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The Structural Engineer

It was purely coincidental that a major theme of my own Chairman’s address to the Scottish Branch on 13 October 1994 was mirrored in the President’s address made only a week before. The fact that we both considered that communication is of vital importance to the future of the Institution was sufficient for me to tidy up the notes made for my address and submit them as a viewpoint article in the journal. Professor H.D. Wright

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Author – Wright, H D

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The Structural Engineer

Mr A. N. Beal (M) (Thomason Partnership) Reducing safety factors in design Codes is rather like tiptoeing towards the edge of an overgrown cliff. Initially, it is safe, but beyond a point there is danger and no one knows exactly where that is. The more that the safety factor is reduced, the greater is the risk that the danger line will be crossed, but we may not know that it has been crossed until many years after the decision, when buildings have ‘matured’ a little and had chance of being subjected to their full design loads. Unfortunately, by that time many thousands of buildings might have been constructed to the new standard, so the possible costs of a mistake are considerable. It should also be borne in mind that, although the risk increases as the safety factor is reduced, there are diminishing returns on the cost-saving side, as nominal reinforcement, defection limits, etc., increasingly govern design.

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The Structural Engineer

The main aim of this paper is to evaluate the structural behaviour of externally bonded, steel plated reinforced concrete beams which were initially exposed to the natural elements for periods of 11 to 12 years and then subsequently tested to failure in the laboratory. Twenty-one beams were tested, of which eight were kept under sustained load and the rest left unloaded during the exposure period. No maintenance or cleansing of the beams was carried out during exposure. The main variables investigated included thickness ofthe adhesive layer, number of steel plates, and number and location of plate laps. The beams were extensively instrumented prior to load testing, and concrete strains, deflection, cracking and steel plate strains were continuously monitored during testing through to failure. The structural behaviour of the beams at the service and ultimate limit states and their failure modes are reported and analysed. Judged on the basis of stiffness and failure loads, the performance of the beams after 11-12 years' exposure was very satisfactory. Many beams retained their structural integrity and ductility, and all the beams failed at loads of 1 % to 29% higher than those of the equivalent control plated beams. Professor R.N. Swamy, Professor B. Hobbs and M. Roberts

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Author – Swamy, R N;Hobbs B;Roberts, M

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The Structural Engineer

Timber truss design Various contributions have been received on aspects of timber design. Jack Baird, a well known Member, writing from Gainshorough in Lincolnshire is concerned about the mismatch between truss rafter design and Code span tables. He writes: Trussed rafters have been a remarkable success for the use of structural timber in the UK, though the thousands of man-days spent on discussing trussed rafters, if spent on other uses of structural timber, might have evolved other more exciting engineering uses. Verulam

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