Seismic collapse fragility of low-rise steel moment frames with mass irregularity based on shaking table test

The collapse risk of building structures has been one of the major factors causing casualties and huge economic losses for earthquake disaster prevention. This paper presents a shaking table test on low-rise steel moment frames with consideration of mass irregularity in the elevation direction. The frames are subjected to naturally observed and artificial seismic waves. As indicated from the test results, the specimen with the irregularity of additive-mass (additional 5% of the roof mass) on the top floor showed considerable amplification on the acceleration and drift responses at the bottom storey when subjected to over-design earthquakes. A numerical model with degraded stress–strain relation is built in terms of fiber elements and calibrated by test results. Incremental dynamic analyses are performed to evaluate the probabilities exceeding three limit states related to immediate occupancy, life safety, and collapse prevention. The seismic fragility curves through a suite of near-fault ground motions in the Uemachi area of Osaka are obtained for the numerical models with and without mass irregularity on the roof, and the vertical mass irregularity tends to play significant roles in the seismic design for collapse prevention.