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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.
This paper describes three contracts whose brief, in each case, demanded a very rapid completion. Factors influencing the design, design control, some of the detailing, and the manner in which the jobs were organised in order to speed up progress are described. Some general conclusions are drawn. The paper concentrates on those matters related to the design process and the interaction of design with construction. It does not deal with equally important considerations related to the actual construction.