High-Strength Concrete Columns With Longitudinal Reinforcement of Mixed Steel Grades

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Title: High-Strength Concrete Columns With Longitudinal Reinforcement of Mixed Steel Grades

Author(s): H. Tanaka, Y. Sato, R. Park, and N. Kani

Publication: Special Publication

Volume: 149

Issue:

Appears on pages(s): 391-412

Keywords: columns (supports); compressive strength; confined concrete; ductility; earthquake-resistant structures; high-rise concretes; high-strength steels; loads (forces); moment-curvature relationship; shear strength; Structural Research

Date: 10/1/1994

Abstract:
In recent years, moment-resisting frames built using high-strength concrete have been used for high-rise buildings, primarily for economic reasons. When such high-rise buildings are subjected to severe earthquakes, cyclic horizontal and axial loading can be imposed on the exterior columns. The ductile behavior of such columns needs to be insured. In this study, improvement of the flexural ductility of high-strength concrete columns under high axial compressive load is attempted by arranging longitudinal bars with mixed steel grades. The basic concept of this method is to achieve the gradual attainment of yield of longitudinal bars, from low- to high-strength steel, as the column deflection increases, and thus to delay the column reaching the maximum moment capacity until the column deflection attains the required level. To verify the adequacy of the preceding design concept, six cantilever columns with 400-mm-square cross section have been constructed and tested under simulated severe seismic lateral loading with axial compressive load of either 0.3 f' c or 0.6 f' cA g. The compressive strength of concrete f' c was 65.7 MPa on average, and steels with yield strengths of 442 and 1033 MPa were used for longitudinal reinforcing bars. The adequacy of the preceding design concept was verified from the test results, and it was found that the New Zealand concrete design code could provide a good guideline for its application to design.