API 650 Annex B · 14th edition (August 2025)

Ringwall Foundation — User Manual

Oct 12, 2026@Shamit
Contents

    About this tool

    The Ringwall Foundation calculator checks a concrete ringwall under a flat-bottom tank to API 650 Annex B (B.4.2): its width, depth and projection; the soil bearing under it for the empty, operating, hydrotest, wind and seismic cases; the hoop tension from the lateral pressure of the confined fill and product; the ring moment from the eccentric loads; the hoop and vertical steel against the Annex B minimums; and, for an anchored tank, the counterbalancing weight (5.12.14) and overturning stability (E.6.2.3).

    Annex B has no equations

    Annex B is informative: it says what the ringwall must resist, not how to calculate it. The tool applies the plain statics its text describes and ACI 318 strength design for the steel, and says so at each step. The allowable bearing comes from your geotechnical engineer (B.2). This is an indicative check, not a certified design.

    Quick start

    Press Load illustrative sample: TK-101 from the Annex L sample, 36 m × 13.8 m, G 0.9, self-anchored, on a 0.7 m × 1.2 m ringwall 0.45 m above grade with 28 hoop bars of 20 mm. Nothing fails; the seismic bearing is the closest at 93 %.

    1. 1
      Identification: enter the Tank ID and press Fill from the other tools to bring in D, H, G, the shell weight, the moments, the test level and the anchor uplift from your saved calculations.
    2. 2
      Section 1: check the tank values and the line loads at the shell (weight ÷ π D).
    3. 3
      Section 2: the ringwall width, depth, projection above grade, any footing, concrete and fill weights, f′c and the lateral pressure coefficient K.
    4. 4
      Section 3: the allowable bearing from the geotechnical report, for operation and, if higher, for the hydrotest and with wind or seismic.
    5. 5
      Section 4: fy, the hoop bars and the vertical bars.
    6. 6
      Section 5: wind and seismic moments and, for an anchored tank, the net uplift.
    7. 7
      Tick the record items, then Calculate & Save and Print / PDF report.

    Filling from the other tools

    ToolWhat it gives
    Shell DesignD, design liquid level H, G, design internal pressure; shell weight per metre
    Roof DesignSelf-supporting roof weight added to the shell; roof live load Lr × D / 4. A column-supported roof adds nothing: enter the share the shell carries.
    Seismic DesignMrw, Ms, Av; self-anchored or not
    Wind DesignMw
    Tank AnchorageMechanically anchored; the largest net uplift of the wind and pressure cases and of the seismic cases; its ringwall width and depth
    Hydrostatic Test, Bottom & Annular PlatesTest water level; bottom plate thickness
    • The latest 2025-edition record of each tool for the same company and Tank ID is used; the table under the button says which record filled what.
    • Values are filled in the boxes, so you can change any of them afterwards.

    Ringwall proportions (section 2)

    finished gradeproduct, q = G γw H = 121.8 kPaK q = 60.9 kPaK (q + γs h) = 71.7 kPaoperating bearing N / b = 114.7 kPah = 1.2 mb = 0.7 m
    The sample section, operating case.
    CheckRuleSample
    Width (B.4.2.2)At least 300 mm (12 in.)700 mm ✓
    Depth (B.4.2.2)Bottom at least 0.6 m (2 ft) below the lowest adjacent finished grade, and below the frost line0.75 m ✓
    Projection (B.3.1)Grade at least 0.3 m (1 ft) above the surrounding ground0.45 m ✓
    Concrete (Figure B.1 Note 2)f′c at least 20 MPa (3000 lbf/in²) at 28 days28 MPa ✓
    Footing (B.4.2.3 f)Over 460 mm (18 in.) wide: a footing should be considerednote
    Annular slab (B.4.2.3 e)Wider than deep: design as an annular slab with radial flexure—
    • The shell normally sits on the ringwall centreline (B.4.2.2). Enter e when it is offset outward, for anchor bolts or bearing; the ring radius becomes D/2 − e.
    • An inverted-T footing is centred under the wall. Its ledges carry fill and product inside and soil outside, and these count in the bearing and in the counterbalancing weight (5.12.14.2).

    Soil bearing (section 3)

    Per metre of circumference, the base carries the shell line load, the product on the part of the ringwall inside the shell, the ringwall and footing, and the soil on the ledges. The ring is stiff in torsion, so the pressure under the base is taken as uniform: q = N / base width.

    CaseShell loadProductAllowableSample
    Empty (new)ws—qa50.9 kPa
    Operatingws + roof liveG γw H + Piqa114.7 kPa
    Hydrostatic testwsγw × test leveltest allowable (B.2.2 b)118.6 kPa
    Operating + windws + 1.273 Mw / D²operatingwind / seismic allowable (B.2.2 c)115.1 kPa
    Operating + seismicPf = ws (1 + 0.4Av) + 1.273 Mrw / D² (E.65)operating × (1 + 0.4Av)wind / seismic allowable185.5 kPa (Pf 67.5 kN/m)
    050100150200Empty (new)51Operating115Hydrostatic test119Operating + wind115Operating + seismic186bearing under the ringwall, kPa; amber: the allowable for the case
    Sample bearing by case.
    • The pressure under the tank bottom at the same depth (q + γs h + bottom plate, 144.0 kPa) is shown beside each case. B.4.2.1 a) wants a more nearly uniform soil loading; the width that makes the two equal, with the shell on the centreline and no footing, is b = 2 w / (q − 2 h (γc − γs)) = 0.33 m for the sample.
    • B.2.2 suggests factors of safety of 2.0 to 3.0 in operation and 1.5 to 2.25 for the hydrotest and with wind or seismic, so the last two allowables may be higher. Left blank, they take qa.
    • The Pf for the sample matches the ringwall load from the Seismic Design record of TK-101 (67.56 kN/m).

    Hoop tension and reinforcement (section 4)

    shell load at eproduct on the inner part,centroid b/4 inside the centrelineuniform soil pressure, resultant at the centrelinem = Σ F xring moment m Rcsample, operating:m = −42.6 × 0.175 = −7.46 kN·m/mm Rc = 134 kN·m
    The uniform moment load (B.4.2.3 b).
    StepRuleSample (hydrotest governs)
    Lateral pressure (B.4.2.3 a)K × the vertical pressure of product and fill: K = 0.5, or 0.3 for granular backfill, or from a geotechnical analysisK = 0.5
    Hoop tensionT = K Rc (q h + γs h² / 2), the pressure integrated over the full depth1,578 kN (operating 1,432 kN)
    Ring moment (B.4.2.3 b)m = Σ F x about the base centreline; M = m Rc8.29 kN·m/m → 149 kN·m
    Steel for the loadsTu / φfy + 2 Mu / (φfy 0.9 d), φ 0.9, load factor 1.66,680 + 2 × 636 = 7,953 mm² (d = 1103 mm)
    Minimum (B.4.2.3 d)0.0025 × vertical cross-section2,100 mm²
    ProvidedHoop bars × bar area28 × 20 mm = 8,796 mm² ✓
    Vertical steel (B.4.2.3 e)0.0015 × horizontal cross-section, per metre of ring1,050 mm²/m; 2 legs of 12 mm at 200 = 1,131 mm²/m ✓
    • The moment changes sign between the empty and the full tank, so the steel for it goes on both faces: the total is the tension steel plus twice the moment steel.
    • The empty case still has hoop tension from the fill alone; the operating and test cases add the product. The case needing the most steel governs.
    • Figure B.1 Note 3: a ringwall over 300 mm (12 in.) wide has bars on both faces; the section graphic shows them that way.
    • The load factor applies to both T and M. ACI 318 uses 1.6 for lateral earth pressure; use your project’s factor if it differs. Crack control to ACI 350 is not checked.

    Wind, seismic and uplift (section 5)

    CheckRule
    Counterbalancing weight (5.12.14.1)Ringwall + footing + soil over the footing (5.12.14.2) ≥ largest net uplift of the wind and pressure cases of Table 5.20
    Seismic uplift (E.6.2.3)The same weight, plus, if ticked, the product directly over the ringwall and footing × (1 − 0.4Av), ≥ the largest seismic net uplift
    Overturning stability (E.64)0.5 D (WT + Wfd + Wg) / Ms ≥ 2.0 for a mechanically anchored tank; WT = shell load × π D + product + bottom plate
    Self-anchored (E.65)No uplift on the foundation; the ringwall is proportioned for the bearing under Pf (section 3)
    • Product load is never used to reduce the anchor load (E.6.2.3); it only helps the foundation resist the anchor pull.
    • B.4.2.3 c) also wants the bending and torsion from wind or seismic loads applied eccentrically, found by a rational analysis that includes the foundation stiffness. That is a record item.

    The checks

    CheckClause
    Ringwall width; depth below grade; shell on the ringwallB.4.2.2
    Tank grade above the groundB.3.1
    Concrete strengthFigure B.1 Note 2
    Footing; annular slab behaviour; K below 0.3B.4.2.3 a, e, f
    Soil bearing, five casesB.4.2.2, B.2.2, 5.12.14.1, E.6.2.3
    Hoop reinforcement; vertical reinforcementB.4.2.3 a, b, d, e
    Counterbalancing weight; seismic; overturning stability5.12.14.1, E.6.2.3 (E.64)
    Design recordAnnex B

    Design record (section 6)

    • Geotechnical investigation and factors of safety (B.2.1, B.2.2); special site conditions (B.2.3, B.2.4).
    • Sand layer (B.3.2); crown or slope to a sump (B.3.3, B.3.4); top level and smooth, staggered splices, bars on both faces (Figure B.1); recesses (B.4.2.2).
    • Eccentric wind and seismic bending and torsion (B.4.2.3 c); backfill control (B.4.2.3 g); support under the shell (B.4.2.4); levelness of 7.5.5.2; eccentric anchor uplift (E.6.2.3); elevated temperature (B.6).

    How the numbers were checked

    • The test suite recomputes every case of the sample by hand: the line loads and their positions, the moment load, the bearing, T, the steel and the minimums. This manual’s build does the same.
    • At the equal-pressure width the tool gives exactly equal pressures under the ringwall and the tank bottom.
    • A footing with an offset shell is checked load by load; the E.65 load matches the Seismic Design tool; the uplift, product credit and E.64 ratio are checked for an anchored tank.
    • Every check was made to fail in turn: narrow, shallow, weak concrete, low allowable, too few bars, too much uplift.

    Saving, reopening and reports

    • Inputs are kept on this device and offered back; Calculate & Save stores the record in SI; Show saved calculations lists and reopens them.
    • Print / PDF report: summary, section graphic, bearing table and chart, hoop table, uplift, record and inputs.
    • The Annex L Data Sheet reads the saved record for line 13: foundation type, allowable soil bearing, wind ring force and projection above grade.

    Limits and interpretations

    • Concrete ringwalls only. Crushed-stone ringwalls (B.4.3), slabs and pile caps (B.4.4) are not covered.
    • Bearing is uniform under the base; the ring carries the moment in torsion. A flexible or very wide ring needs an analysis with the soil stiffness.
    • Bending and torsion from eccentric wind or seismic loads (B.4.2.3 c), and the elevated-temperature moment (B.6 e), are record items.
    • The hoop moment lever arm is 0.9 d with ACI 318 tension-controlled φ 0.9; development, lap lengths and cover follow ACI 318.
    • Wind bearing uses 1.273 Mw / D² on the compression side of the shell, the same form as E.65 for seismic.

    Glossary

    b, hRingwall width and total depth.
    wsShell, roof and attachments carried by the shell, per metre.
    qPressure on the tank bottom from the product (and internal pressure).
    KLateral pressure coefficient.
    RcRadius of the ringwall centreline.
    mUniform moment load per metre (B.4.2.3 b).
    PfSeismic shell load on the ringwall (E.65).
    Wfd, WgFoundation weight and soil over the footing.