Design of water tanks

In short

A water tank is a liquid-retaining structure whose governing requirement is not strength but water-tightness — the section must not crack enough to leak, so steel stresses and crack widths are kept far below those allowed in ordinary RCC. Indian practice follows IS 3370 (Parts 1 to 4) for concrete liquid-retaining structures, with IS 456 for general RC design, IS 1893 (Part 2) for seismic design of liquid-retaining tanks and IS 11682 for RC staging of overhead tanks. The topic is a standing favourite in university RCC/Design-II papers (a full-marks question on a circular or rectangular tank, or an Intze tank) and appears in GATE Civil through hoop tension, dome forces and uplift checks. On the job it is core work for water-supply consultants, PHE/Jal Jeevan Mission projects, municipal corporations, L&T, Tata Projects, NCC, VA Tech Wabag and every ULB that builds an elevated service reservoir.

Explained

Types, parts and the code framework

• By position: (a) resting on ground — service reservoirs, clarifier and settling tanks; (b) underground — sumps, septic and fire tanks; (c) elevated (overhead) — ESR on staging, needed to deliver residual pressure in the distribution network.
• By shape in plan: circular (most efficient — walls carry hoop tension, no corners), rectangular/square (easier shuttering, used for small capacity and where space is rectangular), Intze, conical and spherical (large elevated capacities).
• Parts of an elevated Intze tank, top down: top (roof) dome → top ring beam → cylindrical side wall → bottom ring beam → conical dome (truncated cone) → bottom spherical dome → circular (bottom) girder → columns with bracings (the staging) → annular raft or pile foundation.
• Codes and references: IS 3370 Part 1 (general requirements — materials, grade, cover, joints, testing), Part 2 (reinforced concrete members), Part 3 (prestressed concrete), Part 4 (design tables and moment/tension coefficients); IS 456 for section design, IS 875 Part 3 for wind, IS 1893 Part 2 for seismic, IS 11682 for RC staging, IS 3370 Part 3 / IS 1343 for prestressed tanks. Parts 1, 2 and 4 were substantially revised (2009 and later); always design to the current edition, since the 2009 revision shifted RC tanks from pure working-stress "no-crack" design to limit-state design with explicit crack-width control — many university courses still teach the older working-stress method, so check which your paper expects.
• Capacity and layout of service reservoirs follow the CPHEEO Manual on Water Supply and Treatment (typically one-third to one-half of the day's demand for balancing storage) — the structural designer receives capacity, staging height and site soil data as inputs.

Water-tightness: what the design actually controls

• Crack width limits (IS 3370 Part 2): generally 0.2 mm on the liquid face; 0.1 mm where appearance or severe exposure demands it. Steel is therefore under-stressed and bar spacing kept close; many bars of small diameter beat a few large bars.
• Working-stress ("no-crack") alternative, still examined: the section is treated as uncracked, concrete carrying tension. Permissible tensile stress in concrete — direct tension about 1.2, 1.3, 1.5 N/mm² for M20, M25, M30 and in bending about 1.7, 1.8, 2.0 N/mm² respectively. Steel stress limited to about 115–150 N/mm² (mild steel/HYSD) against 230–275 N/mm² in ordinary RCC.
• Concrete: minimum grade M30 for liquid-retaining members, low water-cement ratio, adequate cement content, good compaction and long curing; dense impermeable concrete is the real water-bar.
• Cover: generally 45 mm (severe exposure) on the liquid face, not less than the bar diameter; excessive cover increases crack width, so do not "play safe" by over-covering.
• Minimum reinforcement: about 0.35% of gross section (0.24% for high-strength deformed bars), reducing with thickness between 100 mm and 450 mm, provided in each direction on each face to control shrinkage and thermal cracking. Minimum wall thickness about 150 mm; domes 100–120 mm.
• Movement joints (IS 3370 Part 1): expansion joint (full discontinuity, free movement), complete contraction joint (steel and concrete discontinuous), partial contraction joint (steel continuous), sliding joint (wall free to slide on base — used to get pure hoop tension), and construction joints (no movement intended). Three design options: fully continuous, semi-continuous with joints at ~7.5–15 m, or free-movement with closely spaced joints. Joints use PVC water bars and sealants.
• Testing: fill with water in stages, allow 7 days for absorption; the drop in surface level over the next 7 days must not exceed 1/500 of the average water depth or 10 mm, whichever is less (corrected for evaporation and rainfall). No visible leakage or damp patches permitted.

Tanks resting on ground

Circular tanks

• Base free / sliding joint: wall acts as pure hoop ring. Hoop tension per metre height at depth h, T = γ·h·D/2; maximum at the base. Hoop steel on both faces; vertical steel is nominal (minimum).
• Base fixed or hinged: the base restrains hoop expansion, so hoop tension near the base falls and vertical bending moments appear; the wall is designed using IS 3370 Part 4 coefficients tabulated against H²/(D·t). Smaller H²/Dt means more cantilever-like behaviour (a very large diameter tends to a flat cantilever wall); larger H²/Dt means hoop/membrane action dominates and the base restraint effect stays local.
• Thickness is fixed by the no-crack check on hoop tension (uncracked composite section) or by crack-width control, plus a practical minimum for placing and compaction.

Rectangular tanks (classified by L/B and H)

• L/B ≤ 2: walls act as a closed horizontal frame; analyse by moment distribution or IS 3370 Part 4 tables for pressure at the critical level. Walls carry bending plus direct (pull) tension transferred from the perpendicular walls.
• L/B > 2: the long walls behave as vertical cantilevers from the base; the short walls span horizontally between the long walls. The common simplification: the bottom H/4 or 1 m, whichever is greater, is designed as a cantilever, and the portion above as a horizontal slab/frame.
• Corners need haunches and properly anchored corner steel — corner moments are the largest action and the usual leakage point.
• Base slab: on firm ground, a nominal 150–200 mm slab with minimum steel on both faces over a levelling course and polythene separation layer; on poor soil, design as a raft for net upward soil pressure. Provide slopes and a sump for de-watering.
• Roof: flat slab, beam-slab or dome; live load typically 0.75 kN/m² if inaccessible and 1.5 kN/m² if accessible, with a manhole, ladder, vent pipe, overflow and level indicator.

Underground tanks and sumps

• Design for two separate worst cases: (i) tank full, no soil support outside → internal water pressure; (ii) tank empty with saturated soil outside → earth pressure at rest/active plus surcharge plus water-table pressure pushing inward. Reinforcement must be provided on both faces.
• Flotation (uplift) check when empty: uplift = γw × base area × depth of base below water table. Required factor of safety ≈ 1.2 against the dead weight of walls, base, roof and any soil on projecting ledges. Remedies: thicken the base slab, project the base as a ledge loaded by submerged soil, provide tension piles or anchors, or install a relief/pressure valve.
• Keep the structure above the shallow water table where possible, provide sub-soil drainage, waterproof the external face, and locate sumps at least the statutory distance from soak pits and drains (local building bye-laws / CPHEEO guidance).

Elevated tanks: the Intze principle and the staging

• Why Intze: a plain cylindrical elevated tank with a flat bottom needs a very thick bottom slab. The Intze tank replaces it with a conical dome plus a bottom spherical dome. The conical dome pushes outward on the bottom ring beam; the bottom dome pushes inward. The Intze condition is that the two horizontal thrusts balance so the bottom ring beam carries zero hoop tension and needs only nominal steel — that is the whole point of the shape.
• Top dome: membrane action — meridional thrust (compression) and hoop force; thickness 100–120 mm with nominal steel both ways. Its meridional thrust's horizontal component puts the top ring beam in hoop tension.
• Cylindrical wall: hoop tension from water pressure plus self weight; designed like a ground circular tank wall but with hoop steel in both faces.
• Circular girder (bottom girder): rests on the staging columns and carries the entire tank. Analysed as a circular beam on N equally spaced supports — design for support (hogging) moment, mid-span (sagging) moment and torsion at the quarter points; coefficients depend on the number of columns.
• Staging: columns (usually 6, 8 or 12) with horizontal bracings at 3–5 m intervals; designed for vertical load plus wind (IS 875 Part 3) or seismic (IS 1893 Part 2), whichever governs — bracings reduce the columns' effective length and carry shear and moment. Shaft (hollow cylindrical) staging is used for tall or seismically critical tanks.
• Seismic design (IS 1893 Part 2): the water is idealised as a two-mass system — the impulsive mass moving with the tank and the convective (sloshing) mass on a spring — giving two periods and two base-shear contributions combined by SRSS. Check tank empty and tank full; provide freeboard not less than the computed sloshing wave height. Elevated tanks are assigned low response-reduction factors (of the order of 1.8 to 2.5 depending on staging type — read the current table), and the importance factor is high (water tanks are lifeline structures).
• Foundation: annular/circular raft for good soil, pile cap and piles for weak soil; check differential settlement, which cracks a tank far more readily than a building.

Loads, other materials and practice

• Loads: water 9.81 kN/m³ (10 kN/m³ in design), RCC 25 kN/m³, soil as per site tests, wind, seismic, roof live load, and thermal/shrinkage effects. Load combinations per IS 456 / IS 1893, with serviceability combinations governing crack checks.
• Prestressed concrete tanks (IS 3370 Part 3): circumferential prestressing keeps the wall in compression under full water, giving a truly crack-free thin wall — used for large-capacity reservoirs and digesters; needs specialist wire-winding.
• Other materials: pressed-steel panel tanks, welded steel plate tanks, GRP/FRP sectional tanks and HDPE/LLDPE moulded tanks for domestic terrace storage; masonry tanks only for very small capacity.
• Durability and O&M: smooth internal finish, no honeycombing, epoxy or food-grade cementitious lining if specified, periodic cleaning (usually every 6 months for potable storage), chlorine-resistant materials, and provision of inlet, outlet at a higher level than the scour/washout, overflow and ventilation.

Key points

• Governing criterion - water-tightness and crack control, not ultimate strength; stresses are deliberately kept low.
• Crack width - 0.2 mm generally, 0.1 mm for severe exposure or aesthetic requirement, on the liquid face.
• Minimum concrete grade - M30 for liquid-retaining members, with low w/c ratio, dense mix and long curing.
• Cover - about 45 mm on the liquid face; more cover means wider cracks, not safer design.
• Minimum steel - about 0.35% (0.24% for HYSD) of gross section, each face, each direction, for shrinkage and thermal cracking.
• Hoop tension - T = γhD/2 per metre height, maximum at base; only a free/sliding base gives pure hoop action.
• Fixed base - reduces hoop tension near the base and introduces vertical moments; use IS 3370 Part 4 tables against H²/Dt.
• Rectangular L/B ≤ 2 - closed-frame action with bending plus direct tension; L/B > 2 - long wall cantilevers, short wall spans horizontally.
• Bottom H/4 or 1 m - whichever is greater, designed as a vertical cantilever in the simplified rectangular-tank method.
• Underground tanks - design for tank full and tank empty (earth plus water pressure) separately, steel on both faces.
• Flotation check - FoS about 1.2 of self weight against uplift when the tank is empty and the water table is high.
• Intze condition - conical dome thrust balances bottom dome thrust so the bottom ring beam carries no hoop tension.
• Circular girder - designed for support moment, span moment and torsion; number of columns fixes the coefficients.
• IS 1893 Part 2 - two-mass impulsive plus convective idealisation, sloshing freeboard, both empty and full cases.
• Movement joints - expansion, complete/partial contraction, sliding; PVC water bars and sealants at every joint.
• Water-tightness test - permissible drop over 7 days = 1/500 of average depth or 10 mm, whichever is less.
• Prestressed tanks - circumferential prestress keeps the wall in compression, giving crack-free thin shells for large capacity.

Formulas and examples

Example 1 — Circular tank resting on ground, 200 kL, sliding base. Try D = 8 m, H = 4 m of water. V = 0.7854 × 8² × 4 = 201.1 m³ ✓ (provide 300 mm freeboard, wall height 4.3 m).

• Hoop tension at base: T = 10 × 4 × 8/2 = 160 kN/m.
• Hoop steel (σ_st = 150 N/mm²): A_st = 160 000/150 = 1 067 mm²/m → 12 mm @ 100 c/c = 1 131 mm², placed as 10 mm @ 140 c/c on each face near the base (561 + 561 = 1 122 mm²/m).
• No-crack thickness (M30: σ_ct = 1.5 N/mm², σ_cbc = 10, m = 280/30 = 9.33):

1 000·t = 160 000/1.5 − 8.33 × 1 131 = 106 667 − 9 421 = 97 246 → t = 97 mm.

• Provide t = 200 mm (minimum thickness, cover 45 mm each face, placing and compaction). Hoop steel curtailed in stages up the height (T falls linearly with h); vertical steel = minimum 0.24% = 480 mm²/m total → 8 mm @ 200 c/c each face.

Example 2 — Rectangular tank resting on ground, L = 6 m, B = 4 m, H = 3 m. L/B = 1.5 ≤ 2 → closed-frame action. Design the bottom 1 m as a cantilever; take the frame at 1 m above base, p = 10 × 2 = 20 kN/m².

• Fixed-end moments: long wall pL²/12 = 20 × 36/12 = 60 kN·m; short wall pB²/12 = 20 × 16/12 = 26.67 kN·m.
• Distribution factors at a corner ∝ 1/6 : 1/4 = 0.4 : 0.6. Unbalanced = 60 − 26.67 = 33.33 kN·m.

Corner moment = 60 − 0.4 × 33.33 = 26.67 + 0.6 × 33.33 = 46.67 kN·m (hogging, tension on the outer face).

• Mid-span, long wall: 20 × 36/8 − 46.67 = 90 − 46.67 = 43.33 kN·m (sagging, tension on the water face).

Mid-span, short wall: 20 × 16/8 − 46.67 = 40 − 46.67 = −6.67 kN·m (still hogging — provide the minimum on the water face).

• Direct pull: short wall 20 × 6/2 = 60 kN/m; long wall 20 × 4/2 = 40 kN/m.
• Steel for the corner, wall 250 mm thick, d = 250 − 45 − 8 = 197 ≈ 200 mm, σ_st = 150, lever arm 0.86d:

A_st(bending) = 46.67 × 10⁶/(150 × 0.86 × 200) = 1 809 mm²/m; A_st(direct tension, short wall) = 60 000/150 = 400 mm²/m. Total ≈ 2 209 mm²/m → 16 mm @ 90 c/c on the outer face at the corner, with half the direct-tension steel carried on each face.

• Cantilever at base: M = γH³/6 = 10 × 27/6 = 45 kN·m/m — comparable to the frame moment, so the same bar size continues down into the base haunch.

Example 3 — Top dome and top ring beam of a 12 m diameter Intze tank. D = 12 m, central rise r = 2 m, dome thickness 100 mm.

• R = (6)²/(2 × 2) + 2/2 = 9 + 1 = 10 m; cosθ = (10 − 2)/10 = 0.8 → θ = 36.87°.
• Load w = 0.10 × 25 (self) + 0.75 (live, inaccessible) = 3.25 kN/m².
• Meridional thrust T₁ = 3.25 × 10/(1 + 0.8) = 18.06 kN/m; stress = 18 060/(1 000 × 100) = 0.18 N/mm² — very safe.
• Hoop force at the edge T₂ = 3.25 × 10 × (0.8 − 1/1.8) = 32.5 × 0.2444 = 7.94 kN/m (compression) → nominal steel 0.24% = 240 mm²/m both ways, 8 mm @ 200 c/c.
• Ring beam hoop tension F = 18.06 × 0.8 × 6 = 86.7 kN → A_st = 86 700/150 = 578 mm² → 6 nos 12 mm (678 mm²) in a 250 × 250 beam with 8 mm stirrups @ 150 c/c.

Example 4 — Flotation check on an underground sump. Internal 6 × 4 m, water depth 3 m, walls 250 mm, base 300 mm, roof 150 mm, water table at ground level, base soffit 3.45 m below it. External plan 6.5 × 4.5 = 29.25 m².

• Uplift U = 10 × 29.25 × 3.45 = 1 009 kN.
• Weights: walls = perimeter (mid-line 2 × (6.25 + 4.25) = 21 m) × 0.25 × 3.15 × 25 = 413 kN; base = 29.25 × 0.30 × 25 = 219 kN; roof ≈ 29.25 × 0.15 × 25 = 110 kN. Total W = 742 kN.
• FoS = 742/1 009 = 0.74 < 1.2 → the empty sump floats. Required W = 1.2 × 1 009 = 1 211 kN; deficit = 469 kN.
• Remedy: project the base slab 0.75 m beyond the walls and load the ledge with submerged soil (submerged unit weight ≈ 10 kN/m³), or thicken the base — note that a thicker base sits deeper and raises the uplift too, so iterate the check.

Example 5 — Water-tightness test acceptance. Tank of average water depth 4.0 m. Permissible 7-day drop = min(4 000/500, 10) = min(8, 10) = 8 mm. Measured drop 15 mm with 3 mm evaporation from the pan = net 12 mm > 8 mm → test fails; investigate joints and construction joints, grout/seal and retest.

Where it is used

• Water supply and PHE projects: elevated service reservoirs, ground-level and underground sumps under Jal Jeevan Mission, AMRUT and state water-supply boards (KWA, MJP, TWAD, UP Jal Nigam) — designed to CPHEEO and IS 3370.
• Water and sewage treatment plants: clariflocculators, aeration tanks, filter beds, chlorine contact tanks and digesters — the same liquid-retaining rules, often with chemical-resistant linings (VA Tech Wabag, Ion Exchange, Thermax).
• Industrial and power plants: cooling-tower basins, process and DM water tanks, fire-water storage sized to TAC regulations and NBC 2016 Part 4 fire requirements for high-rise buildings.
• Buildings and housing: terrace and basement tanks in every apartment project — structural designers at L&T, Tata Projects, Shapoorji Pallonji and municipal corporations check capacity, uplift and waterproofing.
• Job roles: structural design engineer (water retaining structures), PHE/public-health engineer, site engineer for ESR construction and testing, third-party quality auditor, and state PWD/Jal Nigam AE/JE posts whose exams ask this directly.
• Rural and irrigation works: village storage tanks, check-dam collection chambers and farm sumps under the rural employment guarantee (VB-G RAM G, formerly MGNREGA) and watershed programmes.

Interview questions

• Q: Why is the design of a water tank different from ordinary RCC design? A: The controlling requirement is water-tightness, so crack width is limited to 0.2 mm (0.1 mm in severe cases) and steel stresses are kept to roughly 115–150 N/mm², giving more steel at closer spacing than strength alone would need.
• Q: Why is a circular tank preferred for large capacity? A: Its wall carries load in pure hoop tension with no bending corners, so it needs the least material and has the fewest leakage-prone details; the penalty is costlier curved shuttering.
• Q: What is hoop tension and where is it maximum? A: The circumferential tensile force in a circular wall, T = γhD/2 per metre height, maximum at the base of the water depth when the base joint is free or sliding.
• Q: Why does a sliding joint at the base change the design? A: It lets the wall expand radially, so the whole water pressure is carried as hoop tension and no vertical cantilever moment develops at the base.
• Q: State the Intze condition. A: The horizontal thrust from the conical dome is made equal and opposite to that from the bottom spherical dome, so the bottom ring beam carries zero hoop tension and needs only nominal steel.
• Q: How is a rectangular tank with L/B = 3 analysed? A: As two separate actions — the long walls as vertical cantilevers from the base, and the short walls spanning horizontally between them, with the bottom H/4 or 1 m (whichever is greater) taken as cantilever.
• Q: What governs the thickness of an underground tank wall? A: The worse of two cases — tank full with no external soil support, and tank empty with saturated earth pressure plus surcharge outside — which is why steel is needed on both faces.
• Q: How do you check an underground tank against flotation? A: Compare the dead weight of the empty structure plus soil on any projecting ledge with the uplift γw × base area × depth below the water table, requiring a factor of safety of about 1.2.
• Q: What is the acceptance criterion for the water-tightness test? A: After filling and a 7-day settling period, the drop in water level over the next 7 days must not exceed 1/500 of the average water depth or 10 mm, whichever is less, with no visible leakage.
• Q: How is water modelled in the seismic design of an elevated tank? A: As a two-mass system per IS 1893 Part 2 — an impulsive mass rigidly attached to the tank and a convective (sloshing) mass on a spring — analysed for both empty and full conditions with freeboard for the sloshing wave.
• Q: Why does the circular girder of an elevated tank need torsion design? A: Because it is a curved beam on discrete column supports, so the bending moment vector changes direction along the arc and produces torsion, maximum roughly at the quarter points between columns.
• Q: Does increasing the cover make a tank wall safer against leakage? A: No — larger cover increases the surface crack width for the same steel strain, so cover is kept at the specified value (about 45 mm) and water-tightness is achieved through grade, compaction, curing and closely spaced bars.

Common mistakes

• Using IS 456 permissible/limit-state steel stresses instead of the restricted stresses and crack-width limits of IS 3370 — the section then leaks though it is "safe".
• Forgetting the direct tension in rectangular tank walls and designing for bending only; the pull transferred from the perpendicular wall is substantial.
• Taking the hoop tension as constant over the height, or designing the whole wall for the base value, instead of curtailing hoop steel in stages.
• Treating a fixed-base circular tank as a free-base one — missing the vertical base moment, which is where many tanks actually crack.
• Designing an underground tank for internal water pressure only and providing steel on the water face alone; then the empty tank fails under earth pressure, or floats because the uplift check was never done.
• Mixing units — γ in kN/m³ with dimensions in mm, or moments in kN·m with stresses in N/mm² — and getting steel areas out by 10³.
• Ignoring movement joints, water bars and curing on site, and expecting waterproof coating to compensate for porous, badly compacted concrete.

Related topics

Reinforced concrete design (IS 456) · Design of RCC foundations · Prestressed concrete basics · Earthquake-resistant design · Structural analysis: determinate and indeterminate structures · Repair and rehabilitation of structures