British Standards Relevant to Agriculture

British Standard Summaries

BS5061: 1974(a)
Cylindrical Forage Tower Silos
and Recommendations for their Use

The text herein is not a a full reproduction of the British Standard. It is summary based upon interpretation of the original text and not intended as a replacement for the full text. It should be used for general guidance only.

This Standard sets out data for the basic design of tower silos used for the conservation of grasses as silage under normal UK farming conditions. From the data contained in the Standard, silos may be designed, provided that the design is in accordance with accepted structural engineering practice. Additional considerations need to be applied where the grasses contain additives, admixtures or other herbages. The Standard applies to all tower silos with over 3m of effective depth of storage.

Silo dimensions

  • Provided that all structural requirements are met, the height is limited by the necessity of having to wilt consistently to moisture contents which avoid undue risk of fluid pressure developing and effluent being exuded.
  • The limit to diameter of towers is set by the need to ensure that the required rate of consolidation can be achieved with the system of field machinery planned, and that the required rate of unloading can be maintained.


  • The foundations are considered as an integral part of the design, and should be constructed with the relevant recommendations of BS 8004.
  • The foundation shall provide uniform contact pressure on the bearing sections of the silo wall.
  • Whether structural or non-structural, the concrete liable to come in contact with silage effluent shall not be of less than 150 mm vertical thickness and shall be dense concrete of at least 30 N/mm2 compressive strength with a minimum cement content of 330 kg/m3.


  • The silo shall be securely anchored to the foundation when its dead weight is insufficient to resist the upward forces of wind overturning and silage uplift.


  • The distance between the wall of the silo and the nearest domestic dwellings shall not be less than 1.25 times the height of the cylinder, measured from floor level to the top of the cylinder.


  • In calculating the loadings for High Dry Matter (HDM) and general purpose silage towers the following must be considered: imposed roof load, wind load, internal gas pressures, machinery loads, density of silage, and pressures exerted by silage including lateral pressure, vertical frictional load and floor load for top-unloaded silos and bottom unloaded silos.
  • Further details of the calculations involved are available in the full Standard.

Design and materials

  • The design of the structure shall be such that it is able to carry its own weight and resist the specified loading and wind forces.
  • Special consideration should be given to areas of high stress such as around doorways, hatches and in components to which machinery is attached.
  • No material or paint treatment which has toxic or deleterious effect on humans or animals shall be used on any surface in contact with the forage.

Silo wall

  • The Standard gives equations for calculating the maximum permissible compressive stress and minimum silo wall stiffness to prevent buckling under internal suction. The construction of steel stave silos is also considered in this section of the Standard.
  • For concrete stave silos the Standard states that precast concrete staves shall have a flexural strength of not less than 5.0 N/mm2, as should concrete door frames.
  • Steel hoops shall be provided to resist the loads arising. Hoops shall be steel rods with rolled threads with a minimum tensile strength of 415 N/mm 2. The minimum elongation of such hoops shall be 20% on 200 mm gauge length.


  • When erected, in no case is it allowable for more than 2% of the total number of staves to be cracked across the width.
  • In any round not more than 5% of staves shall be cracked, although for staves 12 m or more above ground level this limit can be waived provided that cracks are rebonded by the injection of an approved epoxy (or similar) crack filling and bonding preparation to the full depth of the crack.
  • Not more than 1% of the staves shall have chips exceeding 25 mm in any direction.


  • Where timber is used for the construction of forage towers, the structure shall be designed in accordance with BS 5268.
  • Allowance shall be made for the swelling of timber due to moisture absorption as the silo is filled and for shrinkage as the wood dries out during unloading.


  • All hatches, loading and unloading points, gas vents, fire ports and other parts of the construction which require regular inspection and maintenance shall be located adjacent to the access equipment or working platforms.
  • According to the degree of likely exposure, materials and components shall either be resistant to or protected against the action of silage acid, frost or weathering, or corrosive atmospheres.
  • The silo roof shall be weatherproof. Where the method of bottom unloading necessitates the roof of the silo shall be gas tight but vented against large pressure differences.

Fluid pressure relief

  • To ensure that there is relief of fluid pressure and that there is no accumulation of effluent against the inner surface of the floor/wall joint there shall be provided an internal system of perimeter drainage.
  • The effluent shall be drained away clear of the silo in such a way as to ensure that no effluent comes into contact with the underside of the foundations.

Access equipment

Fixed ladders, working platforms and other access equipment shall be designed and maintained in accordance with the following:

  • Except where stated, fixed ladders other than those within chutes shall comply with BS 4211.
  • The sections shall have sufficient strength and rigidity to withstand the weight of two persons, each of 100 kg, at different points on the ladder.
  • Rungs shall have not less than 380 mm clear length and shall not be spaced at more than 300 mm centres and not less then 150 mm from the silo.
  • Each rung shall be capable of supporting 150 kg over the central 100 mm without undue stress or deflection.
  • Hoists and lifts shall comply with the Offices Shops and Railways Act Statutory Instrument 849 : 1968, and where applicable SI 863 : 1968.
  • Working platforms and stairways (roof ladders etc.) shall have non-slip working surfaces of adequate size to permit two operators to work side by side.
  • The perimeter of the access area shall be guarded by a hand rail consisting of at least two horizontal rails, of which one shall be at 1 m and the intermediate rail at 0.5 m above the working level, in accordance with the Agriculture (Safeguarding of Workplaces) Regulations 1959.
  • The top of the lowest rung of any permanent ladder shall be not less than 3 m from the base of the structure at the time of construction. Any ladder below that height shall be easily detatchable.
  • Safety hoops shall commence no higher than 1 m above the lowest rung of a permanent ladder.
  • Provision shall be made to ensure that any chutes used for access can be adequately ventilated.
  • Provision shall be made for adequate fixing points for any portable equipment used for hoisting equipment or working platforms for servicing or inspecting the exterior of the silo.

Notice boards

Suitable durable notices shall be securely fixed to the silo at a height of between 1 and 2 m. Additionally, notices warning of hazards shall be provided at each point of entry in the silo. The notices shall be either:

  • not less than 300 mm by 200 mm and contain
    • an identification of the hazards such as CO2 and oxides of nitrogen to be expected in the operation of the silo and recommendations for the operators (white letters on a bright red background)
    • guidance on the most satisfactory method of filling the silo (white letters on a green background)


  • not less than 50 mm by 25 mm and include the name of the manufacturer, date of manufacture, the BS number (i.e. BS 5061), and the type of silo.

A British Standard nor this summary does not, necessarily, include all the necessary information for correct implementation of the Standard to any specific application. This is purely the responsibility of the user. Standards are updated by either amendment or revision. Users should ensure that they are using the latest version.

The full text of this Standard can be obtained from the British Standard Institution

ADLib logo Content provided by the Agricultural Document Library
© University of Hertfordshire, 2011