High performance cementitious composites have been increasingly used for a range of structural applications in many countries. More recently, a notable interest has been focused on structural performance under seismic loading. However, a critical lack of coherent information and experimental/numerical data available in the literature has to be recognized along with the absence of specific and well-accepted code-guidelines for use of FRC in seismic applications. More specifically, when dealing with seismic resistant frame systems, few researchers have investigated in the past the seismic response of beam-column joints reinforced with steel fibres. These preliminary experimental tests have shown that adding steel fibres in joints is an effective method for improving joint behaviour and energy absorption capacity as well as enhancing the damage tolerance of joints and reducing the number of stirrups in seismic joints. However, due to the limited number of experimental tests as well as of the wide dispersion in the type and mechanical properties of the fibres adopted in these independent researches, the actual contributions of concrete, steel fibres and stirrups to the overall joint shear capacity has not yet been clearly identified and understood. This research aims to investigate the seismic behaviour and failure modes of beam-column joint subassemblies reinforced with steel fibres with the intent to provide preliminary suggestions for a simple but rational analytical procedure to evaluate the joint shear strength when either fibres and/or stirrups are adopted. As part of a more comprehensive on-going research campaign on the seismic behaviour of FRC members and systems, six 2-D exterior beam-column joint subassemblies were tested under simulated seismic loading (quasi-static cyclic loading regime) at the Civil Engineering Laboratory of the University of Canterbury. In order to assess the contribution of steel fibres to the joint (panel zone) shear strength, both under-designed systems (with no transverse reinforcement in the joint, following older practice before the pre-1970s) and well designed systems (following the NZ concrete design standard NZS 3101:1995) were adopted as benchmark specimens. The performance of steel fibre reinforced beam-column joints were compared with that of conventional joints. Results showed that using steel fibre reinforced concrete (SFRC) within beam-column joints can significantly enhance the shear resistance capacity of joints. However, using steel fibre reinforcement alone can not prevent buckling of the reinforcing bars when joints are under high intensity seismic loading. Furthermore, the test results also showed that using steel fibre reinforcement is an effective method to reduce the lateral reinforcement in the beam plastic hinge region. As part of the analytical investigation, a simplified procedure to evaluate the joint shear contribution provided by different amounts of fibres with or without the presence of stirrups has been also introduced. Influence of the axial load on the joint nominal shear capacity has been accounted for by adopting principle stresses. Tentative strength degradation curves (principle tensile stress vs. shear deformation) have also been calibrated on the experimental data which confirmed that a tentative relationship between the joint shear contributions provided by concrete, stirrups and steel fibres was a viable tool for designing SFRC joint. Furthermore, joint shear resistance coefficient contributed by steel fibres has been compared with previous experimental test available in literature to obtain an appropriate value for SFRC joint design guidelines. M_N performance based domain visualization has also been used to evaluate the hierarchy of strength and sequence of events of beam-column joint subassemblies.
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