Analysis of slab action on the seismic behavior of external RC beam-column joints

The correct evaluation of beam flexure capacity is of paramount importance in the capacity design process of reinforced concrete (RC) frame structures. Even though few experimental studies on beam-column joints have considered the presence of slab, its action as a tension flange in modifying beam-column strength hierarchy has been clearly found. This is mainly due to the increased hogging moment capacity of beams. Nevertheless, European and Italian seismic codes do not explicitly consider slab action as tension flange nor indicate proper values for effective slab width. To better understand this action, an accurate 3D finite element model has been calibrated based on the results of past experimental tests carried out on a full-scale beam-column joint. Afterwards, the calibrated model has been equipped with a portion of the adjacent spandrel beam and slab in order to evaluate their effect on the beam-column connection's behavior through numerical analyses. Alike previous studies, numerical results show that the tension flange effect of slab can significantly increase hogging (negative) moment in beams framing into beam-column joints. Additionally, it has been found that this effect is a function of slab steel amount, also highlighting the role of slab reinforcement orthogonal to the beam. Based on the above, practical indications have been provided for the evaluation of hogging moment in beams specifically concerning effective slab width to be considered in resisting moment calculations as a function of slab steel amount.