The establishment of plastic design in the UK

Tests were made on steel-framed buildings under construction in London in the early 1930s and they revealed startling discrepancies between the values of stresses calculated by conventional elastic designers and those actually measured. It became apparent that very small defects in manufacture of construction could have a great influence on the actual state of a structure; moreover, these defects could not, by their nature, be predicted. It was at this time, however, that results were published on the collapse behaviour of steel structures—initial defects were 'wiped out' by plastic deformation, and the final collapse loads appeared to be independent of the defects. The 'plastic' method was investigated in the UK (in Bristol and Cambridge) from 1936 onwards, and by 1948 a clause was inserted into the relevant British standard permitting plastic design in steel. Further advances in the second half of the 20th century revealed the theoretical basis for simple plastic design. Elastic analysis is necessary for the calculation of deflexions and the investigation of stability, but the examination of 'plastic' strength uses only the equilibrium equations and knowledge of the yield stress of the material. The plastic design method could be renamed that of 'static equilibrium', and it is applicable to structures of reinforced concrete, of masonry, and indeed of any ductile material that a designer would consider safe in practice.