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AD 323: Brittle fracture requirements for baseplates

When BS 5950-1 was first published in 1985, the limits on thickness in 2.4.4 Brittle fracture were not applied to grade 43 (S275) base plates. When BS 5950-1 was revised in 2000, this exemption was removed. Questions have been asked whether the removal of the exemption for S275 grade base plates was related to recorded failures. This AD gives some background information and the reason for the change.

When BS 5950-1 was originally drafted in the 1970s, thick slab bases for columns were frequently made from “blooms”; these were only available in grade 43A and there were no specified toughness requirements for this grade. In practice such material was usually of UK origin and had good toughness properties. Additionally, columns needing thick base plates were commonly designed in simple construction and had no base moments; the failure of such a base plate would then only cause local serviceability problems, rather than overall structural failure.

Today more columns have fixed bases and significant base moments, and thick plates are now available on the international market in various grades from a range of process routes, with corresponding variations in toughness properties. It was therefore judged that the toughness should be verified.

In practice, it would be unusual to have a high level of tensile stress on the welded upper face of a thick base plate; the K factors according to Table 3 would thus be 1.5 (for some modest tension) or 2 (for no net tension). For internal conditions (which applies to most base plates) grade S275JR would then be adequate up to 54 mm thickness (for K = 1.5) or 72 mm (for K = 2). Similarly, grade S275J0 would be adequate for base plates up to 97 mm or 130 mm thick.

Contact: A S Malik
Tel: 01344 636525

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