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The Structural Engineer, Volume 84, Issue 23, 2006
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
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