Reinforced concrete knee joints are generally seen at the roof level of moment resisting frame and bridge bent frame structures. The knee joint core confinement provided by the surrounding members is less in comparison with interior and exterior joints. Both the column and beam members in a typical knee joint sub-assemblage are discontinuous, generating undesired axial forces in members which are compressive in closing and tensile in opening. Under the action of seismic loading, knee joints undergo successive closing and opening cycles, leading to vulnerable shear behaviour. Therefore, the shear strength of knee joints is generally expected to be lower than those of conventional interior and exterior joints. However, current seismic design codes of practice simply extend the des...[ Read more ]

Reinforced concrete knee joints are generally seen at the roof level of moment resisting frame and bridge bent frame structures. The knee joint core confinement provided by the surrounding members is less in comparison with interior and exterior joints. Both the column and beam members in a typical knee joint sub-assemblage are discontinuous, generating undesired axial forces in members which are compressive in closing and tensile in opening. Under the action of seismic loading, knee joints undergo successive closing and opening cycles, leading to vulnerable shear behaviour. Therefore, the shear strength of knee joints is generally expected to be lower than those of conventional interior and exterior joints. However, current seismic design codes of practice simply extend the design rationale from the conventional exterior connections to knee joints. This is primarily due to the lack of sufficient experimental work on the knee joints. An increasing number of failures of the knee joints call for a reconsideration of the validity of designing knee joints as per current design methods. As a result, there is an urgent need to conduct more investigations on the knee joints to study and understand joint seismic behaviour to provide more rational guidelines for seismic design.

In this thesis, a systematic investigation, comprising of a combined experimental and analytical studies on the behaviour and strength of reinforced concrete knee joints is presented. Eleven large-scale knee joint sub-assemblages were designed, constructed and tested under reversed cyclic loading as a part of this study. The effects of variable relative closing and opening joint shear inputs, joint geometry and beam-column eccentricity on the joint behaviour were studied in closing and opening actions. The overall behaviour of each specimen was studied in terms of several parameters namely joint shear capacity, energy dissipation and joint shear cracking. The detailed analysis showed that the shear capacity of experimental specimens, in both closing and opening actions, fell short of the joint capacity suggested by major seismic design codes.

Furthermore, specimens with relatively high opening shear input failed prematurely with lower closing joint shear capacities. Based on the experimental observations, suitable design guidelines for knee joint design were proposed. The contributions of the joint diagonal concrete struts to closing and opening actions were assessed through joint expansion measured through the strain in joint transverse reinforcement.

Based on the observations from the test results, a modified softened strut-and-tie analytical model was proposed to predict the shear capacity of knee joints. In the proposed model, due to their distinct force resisting mechanisms, closing and opening actions were treated separately. Several experimental findings were incorporated to ensure the suitability of the softened strut-and-tie model for knee joints. The model also satisfies force equilibrium, strain compatibility and constitutive relationships of cracked concrete. The comparison of predicted shear capacities with the existing experimental database showed the reliability and superior accuracy of the proposed analytical model for prediction of closing and opening shear capacities in knee joints.