Author(s)

Anis Farah

Abstract

Column-like components such as elevator shafts, stairwells and columns have significant impact upon the structural integrity as well as the cost of tall buildings. The main costs include that of concrete (furnish and place), steel reinforcement (furnish and place bars, splices, dowels and ties) and formwork (furnish and place). For typical columns designed economically according to ACI Code, the concrete contributes around 40% of the cost of square (55% of circular) columns and the corresponding contributions of reinforcement and formwork are around 25% for square (25% for circular) columns and around 35% for square (20% for circular) columns respectively. While the above data is considered reasonable for the United States and Canada, it is expected to vary in other countries where the relative labour and material costs are likely to be different. Due to the large number of combinations of column sizes, reinforcements and concrete compressive strengths, it is beneficial to choose a design that minimizes the cost while meeting the building code requirement. In this paper, an efficient numerical technique is described for the analysis and design of column-like reinforced concrete components under load and biaxial bending. When combined with the costs of labour and material, the procedure yields design and cost curves which enable the engineer to arrive at an economical design. Based on available data, it is shown that for a given concrete compressive strength, the cost/unit concrete area of the column behaves as a linear function of the square root of the vertical reinforcement. It was also observed that for various shapes of columns, the lowest cost corresponds to a vertical steel ratio in the neighbourhood of 1%. Cost optimization examples are given for square, rectangular and circular columns in addition to an elevator shaft in a tall building.

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