INTRODUCTION AND LITERATURE REVIEW
1.2 LITERATURE REVIEW
1.2.2 Rehabilitation / strengthening of damaged RC beam-column joints and connections
1.2.2.1 Concrete Jacketing
The literature survey revealed that the test on beam-column joints and connections were initiated as early as 1967. Since then, other research studies were carried out to improve the performance for beam-column joints and connections. Experimental studies were conducted on both exterior and interior joints/connections on full scaled as well as scaled down specimens. Quasi static loading were used by many researchers to simulate the seismic actions. These frequencies are however substantially lower than those corresponding to the frequencies of actual seismic excitations. Characteristic of loading such as number of repeated loading cycle of displacement, frequency of excitation and the level of excitation plays a vital role in the behavior of the joints and connections.
However, very little informations were reported on the performance of beam-column
Corazao and Durrani [1989] tested three single (two exterior and one interior) and two multiple connections, some including a floor slab by jacketing the column, the joint region and some portion of the beam damaged by earthquake. Due to the difficulties experienced with in-place bending of the crossties hooks in the joint region, the additional joint reinforcement was modified to a set of dowel bars with a hook. The repaired specimens including the enlargement of section and addition of rolled steel elements. The repaired techniques were successful in restoring strength, stiffness and energy dissipation capacity of all three single joints. In some of the specimens, the damage was successfully moved away from the column face due to the added beam bottom bars hooked both in the joint. However for a multiple joint the technique was unsuccessful in improving the behavior. The authors concluded that load transfer mechanism between beams and column should be well addressed for a multiple joints; simply jacketing the column alone will not improved the performance of the joints.
Alcocer and Jirsa [1993] performed experiments to study the suitability of jacketing of non-ductile frame elements as a rehabilitation technique. The need to drill holes through the beams for placing joint confinement reinforcement was eliminated. This was achieved by welding a steel cage around the joint. The cage was made by steel angles and flat bars.
It was reported that the steel cage and the cornet ties confined the beam-column joint satisfactorily up to 4% drift. Important findings indicated that the shear strength of jacketed joints could be estimated using current recommendations for the design of beam- column joints in new construction. It was suggested that the current criteria on bar development should be met by longitudinal reinforcement in the jackets.
Stoppenhagen et al. [1995] repaired a two-third scale model of two bays and two stories of an exterior moment-resisting frame with heavily damaged columns. The damage was
between the spandrel beams. In addition to repairing the damaged, the new columns were designed to increase the lateral capacity of the frame and to shift the mode of failure from shear in the columns to flexural hinging in the beams. The test results under applied cyclic loading indicated that the columns were successfully repaired and that the governing failure mechanisms of the frame were successfully achieved. The results also indicated that the encased columns behaved monolithically and that the lateral strength of the frame was substantially increased.
Hakuto et al. [2000] tested three one-way interior beam-column joints with no joint reinforcement. The damaged specimens were rehabilitated by RC jacketing provided at the beams, columns and joints. The core of the joint strengthened by plain circular hoops, which was made up of two circular U-shaped ties placed through holes drilled in the beams and welded in place. The undamaged specimens were strengthened by providing column jacket only. The strengthening scheme changed the behaviour of the joint in ductile mode with formation of plastic hinges in beam, except for the specimen in which only column jacket was provided. The major drawback of this technique was the addition of joint core hoops was very labor-intensive.
Tsonos [2002] studied the effectiveness of RC jackets applied in areas which were inaccessible due to presence of adjacent structures in one or more sides of the columns and beam-column connections. A two sided-concrete jacket with strength of 60 MPa was used for repairing. Additional joint ties were placed by coring the beam and short bars were placed in a transverse direction inside the hooks of the beam. The author reported that the mode of failure before jacketing which involved severe loss of joint core was significantly improved. A formation of a beam hinge and buckling of beam bars after jacketing were also observed.
Bligh et al. [2005] used concrete jackets as a rehabilitation scheme to enhance the confinement that would be provided by framing members on all four sides of the connections. Due to concrete jacketing, the clear span of beams and columns were reduced which led to increase in shear demand in beam and column. Their study suggested that re-checking of the shear strength must be done for structural components after repaired with a concrete jacket.
Karayannis et al. [2008a] rehabilitated ten exterior beam-column connections where a thin RC jacket with light and dense reinforcement of small diameter has been provided locally at the damaged joint region and partly at beam / column. The jackets were made of a high strength, nonshrink and flowable cement mortar. The authors observed that apart from exhibited higher values of load carrying capacity and hysteretic energy dissipation, locally applied jacketing improves the damage behaviour of the joint by changing the brittle failure mode to a ductile one. Furthermore, the authors indicated that the structural geometry and building mass were not modified as the damaged regions were encased with a thin jacket. Therefore, the dynamic characteristic of the structures remain practically unaffected. They also suggested that RC jacket enhances the beam flexural strength due to beam rehabilitating and shear strength capacity accordingly increase. Thus, in real structures an after-repair shear capacity check has to be performed in order to re-establish the capacity design concept for the whole building.