Seismic performance of reinforced concrete beam-column joints strengthened by ferrocement jackets

Abstract

Beam-column joints are critical members for transferring forces and moments among the beams and the columns. Failure of beam-column joints may cause catastrophic collapse of buildings. As buildings located in areas of low to moderate seismic risk are traditionally designed without seismic provision, one of the characteristics of joints in these buildings is with no or insufficient transverse reinforcements. Such joints have limited shear capacity. Vulnerability of the joints has been demonstrated by post-earthquake reconnaissance. Thus, it is necessary to upgrade the deficient joints in order to survive under earthquake attack. The objective of this study is to develop efficient strengthening schemes for beam-column joints without seismic provision using ferrocement jackets. Three strengthening schemes have been proposed to enhance shear resistance of interior and exterior joints. In total, 16 specimens, including 5 control specimens and 11 strengthened specimens in 1/2- and 2/3-scale, were prepared and tested to failure under cyclic loading. The three strengthening schemes have been proved to be effective for enhancing the seismic performance of joints including improvement on peak strength, energy dissipation and stiffness. The first scheme is developed for interior joints. Ferrocement jackets are applied to enhance shear resistance of the joints while diagonal reinforcements are used to reduce the force transferred to the joint cores. Strength of mortar is the vital factor affecting the performance. Increasing axial load from 0.2fc'Ag to 0.6fc'Ag has detrimental effect on shear strength of the joints. The second scheme is developed for exterior joints using ferrocement jackets with skeleton reinforcements. Skeleton reinforcements are provided in the form of grid or diagonal reinforcements to reinforce the joints. High axial load (0.4fc'Ag) has a detrimental effect on the horizontal load carrying capacity, but its influence on joint shear strength is not significant. Further, specimens with diagonal reinforcements in ferrocement perform well as compared with that using grid reinforcements. In the last scheme, ferrocement jackets with one or two chamfer(s) are introduced to protect exterior joints through enlarging the effective joint area. Failure mode is shifted from joint shear before strengthening to beam flexural after strengthening. Ferrocement jacket with one chamfer is recommended due to less demand on space and with slight decrease in peak strength and energy dissipation as compared with using two chamfers. To predict the shear strength of beam-column joints, existing models overestimate the shear strength of joints under high axial load and ignore the contribution by ferrocement jackets. Using the experimental results, the softened strut-and-tie model is modified by adjusting the concrete strut. The modified model provides accurate prediction on the shear strength of joints strengthened by the proposed schemes including those subjected to high axial load. Finally, shear stress-strain envelopes for joint panel zones are proposed based on experimental results and shear strength model. The proposed envelops take into account of joint types, axial load ratios and strengthening. They are implemented in a macro joint element for simulation. The model with the proposed envelops for joint panel zones is validated by accurately predicting load-displacement responses for the joints with and without strengthening.