Experimental Assessment of Unreinforced Exterior Beam-Column Joints

Abstract eng:
Reinforced Concrete (RC) buildings designed according to obsolete seismic codes or for gravity loads only are widespread in Italian and Mediterranean building stock. In seismic performance assessment of these typical non-conforming buildings, collapse safety might be significantly affected by non-linear behavior of joints that are involved in the failure mechanism, especially if they are characterized by poor structural detailing, such as the lack of an adequate transverse reinforcement in the joint panel. Few reliable approaches for modeling all sources of nonlinearity are proposed in literature for poorly designed beamcolumn joints because of relatively poor information from experimental tests. Even if a quite significant amount of research on seismic performance of unreinforced joints has been carried out in last years, a very few portion of them handled with specimens with plain hook-ended longitudinal bars. A small number of experimental results include also the analysis of local aspects, such as the evaluation of joint shear strain. Therefore, a higher number of tests is necessary to deeper understand joint seismic response in order to validate the existing models or calibrate new ones. The current study aims at improving the understanding of seismic performance of exterior joints without transverse reinforcement in non-conforming RC buildings through experimental tests. Four full-scale exterior unreinforced beamcolumn joint sub-assemblages are tested under cyclic loading. The specimens are different for beam longitudinal reinforcement ratio and for typology of longitudinal bars (plain or deformed). Two different kinds of joint failure are expected, with or without the yielding of the adjacent beam. Strain gauges located on beam bars and displacement transducers on the joint panel allow the complete definition of both the main deformability contributions, namely fixed-endrotation and shear strain of joint panel, highlighting the difference between the observed failure modes. Design criteria, adopted setup and experimental results are described and discussed. From the analysis of the global experimental responses, it is observed that the higher the beam longitudinal reinforcement ratio, the higher the joint shear strength, and that the post-peak degrading behavior is always controlled by the response of the joint panel. The analysis of local responses highlighted that the sum of the main deformability contributions due to joints (joint panel shear strain and rotation at beam/joint interface) represents the major part of the imposed drift for all tests. In particular, in tests for which the beam reaches the yielding moment, the contribution of the deformation mechanism associated to the rotation at beam-joint interface is predominant, particularly in the case of plain longitudinal reinforcement.

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