Behaviour of RC beam-column connections retrofitted with FRP strips

Publication Type:
Thesis
Issue Date:
2009
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Reinforced concrete (RC) buildings designed prior to the implementation of seismic design codes were largely designed without structural details required for satisfactory seismic performance. These buildings are therefore more susceptible to damage in case of an earthquake and hence may require strengthening to meet current design standards. For instance, the majority of buildings that may require strengthening in Australia are those designed prior to the implementation of Australia’s first earthquake design standard in 1979. Non-seismic load designed RC framed structures may possess several inherent weaknesses when subjected to a seismic attack. One major design deficiency is inadequate or no transverse reinforcement (otherwise known as stirrups) in the joint region of the RC beam-column connections (herein referred to as connections) which may lead to joint shear failure in a seismic attack. It is important here to establish that a joint refers to the intersecting region of the column/s and beam/s while a connection refers to the joint region plus surrounding beam/s and column/s. Another inadequacy is the practice of not anchoring the bottom beam longitudinal bars which may lead to bar pull-out due to load reversal in the case of a seismic attack. The proportioning of the beam and column elements framing into the connection region may lead to the undesirable formation of a strong-beam-weak-column mechanism. In addition, plastic hinging may form in the beam or column adjacent to the joint region thus compromising the strength and integrity of the joint. Past earthquakes such as El Asnam (1980), Mexico (1985), San Salvador (1986), Loma Prieta (1989) and Turkey (1999) have shown the vulnerability of RC framed structures with inherent non-seismic load designed weaknesses to catastrophically collapse due to connection failure. The proper strengthening of connections with such inherent weaknesses is in urgent need in order to ensure the safe performance of RC connections under seismic attack. Over the past decade or so, fibre reinforced polymer (FRP) composites have emerged as a viable solution for strengthening structures due to their superiority in strength-to- weight ratio, ease of handling and forming into shapes, and corrosion resistance whencompared with more traditional construction materials such as steel. Many experimental, analytical and numerical studies have been conducted on the strengthening of RC beams, columns, slabs and walls as well as various other structural elements with FRP. Many field applications have also been reported around the world. Significantly less research and field applications by comparison have been reported on the strengthening of RC connections with FRP composites. Experimental studies on FRP-strengthened exterior and interior connections have demonstrated the ability of externally bonded FRP to rectify the inherent weakness of non-seismic load designed RC connections by enhancing the joint shear capacity, in addition to enhancing the flexural capacity and to also relocate the possible formation of plastic hinges in the beam away from the joint region, as well as to promote a strong- column-weak-beam failure mechanism. Most of these experimental studies, while extremely useful, have been concerned with the behaviour of the strengthened connection as a whole with less attention paid to the behaviour of the FRP strengthening itself. In addition, the distinct lack of numerical simulations and analytical models, by comparison, are hindering better understanding and the more widespread rational design of FRP-strengthened for RC connections. The research reported in this dissertation focuses on the commonly occurring case of a shear strength deficient connection. Such connections are retrofitted or repaired with FRP strips however from herein such application of FRP will collectively be referred to as strengthening. The key aims of this project are to (i) experimentally observe and quantify the behaviour of FRP strengthening in FRP-strengthened connections, (ii) accurately simulate the experimental results produced in aim (i) using finite elements and identify the strengths and weakness of such numerical modelling, (iii) perform parametric studies with the calibrated numerical models, (iv) develop an analytical model which can rationally and reliably predict the behaviour of the FRP-strengthened connections, and (v) formulate a design approach which can be easily incorporated into future versions of existing FRP-strengthening design guidelines as well as new guidelines. Initially a state-of-the-art review is conducted on the current state of knowledge pertaining to FRP-strengthened RC connections. Existing research is also systematically categorised for ease of reference and comparison and for identifying the various issues still requiring research attention. The experimental program then forms the heart of the project and considers the shear strengthening of virgin connections as well as the repair of damaged connections. A strength hierarchy dictating shear failure in the joint region (even after application of the FRP) is deliberately chosen in order to assess the effectiveness and limitation of the FRP strengthening. All test specimens are extensively instrumented and the majority of connections are tested under mono tonic load which greatly enhances the ability to effectively monitor the behaviour of the FRP as well as the cracking behaviour of the connection. Two connections are also tested under cyclic loading in order to assess the energy absorption characteristics of the strengthening schemes. Finite element models are developed for plain (RC connection without any FRP strengthening) and FRP-strengthened RC connections which are then used to perform parametric studies to analyse various parameters affecting the behaviour of the connections such as amount of FRP, location of the FRP strips and strength of concrete. Also, a simple analytical model is presented based on the finding of the tests which can accurately estimate the contribution of the FRP strengthening to the connection shear strength. Finally, recommendations are made for design of FRP strengthening of shear deficient RC connections and future research needs are identified.
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