Performance Based Earthquake Design/Analysis Methods for Reinforced Concrete Beam-Column Connections

Abstract

In reinforced concrete moment-resisting frames subjected to cyclic loading, the cyclic response, including stiffness degradation, strength degradation, and energy dissipation, is significantly affected by the behavior of the beam-column joints. In the present study, for performance based earthquake design methods of reinforced concrete structures, analytical and experimental studies were performed to evaluate the earthquake response and seismic performance of the beam-column connections.

The cyclic behavior of the beam-column connections is significantly affected by bar bond-slip and joint shear deformations. An experimental study was performed to evaluate the seismic performance of beam-column connections using grade 600 MPa bars for beam flexural reinforcement. Full scale four interior connections and three exterior connections were tested under cyclic lateral loading. The specimens were designed according to the special seismic provisions in ACI 318-11. The structural performance of the specimens with 600 MPa D22 and D25 bars was directly compared with that of the specimen with 400 MPa D25 bars. In the case of the interior connections, the load-carrying capacity and maximum deformation were close to those of the specimen with 400 MPa bars. On the other hand, the energy dissipation capacity of the specimens with 600 MPa bars decreased by a maximum of 25% due to the increased bond-slip at the joints. In the case of the exterior connections, significant bond-slip occurred at the beam bottom bars due to insufficient development length, which decreased the deformation capacity and energy dissipation capacity of the specimens.

To predict the bond-slip of beam flexural bars in the joint, a bond-slip model was developed. The proposed model estimated the bond-slip relationship using simplified bond strength and bar strain distribution in the beam-column joint. The bond strength was defined from the existing test results of beam-column connections that showed complete bond failure. For verification, the prediction of the proposed model was compared with existing test results of concrete block specimens for bond-slip and beam-column connection specimens. The result showed that the proposed model predicted well the bond strength degradation and bond-slip in the beam-column joints.

On the basis of the bond-slip model, a joint shear strength model addressing the effect of bond-slip of beam re-bars was developed. The proposed model consists of truss mechanism and diagonal strut mechanism. The developed bond-slip model of beam re-bars was implemented in the proposed model. For verification, the predictions of joint shear capacity and deformation capacity were compared with existing test results of 64 interior beam-column connections. The result showed that the proposed model predicted the joint shear strength degradation and deformation capacity with reasonable precision.

Using existing test results of 69 interior and 63 exterior connections, the variation of energy dissipation (per load cycle) according to the bond-slip and joint shear strength was statistically investigated. The results showed that the energy dissipation correlated with the parameters of the bar bond-slip, better than with the joint shear strength. On the basis of the result, the energy dissipation of beam-column connections was defined as the function of the bond parameters. By using the energy function and the Pinching 4 model of OpenSees, an energy-based hysteresis model was developed, such that the area enclosed by the cyclic curve is the same as the predicted energy dissipation. The proposed model was applied to existing test specimens. The predictions were compared with the test results, and showed good agreement.

On the basis of the developed energy-based model with various energy dissipation capacities (κ = 0.2, 0.4, 0.6, and elasto-perfectly plastic for beam-column connections

and κ = 0.4 and elasto-perfectly plastic for columns on ground), nonlinear dynamic analysis was performed for three types of low-rise moment frame structures. The results showed that the energy dissipation capacity degradation in the joint increases the lateral drift and ductility demand of moment frame structures. Furthermore, in the moment frame structures with shorter natural period, the lateral drift was increased. For larger yield strength reduction factor, structure performance was greatly affected by the energy dissipation capacity of the structure

Finally, to restrain the bond-slip and improve the structural performance of beam-column connections, relocated plastic hinge methods were proposed. Cyclic load tests were performed for beam-column connections strengthened with 45° bent bars and 90° hooked bars to confirm the effects of plastic hinge relocation. The test results showed that despite small hc /db values less than 20, by using the strengthening methods, the bar bond- and shear strength-degradations in the joints were substantially decreased. To address the enhanced performance, the bond resistance of the beam flexural bars and the joint shear strength were redefined considering the details of the strengthening methods so that engineers can design the strengthening methods according to current design codes for conventional beam-column connections. On the basis of existing test results, the seismic design and detailing of beam-column joints with strengthening bars were recommended.