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The stresses which in theory can occur in a simply supported structure owing to temperature distributions are, as demonstrated by the author, significant in terms of the total load effects. However, since the stresses are caused by the self-restraint of the section, it would seem logical to expect some reduction of stress with the onset of cracking, whether caused by the effects of the temperature distribution itself or by external loading. The method by which allowance for cracking should be included in the temperature stress calculations is not clear, although the technique
adopted would seem to be a logical approach. Indeed, if the stresses obtained (Fig 5(h)) are compared with those calculated assuming an uncracked section, there is a reduction in the top surface compressive stress of about 13% using the cracked section. However, if the stresses calculated at a depth of 360 mm using the two methods are compared, it can be shown that using an uncracked section results in a tensile stress of 0.6 N/mm2 compared to 1.1 N/mm2 for the cracked section. It would
seem that there is a need for further research in this area.
A description of the development of an interactive computer program to design and detail reinforced concrete structures is given. The program provides facilities that enable complete reinforcement details to be generated in situations in which details cannot at present be completed by automatic programs. An example of the use of the program is included. The work forms part of a continuing research project, and future developments are outlined.
P. Mills and D.M. Brotton
A requirement to check the widths of load-induced cracks is now a feature of current British Codes for structural concrete. However, the theoretical background to the procedures given in the Codes has not been published in a readily available and reasonably condensed form. This paper attempts to rectify this situation by presenting the derivation of a theory for the prediction of cracking in hardened concrete. This theory is shown to be a logical development of earlier theories, and is based on the extensive research program carried out at the Cement and Concrete Association over the last 14 years. The theory forms the basis of many Code crack prediction equations, and the derivation of these is discussed.