Seismic performance and shear strength of reinforced concrete beam-column knee joints

Abstract

Over the last three decades, the discipline of earthquake engineering has witnessed tremendous improvements and innovations in the seismic design of reinforced concrete (RC) frame structures. Notable advances have been achieved in the seismic behaviour and design of RC beam-column connections, in particular conventional interior and exterior beam-column joints. However, during the past two decades few investigations have been conducted on the seismic performance of knee connections, which are normally seen at the roof level of frame buildings and pier bents of an RC bridge. The structural response of RC knee joints subjected to earthquake-induced loading is presently giving a lot of concern to engineers as a result of their questionable performance. Presently, codes of practice (EC2&8, ACI 318-14, etc.) specify rules for the design of beam-column joints, where the guidelines provided are only for conventional interior and exterior joints, with little or no mention of the knee joints. An increasing number of failures of knee joints before the corresponding inter-storey beam-column joints have been perceived under earthquakes. Therefore, it is imperative to re-consider the validity of designing knee joints directly as conventional beam-column connections as per current design methods. This thesis presents a combined experimental and theoretical study on the seismic performance and shear strength of reinforced concrete beam-column knee joints. The experimental study consists of a series of reinforced concrete (RC) beam-column knee joints with different configurations including the different detailing techniques of joint reinforcement and the dimensions of the joint region. A Total of 10 large-scale beam-column knee joint specimens, which are fabricated to simulate those in as-built RC frame buildings designed in accordance to the provisions of ACI 318-14 and ACI-ASCE 352R-02, were tested under reversed cyclic loading. It was shown that the area of transverse reinforcement and member sizing of connecting beam and column had a significant effect on the shear strength and the ductility of the joints. However, none of these specimens had shown sufficient ductility energy dissipation capacity. Tested results are also compared with those predicted by five seismic codes of practice, namely, ACI 318, EC8, NZS 3101, GB50010, and AIJ. In general, the present seismic design codes cannot accurately predict the shear strength of beam-column knee joints under earthquake-type loading. Finally, an analytical model is also presented for analysing the shear performance and predicting shear resistance of reinforced concrete (RC) knee joints. The proposed method, termed as Modified Soften Strut-and-Tie Model (MS-STM), is partly derived from the experimental observations of knee joint specimens and partly modified from the soften strut-and-tie model. In the proposed methodology, the new strut-and-tie model satisfies different equilibrium conditions including all force components (axial forces, shear forces and moments) transferred from the beam and column framing into the joint, as well as the compatibility and the constitutive relationships of cracked reinforced concrete. The proposed model not only revealed the different mechanisms on resisting the input forces but also quantified the shear strength of knee joints under both closing and opening actions. The accuracy of the proposed model is validated by comparing the calculated shear strength with the experimental data available in the previous literature and a good agreement was observed. The proposed model can provide crucial insights in to the shear performance of knee joints under seismic loadings.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology