Seismic fragility of low-rise piloti buildings based on 2017 Pohang earthquake damage

Due to the 2017 Pohang earthquake, many low-rise piloti buildings suffered minor to severe damage. Low-rise piloti buildings in Korea are generally both vertically and horizontally irregular systems. The first floor is a weak and soft story, and the core wall is located at the corner or edge of the plan creating a horizontal irregularity. Furthermore, the damage investigation indicates that the insufficient amount of column hoops and poor detailing of reinforcement. It was presumed that the combination of soft story, torsional behavior and poor detailing brought brittle failure at the columns located at the furthest distance from the core. Such a brittle and irregular nature makes it difficult to predict damage with a HAZUS-type approach, which uses seismic fragility functions defined in terms of the spectral displacement and the capacity spectrum method based on pushover analyses. Comparison of pushover analysis results of piloti buildings indicated that location of peak and overall deformation capacity differed with the intensity of the torsional behavior and failure mode of the column. Therefore, this study aimed to develop an improved seismic fragility function for piloti buildings. To reflect the observed damage, the use of damage criteria based on member performance, including stress and strain, and spectral acceleration was proposed. Fragility functions for the piloti buildings were constructed by using threedimensional incremental dynamic analyses. Damage estimates for piloti buildings in the target area that were obtained by using obtained fragility functions showed good correlations with damage survey data.