Storage Structures — Bins & Silo Design — Drying & Storage of Crops Notes

Last reviewed 1 Oct 2026 · 7 min read

Classification of storage structures

Class	Examples	Capacity
Traditional (household)	Kothi, bukhari, morai, mud bins, bamboo bins, gunny bags	Up to a few tonnes
Improved (small farm) bins	GI-sheet bins (PAU bin, Hapur tekka), Pusa bin (with polythene film), brick/ferro-cement bins	About 0.5–5 tonnes (varies by model)
Bag storage (godowns)	Warehouses of FCI, CWC and state warehousing corporations; CAP (cover and plinth) storage	Hundreds to thousands of tonnes
Bulk storage — bins and silos	Steel (corrugated galvanised, bolted) and RCC silos; flat storage; vertical silos	Hundreds to tens of thousands of tonnes
Underground	Pits, tunnels (limited use now)	Local

Requirements of a good grain store

Dry (damp-proof floor and roof), rodent- and bird-proof, insect-tight (or capable of fumigation/hermetic sealing), well-ventilated or aerated where needed, cool (insulation/white roof), strong, easy to load and unload, easy to clean and inspect, and economical.

Bins versus silos

Feature	Shallow bin (low-height)	Deep bin (silo)
Height-to-diameter (H/D) ratio	Less than about 1.5 (or ≤ 2)	Greater than about 1.5–2
Pressure theory	Rankine / Coulomb (the whole wedge of grain loads the wall)	Janssen (wall friction supports part of the grain weight)
Discharge	Often by flat floor and sweep/auger	Hopper bottom with gravity flow
Typical use	Warehouse-type flat storage, farm bins	Large commercial storage, ports, flour/rice mills

Grain properties for design

Property	Meaning	Typical values (indicative)
Bulk density $γ$	Mass per unit volume of the grain mass	Wheat about 770–820 kg/m³; paddy about 540–580; rice about 800–850; maize about 720–750; soybean about 720–780; pulses about 780–850
Angle of repose	Slope angle of a heap	About 20–35°; paddy about 35–38°
Angle of internal friction $ϕ$	Shear strength of grain mass	About 20–30°
Coefficient of wall friction $μ^{'} = tan δ$	Friction between grain and wall (smooth steel lowest; concrete highest)	About 0.25–0.45
Janssen coefficient $K$	Ratio of horizontal to vertical pressure	About 0.35–0.5 (Rankine: $K = \frac{1 - sin ϕ}{1 + sin ϕ}$ )
Flowability	Depends on moisture, fines and particle shape	—

Pressures in a silo — Janssen's theory

At depth $z$ below the top of the grain, the vertical pressure is less than the overburden $γ z$ because wall friction carries part of the weight:

FormulaJanssen's equations

p_{v} = \frac{γ R}{μ ^{'} K} [1 - exp (- \frac{μ ^{'} K z}{R})] p_{h} = K p_{v}

where $R = \frac{A}{U}$ is the hydraulic radius (plan area ÷ perimeter; $R = D /4$ for a circular silo of diameter $D$ , and $R = \frac{ab}{2 ( a + b )}$ for a rectangular silo $a \times b$ ), $μ^{'}$ the wall friction coefficient and $K$ the pressure ratio.

At great depth, $p_{v} \to \frac{γ R}{μ ^{'} K}$ (pressure becomes constant).
Wall friction force per unit perimeter is $μ^{'} p_{h}$ — the vertical load on the wall — which must be carried by the wall and foundation.
Design codes (IS 4995, ISO 11697, Eurocode) add overpressure factors for discharge (dynamic pressure at emptying, with eccentric discharge greater than static).

Worked ExampleExample — pressures in a circular silo

A silo of diameter $D = 6$ m stores wheat with $γ = 7.65$ kN/m³ ( $780 kg/m^{3} \times 9.81$ ), $ϕ = 2 5^{\circ}$ and $μ^{'} = 0.30$ .

$R = \frac{6}{4} = 1.5$ m.
$K = \frac{1 - sin 2 5 ^{\circ}}{1 + sin 2 5 ^{\circ}} = \frac{1 - 0.4226}{1 + 0.4226} = 0.406$ .
$μ^{'} K = 0.30 \times 0.406 = 0.1218$ ; $\frac{γ R}{μ ^{'} K} = \frac{7.65 \times 1.5}{0.1218} = 94.2$ kPa.
At $z = 10$ m: exponent $= \frac{0.1218 \times 10}{1.5} = 0.812$ ; $exp (- 0.812) = 0.444$ .
$p_{v} = 94.2 \times (1 - 0.444) = 52.4 kPa$ (compared with $γ z = 76.5$ kPa without friction).
$p_{h} = 0.406 \times 52.4 = 21.3 kPa$ on the wall at 10 m depth.

The limiting vertical pressure at great depth would be 94.2 kPa. Hoop tension per metre height at that depth $= p_{h} \times \frac{D}{2} = 21.3 \times 3 = 64 kN/m$ — the wall (steel or RCC) is designed for this.

Pressures in shallow bins

For a shallow bin the Rankine active pressure $p_{h} = K γ z$ (or Coulomb with wall friction) is used; the pressure increases linearly with depth, so the walls need the stiffest section at the base and tie rods/buttresses may be provided.

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