People: Structure: Engineering - Standing on the Shoulders of Giants

Hipped Roofs and Rafters This is a subject clearly of continuing interest to members. Mr M. Bowden, from Bromley in Kent, writes: Having read the two latest comments, I find that my analysis of the forces involved are quite different to their proposals: e.g. I cannot see how a purlin force applied perpendicular to a rafter can induce an axial force in it. Could I suggest that the use of purlins makes the triangular shape of the roof a redundant structure? My reason for saying this is as follows. With normal 4in x 2in (yes: imperial: do you remember it?!) rafters, the triangular structure is stable, the compression forces are small, but the span from the ridge to the eaves gives great bending overstress and deflections in the rafters. These are reduced by propping the centre of the rafter with a purlin. The carpenter has to position the purlin carefully, otherwise he will be forcing the rafter outwards and it will become disjointed at either the eaves or the ridge. Obviously, the purlin would be positioned before the load is applied, but the need for careful positioning is the same. (See Figs 1,2.)

This paper describes the inspection, assessment and strengthening design of the Puente Duarte Suspension Bridge, which is undergoing a major rehabilitation and strengthening programme. The bridge carries a four-lane highway and comprises a conventional steel truss and concrete deck, supported by steel towers. Significant corrosion was found on primary structural elements such as the towers and main cables which, combined with strengthening requirements, has resulted in an extensive reconstruction project. H.R. Hobbs

The history of structural engineering in the last 100 years divides neatly into two periods separated by World War 2. Each period is best treated separately because, although there was innovation in each, the changes in the last 50 years greatly exceeded those in the first 50 years. R.J.M. Sutherland