The President 1969-70

Mr. M. H. Rees: 'This is an extremely interesting structure and I have really just three questions out of the very numerous thoughts that have occurred to me, mostly concerned with the staging of prestressing and the adjustment of arch thrust on decentering. Both of these go to make tools invaluable to the designer in making large span concrete structures economic.'

Mr. F. C. Greenfield : ‘The design-or examination of another’s design - of a modern structure calls for “professional excellence” comparable with any other of man’s skills; it is high time the layman was aware of this. Legislation is badly needed to ensure that the public are protected through structural design, examination of structural design (on behalf of the local authority) and also supervision on site-all under the direction of Chartered Engineers recognised in their profession for their skill in this field. Local authorities should ascertain that they have the necessary professional excellence in their staff; if they do not they should either attract the right men or engage Consulting Engineers-or do both. For those who rely entirely on their own staff, full responsibility must be shouldered by the engineer-Chief Officer.'

It is shown that an examination of the deformation of a free cable suggests an array of diagonal stays or braces as a means of reducing the deflections. The braces prestress the cable which, since it behaves nonlinearly, produces an increase in the stiffness. Bracing is similar to gravity loading in that it increases the vertical forces acting on the cable. The increased stiffness is thus analogous to gravity stiffness. A theory is presented for analysing this type of structure, and a numerical method of solving the governing equations is given. An experimental investigation verifying the theory is included and finally a theoretical discussion of the significance of the main parameters is given. T.J. Poskitt and P.R. Stott