Global Seismic Damage Evolution And Collapse Safety Margin Of Concrete Structures Subjected To Strong Earthquakes

Development of reasonable global seismic damage model for the assessment of damage degree and the proper evaluation of collapse safety margin of structures under strong earthquake is of much positive significance for updating seismic design provisions. It is also meaningful to utilize collapse margin ratio (CMR) to assist collapse resistance design of structures, retrofit design of corroded structures as well as controlled structures with energy dissipation devices, etc. Global seismic damage and collapse safety margin play act as main thread in this dissertation, based on which some problems are discussed, e.g. parameters associated with the definition of CMR (spectral intensity index, earthquake wave, etc), ultimate story drift with the consideration of collapse safety margin, CMR-based retrofit for controlled and corroded structures. Some findings can be achieved as follows:(1) On the basis of the background of nonlinear Pushover analysis and the relationship between effective stiffness and vibration period, a modified version of the Ghobarah’s model is proposed to consider the effect of multiple vibration modes on global seismic damage of structures. This modified model has bridged the Ghobarah’s model and the final softening method, considering simultaneously the influence of the changes in vibration modes and vibration periods on global seismic damage. The results indicate that, the effect of higher vibration modes on seismic damage would be gradually generated as structural height increases and nonlinearities developing in structures under strong earthquakes. Hence, it is necessary to include such effect during global seismic assessment. Through detailed comparisons, the proposed modified model is proved to be reasonable and easy for use, especially for damage assessment of structures under strong earthquakes.(2) It is pointed out that selection of earthquake spectral intensity in the assessment of structural collapse safety margin should be customized for structures with different transition range of vibration periods. The CMR should be also adjustable according to the annual probability of exceedance of earthquakes and the shape of their corresponding response spectra. The analysis indicates that, for structures with medium and short periods, spectral acceleration, Sa(Ti) should be replaced by its modified version, Sa*to take into account the elongation of vibration period; Both Sa(T1) and Housner’s spectral intensity, SI, are applicable for structures with medium periods while SI is more advantageous for structures with long periods. In the case of important structures with long vibration periods, the determination of CMR should be combined with site-specific response spectrum, or the value of CMR is overestimated.(3) Collapse resistance design method for the weakest floor with the consideration of collapse safety margin is proposed based on the modified multiple-mode-based global seismic damage model. The transition of the weakest floor with increasing level of peak ground acceleration (PGA) is discussed as well as the uncertainties associated with the maximum story drift and the place where it occur corresponding to the critical collapse state of structures under strong earthquakes. The analytical data indicate that, for those structures designed strictly in accordance with mandatory code provisions, the intervene of higher modes in structural responses make the ultimate story drift differ greatly. It could be an effective way to promote collapse safety margin of structures by control the weakest floor in the critical collapse state. In addition, a positive relation is found, by the example study, between the maximum story drift corresponding to the critical collapse state and CMR.(4) Determination of CMR using Pushover analytical procedures is proposed by considering the characteristics of structures with passive energy dissipation devices, in which the spectral intensity corresponding to collapse state is evaluated by the demand spectrum of controlled structures and the inelastic capacity spectrum generated from Pushover analysis. The analysis indicates that, in the determination of spectral acceleration of controlled structures under rare earthquakes, it is necessary to consider the change in fundamental vibration period and total equivalent damping ratio. The procedures are found to be reasonably conservative and computationally efficient. It is also verified that the addition of passive energy devices could yield enhancement both in earthquake resistance and collapse safety margin.(5) The procedures for determining the collapse safety margin of corroded structures are discussed based on the quantitative analysis of damage development in corroded structures and on the consideration of the effect of residual service life on spectral acceleration corresponding to rare earthquakes. Then, seismic retrofit method using fiber reinforced plastics (FRP) for corroded structures is suggested where time-dependent characteristics of CMR. The analysis indicates that damage for corroded structures evolves much quicker than those uncorroded ones due to corrosion-induced initial damage and deterioration in bond between reinforcement and concrete. The CMR of corroded structures would be overestimated if the effect of residual service life on the determination of spectral acceleration corresponding to rare earthquakes is not taken into account. Reasonable retrofit for corroded structures using CMR as performance index could be realized.