Duration of Strong Motion Exceeding Bridge Design Spectra in the 2016 Kumamoto Earthquake

The duration of strong motion exceeding the bridge design spectra in the bridge design specification in Japan is evaluated for the 2016 Kumamoto Earthquake. The structural response duration is defined as the total threshold interval of the velocity response envelope calculated for a viscously damped (5%) single-degree-of-freedom oscillator with various natural period. The prescribed design spectra are employed as frequency-dependent thresholds. The ground acceleration records observed at the K-NET and KiK-net stations during the Kumamoto earthquake are used for the analysis. Spectrum representations of the duration and number of cycles are obtained for four representative ground motion observation stations. The distribution maps of the duration are also presented for the strong motion observation stations in the Kumamoto prefecture.