Seismic risk of substandard RC frames with foundation settlement

Abstract

Foundation settlement is one of the most common problems in buildings. Situations such as poorly administered deep excavations and soil liquefaction cause ground deformation and thus trigger the settlement of nearby buildings. Substandard RC buildings may suffer substantial damage from foundation settlements, and seismic effects exacerbate the problem. Although the seismic risk of buildings exposed to settlement has been studied by some researchers, in the majority of them, inter-story drift ratios (IDRs) have been used as the key response parameter. However, because settlement causes considerable strain on structural elements, the stresses in critical regions of structural elements may approach their flexural capacity even when lateral deformations are very low. Therefore, the strain-based approach was employed in the study. As the case study building, a substandard RC frame with poor concrete quality and inadequate transverse reinforcement is studied. It is numerically modelled using OpenSees software framework, and the simulated response is validated using data from an earlier experiment. Elements are defined using a force-based approach, and fiber-sections are used in the section definitions to capture the axial force-moment interaction. A number of settlement profiles are applied to the frame, each with a specific range of settlement amounts. First, the redistribution of internal forces following settlement is examined. The findings of the static analysis show that the internal force distributions change substantially following the settlement. Members near the settled area experience a significant increase in axial, shear, and bending moment demands. In some cases, flexural and shear demands exceed capacity. The results of the pushover analysis lead to some remarkable observations as well. The direction of pushover loading has been demonstrated to have a considerable effect. The type of failure and the collapse mechanism are demonstrated to be dependent on the settlement profile. It is shown that the maximum base shear and drift ratio at ultimate displacement capacity decrease significantly as the settlement amount increases. Incremental dynamic analyzes (IDA) are performed using a ground motion set scaled to a specific intensity range. To construct seismic fragility curves, the amount of settlement is added as a new independent parameter to the conventional functional form. A set of damage states are defined based on resultant strains. Two sites with different seismic activity rates are chosen as the case study sites, and the seismic hazard associated with each is identified. Seismic fragility analysis data and seismic hazard information are combined to calculate the annual probability of collapse. The presence of a vertical excitation component as well as consideration of shear failure are found to have a significant influence on the seismic fragility analysis results. The median collapse capacity decreases by up to 26% due to foundation settlement. The annual probability of collapse increases by 0.5% as an outcome of this reduction in the median capacity.