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Issue 23/24

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The Structural Engineer

A parametric study has been carried out to investigate the range of possible bending moments that can be obtained when back-analysing field data from a concrete diaphragm wall. A recent opportunity to monitor the performance of a diaphragm wall in Cambridge has yielded data which has been used here as an illustrative example. The chief uncertainties are found to arise from the choice of concrete model, the evaluation of concrete parameters, and the difficulties in interpreting field measurements. A factor of nearly two has been found between the highest and lowest values of bending moment that can be inferred from these particular field measurements. The study has clear implications for future monitoring work. C.W.W. Ng, M.L. Lings and D.F.T. Nash

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The Structural Engineer

‘Evolution is a most efficient engineer’ said The Economist (10 November 1990, p136). Evolution, with natural selection (NS) as hypothesis for how it happens, amplified by genetic theories of heredity, is a biological process; it might well have value in engineering design, but it will be by analogy and not by imitation. Let us see what, as engineering designers, we are trying to do and whether and where this analogy might be of use to us; like the duellist in the blacked-out room who, out of humanity, discharged his pistol up the chimney and brought down his adversary nonetheless, we can profit from indirect approaches. Professor Sir Alan Harris

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The Structural Engineer

Mr A. C. G. Hayward (F) (Cass Hayward & Partners) Adhesive bonded stiffeners could well be a viable alternative to welding where fatigue loading governs design, as in bridge structures. Mere presence of welded stiffeners on a girder web automatically downgrades the bending stresses which can be used to those of a class ‘F’ detail under BS 54OO:Part 10. If such stiffeners could be attached by bonding (or other method which does not cause stress concentration), fatigue should rarely be critical, the classification being raised to at least ‘D’ because of presence of longitudinal web-to-flange welds or any butt welds to the flanges. Depending on stress levels, this would increase the fatigue life of the member by a factor at least 2, such that, if the predicted endurance was 60 years as controlled by class ‘F’ welded stiffeners, it would increase to 120 years if the stiffeners were bonded. Although fatigue is rarely critical in plate girder highway bridges designed to the fatigue spectra in BS 5400, it usually governs the working stresses in short-span rail bridges. But the situation with regard to highway bridges is worthy of review because load spectra in BS 5400 are becoming outdated owing to recent increases in intensity of road traffic.

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The Structural Engineer

The design and construction of buildings in the unused space above existing activities and structures has become an acceptable method of creating development sites in city centres and other areas where space is at a premium. The creation of these buildings is heavily influenced by the engineering. The ability of the engineer to respond in a creative way to demands that are outside the boundaries of normal commercial development is an essential ingredient to the success of any such project. The development of the design of the building over Charing Cross Station and the resolution of some of the design issues provides an interesting example of these engineered designs. M.A. Barrie and G. Weston

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The Structural Engineer

Conventional limestone concrete airfield pavements are prone to spalling as a result of jetblast from vertical takeoff and landing aircraft. This paper describes a collaborative research programme aimed at developing jetblast-resistant pavement quality concretes incorporating alternative cementitious materials and aggregates. Ten alternative mixes were evaluated by subjecting slabs to simulated Harrier jet-engine blast on the ground erosion test facility at British Aerospace, Warton. S.A. Austin, P.J. Robins and M.R. Richards

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The Structural Engineer

A series of experiments has been carried out in a purpose-built sand tank to assess the foundation/sand behaviour under load where the investigation is confined to the non-failure characteristics of the soil. The semi-full-scale foundation is of bolted steel box construction, instrumented to provide the contact stresses at selected points. Stresses and displacements are also measured automatically at various depths in the sand, using a dedicated computer system with a specially developed management suite of programs. The variation of displacements and stresses, once processed, are plotted against depth. The results are then compared with data from linear finite element method, using back-analysis techniques. Various parameters are then established and ascertained. R. Delpak, G.O. Rowlands and A. Siraty

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The Structural Engineer

Reinforced concrete columns are now often designed by computer, but design charts are still a popular alternative. Many sets of charts have been published for different Codes of Practice over the years, by various authors, but most of those for rectangular columns (e.g. BS81lO:Part 3' and the IStructE manual²) cover only columns that are reinforced with four bars placed near their corners. A.N. Beal and F.N. Pannell

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The Structural Engineer

Approval for Building Regulations - and the use of ‘structural design programs’ Mr R. R. Stagg, on 20 October, expressed concern regarding what he saw as the excessive reliance, in submissions for building control purposes, on calculations as presented by computer printouts from ‘structural design programs’. This topic has been taken up in a letter from Denis Wall of Dalkey in County Dublin: Being a late entrant to building control in the Dublin area a dozen years ago, I considered that the ‘checking of calculations’ was a very barren exercise, given that the builder never sees them. Verulam

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