Non-seismically designed reinforced concrete beam-column joints strengthened by unsymmetrical chamfers

Abstract

Although Hong Kong is a region with moderate seismic risk, there was no seismic consideration of low-rise reinforced concrete structures designed according to pre-2004 Concrete Code. Beam-column joints (BCJs) as the most critical component of frame structures have several deficiencies, in particular absence of joint shear reinforcements. As the result, old buildings may not have sufficient joint shear capacity which may lead to collapse of the whole buildings. Thus, it is necessary to develop practical ways to strengthen the existing buildings, especially for lifelines facilities like hospital, public transportation stations, fire stations, etc. They will have to be upgraded to meet the soon-to-be-enforced seismic requirements on old buildings. In this study, a strengthening strategy for BCJs is proposed by installing unsymmetrical chamfers on the soffit of beams. There is no alteration to the building plans and chamfers can be constructed easily within hours. To investigate structural behaviour of non-seismically designed BCJs strengthened with chamfers, both experimental studies and numerical studies were conducted to identify the load-transfer mechanism. In the experimental studies, seventeen 2/3 scale specimens of interior and exterior BCJs include control specimens, strengthened specimens, specimens with joints shear reinforcements and joints with column eccentricity were prepared and subjected to cyclic horizontal displacement under constant vertical load until failure. As shown by the experimental results, unsymmetrical chamfers can effectively improve joint shear capacity and hinder the joint shear failure. Chamfer contributes to joint shear capacity when it is under compression. Chamfer size is the critical parameter influencing the structural beahviour and chamfer can be unreinforced. Eccentricity generated by unequal dimensions of column has an opposite influence on joints when subjected to cyclic loading. Numerical studies were conducted through nonlinear finite element analysis to provide thorough understanding of load-transfer mechanism of BCJs with chamfers. Based on the principal stress contours, an additional strut is observed inside chamfer in additional to strut within joint area. Based on parametric studies on chamfer size, width of the additional strut increases with increasing chamfer size. It effectively improves diagonal strut of which is the primary mechanism of unreinforced BCJs to resist joint shear force. Based on the above, a modified softened strut-and-tie model to estimate joint shear strength of BCJs with chamfers is proposed. Depth of compression zone of beam is introduced to compute the diagonal strut. Finally, design rules are recommended for the strengthened strategy and load-transfer mechanism of both interior and exterior BCJs with chamfers are formulated.