Construction, Insulation and Weatherproofing of Flat Roofs

Steel purlins in BS 449 -empiricism? Mr R. C. Hairsine has written from Dorset seeking the advice of our readers on the origin of a formula in BS 449. His letter reads: Verulam

Some of the long-standing teaching problems in structural engineering degree courses are associated with the principle of virtual work. This paper presents a simple proof of the principle for a general deformable body. The proof, which is sufficiently simple for undergraduate teaching, is based on two simple results in engineering mechanics and the exploitation of the concept of virtual displacements to include the use of physically impossible displacements. It is hoped that the paper will contribute towards the teaching of the theory of structures in the universities and polytechnics, and help to remove the misunderstanding of the principle of virtual work which is, at present, widespread among structural engineering students. F.K. Kong, J.M. Prentis and T.M. Charlton

Mr T. Lawson (Dept. of Aeronautical Engineering, University of Bristol): I should like to congratulate the authors on a most interesting paper. Could Mr Croft tell us how he separated typhoon and non-typhoon meteorological data and about his approach to joint populations.