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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
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