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Shear Strength and Deterioration of Short Reinforced Concrete Columns under Cyclic Deformations

H. Umehara and J. O. Jirsa

1983

Short column failures have been observed following many earthquakes. Short columns can generally be limited to canes where the shear span-to-depth or thickness ratio is less than about 3. There has been research conducted on such members but many variables have not been evaluated systematically.

A series of ten short columns with rectangular sections (9x16 in.) were tested in this study and compared with the results of square columns (12x12 in.). Loading history and level of axial loads were the main variables. Columns with sqaure sections were tested under cyclic bidirectional lateral loadings and reported previously by other researchers. Based on the tests, it was conclude that the maximum capacities of columns subjected to diagonal unidirectional loading along the principal axis. In addition to strength considerations, short columns must be evaluated in terms of their energy dissipating (hysteric) characteristics.

An empirical equation for the shear strength of columns was derived using data from continuous beams falling in shear. The equation was calibrated using results of short column tests. The equation is a function of the shear span-to-depth ratio, the concrete strength, the core area of the column and the axial load. The effect of transverse reinforcement was not included because the short column tests indicated that increased amounts of transverse reinforcement had a minimal influence on shear capacity. More than 70 percent of the total shear capacity was contributed by the concrete. To prevent rapid strength degradation, the shear capacity of a column must be greater than the capacity corresponding to the development of flexural hinges at the ends of the columns. Columns evaluated using existing procedures for shear strength did not always exhibit satisfactory strength and energy dissipating capacities were maintained under severe reversed loading conditions.

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