Eurocode 1 — Actions on structures - Silos and tanks

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The second generation of EN Eurocode standards is expected to be published between 2023 and 2026. These documents are being published as soon as they are available, to enable users to prepare for the transition from the first generation to second generation of Eurocodes. As set out in the National foreword of the BS EN second generation Eurocodes, UK adoptions of the first generation of Eurocodes will be withdrawn by BSI on 30 March 2028. That means there is a period of coexistence between 2023 and 2028 during which both first and second generation Eurocodes are available.

Until 30 March 2028, the first generation documents should be considered as the applicable standards for buildings and civil engineering works constructed in the UK unless otherwise specified by the relevant authority or in the specification for a particular project. While the use of provisions in second generation Eurocodes in conjunction with first generation Eurocodes is not precluded, it should be undertaken with care and should only be done when users are satisfied that it will not result in a lower level of reliability than the minimum level set in the first generation Eurocodes and associated UK National Annexes.

1 Scope

1.1 Scope of EN 1991‑4

(1) This document provides rules for calculating actions for the structural design of silos and tanks.

NOTE 1 Silos are used for the storage of particulate solids. Tanks are used for the storage of liquids.

NOTE 2 For limitations on rules for silos given in this document, see 1.3.

NOTE 3 For limitations on rules for tanks given in this document, see 1.4.

(2) This document includes some provisions for actions on silo and tank structures that are not only associated with the stored particulate solids or liquids (e.g. the effects of thermal differentials) but substantially affected by them.

NOTE Liquid loads on tanks are very precisely defined. Many loads on silos are not known with great precision. This document provides guidance for many practical situations for which very limited certain knowledge is available, and the information is derived from the limited experimental and analytical information available, coupled with conclusions drawn from failure investigations. The information is not based on a sound statistical treatment of experimental data.

(3) This document is intended for use with concrete, steel, aluminium, timber and FRP storage structures.

NOTE FRP is the standard acronym for fibre reinforced polymer materials.

(4) This document is also applicable for the structural assessment of existing silos and tanks, unless otherwise specified by the relevant authority or, if not specified, agreed between the relevant parties for the specific project.

NOTE 1 Changes in filling or discharge arrangements, changes in the wall friction of inner surfaces, or in the use of the silo, including storage of different particulate solids, can be reasons for assessing existing silos.

NOTE 2 Differentiation of the liquid stored can be a reason for assessing existing tanks.

1.2 Assumptions

(1) The assumptions of EN 1990‑1 apply.

(2) This document is intended to be used in conjunction with EN 1990‑1, with the other parts of EN 1991, EN 1992, EN 1993, EN 1995, EN 1997, EN 1998 and EN 1999 where relevant to the design of silos and tanks.

1.3 Limitations on silos

1.3.1 Geometrical limitations

(1) The following geometrical limitations apply to the design rules for silos and silo batteries (see 3.2.59 and 3.2.60) covered by this document:

the silo planform cross-section shapes are limited to those shown in Figure 1.1c.

NOTE 1 Further information concerning planform cross-section geometries is given in Clause 7.

NOTE 2 For the determination of the effective diameter d_c of the silo see Figure 1.1c;

the following dimensional limitations on the aspect ratio for free-standing single cell silos h_c/d_c, the overall height h_b and the effective diameter d_c apply (see Figure 1.1):

h_c/d_c < 10

(1.1)

h_b < 100 m

(1.2)

d_c < 60 m

(1.3)

NOTE 3 See Figure 1.1 for h_c, d_c and h_b.

the structural transition lies in a single horizontal plane (see Figure 1.1a).

(2) Only hoppers that are conical (i.e. axisymmetric), rectangular pyramidal with a/b ≤ 1,5, wedge-shaped (i.e. with two vertical end walls on opposite sides) or oblique are covered by this document. Other hopper shapes and hoppers with internal structures require special considerations.

(3) Silos with an oblique conical hopper used to achieve an eccentric outlet are covered by this document.

(4) Silos with an oblique hopper are covered, but generally silos with a systematically non-symmetric geometry are not specifically covered by this document. These situations include a chisel hopper (i.e. a wedge hopper beneath a circular cylinder) and hoppers with an elongated outlet other than wedge shaped.

1.3.2 Limitations on the stored particulate solids

(1) The following limitations on the stored particulate solids apply to the design rules for silos contained in this document:

each silo is designed for a defined range of particulate solids properties;
the stored particulate solid is either naturally free-flowing, or can be guaranteed to flow freely within the silo container as designed, or is unloaded from the top of the silo;
for stored particulate solids in Flow Group C (see Annex C), special provisions can be necessary (see 5.4);
stored particulate solids in Flow Group D (see Annex C) are excluded, except where they are stored in silos unloaded from the top (see Figure 5.3a and 7.2.2);
each silo is designed for a defined mode of operation;
the maximum particle dimension in the stored particulate solid is not greater than 0,03d_c (see Figure 1.1c).

NOTE 1 The relevance of the possible range of the particulate solids parameters to the design of a specific silo can be agreed by the relevant parties (see also Annex D).

NOTE 2 Where stored particles are large compared to the silo effective diameter d_c, significant problems can occur due to mechanical interlocking that this document does not address.

NOTE 3 Where particles are large compared to the silo wall thickness, a single particle can apply a large local force on the wall. This aspect is outside the scope of this document.

NOTE 4 Where stored particulate solids are very sensitive to specific environmental conditions, interstitial air humidity and wall surface temperature, the particulate solid properties and wall friction coefficient can change to a degree that is not covered within the scope of this document.

NOTE 5 Where attainment of the dew point at the wall causes particulate solid to stick to the wall an erratic and eccentric flow can occur during discharge of the silo. This aspect is outside the scope of this document.

(2) Where large particles are stored, the outlet dimension is required to be sufficiently large to ensure that mechanical interlocking does not occur.

(3) The phenomena of particulate solid-induced vibrations, oscillations and shocks (quaking, honking, banging, etc.) are outside the scope of this document.

NOTE Silos that are used to store particulate solids that can induce these effects can require special load assessments or special design considerations beyond the scope of this document.

1.3.3 Limitations on filling and discharge arrangements

(1) The following limitations on the filling and discharge arrangements apply to the design rules for silos:

filling and/or discharge involves only negligible inertia effects and impact loads;
where discharge devices are used (for example feeders or anti-dynamic tubes), the particulate solids flow is known to be smooth and reliable.

NOTE 1 This document includes the filling pressures in square or rectangular silos that contain internal structural ties between the vertical walls.

NOTE 2 Extreme rates of filling and discharge can give rise to dynamic loads that are not considered in this document (see also 1.3.3(6)).

NOTE 3 Elongated outlets present special problems. Where a feeder is used to control the discharge of the particulate solid from the silo, its design can affect the particulate solids flow pattern in the silo. This can produce either mass flow or fully eccentric mixed flow, or fully eccentric pipe flow in the silo.

(2) It can be necessary to use discharge aids to achieve free flow (see 3.2.23 and Annex C). This document does not provide rules for the following possible features of a silo: discharge feeders, anti-dynamic tubes, cross-beams and structural elements inserted into the particulate solids flow domain.

(3) Variations that can occur in the discharge of particulate solids arising from fluctuations of particulate solid properties caused by changes of humidity and temperature inside the silo and the wall surface leading to disturbances of the flow pattern are outside the scope of this document.

NOTE Special conditions can lead to moisture deposition if the dew point is reached at the wall surface. Wet surfaces can cause particulate solid to stick to the wall, leading to flow problems and higher local pressures or non-symmetric load patterns that are not considered in the design situations of this document and are outside the scope of this document.

(4) The loads induced by discharge arms, stationary and rotating feeders, and similar equipment are outside the scope of this document. Information concerning such loads can be obtained from the manufacturers of the equipment.

(5) The functional design of silos is outside the scope of this document, but consideration should be given to the blockage of the outlet if particle sizes exceed d_o/8, where d_o is the characteristic dimension of the outlet.

(6) Loads that arise from silo quaking, shocks, honking, pounding and silo music are not defined in this document. However, the rules of this document may be used for silos that can be susceptible to these phenomena, but this specific aspect of design is classified as Action Assessment Class AAC4.

NOTE These phenomena are not well understood but are believed to be a consequence of the pattern of particulate solids flow. The use of this document cannot ensure that these phenomena will not occur, or that the designed structure will be adequate to resist the forces they can induce.

(7) The design of silos for the storage of silage and haylage is outside the scope of this document.

(8) Silos in Action Assessment Class AAC4 or Consequence Class CC4 can be designed according to the rules of this document, but further information can be required to guarantee the safety of the design.

(9) The actions on the silo structure from feeders and gates are not specified in this document. These include those from unattached feeders that can transfer loads to the silo structure through the stored particulate solid.

(10) The actions on the catwalks, ladders, gantries, feeders, conveyors and other structures connected to a silo are outside the scope of this document.

1.4 Limitations on tanks

(1) The rules in this document apply to ground supported or elevated (pedestal and/or column or girder supported) tanks (Figure 1.2).

(2) The rules for tanks apply only to tanks storing liquids with only small vapour pressures above the surface. The upper limit for the pressure in the vapour space above the liquid in a tank is 500 mbarg (= 50 kPa).

(3) The actions on the catwalks, ladders, gantries, pipework and other structures connected to a tank are outside the scope of this document.