Standards Development

Scope

1.1 Scope of EN 1991-4

(1) EN 1991‑4 provides guidance 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) EN 1991‑4 includes some provisions for actions on silo and tank structures that are not only associated with the stored 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 standard 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) EN 1991‑4 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) EN 1991‑4 may be used for the structural assessment of existing construction, in developing the design of repairs and alterations or for assessing changes of use.

NOTE Where the structural appraisal of an existing structure is being considered, reference can be made to the National Annex and to the client concerning the relevance of the current standard.

1.2 Assumptions

(1) The assumptions of EN 1990 apply.

(2) EN 1991‑4 is intended to be used in conjunction with EN 1990, 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 covered by this document:

• the silo here defined is either an isolated structure or can be part of a battery of silos. For a silo battery, the term silo is used throughout this standard to refer to a single cell within the battery;

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

NOTE 1 Minor variations to these shapes can be accepted provided the structural consequences of the resulting changes in pressure are expected to be considered. Further information concerning planform cross-section geometries is given in 7;

NOTE 2 Further information concerning planform cross-section geometries is given in Clause 7.

• the relevant overall height of the silo hb (Figure 1.1a) is measured from the level of the equivalent surface of the stored solid (see 3.2.17) when the silo is filled to its maximum capacity, down to the apex of the cone of the hopper or to the flat base where there is no hopper;

NOTE For the evaluation of ho to calculate hb, see (2).

• the effective diameter dc of the silo should be determined as indicated in Figure 1.1c;

• the following dimensional limitations on the overall height hb and aspect ratio hb/dc apply (see Figure 1.1):

hb/dc < 10 (1.1)

hb < 100 m (1.2)

dc < 60 m (1.3)

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

• the relevant cylindrical section height of the silo hc (Figure 1.1a) should be measured from the level of the equivalent surface of the stored solid (see 3.2.17) when the silo is filled to its maximum capacity, down to the structural transition (see Figure 1.1a) or to the flat base where there is no hopper;

(2) For a symmetrically filled circular silo of diameter dc, h0 should be determined as:

 (1.4)and for a symmetrically filled rectangular silo of characteristic dimension dc, h0 should be determined as: (1.5)where:ϕr is the angle of repose of the solid (see Table C.1).

NOTE For solids that can become fluidised on filling, the value of ϕr can also be zero.

(3) The value of ho for a powder may normally be taken as ho = 0 because the solid is naturally aerated on deposition. Silo design for storing powders should allow for higher filling levels arising from this effect.

Please see draft for Figure 1.1 — Silo forms showing dimensions and pressure notation

(4) Only hoppers that are conical (i.e. axisymmetric), rectangular pyramidal, wedge-shaped (i.e. with vertical end walls) or oblique are covered by this standard. Other hopper shapes and hoppers with internal structures require special considerations.

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

(6) Silos with an oblique hopper are covered, but generally silos with a systematically non-symmetric geometry are not specifically covered by this standard. These situations include a chisel hopper (i.e. a wedge hopper beneath a circular cylinder). a diamond-back hopper and a transition hopper that transforms a rectangular into a circular cross-section.

NOTE A transition hopper for a circular silo transforms the circular cross-section into a rectangular shape, or for a rectangular silo it transforms a rectangular into a circular cross-section from the transition to the outlet. These forms are not explicitly treated in this standard, but the provisions for circular hoppers are believed to be suitable for structural design purposes.

• a cylindrical silo does not contain internal structures other than an inverted cone at the base.

• the loads induced by discharge arms, stationary and rotating feeders, and similar equipment are outside the scope of this standard. Information concerning such loads should be obtained from the manufacturers of the equipment.

1.3.2 Limitations on the stored solids

(1) The following limitations on the stored 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 solid is either naturally free-flowing or can be guaranteed to flow freely within the silo container as designed;

• for stored solids in Flow Group C (see Annexes C and D), special provisions can be necessary (see 5.4);

• stored solids in Flow Group D (see Annexes C and D) 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 solid is not greater than 0,03dc (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. Where any uncertainty exists, the client can obtain specialist advice from an appropriate testing laboratory (see also Annex D).

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

NOTE 3 Where stored particles are large compared to the silo characteristic dimension dc, significant problems can occur due to mechanical interlocking that this standard does not address.

NOTE 4 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 standard.

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

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 solids flow should be known to be smooth and reliable.

NOTE 1 This standard 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 (15)).

(2) It can be necessary to use discharge aids to achieve free flow (see 3.2.25 and Annex D). This standard does not provide rules for the following possible features of a silo: discharge feeders, anti-dynamic tubes, chinese hats, cross-beams and structural elements inserted into the solids flow domain.

(3) Variations that can occur in the discharge of 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 standard.

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

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 standard and are outside the scope of this standard.

(5) The silo loads defined in this standard are premised on the assumptions of isotropic or orthotropic shell theory, with membrane and bending resultants in the silo wall. Concrete silos are generally considered to be thick-walled silos, whilst metal and FRP silos are generally to be considered to be thin-walled silos. In other structural forms, such as wooden stave silos, the load conditions should be carefully considered, with special attention to the consequences of non-symmetrical pressures (e.g. the proxy load rules).

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

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

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

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

(9) Silos in AAC4 or Consequence Class 4 may be designed according to the rules of this standard, but further information can be required to guarantee the safety of the design.

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

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

1.4 Limitations on tanks

(1) The rules in this standard apply to on-ground 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). 

Please see draft for Figure 1.2 — Tank forms showing dimensions and pressure notation

NOTE For simplicity and clarity in Figure 1.2b, only the liquid level MDLL has been identified with its dimensional location hL with its corresponding depth coordinate xL. The other defined liquid levels, MNOL and min. NOL, are marked to indicate their locations.

(3) The actions on the roofs of tanks defined in EN 1991‑1‑1, EN 1991‑1‑3 to EN 1991‑1‑7 and EN 1991‑3 should be considered.

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

(5) This standard does not cover tanks used to store liquids whose vapours are susceptible to explosion (CC4).

NOTE 1 Loads exerted on structures near a tank as a result of an explosion within it can be evaluated according to EN 1991‑1‑7.

NOTE 2 The National Annex can give guidance on the pressure exerted on structures near the tank as a result of an explosion within it.