Strengthening of RC Structures

Seismic strengthening of beam-column joints with FFHRS

Beam-column joints being the crucial components, often govern the seismic behavior of a non-seismically designed reinforced concrete structure. The primary reason for the vulnerability of the beam-column joints is the high shear force that comes into the joint core due to high bending moments of opposite signs in the beams and columns framing into the joint. The new designs demand to provide additional joint shear reinforcement in the core to cater for these high shear forces. However, this joint shear reinforcement is absence in the joints of old, non-seismically designed structures. Therefore, the joints of such structures often need a certain strengthening to be able to withstand high shear forces in the event of an earthquake.

Various strengthening schemes that can be used for the strengthening of beam-column joints are based on providing aditional shear reinforcement to the joint core. These strengthening schemes include FRP wraping, concrete jacketing, steel plating etc. Although these strengthening methods are quite effective as shown through several laboratory tests, their use in practice is often met with problems of accessibility of the sides of the joint core due to the beams and slabs framing into the joint. This severely affects the effectiveness of the strengthening used for the joints.

Within the framework of my PhD, I worked on development of fully fastened haunch retrofit solution (FFHRS) for strengthening of beam-column joints of non-seismically designed structures. The principle of this retrofit solution is not to provide additional shear reinforcement but to protect the joint by reducing the shear force coming on to the joint core. The strengthening solution involves in providing steel diagonal elements (known as haunch elements) at the interface of the beam and the column and connecting it to the frame members using post-installed anchors. The use of post-installed anchors render this solution low-invasiveness and the solution is equally effective even if all the sides of the joint are not accessible. The efficiency of this retrofit solution largely depends on the performance of the anchorage used to connect the haunch elements to the frame members. If the anchorage serves its function well, the retrofit solution leads to the re-distribution of forces around the joint core and with proper design, result in changing the failure mode from brittle joint shear to ductile beam flexure. The re-distribution of the forces leads to reduction in bending moment at the face of the joint but also leads to an increase in the shear force in the frame members (beams and columns). Therefore, the design of the retrofit solution must consider the sufficiency of the shear capacity of the frame members.

Strengthening of full-scale structure using FRP

Strengthening of real-life structures in the field is much more challenging than the strengthening of components or sub-assemblies in the laboratory. The shear size of the structure, limited access to the critical areas, interference due to the internal reinforcement etc. are some of the problems that may not allow an optimum application of strengthening. Therefore, a particular strengthening method can be best proven through full-scale structural level tests. This was one such rare opportunity where a full-scale structure previously tested under pushover loads was repaired and strengthened using fiber reinforced polymers (FRP) and re-tested under pushover loads until failure. The strengthening involved repair and restoring of the damaged section, flexural strengthening with FRP laminates anchored in the slots and confinement by FRP wrapping. The strengthening method was motivated by the previously performed research on the strengthening of beam-column joint sub-assemblies. The strengthening was performed for the beams, columns and beam-column joints of the structure. With an economic design of the strengthening, the retrofitted structure could resist 90% of its ultimate load-carrying capacity under as-built condition. However, due to the heavy damage due to previous testing, the stiffness of the structure could not be recovered.

Shake table tests on structures with joints strengthened using FFHRS

For any structure subjected to seismic loads, the type of test closest to reality is the shake table test. In order to verify the suitability of the fully fastened haunch retrofit solution (FFHRS) for retrofitting of the joints of structures when subjected to dynamic earthquake loading, an exact replica of the frame tested earlier was retrofitted with FFHRS and tested under dynamic loads. The objectives of this test were: (i) to understand the seismic behaviour of RC structures with joints retrofitted with FFHRS under real seismic situation; (ii) to evaluate the efficacy of the FFHRS when subjected to dynamic seismic loads; and (iii) to provide database for validation of the spring models developed for the assessment of joints strengthened with FFHRS at a structural level. The tests were the first verification tests for haunch retrofit solution to be performed under realistic seismic excitation and clearly proved the effectiveness of FFHRS in improving the seismic performance of non-seismically designed structures.

Investigating hybrid strengthening using FRP

Investigating the efficiency of hybrid strengthening using a combination of near surface mounting of laminates and confinement through wrapping on RC structural elements under different loading conditions. The work is performed within the framework of the collaboration with IIT Hyderabad (Prof. Suriya Prakash). Corresponding computational modeling and numerical analysis at the University of Stuttgart augment the tests and provide in-depth information on the various mechanisms.

Modeling of beam-column joints strengthened using FFHRS

Development of spring based numerical models to simulate the seismic behavior of beam-column joints strengthened using FFHRS considering the nonlinear behavior of anchorages. The spring based models are able to simulate the behavior of the joints with FFHRS with a good accuracy while at the same time being practical enough for use at the structural level.

Nonlinear dynamic analysis of structures strengthened using FFHRS

Developing numerical modeling and analysis procedure for RC structures strengthened using FFHRS subjected to dynamic (shake table excitation). The model gives due consideration to the anchorage behavior and are able to capture the right failure modes. With the models, the locations requiring strengthening could be identified and an optimum strengthening solution can be designed for the structure.