pi × D² × 785] − W1 | 312.5 | 37.6 / 5/12 × 380 = 158.3 |
| Test pressure | [Pt × D² × 785] − W3 | 387.7 | 46.7 / 5/9 × 380 = 211.1 |
| Wind load | PWR × D² × 785 + 4 MWS / D − W2 | 153.9 | 18.5 / 0.8 × 380 = 304.0 |
| Seismic load | 4 Mrw / D − W2 (1 − 0.4 Av) | 373.2 | 44.9 / 0.8 × 380 = 304.0 |
| Design pressure + wind | [(Fp Pi + PWR) × D² × 785] + 4 MWS / D − W1 | 401.2 | 48.3 / 5/9 × 380 = 211.1 |
| Design pressure + seismic | [Fp Pi × D² × 785] + 4 Mrw / D − W1 (1 − 0.4 Av) | 620.5 | 74.7 / 0.8 × 380 = 304.0 |
| Frangibility | [3 × Pf × D² × 785] − W3 | — | Fy (1.0) |
- MWS = PWS × D × H² / 2: 860 × 15 × 12² / 2 = 928,800 N·m. Four times a moment over the diameter is the extra load on the windward or seismic side.
- Fp (5.2.2) is at least 0.4; a lower entry is raised to 0.4 with a Note.
- Av is entered as a fraction of g (0.10, not 10). Table 5.20 says “% g”, but (1 − 0.4 Av) only makes sense with a fraction, which is how Annex E uses it.
- For a supported cone roof meeting 5.10.4, tick the box: PWR is taken as zero.
- Frangibility applies only to tanks designed to 5.10.2.6 d: tick the box and enter the failure pressure Pf from the Roof Design tool.
- A case with no net uplift is shown as “no uplift”. If no case lifts the tank, Table 5.20 needs no anchors; wind stability (5.11.2) and seismic (Annex E) still decide whether anchors are needed.
- Wind and seismic are never combined (5.12.9). The 33 % increase in 5.12.9 is for the anchorage parts designed to AISC 360, not the Table 5.20 bolt stresses.
Anchors (section 4)
| Item | Rule | Sample |
|---|---|---|
| Load per anchor (5.12.2) | Tb = U / N, N equally spaced, at least 4 | 620.5 / 16 = 38.78 kN |
| Spacing (5.12.3) | π (D + 2 t) / N ≤ 3 m (10 ft) at the shell outside | π × 15.016 / 16 = 2.95 m |
| Tensile area (5.12.5) | Lesser of the net root area and the corroded shank area; CA on the shank, not the thread (5.12.6) | M30 × 3.5: root 30 − 1.226869 × 3.5 = 25.706 mm → 519.0 mm²; shank 27 mm → 572.6 mm² |
| Bolt Fy (Table 5.20) | Specified minimum yield, not over 380 MPa (55,000 lbf/in²), or the value at the maximum design temperature | 380 MPa |
| Bolt size (5.12.6) | Corroded shank at least 25 mm (1 in.) | 27 mm ✓ |
| Eccentricity (5.12.4) | e ≥ em = 54 + d/2 + 500 Et D T | 54 + 15 + 500 × 1.2×10⁻⁵ × 15 × 70 = 75.3 ≤ 80 mm ✓ |
| Embedment (5.12.13) | Develops Fy × nominal root area | 380 × 519.0 = 197.2 kN |
| Seismic attachment load (5.12.13) | Lesser of that and 3 Tb (seismic cases) | min(197.2, 3 × 38.78) = 116.3 kN |
Straps (5.12.15): choose Hold-down straps and enter the width, thickness, any reduced (contoured) width and the slope. The area is the lesser of the section at the shell and at the reduced width, with at least 1.5 mm (1/16 in.) corrosion on each surface (5.12.6). Straps are at least 6 mm (¼ in.) thick, their Fy may not exceed the shell’s Fty (Table 5.20), they are not capped at 380 MPa, and they may slope no more than 5° from vertical (5.12.15.5).
- Leave N blank to use the least number the spacing allows: π (D + 2t) / 3 m, rounded up, at least 4. Where a case is overstressed its message gives the number of anchors of that size that would pass.
- Unequal spacing is not modelled: if the anchors are not equally spaced, Tb must be increased (5.12.2), a record item.
- Et is per °C (per °F in US units) and T is the maximum design temperature less ambient.
Anchor chair (section 5)
| Item | AISI Part V | Sample (M30, P = 116.3 kN) |
|---|---|---|
| Design load P | Lesser of the allowable bolt load and 1.5 × the bolt load; for seismic cases not less than the 5.12.13 attachment load | seismic: min(197.2, 3 × 38.78) = 116.3 kN; design pressure: 1.5 Tb = 29.3 kN |
| Reduction factor Z (5-4) | 1 / [0.177 a m / √(Rt) × (m / t)² + 1]; m blank: Z = 1 | R 7,500, t 8 − 1.5 = 6.5, a 200, m 8 mm: Z = 0.340 |
| Local shell stress (5-3) | S = (P e / t²) [1.32 Z / (1.43 a h² / Rt + (4 a h²)0.333) + 0.031 / √(Rt)] | 99.6 MPa ≤ 5/6 Fty = 216.7 ✓ (and ≤ 172) |
| Top plate (5-1), (5-2) | S = P (0.375 g − 0.22 d) / (f c²) ≤ 25 ksi (172 MPa) | g 60, f 35, c 25: 84.6 MPa ✓ |
| Height | At least 6 in. (152 mm); 12 in. (305 mm) on a bottom 3/8 in. or thinner with seismic or wind over 100 mph; at most 3a | 400 mm (305 – 600) ✓ |
| Nut and edge | e ≥ 0.866 d + 0.572 in.; f ≥ d/2 + 1/8 in.; g preferably d + 1 in. | 80 ≥ 40.5; 35 ≥ 18.2; g 60 (55) |
| Side plates | j ≥ max(½ in., 0.04 (h − c)); j k ≥ P / 25 ksi | 16 ≥ 15; 1,280 ≥ 675 mm² |
| Welds (5-5) – (5-8) | WV = P / (a + 2h); WH = P e / (a h + 0.667 h²); √(WV² + WH²) ≤ 9.5 w | 116.3 and 49.8 → 126.6 N/mm ≤ 524 (8 mm) ✓ |
- Each load case with uplift is checked against its own Table 5.20 shell allowable (5.12.10): 2/3 Fty for design pressure, 5/6 for the others, 1.0 for frangibility. The chart shows every case; the summary reports the most utilised.
- AISI recommends 25 ksi (172 MPa) with no increase for wind or seismic. Over that but within the Table 5.20 allowable the check is a Note, not a failure, since 5.12.10 is the code limit.
- If the shell is overstressed: widen the top plate (a), raise the chair (h), use more, smaller bolts, reduce e, or go to a continuous top ring (AISI Figure 5-3) and record that design.
- Leave a or h blank to skip the chair; it then stays as a record item.
Continuous ring and 5.12.8 (section 5)
The Shell at the attachment menu picks how the shell is checked. Separate chairs use AISI (5-3) above. The other two choices replace that check; the top plate, height, side plate and weld checks stay.
| Continuous ring (AISI Figure 5-3) | Rule | Sample: chair a 120, h 250; ring 100 × 25 mm |
|---|---|---|
| Load on the ring | W = P e / h outward at each of the N chairs (and inward on the bottom) | 116.3 × 80 / 250 = 37.23 kN |
| Section | Ring plate + the shell within 16 t either side: 32 t + c of shell | 100 × 25 + 233 × 6.5: A 4,014 mm², I 4.76 × 10⁶ mm⁴ |
| Moments, α = π / N | At a chair (W R / 2)(1/α − cot α); midway (W R / 2)(1/sin α − 1/α) | 9.20 and 4.61 kN·m (R 7,530 mm) |
| Hoop force | W / (2 sin α) midway, W / (2 tan α) at a chair | 95.4 kN |
| Stress | T / A + M y / I, the larger of the two sections, against the case’s Table 5.20 shell allowable | 158.7 ≤ 216.7 MPa ✓ |
| 5.12.8 evaluation (ASME VIII-2 Part 5) | Limit | Sm 2/3 × 260 = 173.3 MPa (design pressure) |
|---|---|---|
| Pm, general primary membrane | Sm | 173.3 |
| PL, local primary membrane | 1.5 Sm | 260 |
| PL + Pb | 1.5 Sm | 260 |
| PL + Pb + Q | 3 Sm (SPS) | 520 |
- The ring formulas are the thin-ring solution for N equal radial loads; with two loads they give the textbook W R / π at the loads and W R (½ − 1/π) between them, and with many loads the hoop force tends to W N / 2π.
- For 5.12.8, pick the case the analysis was run for and enter its stress intensities (or equivalent stresses) at the attachment. A category left blank is a Note; the analysis reference goes into the record and the report, and a 5.12.8 record item asks that shell deflection and rotation were included.
- From a WRC 107 / 537 run in PV Elite, Compress or CodeCalc, read the vessel stress summation at the attachment: Pm goes to Pm, Pm + Pl to PL, and Pm + Pl + Q to PL + Pb + Q. WRC 107 does not separate primary bending, so PL + Pb may stay blank. Convert kgf/cm² (× 0.0981) or psi to MPa first. Those programs apply their own S, 1.2 S for occasional loads and SPS; here the limits come from the case’s Table 5.20 allowable instead, as 5.12.8 requires.
- The tool does not run the analysis: that needs a finite element model or the WRC 107 / 537 curves, which are outside it.
Counterbalancing weight (section 6)
| Item | Rule | Sample |
|---|---|---|
| Ringwall weight | π D × width × depth × γ (23.5 kN/m³ unless entered) | π × 15 × 0.6 × 1.2 × 23.5 = 797 kN |
| Soil above the footing (5.12.14.2) | Effective weight, entered in kN | — |
| Check (5.12.14.1) | Foundation weight ≥ the largest net uplift | 797 ≥ 620 kN ✓ |
- The ringwall is taken at the tank diameter as its centreline. For a slab or a different layout, enter the effective soil and an equivalent width and depth.
- Overturning stability and soil bearing under the wind or seismic moment (5.12.14.1) are evaluated per Annex B in the Ringwall Foundation tool, which reads this tool’s net uplift through Fill from the other tools.
The checks
| Check | Clause | Rule |
|---|---|---|
| Each load case with uplift | Table 5.20a / 5.20b | Bolt or strap stress ≤ allowable fraction of Fy. |
| Net uplift | Table 5.20 | Pass, with a note, when no case lifts the tank. |
| Number and spacing of anchors | 5.12.2, 5.12.3 | At least 4; ≤ 3 m (10 ft) apart. Not applicable without uplift or entered anchors. |
| Anchor bolt size | 5.12.6 | Corroded shank ≥ 25 mm (1 in.). |
| Anchor bolt eccentricity | 5.12.4 | e ≥ em. |
| Strap thickness, material, slope | 5.12.6, Table 5.20, 5.12.15.5 | ≥ 6 mm; Fy ≤ Fty; ≤ 5°. |
| Shell stress at the anchor chair | 5.12.10, AISI (5-3) | S ≤ the case’s Table 5.20 shell allowable; a Note over 172 MPa (25 ksi). |
| Chair top plate | AISI (5-1) | S ≤ 172 MPa (25 ksi). |
| Chair proportions | AISI Part V | h at least 152 or 305 mm, at most 3a (Note); nut clearance; f. |
| Chair side plates | AISI Part V | j ≥ max(12.7 mm, 0.04 (h − c)); j k ≥ P / 172 MPa. |
| Chair welds to the shell | AISI (5-5) – (5-8) | Resultant ≤ 9.5 w (kips per in., w in in.). |
| Continuous ring at the top of the chairs | 5.12.10, AISI Figure 5-3 | Ring stress ≤ the case’s shell allowable (replaces 5-3). |
| Shell at the attachment, 5.12.8 evaluation | 5.12.8, ASME VIII-2 Part 5 | Entered Pm, PL, PL + Pb and + Q against Sm, 1.5 Sm, 3 Sm (replaces 5-3). |
| Counterbalancing weight | 5.12.14.1 | Foundation ≥ largest uplift. |
| Pressure combination factor | 5.2.2 | Note when Fp under 0.4. |
| Anchorage record | 5.12 | Note until every visible item is ticked. |
Anchorage record (section 7)
| Clause | Item | Shown for |
|---|---|---|
| 5.12.7, 5.12.10 | Chairs or rings designed (AISI or equal); shell stress within the Table 5.20 allowable or evaluated per 5.12.8; top plate overhang no more than ½ in.; a continuous top ring checked for P e / h. | Bolts |
| 5.12.2 | Anchors equally spaced, or Tb increased. | All |
| 5.12.11 | Radial growth of the shell allowed for (P.2.5.1); slotted holes where needed. | All |
| 5.12.12 | Bolts snug-tightened, straps welded with the tank full of test water; nuts locked. | All |
| 5.12.13 | Embedment develops Fy; hooked anchors per E.6.2.1.2; seismic attachment load. | All |
| 5.12.9 | Anchorage parts to AISC 360 (ASD); 33 % increase only for wind or seismic. | All |
| 5.12.14.1 | Overturning and soil bearing per Annex B. | All |
| 5.12.15.1 – 5.12.15.4 | Strap embedment and anchor plate; corrosion near the concrete; splice welds; attachment welds. | Straps |
Reading the results and graphics
✓ PassMeets the requirement.
✗ FailsMore or larger anchors, a heavier ringwall, or a change of detail.
⚠ NoteFp raised to 0.4, or unticked record items.
○ Needs inputNot enough data yet.
- The load per anchor chart has a bar for Tb in each case and an amber line for the allowable stress × area. A red bar is over the line.
- The case table lists U, Tb, the stress, the allowable, the shell allowable fraction and the number of anchors of this size the case needs.
- The layout draws the anchors around the tank with their spacing, and for bolts an elevation of a chair with e against em.
- On a phone the graphics scroll sideways.
How the numbers were checked
- 5.12 and Tables 5.20a/b were read from the 14th edition (August 2025).
- The sample’s weights, the six uplifts, the root area, stresses, spacing, em, embedment and ringwall are recomputed by hand in the test suite and in this manual’s build.
- The US rule set was checked against Table 5.20b with its own constants (4.08, 55,000 psi, 10 ft, 2.125 in.), not by converting the SI result.
- The chair formulas reproduce all four rows of AISI Table 5-1 (top plate at 25 ksi with f = d/2 + 1/8, emin 1.87 – 2.52 in.) and the A, B, WV and WH of an independent worked example.
- Each check was made to fail in turn: 12 anchors (spacing), 3 anchors, an M24 bolt (shank 21 mm), an overloaded M24, a 70 mm eccentricity, a small ringwall, a 5 mm strap, a strap stronger than the shell and a 7° slope; and the edge cases: N blank, Fp under 0.4, a supported cone roof, frangibility, entered W values, no net uplift and a 6 mm bolt CA where the shank governs.
Saving, reopening and reports
1 · TypingInputs are kept on this device.
2 · Calculate & SaveA new record on the server, in SI.
3 · ReopenOpen any earlier record for the tank.
4 · ReportPrint or save as PDF.
- Kept on this device. A banner offers Restore or Discard, in the units you were using. A successful save clears it.
- Show saved calculations for this tank lists records with the rule set, the anchors, the largest uplift and failing checks. Open reloads everything; saving again makes a new record noting what it was based on.
- Print / PDF report: summary, weights, the case table and chart, the anchors, the ringwall and layout, the record list and the inputs.
Messages and what to do
| Message | Meaning and what to do |
|---|---|
| Table 5.20 case overstressed | Use the number of anchors given, a larger bolt, or a stronger grade up to the 380 MPa limit. |
| 5.12.3 spacing | More anchors. |
| 5.12.6 bolt size | Corroded shank under 25 mm: a larger bolt or less corrosion allowance. |
| 5.12.4 eccentricity | Move the bolt out, or check the thermal growth input. |
| 5.12.14.1 counterbalancing weight | A deeper or wider ringwall, or count the soil over a footing. |
| 5.2.2 Fp | Entered under 0.4; 0.4 is used. |
Limits and interpretations
- Tables 5.20a and 5.20b are used with their own printed constants; neither is converted from the other.
- The chair formulas are AISI’s approximations of Bijlaard’s local-load analysis for a chair with two side plates (Figure 5-1 c). An anchor ring other than a top ring on chairs is outside the tool; leave the chair blank and keep it as a record item.
- The continuous ring is a thin ring in its own plane, with W = P e / h at every chair and 16 t of shell each side as AISI allows. The equal inward load on the bottom plate is not checked, as AISI advises, unless the bottom is very light.
- For 5.12.8 the tool does not compute local stresses (that needs a finite element model or the WRC 107 / 537 curves); it classifies and checks the results you enter, with Sm the case’s Table 5.20 shell allowable. As Sm is at least 2/3 Fty, the alternative Sy and 2 Sy limits of VIII-2 never govern. Peak stress and fatigue are not checked.
- AISI misprints, as read: emin is 0.866 d + 0.572 (printed “0.8864 + 0.572”); eq. 5-6 is the horizontal load WH (both printed as WV); Table 5-1 f for the 2 in. bolt is 1⅛ (printed 1). Each reading reproduces Table 5-1.
- The chair is checked against the corroded bottom course (t₁ − CA) at R = D/2; eq. 5-8 is used as printed (9.5 w; the text derives 9.6).
- Av is a fraction of g, read with Annex E, although Table 5.20 says “% g”.
- Straps use one Fy for the section at the shell and the reduced section; where the embedded part is stainless (5.12.15.2), check that section with its own Fy.
- The seismic attachment load uses the larger Tb of the seismic and design pressure + seismic cases.
- Shell loss uses 77 kN/m³ steel; enter W values for stainless or a different build-up.
- Wind and seismic loads, the frangibility failure pressure and the weights come from the Wind Design, Seismic, Roof Design and Shell Design tools.
Glossary
| U | Net uplift for a load case: lift less the resisting weight. |
| Tb | Load per anchor, U / N. |
| W1, W2, W3 | The resisting weights defined under Table 5.20. |
| Fy, Fty | Minimum yield of the anchor, and of the bottom shell course. |
| Fp | Pressure combination factor (5.2.2). |
| MWS, Mrw | Wind moment on the shell; seismic ringwall moment (Annex E). |
| e, em | Bolt eccentricity from the shell outside, and its minimum. |