論文の公開元へPREDICTION OF JOINT DEFORMATION OF REINFORCED CONCRETE INTERIOR BEAM COLUMN JOINTS
概要
Beam-column joints play an essential role in the seismic performance of RC frame structures since the collapse risk of RC moment frames can be considerably increased by joint failure. Thus, the integrity of beam-column joints is essential for structural integrity due to the transfer of loads effectively between beams and columns in moment frames during earthquakes. To ensure joint integrity, the most modern seismic design codes such as ACI (USA), AIJ (Japan), EC8 (Europe), and NZS (New Zealand) have provisions for the seismic design of beam-column joints in RC moment frames based on extensive laboratory test results. These provisions were primarily based on the joint shear strength, regardless of joint deformation. This could be related to the fact that numerous experimental and analytical studies focused on the basic shear strength of beam-column joints over the past few decades. The results of these studies revealed that beam-column joints with high shear stress levels (i.e., heavily reinforced beams) tend to fail in shear, regardless of the amount of shear reinforcement within the joint. Therefore, it was more logical to limit shear stresses within the joint by comparing the shear demand to a nominal shear capacity to prevent the joint from failure.
Despite this significant achievement, detailed investigations on the deformation of beam-column joints and their effects on the lateral response of moment frames have been relatively limited. This could be because the joint failure mechanism was studied separately from the failure mechanisms of the frame members in the majority of the existing studies. In fact, the separation of the failure mechanisms is difficult in many cases since the design parameters of both the members and the joint influence the failure mechanisms. Thus, to address these issues neglected in the first generation of the analytical and experimental studies on RC joints, experimental and analytical investigations were conducted to supplement and refine the existing knowledge as well as to develop a practical approach to evaluate the effect of joint deformation on the seismic response of RC moment-resisting frames.
The first part of this research focused on the identification of main parameters that significantly affect joint behavior. To this end, an analytical study was carried out using the quadruple flexural resistance (QFR) model. According to the results of the analytical research, the column-to-beam flexural strength ratio and joint shear reinforcement ratio were found to be essential parameters apart from concrete strength and the amount of longitudinal reinforcement in the beam. They were selected as key test parameters. Moreover, the effect of joint shear reinforcement on joint behavior was found to be dependent on the quantity of beam longitudinal reinforcement and the flexural strength ratio.
A displacement-controlled cyclic loading test was conducted on eight half-scale interior joint specimens to investigate the combined effect of these two key parameters on the strength and deformation of RC interior beam-column joint connections. The cyclic performance of each test specimen was examined in terms of the lateral resistance, failure modes, strain distributions of the beam, column, and joint reinforcements, joint shear stresses, and deformation components. Some of the critical findings from the test are summarized below.
First, the test specimens with a larger flexural strength ratio of 1.5 exhibited better performance than those with a smaller strength ratio of 1.1, regardless of joint shear reinforcement ratio. Second, the increased amount of joint transverse reinforcement (steel percentage of 0.36% to 0.72%) gave less increase in the lateral strength and deformation capacity. Mainly, this phenomenon happened to the specimens with the largest amount of longitudinal reinforcement in the beam. The results were consistent with those of the analytical study. Third, all the test specimens exhibited joint failure. However, the failure was not caused by joint shear but joint moment because throughout the lateral loading, the shear stress-induced in the joint increased even if the width of the diagonal shear cracks on the beam-column connection increased. Fourth, the degradation of the lateral load resulted from not the degradation of the joint shear stress but the loss of anchorage capacity of the beam longitudinal reinforcement passing through the joint.
Although the experimental study provided valuable information on the behavior of the interior beam-column joints, no clear correlation was found between the test variables and the deformation of the beam-column joints. The final goal of this study is to develop a practical approach to evaluate the effect of joint deformation on the seismic response of RC moment-resisting frames. Therefore, a two-dimensional finite element investigation was carried out to examine the complex behavior of RC beam-column joints. The validation and calibration of the FE models were first done by simulating the test results. Subsequently, parametric investigations varying joint shear reinforcement ratio, strength ratio, joint aspect ratio, and area ratio of adjoining members were conducted on 39 full-scale FE models of the beam-column joint connection. Some of the important findings from the finite element investigation are summarized below.
The results of the FE analysis were consistent with the observations made from the experiment. First, the beam-column joint connections with a larger flexural strength ratio of above 2 exhibited good performance and a relatively full hysteretic loop, although the joint shear reinforcement ratio is 0.3%. Joints remained elastic throughout the lateral loading, and the plastic hinges formed at beam ends. Second, the joint shear strength was increased by 5 to 10% with respect to an increase in the shear reinforcement ratio (0.3% to 0.7%), but the joint shear deformation was reduced approximately three times due to an increase in the joint shear reinforcement ratio. Third, the reduction of joint shear deformation was dependent upon the column-to-beam flexural strength ratio and area ratio of the members framing into the joint. For instance, the largest decrease in the joint shear deformation took place in the specimens with the larger values of the strength ratio and area ratio, and vice versa.
Based on the results of the experimental and finite element studies, three simple equations were developed to predict the joint shear deformation index (SDI) of RC interior beam-column connections corresponding to three different types of failure (i.e., joint failure before beam yielding, joint failure after beam yielding, and beam flexural failure). These equations can be used for predicting the joint deformation contribution to the total story drift of beam-column joints under critical structural deformations. Compared with previously proposed models and theories, this method does not require complex nonlinear numerical analyses of the structure or sub-assemblage.
Finally, a nonlinear time history analysis was performed on three hypothetical RC perimeter frames in which interior joints were designed and detailed to exhibit three different failure modes, as mentioned above. The main purpose of this analysis was to ensure the reliability and effectiveness of the proposed approach in terms of multi-story RC moment-resisting frames under seismic excitation. Some of the important findings from the seismic response analysis are summarized below.
First, the column-to-beam flexural strength ratio of 1.45 or less was found to be insufficient to protect columns from yielding. Also, the joint shear deformation was observed to increase with a decrease in the flexural strength ratio. Second, there was no noticeable difference in the lateral response of the considered three frames for the seismic excitation with PGA of 0.25g and 0.4g corresponding to the serviceability limit state because they behaved elastically. However, as the seismic intensity was increased further, aiming at the ultimate limit state, the difference in the interstory drift existed. Third, Exterior beam-column joints performed much better than interior joints since the shear deformation was significantly smaller than that of interior joints. This better performance was attributed to a larger column-to-beam flexural strength ratio and lower shear stress level in the joint. Fourth, the shear deformation index (SDI) of interior joints of the first and second stories in three frames was predicted by the proposed three equations. A reasonable agreement was found. Despite some disagreement between the predicted and observed SDI values, the proposed simple approach can consider additional interstory drifts due to joint shear deformation. Therefore, the equations are deemed useful and practical to identify inelastic joints in RC moment resisting frames without doing nonlinear static and dynamic analysis.
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