Earthquake resistance of reinforced concrete corner beam-column joints with different column axial loads under bi-directional lateral loading

Abstract

The seismic performance of a corner beam-column joint in reinforced concrete frames was studied by testing two three-dimensional corner beam-column subassemblage specimens without slabs under constant column axial load and bi-directional lateral cyclic load reversals. The column-to-beam flexural strength ratio was varied from 1.4 to 2.3 by changing the magnitude of column axial load. Although a sufficient margin to prevent shear failure was provided to a corner beam-column joint in the test, the subassemblage specimens failed in joint hinging after beam and column longitudinal bars and joint hoops yielded. The ultimate joint hinging capacity of a corner joint under bi-directional lateral loading was enhanced by an increase in column compressive axial load, and can be estimated based on the new mechanism proposed by Kusuhara and Shiohara. INTRODUCTION A new mechanism of joint hinging was proposed by Shiohara [1], a professor at the University of Tokyo, Japan, for a beamcolumn joint in reinforced concrete (RC) moment-resisting frames. The joint hinging mechanism is observed in laboratory tests when an ultimate flexural capacity of a column section is close to that of a beam section in an RC unit frame. A joint hinging model proposed by Kusuhara and Shiohara [2] is shown in Figure 1 for a plane exterior beam-column joint. An exterior beam-column subassemblage is divided into three elements; an upper column, a lower column and a beam. Each element rotates like a rigid body as shown in Figure 1, forming a principal diagonal crack along a diagonal compression strut in a joint and a short diagonal crack developing from a reentrant corner in a tesion side. Recent experimental studies to verify the joint hinging mechanism have been conducted using 2D plane interior [3] and exterior [4] beam-column subassemblage specimens. There are, however, few tests which use 3D beam-columnjoint subassemblages with orthogonal beams to each other which frame into a column such as a corner beam-column joint [5]. The previous study [5] dealt with not joint hinging failure, but beam flexural yielding. For corner columns in RC buildings, a loss of capacity to sustain column axial load resulting from severe damage to a corner joint has resulted in partial story collapse of the buildings in past earthquakes as illustrated in Figure 2 for the 1993 Guam Island Earthquake. The ultimate flexural capacity of a corner column frequently decreases during an earthquake because the axial load on the corner column cyclically increases and decreases by change of direction of lateral loads induced by earthquake excitations. Therefore, it is of great importance to investigate earthquake resistant performance of a corner beam-column joint subjected to tri-directional earthquake loading. Therefore the seismic performance of a corner beam-column joint in RC frames was studied, focusing on joint hinging mechanism, by testing two three-dimensional beam-column subassemblage specimens without slabs under both constant column axial load and bi-directional lateral cyclic load reversals. Figure 1: Joint hinging model for an exterior joint [2]. Figure 2: Failure of corner beam-column joint in Guam Island Earthquake in 1993.