Correspondence on Profiled Steel Sheet/Dry Boarding Composite Floors by Dr. H.D. Wright, Professor H

Mr A. C. G. Hayward (F) (Cass Hayward & Partners): The paper broadly demonstrates that external plate reinforcement glued to a loaded structure is as effective as if it had been firstly relieved of load. This is reassuring to engineers seeking economic strengthening measures. Relieving a structure of load (or stress) before strengthening can be prohibitively expensive. The economy and feasiility of glued plates could be further enhanced if traffic was allowed to use the structure while the glueing and curing process was actually taking place. Could the authors comment on this possibility and whether research is warranted or possible.

Jennings & Gilbert consider the advent of the computer to have been ‘one of the greatest catalysts for change in the revamping of structures curricula’. This is a view commonly shared by structural educators, but there is much less unanimity on the nature of the changes that are to be made. Most have welcomed the impetus given by the computer for the rationalisation and categorisation of analysis techniques into stiffness or flexibility based approaches. There has also been a widespread concern that students should be equipped to carry out and use computer analyses intelligently and appropriately. Opinion has, however, been divided on the most effective means of achieving this. D. Johnson

Lateral-torsional buckling of beams has been a topic of research for many years, yet a correct interpretation of the phenomenon under general conditions of moment and lateral end restraints is not well established. In the literature, Timoshenko's exact solution for the uniform buckling moment of a simply supported I-beam has been treated as a reference solution and the effects of moment gradient and lateral end restraint are modelled via an empirical modification factor which, when multiplied by the reference solution, gives the buckling moment under actual conditions. This approach has been continued for the last 40 years, and various studies have been published regarding the dependence of this modification factor on moment gradient assuming simple end conditions. The results of these studies are more or less similar because of the common underlying assumption of ignoring or approximating the warping rigidity of beams for simplicity of analysis. The need for a simple design formula rather than logic was the obvious motivation for extending Timoshenko's solution to the general cases of loading and end conditions. In the authors' opinion, current approaches simply ignore the fact that the lateral-torsional buckling is a superposition of two fundamental modes of deformation, i.e. lateral bending with warping and twisting, as outlined by Worthington. Thus the effects of boundary conditions regarding lateral displacement and twist combined with the moment gradient factor on the two modes should be studied separately and then superimposed according to certain rules. This interpretation advocates the need for two parameters to describe the effects of the stated factors on the two fundamental modes and their subsequent superposition to obtain the critical moment of the combined mode. This is in contrast to the current approach of using a single factor to incorporate the effects of all the variables. M.D. Pandey and Professor A.N. Sherbourne