Seung Hun Choi

Investigating flow separation regions on the surfaces of three-dimensional bluff bodies in turbulent flows is important because these regions can induce significant aerodynamic loads. Separation bubbles can generate extreme pressures, making the roof components of low-rise buildings vulnerable. In this study, proper orthogonal decomposition (POD) was applied to wind-induced roof pressures to elucidate the physical significance of the dominant modes. Based on the interpretation of the first mode from the POD, the mean reattachment length of the roof separation bubbles on a low-rise building model in turbulent flow was determined. The mean reattachment length derived from the POD was then compared with the length obtained from an aerodynamic database. For the centerline of the roof, the mean reattachment length based on the POD aligned well with that from the aerodynamic database, showing a difference of less than 5%. This study highlights the efficacy of POD as a powerful tool for estimating the reattachment length of separation bubbles on bluff bodies.

In this study, the vulnerability curves for masonry and concrete frame buildings are assessed based on building fragility curves for the debris-flow caused by landslides on mountain slopes. The First-Order Second-Moment (FOSM) method is used to estimate the building fragility curve (expressed as probability of damage exceedance) subjected to debris-flow. In this method, the horizontal displacement of a building impacted by debris-flow and the statistics of resistance (i.e., building displacement) following four different damage states (i.e., slight, moderate, extensive, and complete) are utilized to estimate the building fragility curve. The building vulnerability curves (expressed as mean probability of damage) were evaluated based on the estimated building fragility curve and corresponding mean damage ratio for each damage state and were verified by calculating the root mean square error with datasets obtained from post-disaster damage assessment. In this study, the effects of structural material, type, and height on the building vulnerability curves were also studied. All vulnerability curves of buildings estimated in this study were fitted and databased using parameters of the log-normal cumulative distribution function and can be used to measure the performance of buildings in debris-flow prone areas as well as to provide information for risk and loss assessment.

In this study, fragility functions are developed to estimate expected volcanic ash damage to vinyl greenhouses and livestock sheds among common agricultural facilities in South Korea. The fragility functions are derived through the FOSM (first-order second-moment) analytical approach based on the thickness distribution of volcanic ash observed in the 1980 eruption of Mt. Saint Helens in the USA and on agricultural facility codes and specifications in South Korea. In this study, the volcanic ash fragility functions evaluated represent the conditional probability of failure of vinyl greenhouses and livestock sheds over the full range of volcanic ash loads. For the evaluation, 4 types (i.e., automated, single span, tree crop, and double span types) of multi-hazard resisting vinyl greenhouses and 3 types (i.e., standard, coastal, and mountain types) of livestock sheds conventionally utilized in South Korea are considered. All volcanic ash fragility functions estimated in this study are fitted using parameters of the lognormal CDF (cumulative distribution function), and the obtained parameters are compiled into a database for use in the future. The volcanic ash fragility functions developed in this study are intended to be used to evaluate potential risks and losses from volcanic eruptions around the Korean Peninsula.