Uni‐Directional vs. Tri‐Directional Seismic Fragility Assessment of Low‐Rise RC Frames in High Seismic Regions

ABSTRACT An unpredictable and complicated phenomenon, earthquakes. When an earthquake strikes in nature, seismic instruments record it in three components, Viz. X, Y, and Z. Tri‐directional analysis thus depicts the seismic behavior of any structure during an earthquake occurrence most precisely and realistically. It is in common practice to consider only the single component of the earthquake to perform nonlinear time history analysis. Additionally, vertical acceleration is frequently disregarded or merely considered by code‐recommended V/H ratios without any valid rationale. Even Nepal's building code, NBC: 2020, does not advise separately taking vertical acceleration into account while analyzing the structures. Numerous earlier studies have demonstrated the significance of vertical acceleration, especially in high seismic zones or near‐field excitation. As a result, a more rational approach, i.e., tri‐directional analysis, is used in this study to account for both the vertical and horizontal components of an earthquake. Nepal's Himalayan areas are in a high seismic zone. The level of seismic excitation that a structure encounters in high seismic areas is quite high and frequently exceeds the recommendations and criteria set out by the codes. To examine the seismic performance of structures in Nepal's high seismic zones, an analytical study is conducted using two different types of models (M1 and M2) that typically represent low‐rise RC frames in Nepal. These models are created using the mandatory rule of thumb (MRT), which is the most widely used method of design for low‐rise concrete frame structures in Nepal. While Model M2 is an l ‐shaped model with a vertical irregularity, Model M1 refers to a symmetric model. For both models, analytical fragility functions are created for the unidirectional and tridirectional load scenarios. The findings from the analytical fragility curves imply that when only unidirectional shaking is used to examine the structure, the exceedance probabilities are underestimated. Furthermore, the difference in the exceedance probabilities between uni‐ and tri‐directional seismic shaking was more for the l ‐shape (irregular) model compared to the regular model, demonstrating higher fragility of the irregular model compared to the regular ones during tri‐directional seismic shaking.