In this study, the variability inherent in the performance of RC frame structures to bridge over a column loss was investigated, considering the uncertainties existing in gravity loads, material properties, and construction geometries. Uncertainty and sensitivity analysis were conducted in tandem to provide the influence and significance of the uncertain parameters on RC frames to resist progressive collapse. Quasi-static pushdown analysis was used to assess the residual load-resisting capacity of structures with an initialized damage. A set of structural models were generated for uncertainty analysis by using the correlation-reduced Latin hypercube sampling method. Sensitivity analysis was performed by independently varying parameters with one standard deviation away from their means. Two typical RC frame structures with different span aspect ratios were taken as the study cases. The macromodeling technique, which was validated by the test results, was used for analysis. The study results indicated that structural uncertainties have significant effects on the behavior of RC frame buildings for mitigating progressive collapse caused by the loss of a ground column. Among the studied uncertain parameters, the gravity loads and the properties of reinforcement have the most effects on the residual load-resisting capacities of damaged frames.

The corrosion of reinforcement is one of the major problems affecting the durability of aging reinforced concrete (RC) structures. In this paper, the results of a study concerning the effects of reinforcement corrosion on the behavior of reinforced concrete structures subjected to the scenario of the loss of a middle column are presented. Four typical RC moment frame structures were selected as case studies. The general corrosion process induced by the penetration of chloride ions was considered for an atmospheric exposure environment that would eventually lead to the loss of reinforcement cross-sectional area, the reduction of reinforcement strength and ductility, and the spalling of concrete cover. A macro-modeling technique, which was validated by the test results and the high-fidelity finite element model, was utilized for pushdown analysis. To account for the dynamic effects associated with sudden column loss, a procedure based on energy equilibrium was employed to generate the dynamic load-displacement curves based on the static pushdown responses. The results established that aging structures with severe corrosion are much more vulnerable to progressive collapse than newly constructed ones. After 50 years of corrosion, the ultimate deformation capacity of structures was reduced by over 40%, and the first peak load capacity and ultimate load capacity of the structures were reduced by a range between 20 and 30%. Finally, linear models that depict the time-dependent changes in the load-carrying and deformation capacities of the structures to resist progressive collapse were developed with the aim of providing a simple way to incorporate corrosion effects in the design process.

Analytical fragility analyses are conducted to investigate the impact of seismic design level on the seismic performance of RC moment-resisting frame buildings designed according to the provisions of the current Chinese seismic code. Four groups of buildings of low-to-mid height (3, 5, 8, and 10 stories) are studied, each of which accounts for diverse levels of seismic design, namely, precode, low code, medium code, and high code. Fragility relationships are derived for each building using nonlinear time-history analyses with 100 real ground-motion records. The results show that the buildings designed for a higher seismic code level exhibit less fragility as improving the seismic design level leads to a raised fragility median and a reduced fragility dispersion. Data on the reference seismic hazards with the probabilities of exceedance of 10 and 2–3% in 50 years show that almost all the buildings can satisfy the performance requirements of reparability and no collapse, respectively. However, the damage limitation performance objective, which is associated with 63.2% in 50-year hazard, cannot be satisfactorily achieved by medium-code and high-code buildings. The enhancement of structural resistance caused by the improvement of seismic design level is counteracted by the simultaneously increased seismic action; therefore, the limit-state probabilities of the buildings at the corresponding reference earthquakes are not decreased but increased accordingly. The current Chinese seismic design code seems conservative because the buildings show marginal potential of suffering serious damages endangering life when subjected to the seismic hazard level higher than it was designed for. Thus, it is possible to account for larger anticipated nonlinear behavior in the definition of seismic design force.

A ground motion intensity measure (IM) well correlated with structural demand plays an important role in predicting the damage potential of earthquake records. This study presents a new approach to develop a compound IM through canonical correlation analysis (CCA). This method utilizes the ability of CCA to search for two linear combinations to maximize the correlation between the corresponding random vectors. To illustrate, the nonlinear responses of single-degree-of-freedom (SDOF) systems due to 98 real ground motion records are used. In a log-log scale, a compound IM in terms of a linear combination of six non-structure-specific IMs is developed with a maximum dependence on the linearly combined structural demand of both peak displacement and dissipated energy. Through a comprehensive discussion, the feasibility and performance of the presented approach is investigated for SDOF systems over the entire period range. It is found that the developed compound IM shows significantly high-level correlation with structural demand, and the corresponding correlation coefficients are greater than 0.95 at most periods. This validates the effectiveness of the presented method in improving prediction accuracy for earthquake damage potential through considering multiple IMs.

A simplified approach is proposed to investigate the seismic ductility demand of shear-critical reinforced concrete (RC) columns based on experimental calibration. Incorporating with the Bouc–Wen–Baber–Noori (BWBN) model, the governing equation of the equivalent single-degree-of-freedom (SDOF) system is first presented to describe the typical hysteresis characteristics, where the relationship between the inelastic restoring force and translational displacement is calibrated by comparing with the experimental data of shear-critical RC columns under quasi-static cyclic testing. Based on the developed governing equation, the strength reduction approach is further implemented to estimate the seismic ductility demand of shear-critical RC columns by establishing the constant strength-ductility demand spectral (CSDDS) method. Finally, an empirical expression is regressed to predict the mean and standard deviation of seismic ductility demand of shear-critical RC columns. This study verifies the feasibility of the proposed approach to assess the seismic ductility demand of shear-critical RC columns. The presented idealized SDOF system can simplify the establishment of CSDDS method. The lognormal assumption cannot well describe the probabilistic characteristics of seismic ductility demand if all the uncertainties in model parameters and earthquakes are considered. The developed empirical expression is applicable to investigate the probabilistic seismic risk of shear-critical RC columns under different earthquake excitations.