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Issue 23/24

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The Structural Engineer

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The Structural Engineer

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The Structural Engineer

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The Structural Engineer

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The Structural Engineer

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The Structural Engineer

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The Structural Engineer

My gratitude is tinged with surprise. Looking back, there are the 23 research papers published in our Journal, The Structural Engineer. It’s not long since a Member, unhappy with its content, complained that on average, such papers are read by only three people. Well, the author reads it, and there are a couple of assessors, so he was probably right. Many firsttime authors are amazed by the total silence that greets most publications of results of their research. Another recollection is of over 30 years’ work on Eurocodes. Is there any recent topic less popular with our Members? It seemed more likely that I would face our Disciplinary Committee, rather than this distinguished gathering. Nor can I match achievements of recent medallists. For example, John Roberts provided safely the thrills of rollercoasting; John Burland kept safe one tower, Big Ben, and made safe another, at Pisa; and Leslie Robertson designed some of the world’s tallest buildings. R. P. Johnson MA, FREng, FIStructE, FICE Emeritus Professor of Civil Engineering University of Warwick

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The Structural Engineer

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The Structural Engineer

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The Structural Engineer

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The Structural Engineer

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The Structural Engineer

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The Structural Engineer

This paper makes use of data obtained from tests on simplysupported reinforced-concrete beams with an overhang, which were designed in compliance with the earthquake-resistant design clauses of the European codes EC 2 and EC 8. The test data reveal that increasing the amount of link reinforcement within the critical regions beyond the amount required to safeguard against shear failure does not lead to beam behaviour that satisfies the performance requirements for strength and ductility specified by the codes. This experimental evidence is complemented by an analytical investigation based on nonlinear finite-element analysis, which not only shows that the causes of the measured and observed specimen behaviour relate to the brittle nature of concrete at the material level, but also demonstrates that imparting concrete with strain–softening characteristics – through, for example, the inclusion of fibres – produces a significant increase in both loadcarrying capacity and ductility. Prof. Michael D. Kotsovos Dipl Ing, PhD, DSc Eng Department of Civil Engineering, National Technical University of Athens, Athens 157 73 Prof. Milija N. Pavlovic BEng, MEngSc, PhD, ScD (Cantab), CEng, FIStructE, FICE, FConsE Department of Civil Engineering, Imperial College, London SW7 2BU

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The Structural Engineer

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The Structural Engineer

The web crushing rules in EN 1992 are investigated by comparison with tests. Although they give significant increases compared with past practice, they appear to be largely valid within the range of tests. There are, however, some very marginal values and some slightly below the safe ratio of tests to predictions of 1.0. These are all for cases where the 80% yield rule governs. It is therefore recommended that this is made slightly more conservative. The rules allow greater web crushing stress with inclined links. With links at 45° the maximum force is doubled and can be as much as 3.5 times that allowed by BS 5400 Part 4. The rules also allow the full web crushing stress irrespective of slenderness. There do not appear to be tests to justify these large stresses. Further testing is recommended and suggestions for restrictions on shear stresses in slender webs and with inclined links are proposed for the interim. The proposals from the work modifying the rules for web crushing have been adopted in the National Annex to Parts 1 and 2 of EN 1992. Paul Jackson BSc, PhD, CEng, FIStructE, MICE Technical Director, Gifford Stephen W. Salim BEng (Hons), PhD Scott Wilson, ex-Gifford

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The Structural Engineer

Current knowledge of the behaviour of reinforced concrete appears to be not good enough to enable us to design basements which do not crack and leak. The paper discusses the possible reasons for this, and makes proposals for better designs in future. This starts with the life cycle of concrete and the importance of restraint. Formulae for minimum reinforcement contents to control cracking in both immature and mature concrete are then derived. This is illustrated by a flow chart. The misleading advice in codes of practice and industry guides is discussed, and the true tensile strength of concrete is analysed. Although the outcome is a recommendation for higher reinforcement contents than are currently employed, it is based on a sound theoretical approach backed up by the evidence of basements which still crack and leak. Furthermore, if cracking is not controlled, all the crackcontrol reinforcement that is in the concrete has been wasted. A reversal of traditional design philosophy is proposed, with the concrete section first selected at minimum thickness and then adequate reinforcement provided. Stuart J. Alexander MA, CEng, FIStructE, FICE, MCMI WSP Group

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