ACCEPTABLE STRUCTURAL COLLAPSE SAFETY MARGIN RATIOS BASED ON ANNUAL COLLAPSE PROBABILITY

The relationship between average annual collapse probability and collapse margin of structures is derived and a method used to determine acceptable collapse margin ratio (CMR) with certain annual collapse probability is then proposed. Annual collapse probability is obtained through the combination of collapse vulnerability analysis with seismic hazard analysis under the framework of total probability theory. Further, the relation between annual collapse probability and CMR is established by using the median of collapse earthquake intensity. Then, acceptable CMRs are determined based on the lower limit of annual collapse probability which comes from the collapse probability corresponding to a rare earthquake. With the design acceleration response spectrum specified in Chinese seismic design code, a so-called CMR spectrum is developed and some annual collapse probability limits and corresponding acceptable CMRs are proposed for buildings with seismic precautionary category (SPC) of B and C for several seismic fortification intensities (SFI). The investigation indicates that acceptable CMRs tend to increase as acceptable annual collapse probability decreases, following a mathematical power function relation between each other. Acceptable annual collapse probability is associated with rare-level earthquake intensity, seismic hazard and global collapse uncertainty. In accordance with Chinese aseismic design background, the CMR spectra developed with an acceptable annual collapse probability can reflect the variation of acceptable CMRs with different global uncertainties. Theoretically, these spectrums can be applied for a quick assessment for structures having different fundamental vibration periods. On the basis of acceptable collapse probabilities under a rare earthquake as suggested in CECS 392 guideline, the acceptable annual collapse probabilities for buildings with SPC of B are far below than those of C. And, the acceptable CMRs for all buildings exhibit a tendency to increase as seismic fortification intensity increases.