The original curves (as per O'Rourke and So (2000)) pertain to crossing cumulative curves between consecutive damage states, DS, which results in (meaningless) negative discrete damage probabilities. These fragility curves need to be corrected to be used for damage assessment purposes.
Original parameters of fragility curves (log-normal distribution):
DS2: Medial = 0.49, Standard Deviation = 0.55
DS3: Medial = 0.86, Standard Deviation = 0.39
DS4: Medial = 0.99, Standard Deviation = 0.27
DS5: Medial = 1.17, Standard Deviation = 0.21
The displayed fragility curves have been corrected according to the guidelines by Porter (2021), by forcing fragility curves not to cross through the computation of a new median PGA and a new logarithm standard deviation that is constant for all damage states as per equation (37) in Porter (2021).