BS Strength Classes

Since it was published by BSI in 1992 the BS 7611: Potato storage boxes for mechanical handling has become the yardstick for potato box design in the UK. Most new boxes are sold with claims that they meet one of the three BS strength classes that limit height of stacks to the strength of the box, namely for four, six, or eight high. The tough load tests in the British Standard have proved to manufacturers who test their boxes, the importance of nailing, bracing and timber thickness.

Nailplate hydraulically pressed into a
potato box side to increase racking
strength to BS 7611 requirement

With the possible exception of large wood packing cases for export, the end-use of a 1000 kg potato box in stack is the toughest task of any wood packaging product. Very large stresses are commonplace needing modern solutions and the industry has benefited from the development work of a leading English box manufacturer who during the 1990s pioneered structurally efficient boxes based on the punched steel nailplate shown in the illustration. The stresses on a box with 7 tons of compression load above it are self evident, but our surveys have shown there are a small number of box manufacturers who have little regard for adequate specification in their designs and few buyers have the experience to reject boxes that are of inadequate design. We are often asked if BSI, Kitemarking or ISO 9000 offer a remedy to combat misleading claims or poor design and we have covered these issues in Datasheet 34a below.

If box buyers are procuring BS Class 6 or 8 boxes they are advised to insist on seeing the laboratory testing certificate to prove a manufacturer’s box design meets BS 7611. The certificate should contain details of thicknesses, species, nailing details and plate sizes on each of 4 sides and should be disregarded if not relating to the design in hand.

Just two examples of key points are - nails of a minimum diameter and bending strength have proven essential to meet BS Classes 6 and 8 and - for timber diagonal V or X braced designs - if timber for posts and boards is not kiln dried to 16 to 17% moisture content or below before assembly, shrinkage in the early months of box life will drop performance below Class 6 (Punched steel nailplates are much more forgiving of high timber moisture content than diagonal wood bracing).

The PalletLink designs given in Datasheet series 35 and 36 below are fully tested and give the full timber, sheet material, nail and nail-plate details for those deciding to use the PalletLink design route. Nevertheless manufacturers need to test their designs before BS marking is applied. 

Manufacturers attempting to achieve Class 6 or 8 performance without testing  are unlikely to succeed. Being in possession of the all-important testing certificate is essential, without this, in the case of an accident to personnel the manufacturer is directly liable for the design used and even without accident weak boxes mean shortened box life, often by several years. There are several essential steps to assess designs against the test requirements of BS 7611, PalletLink Datasheet 34b summarises these.

A steady interest in tightening of specifications has led to calls for revisions to be made to the 1992 edition of BS 7611. In advance of this revision a survey by PalletLink has been completed, including the contentious issues of repair-kits to upgrade old boxes, wood preservatives, whether new designs should include increased payloads and the specification of nailplates. (Survey details in 31c below).

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PDF31b Background to BS 7611 Potato storage boxes. (PDF 124kb)
PDF31c Proposed revision to BS 7611. (PDF 46kb)
PDF31h Potato box payloads exceeding 1150 kg. (PDF 41kb)
PDF32a Summary of Potato box sizes, capacities, listed in BS 7611. (PDF 71kb)
PDF35b1 Potato Box Specification BS 7611 Class 8 Timber X Braced 8TXA4.. (PDF 197kb)
PDF79n Stiffness properties of softwoods undried or dried. (PDF 69kb)
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