- Current bridge design codes do not account for the effects of duration of mega-thrust subduction zone earthquakes. Furthermore, codes and even recent performance-based design methodologies only make use of the peak response quantities (forces, displacements, and ductility demands) and do not account for the increased number of inelastic cycles that the structure is expected to experience due to the mega-thrust long-duration subduction zone earthquake ground motions.
The main goal of this research is to investigate the effect of subduction zone earthquake ground motions on bridge structures as a first step towards providing basic information for adapting current codes and performance-based design guidelines. To achieve this goal, eighty (80) subduction zone earthquake ground motions are processed and motions from shallow crustal earthquakes with similar spectral shapes are selected. Three simple nonlinear single-degree of freedom (SDOF) models are subjected to the 160 (80 x 2) ground motions and seismic demands are estimated. The SDOF models analyzed include an elasto-plastic with hardening model, a Takeda model, and a model with isolation bearings. The models are representative of recent bridges in California and the Pacific Northwest. Effects of varying the parameters that describe these SDOF models are studied. Effects of P-Delta are also investigated. Four measures of seismic demand are assumed to be correlated to damage that will be sustained by a bridge The seismic demands monitored in these analyses are ductility, energy dissipated (yielding and damping), number of inelastic excursions, and normalized cumulative absolute inelastic displacements. These four measures of seismic demand are assumed to be correlated to the damage that will be sustained by a bridge.Statistical correlation coefficients between ground motion intensity measures that are sensitive to duration (cumulative absolute velocity, significant duration, and arias intensity) and the four measures of seismic demand described above provide for new insight on the effects of duration on bridge structures.
To study the effect of the long-motion duration, a parametric sensitivity analysis of simplified nonlinear SDOF models corresponding to cantilever single-column bridge bents is performed. Three different SDOF models are analyzed: an elastoplastic SDOF model, a Takeda-type (stiffness degrading) model, and an isolated bearing system model. For the elastoplastic and Takeda type models, the input parameters of this parametric analysis include: (i) R-factor (26 levels); (ii) hardening ratio (4 levels); (iii) two earthquake types (80 Crustal earthquakes records, 80 Subduction zone earthquakes records); (iv) structural periods (3 levels); (v) P-Delta effect (2 levels). For the SDOF system with the isolation bearing, the input parameters are: (i) period of the bearing (3 levels); (ii) friction coefficient (24 levels); (iii) two earthquake types (80 Crustal earthquakes records, 80 Subduction zone earthquakes records); and (iv) P-Delta effect (2 levels).
The Open System for Earthquake Engineering Simulation (OpenSees) software is used to compute the nonlinear dynamic response of each SDOF model and a batch queue system (HTCondor, Thain 2005) is used for running the very large number of simulations efficiently. Comparative statistics of nonlinear response quantities of SDOF systems subjected to crustal earthquakes and subduction zone earthquakes are presented in terms of seismic capacity (normalized strength), and seismic demands (ductility factor, cumulative absolute inelastic displacement, number of inelastic excursions, ratio of damping energy and yielding energy). In addition, for the system with isolation bearings, the effect of the friction coefficient on the isolation bearing displacement, pier base shear, and deck accelerations are studied under these two earthquake types. Lastly, the linear correlation coefficient between the seismic demands and the ground motion intensity measures is presented.
In conclusion, this parametric analysis provides insight on the expected damage (and its uncertainty) as a function of various ground motion characteristics of long-duration earthquake records and provides data that can be used later to calibrate design equations that have been developed for crustal ground motions.