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.

3.2_400
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 the answer is no. We have covered Kitemarking in Datasheet 34a 

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. 

Nails of a minimum diameter and bending strength have proven essential to meet BS Classes 6 and 8 together with steel nailplates (much more forgiving of high timber moisture content than diagonal wood bracing). However manufacturers need to test their designs before BS marking is applied. Manufacturers attempting to achieve Class 6 or 8 performance using clenched nails in wood diagonals (V or X braced designs) are unlikely to succeed. Contentious issues of repair-kits to upgrade old boxes, wood preservatives, whether new designs should include payloads above 1 tonne 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 66kb)
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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