Silo Slenderness Ratio
Calculate a silo's slenderness ratio, λ = H ÷ D, from the stored product height H (m) and the silo diameter D (m). This simple ratio is the fundamental criterion that CLASSIFIES silos and determines how their pressures behave and how codes treat them. Silos with HIGH slenderness (typically H/D ≥ 1.5-2, called slender or 'tall') are dominated by the Janssen wall-friction effect: pressure saturates quickly, most weight transfers to the walls, and the bottom receives a much lower pressure than the product column would suggest. Silos with LOW ratio (H/D < 1.0-1.5, called squat) behave intermediately between Janssen and a tank: wall friction has less extent to act, and a larger fraction of weight reaches the bottom. This distinction changes the applicable pressure formulas, the discharge overpressure factors and even the expected flow type. The slenderness ratio is thus the first decision in silo analysis — it sets which load model to use and influences the whole structural concept, from foundation to walls. Enter the product height and the silo diameter.
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Silo slenderness ratio
The slenderness ratio of a silo, λ = H ÷ D, is the fundamental criterion that classifies silos and determines how their pressures behave and how the design codes treat them. Simple though it looks — merely the ratio between the height of the stored product and the diameter — this ratio is the first decision in the analysis of any silo. Silos with a high slenderness ratio (typically H/D ≥ 1.5 to 2, the slender or 'tall' silos) are dominated by the Janssen wall friction effect: the pressure saturates quickly, most of the weight is transferred to the walls, and the bottom sees a much lower pressure than the column of product would suggest. Silos with a low ratio (H/D < 1.0 to 1.5, the squat or short silos) behave in an intermediate way between Janssen and a liquid tank: wall friction has less vertical extent in which to act, and a larger share of the weight reaches the bottom, approaching the behaviour of a liquid. This distinction changes the applicable pressure formulas, the discharge overpressure factors and even the flow pattern expected at discharge (mass flow or funnel flow). That is why the slenderness ratio shapes the whole structural concept — from the foundation to the walls — and is the starting point for choosing the right loading model. Enter the product height and the silo diameter.
Related Tools
Silo Vertical Pressure (Janssen)
Calculate the vertical pressure of stored product at a silo cross-section by the Janssen equation, p_v = (γ·D)/(4·μ·K)·(1 − e^(−4·μ·K·z/D)), from the product unit weight γ (N/m³), silo diameter D (m), product-wall friction coefficient μ, lateral pressure ratio K and depth z (m). The Janssen equation (1895) is the basis of silo structural design and reveals a counterintuitive, fundamental fact: pressure at the bottom of a silo does NOT grow indefinitely with product height like a liquid (p = γ·h). Instead it tends to a LIMIT (asymptotic) value. This is because granular product (grain, cement, ore) transmits part of its weight LATERALLY to the walls, and product-wall friction 'holds' that load, relieving the bottom. The deeper it goes, the larger the fraction of weight carried by wall friction, until all added weight is absorbed by the walls and bottom pressure stabilizes. So silos can be very tall without bottom pressures proportional to height. This arching and wall-friction effect is the heart of silo design. Enter the unit weight, diameter, friction coefficient, lateral pressure ratio and depth.
Silo Discharge Overpressure
Calculate the DISCHARGE horizontal pressure in a silo, p_d = C_d · p_h, from the discharge overpressure coefficient C_d and the static horizontal pressure p_h (from Janssen for the full silo at rest). One of the most important phenomena — historically responsible for many silo failures — is that wall pressures during DISCHARGE are SIGNIFICANTLY HIGHER than static pressures with the silo merely full. When the product starts flowing toward the outlet, flow zones and dynamic arches form, and stress redistribution generates pressure peaks (overpressures) on the wall, especially at the transition from the cylindrical body to the hopper. The overpressure coefficient C_d (typically 1.3-2.0 or more, per the code, flow type — mass or funnel — and geometry) amplifies the static pressure to cover these dynamic peaks. Silo design codes (such as EN 1991-4 / Eurocode and ANSI) prescribe these factors precisely because designing a silo only for static loads, ignoring discharge overpressure, is a classic cause of structural collapse. Enter the overpressure coefficient and the static horizontal pressure.
Silo Horizontal Pressure (Janssen)
Calculate the horizontal pressure the stored product exerts on a silo wall by the Janssen equation, p_h = (γ·D)/(4·μ)·(1 − e^(−4·μ·K·z/D)), from the unit weight γ, diameter D, product-wall friction coefficient μ, lateral pressure ratio K and depth z. Horizontal pressure is the outward thrust grains apply against the silo walls — the load that sizes the wall for hoop tension (in cylindrical silos, the wall acts as a ring under internal pressure). It relates to vertical pressure by the lateral pressure ratio K (p_h = K·p_v), typically 0.3-0.6 for granular products and depending on the product's internal friction angle. Like vertical pressure, horizontal pressure tends to an asymptotic value with depth, by the same wall-friction effect of Janssen theory. Horizontal pressure is decisive for the thickness and reinforcement of concrete silo walls and the plating of steel silos, and rises significantly during DISCHARGE (dynamic overpressure), which codes handle with amplification factors. Enter the unit weight, diameter, friction coefficient, lateral pressure ratio and depth.
Silo Asymptotic Pressure
Calculate the asymptotic (saturation) vertical pressure of a deep silo, p_∞ = (γ·D) ÷ (4·μ·K), from the product unit weight γ, diameter D, product-wall friction coefficient μ and lateral pressure ratio K. This is the LIMIT value the Janssen vertical pressure tends to at great depth — the maximum bottom pressure a silo can reach, however tall the stored product. It is Janssen's most striking result: while in a liquid pressure would grow without limit with height (p = γ·h), in granular product WALL FRICTION absorbs all added weight beyond a certain depth, making bottom pressure SATURATE. So a 30 m silo of grain may have a bottom pressure equal to only a few metres of product column. This asymptotic pressure is fundamental in design: it sets the maximum bottom and wall load the structure must bear, regardless of height, and explains why silos can be built slender and tall with relatively modest foundations. Note it is proportional to diameter and inversely proportional to friction — wide, smooth-walled silos generate higher pressures. Enter the unit weight, diameter, friction coefficient and lateral pressure ratio.
Janssen Characteristic Depth
Calculate a silo's Janssen characteristic depth, z₀ = D ÷ (4·μ·K), from the diameter D, product-wall friction coefficient μ and lateral pressure ratio K. The characteristic depth governs how fast silo pressures approach their asymptotic (limit) value: in the Janssen equation, it is the depth at which pressure reaches about 63% (1 − 1/e) of the maximum. Depths of a few times z₀ practically reach the limit pressure. Conceptually, z₀ shows how 'deep' the silo must be for wall friction to dominate: silos with small z₀ (small diameter, high friction) quickly reach the constant-pressure regime and behave as slender (tall) silos; silos with large z₀ (large diameter) saturate slowly and behave more like squat silos, where much of the weight still reaches the bottom. The characteristic depth is thus a natural measure of the vertical 'scale' of the silo's pressure behavior, useful to classify it and understand its load profile. Enter the diameter, friction coefficient and lateral pressure ratio.
Wave Steepness
Calculate the steepness of a wave, s = H ÷ L, dividing the wave height H by the wavelength L. The dimensionless result is the relative steepness of the wave — the taller it is for its length, the steeper. Steepness has a physical limit: deep-water waves break when s exceeds about 1/7 (0.143), as the crest becomes unstable (120° angle). Young storm waves are steep; swell that travels long distances is gentle (low steepness). Steepness governs wave stability, breaking and vessel comfort. Enter the wave height and wavelength.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.