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In most design codes (1,2,3) provisions for the design of confinement reinforcement contain empirical constants that were based on the experimental data available in the literature. Most of the data used was from tests in which normal strength concrete columns with square cross sections were used. Only recently, a limited amount of experimental data on high strength concrete (HSC) columns bas become available. Experimental data on rectangular HSC column behavior, on the other hand, is rarely found in literature, especially on large size HSC specimens tested under moderate to high axial load levels and subjected to large inelastic displacement excursions. This paper presents results from a continuing research program which aims to study confinement of concrete by lateral reinforcement. The current work deals with the experimental behavior of HSC (52 MPa < fc’< 112 MPa) columns having rectangular cross sections. Large-size columns (250 x 350 x 1473 mm) with heavy stubs (508 x 762 x 813 mm) were tested under moderate to high axial load levels and reversed cyclic displacement excursions. Effects of several variables such as section geometry, axial load level, and amount of lateral reinforcement on the behavior of these specimens are studied, and the responses of the rectangular specimens are compared to those of specimens with square cross sections. It is concluded that HSC columns having square and rectangular cross sections can be designed to behave in a ductile manner, provided that sufficient amount of confinement reinforcement is used in an efficient configuration.

Synopsis: Components of most concrete structural systems, such as slabs, long span thin shells, containment vessels and protective structures are stressed in multiaxial states of stress. This study explores the behavior of high strength high performance concrete under hiaxial loading in comparison to uniaxial loading conditions, and to propose a modified Elastic Modulus expression for concretes under biaxial loading for cylinder compressive strengths above 12,000 psi (82 MPa). In excess of 100 high-strength cube specimens in several series were tested to failure under uniaxial and biaxial compression. Ratios of the minor to major principal stresses (o2/o1) were selected as a major test variable. From the test results, it is shown that confinement stress in the minor principal direction has a pronounced effect on the strength and deformational behavior in the principal direction. Both the stiffness and ultimate strength of the concretc increased to a value of approximately 30 percent. Crack development in the tested specimens under biaxial compression progressed into asymptotic tensile splitting cracks along the o2 direction. A mathematical model and an empirical equation were developed for the elastic modulus ofconcrete under biaxial loading as a result of these tests.

This paper presents the research work, experimental and analytical, on precast hybrid concrete beam-to-column connections at the National Institute of Standards and Technology and its contribution to the development of seismic design provisions for precast concrete frame structures.

The use of high-strength concrete in construction industry has expanded in recent years for its superior strength and performance. However, many aspects of structural design for high-strength concrete columns remain to be developed. Of fundamental importance is the development of a rectangular stress block that is applicable to high-strength concrete. The current rectangular stress block parameters, intended for normal-strength concrete, are not suitable for use in high-strength concrete columns. A new rectangular stress block is presented in the paper for the computation of column strength under combined flexure and axial compression. Strength and ductility of concrete are inversely proportional. Therefore, high- strength concrete columns exhibit brittle characteristics, developing sudden and explosive failures under concentric compression. Therefore, the design of high-strength concrete columns becomes a challenge, especially for seismically active regions. While column ductility can be increased through confinement, the ACI 318-02 (1) confinement requirements are intended for normal-strength concrete columns and are not applicable to columns cast from high-strength concrete. A displacement based design procedure has been developed for the confinement of high-strength concrete columns. The procedure is presented in the paper with related design expressions. Summary of experimental findings on inelastic deformability of high-strength concrete columns is also presented with the effects of design parameters highlighted.

Design requirements for composite construction in steel and concrete as practiced in the United States are reviewed. Included are buildings and highway bridges. After a brief account of the origins of composite construction in America, an emphasis is placed on the early rules issued by ACI and AISC for composite columns in buildings, by AASHO - the predecessor of AASHTO-for composite beams in highway bridges and by AISC for composite beams in buildings. All four sections include outlines of subsequent changes that have taken place over the years. The paper is concluded with a discussion of a potential decrease in the strength of a stud shear connector located in the trough of a steel deck.