THESIS
2019
xxi, 249 pages : illustrations (some color) ; 30 cm
Abstract
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...[
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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.
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