STATIC BEHAVIOR OF RC COLUMN JOINTS AS AFFECTED BY CHANGING OF ITS GRADE OF CONCRETE AND DETAILS OF REINFORCEMENT

Abstract

The columns of high-rise structures are usually constructed from high strength concrete and the connecting beams and slabs at floors are of normal strength concrete. Consequently, the beam column joint is of normal strength concrete. The purpose of the study is to investigate the strength of high strength concrete columns as affected by the low strength concrete of connecting beams, and to study the effect of reinforcement details of the joint of high strength concrete column and normal strength beam on failure mode of such column and its strength. Experimental study was carried out to investigate the effect of changing the grade of concrete on the behavior of RC column and its joint considering the interaction of the column with the floor beams. Floor beams and the joint are constructed from normal strength concrete, while the other parts of the column below and above the joint are constructed using different grades of high strength and normal strength concretes. Another significant parameter affecting the behavior of such columns is the details of reinforcement of the joint represented by horizontal or inclined stirrups. Twelve specimens of RC columns and intersecting beams are prepared and tested under axial compressive loading on the column as well as under vertical load on the beam. The results include the failure mode, strain distribution, ultimate load and strength of the columns considering the change of concrete strength and the details of reinforcement. It has been found that failure mode of such structures occurs at the joint because of its low strength which has been constructed with the floor beams. It has been shown that inclined stirrups at joints do not change the failure mode even it reduces its severity. Other steel arrangement is still needed to change the failure mode. However, inclined stirrups have significant influence on increasing the ultimate strength of the columns. This is because the inclined stirrups cause an increase on the shear strength of the column resulting in the increase of the strength. The strain distribution is highly affected by the interaction of beam and the column. Maximum strain occurred at the joint due to combined state of stress. An equation was deduced to estimate the strength of axially loaded columns considering the interaction of floor beams, variation of concrete strength and details of reinforcement. The deduced equation is applicable to normal strength and high strength concrete.