Impact of support settlement on the seismic fragility of a substandard RC frame with plan irregularity

Abstract

Substandard reinforced concrete (RC) frames are vulnerable when subjected to ground shaking. In some seismically active regions, buildings without sufficient earthquake resistance represent a large portion of the existing building stock. Foundation settlement is induced by factors such as adjacent deep excavations, soil liquefaction, and poor soil conditions. As a result, damage to structural elements immediately reduces the ability of the structures to resist lateral loads. However, engineers tend to overlook this effect in the performance assessment of RC structures. The primary objective of this thesis is to investigate the impact of the support settlement on the seismic fragility of substandard RC buildings. Along the way, the influence of settlement on aspects such as the redistribution of internal forces, the seismic capacity, and the change in the in-plan eccentricity, is studied as well. A 3 story substandard RC building with plan irregularity was considered as the case study building. The case study building had poor seismic resistance, lacked ductile reinforcement detailing, and had a weak column strong beam type of failure mechanism. Moreover, it had plain reinforcement bars and its concrete strength was lower than the strength required in the existing codes. In brief, it had the deficiencies that are common to most of the substandard buildings. The finite element model of the case study frame is created using the OpenSees software framework. Settlement scenarios in which specific axes (X1, Y1, and X1Y1) settle by 1 cm, 2 cm, and 3 cm, are applied to the frame. To study the impact of the settlement, static analysis, pushover analysis, and incremental dynamic analysis (IDA) are carried out. The findings reveal that the columns in the settled frame and in the adjacent frame are greatly influenced by the settlement. Moreover, a significant increase in the shear and curvature demands is observed in the beams that span along the perpendicular direction of the settled frame. The maximums of those changes are observed in the case when two exterior axes (X1Y1) settle by 3 cm. In this case, the axial force surges by 84% for the critical column C3 and the yield curvature capacity is surpassed at six locations along the beams. As for the effect of the settlement on the local behavior of the sections, moment curvature (M-C) analysis findings indicate that the settlement greatly influences the strength and the curvature ductility for the columns in and near the settlement area. As for the pushover analysis, the findings indicate that the settlement of an axes of a frame poses initial stressing and leads to early yielding of the structural members that are in the opposite direction. Therefore, it is noticed that the settlement leads to global stiffness degradation in the perpendicular direction to the settlement. It is observed that the amount of reduction of ultimate roof drift capacity of the frame is sensitive to direction of loading, location of the settlement axes, as well as the settlement amount. It is revealed that the impact of the settlement on the ultimate roof drift capacity is more pronounced when the pushover loading is applied in the +Y direction. The drop reached up to 21%, 25%, and 18% for scenarios X1, Y1, and X1Y1 respectively. On the contrary, it is observed that the impact of the settlement on the deformation capacity in the weak direction (+X) is less. Moreover, the findings suggest that the settlement caused a change in the global failure mechanism. Most often global failure of substandard frames is triggered by failure of one column since the redundancy is very limited. It is observed that the critical column that triggers failure of the frame tends to differ depending on the type and extent of settlement. In particular, the reduction in the axial force demands for the critical columns before any settlement results in them becoming less critical while another column starts becoming the critical column that triggers the initiation of global failure. Further, the hinging mechanism of the frame changes due to settlement as well. The findings show that more plastic hinges are formed in beams that are in and near the settled axis. This is a result of settlement induced strains consuming some major portion of the deformation capacity up to or beyond the yield limit prior to any seismic action. The impact of the settlement on the change in the in-plan eccentricity ratio is also investigated in this study. The findings suggest that the settlement leads to remarkable changes in the stiffness distribution due to reducing of the stiffness contribution of the settled frames. For the studied settlement scenarios, this change in the stiffness distribution resulted in decreasing of the in-plan eccentricity ratio. The center of rigidity (CR) shifted towards the center of mass (CM). As a result, it is found that for Y1 = 3 cm, the direct distance between the CM and CR drops by 40% due to settlement. The computed seismic fragility curves indicate that for all the considered scenarios, seismic fragility of the frame is sensitive to the settlement amount. It is found that 1 cm of differential settlement is sufficient to induce slight damage without any seismic action. As for the moderate damage state, the findings reveal that the settlement caused minimal variations in the median capacity. Meanwhile, marked reductions by up to 8% in the median collapse capacity are observed when the settlement amount reached 3 cm.