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The in situ performance of several reinforced concrete columns of an eight-storey structure during construction and later during occupancy and service over a period of 6 years is reported. The time-dependent movements occurring in the columns are discussed and the creep strains are predicted by two methods from a knowledge of the stress history of the columns and control tests exposed to the same environment as the structure. The predicted and measured in situ movements show good agreement. The implications of the long-term concrete movements in relation to the stresses and loads carried by the concrete and steel reinforcement are discussed. It is shown that the concrete stresses arising from imposed loads amounted to less than 15% of the dead load stresses induced during construction. Almost all of the concrete stresses from imposed loads were lost owing to stress relaxation and load transfer to the reinforcement within the subsequent 3 years of the structure's life. During the same time, the stress in steel increased by amounts varying from 44% to 106% of that due to the dead loads imposed during construction, resulting in four of the columns exceeding their steel design stresses. After 4.2 years in service, the load carried by the concrete decreased by amounts varying from 6% to 16%, whereas the load carried by the steel increased by 24% to 48% compared to their loads at the end of the construction period. It is shown that in situ behaviour of reinforced concrete columns is very complex and that it should be considered at the design stage.
R.N. Swamy and P. Arumugasaamy
I would first like to congratulate Dr. Brohn and Dr. Cowan on their excellent paper and the stimulation it has evidently brought about. Before proceeding to comment I should give some background about the educational institution with which I am connected.
In May we included a letter from Mr. N. Thomas, a South African member, in the continuing discussion about the judgement in the Anns case. He quoted section 23 of South Africa S National Building Regulations and Building Standards Act, 1977, which we took to indicate that local authorities there were absolved from responsibility for their duties in situations similar to Anns. We were mistaken. Mr. Thomas now writes to correct us: Thank you for publishing my letter re. our new building law here in the Republic. With regards to your comments/queries I must point out that our section 23 does not exempt a local authority from having to give redress for negligence. Common law still prevails. You asked what sanction is there to ensure that the local authority will carry out its duty with proper care? Here I refer to our section 27 which lays down that the Minister, after consultation with the Council of the South African Bureau of Standards and the Administrator of the Province in question, is enabled to order a local authority to apply any provision of the Act. If the local authority 'without reasonable cause fails to comply within a reasonable time' the Minister (after further consultation) can deprive that local authority of any power . . . conferred upon or entrusted to it by the Act. This deprived power is then conferred to 'any person, including the Administrator', and that person then becomes, for the purposes of such power, the local authority.