Year

2014

Department

School of Civil, Mining and Environmental Engineering

Abstract

T connections (exterior beam-column joints) have long been recognised as the most critical components of reinforced concrete (RC) structures, especially structures that are subjected to reverse cyclic lateral loads derived from earthquakes. Recent earthquakes have showed that failures of connections in general and of T connections in particular, often lead to a partial or total collapse of the whole structure. For this reason, strengthening the beam-column T connections of RC frames built in seismic areas is essential, and why various strengthening techniques have been proposed in recent years. These techniques include epoxy repair, removal and replacement, concrete jacketing, concrete masonry jacketing, steel jacketing, using steel straps, and externally bonded fibre reinforced polymer (FRP). Of them, externally bonded FRP is an ideal technique for strengthening beam-column joints because it eliminates some disadvantages of the remaining methods such as complicated, expensive construction, and corrosion. Various methods of using FRP to strengthen beamcolumn connections have been proposed, but they still have limitations such as debonding the FRP from the concrete surface and a low FRP confinement effect at the core of the joint core which reduces its effectiveness.

Before an effective strengthening method can be developed, a thorough understanding of the behaviour of the as-built one is essential, but because the shear mechanisms in beam-column connections are complicated, the research community still does not fully understand their behaviour. Therefore, to develop an effective FRP strengthening method for T connections, this study commenced by developing an empirical model to predict the joint shear strength and then a theoretical model to understand the joint shear mechanisms. Finally, based on an understanding of the joint shear strength and joint shear mechanisms, a new method of strengthening RC T connections using Carbon fibre reinforced polymer (CFRP) and concrete covers was developed.

To develop an empirical joint shear strength model, 98 RC exterior beam-column connections were incorporated into a database that was then used to evaluate the key factors influencing the RC T connections, and a new parameter for the joint shear strength, referred to as the ―beam bar details index‖ was proposed. The consideration of this parameter significantly improved the accuracy of the proposed model. Although this model is relatively simple, it can predict the joint shear strength of standard and substandard RC T connections quite accurately, and therefore it has implications for the practical design of as-built RC T connections. Moreover, its prediction also helped the designer determine the necessity to strengthen the considered T connection.

In order to understand joint shear mechanisms, a theoretical joint shear strength model was developed, and immediately revealed that columns close to the joint core can resist a significant amount of the shear force created by the tensile forces stemming from the beams’ longitudinal bars. This is a special feature of this proposed model because most existing models assert that this tensile force can be transferred and is only resisted by the joint core. Moreover, this model can not only predict the joint shear strength accurately, it can also predict the failure mode of RC T connections. These will help a designer to double check the joint shear strength and determine which parts of the connection must be strengthened.

Based on an understanding of the joint shear strength and joint shear mechanisms, a new method of strengthening RC T connections using Carbon fibre reinforced polymer (CFRP) and concrete covers was developed and then evaluated with an extensive experimental investigation on four full scale RC T connections, including one control and three strengthened specimens. The results showed that although this method is relatively simple, it can eliminate debonding and/or bulging of FRP from the concrete surface and the low confinement effect of existing FRP-strengthening methods. Therefore, this proposed method is efficient and can be used to strengthen RC T connections.

To apply this method for practical strengthening, a joint shear strength model for FRP-strengthened T connections was developed based on the proposed theoretical model for the as-built connections and then calibrated using a database consisting of 32 FRP-strengthened connections. This model is quite simple but it can predict the joint shear strength of FRP-strengthened T connections quite accurately. Finally, a design procedure for the proposed strengthening method was also established, and by following this procedure the required amount of FRP needed for a particular application can be designed.

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