George P. Mavroeidis

A Mathematical Representation of Near-Fault Ground Motions

A simple, yet effective, analytical model is proposed for the representation of near-field strong ground motions. The model adequately describes the impulsive character of near-fault ground motions both qualitatively and quantitatively. In addition, it can be used to analytically reproduce empirical observations that are based on available near-source records. The input parameters of the model have an unambiguous physical meaning. The proposed analytical model has been calibrated using a large number of actual near-field ground-motion records. It successfully simulates the entire set of available near-fault displacement, velocity, and (in many cases) acceleration time histories, as well as the corresponding deformation, velocity, and acceleration response spectra. Furthermore, a very simplified methodology for generating realistic synthetic ground motions that are adequate for engineering analysis and design is outlined and applied. Finally, it should be noted that the analytical model (along with the scaling laws of its parameters) proposed in the present work has the potential to facilitate the study of the elastic and inelastic response of conventional, nonconventional (e.g., base-isolated), and special structures (e.g., suspension bridges, fluid-storage tanks) subjected to near-source seismic excitations as a function of the model input parameters and thus, ultimately, as a function of earthquake size.

Effect of Fault Rupture Characteristics on Near-Fault Strong Ground Motions

The effect of fault rupture characteristics on near-fault strong ground motions is investigated using a kinematic modeling approach in an attempt to identify physical processes that lead to specific ground-motion patterns. The shear-stress distribution on the causative fault plane of four well-documented seismic events (i.e., 1979 Imperial Valley, 1985 Michoacan, 1989 Loma Prieta, and 1999 Izmit) is calculated based on fault slip models available in the literature using the methodology proposed by Bouchon (1997) for stress field computations. In order to associate the fault rupture characteristics (i.e., slip, rupture velocity, state of stress) of the investigated earthquakes with near-fault ground motions generated by the events, forward ground-motion simulations are performed using the discrete wavenumber representation method and the concept of the S -wave isochrones is exploited. The results indicate that the seismic energy radiated from the high-isochrone-velocity region of the fault arrives at the receiver within a time interval that coincides with the time window of the long-period ground-motion pulse recorded at the site. Furthermore, the near-fault ground-motion pulses are strongly correlated with large slip on the fault plane locally driven by high stress drop. In addition, the local rupture velocity seems to be inversely correlated to the spatial distribution of the strength excess over the fault plane confirming findings of previous studies. For various events the area of the fault that contributes to the formation of the near-fault pulse encompasses more than one patch of significant moment release (subevent) (e.g., 1979 Imperial Valley, 1989 Loma Prieta). This observation explains why a dislocation model with average properties (i.e., slip, rise time, etc.) reproduces successfully near-fault ground motions for strike-slip faults and for dip-slip faults with intermediate-to-large earthquake magnitudes (Aki, 1979). However, for very large earthquakes, such as megathrust events on subduction zones (e.g., 1985 Michoacan), the fault region that contributes to the pulse formation encompasses individual subevents and, consequently, cracklike slip functions (rather than dislocation models) may be more appropriate for the simulation of the near-fault ground motions.

Damping/global energy balance in FE model of bridge foundation lateral response

Abstract A global energy analysis is presented of three static unloading–reloading foundation lateral loading cycles, calculated using the nonlinear finite element (FE) program DYNAFLOW. This simulates seismic action on an offshore pier foundation in the Rion-Antirion Bridge in Greece, located in deep-sea water (65 m). A cyclic horizontal force is applied at a height of 30 m to a rigid raft 78 m in width placed on the surface of an idealized 2-layer soil profile consisting of a 3.5 m man-made gravel layer over soft deep natural clay, with elastic vertical steel inclusions reinforcing the soil. Results of the two-dimensional FE run are used for the energy analysis. It is verified that for the three cycles, the sum of energies associated with the external forces and moments, mostly dissipated through hysteresis loops, is about equal to the sum of the total internal energies dissipated or stored in the system. For the smaller loops almost all energy is dissipated in the soil, while for the largest loop about half of the energy is dissipated by horizontal sliding at the raft-soil interface. Global damping ratios obtained from the areas of the horizontal and rocking moment hysteresis loops are about double of those computed from the corresponding static backbone curves using the Masing criterion.

Multiobjective Design of Supplemental Seismic Protective Devices Utilizing Lifecycle Performance Criteria

AbstractThe cost-effective design of seismic protective devices considering multiple criteria related to their lifecycle performance is examined, focusing on applications to fluid viscous dampers. ...

PARAMETRIC INVESTIGATION OF THE DYNAMIC RESPONSE OF RIGID BLOCKS SUBJECTED TO SYNTHETIC NEAR-SOURCE GROUND MOTION RECORDS

We study the seismic response of rigid block structures against synthetic pulse-like ground motion records. A large number of synthetic ground motion records are systematically produced for various magnitude-distance scenarios. More specifically, we generate pulselike ground motions for a grid of 56 receiver stations assuming a vertical strike-slip fault. The site conditions simulate a NEHRP Class D site, while for every combination of hypocenter, magnitude and receiver location, we generate 100 realizations consisting of lowand highfrequency components. The low-frequency component is based on a four-parameter wavelet, while the specific barrier model is used for the high-frequency component. The synthetic ground motions are used to study the seismic overturning of rigid blocks of various dimensions. Τhe low-frequency pulse is described by four-parameters which refer to the amplitude, the prevailing frequency, the phase angle and the oscillatory character of the record, on top of which the high-frequency component is added. This description allows to parametrize the seismic response and thus improve our understanding on the effect of base motion characteristics on the overturning of rigid blocks.

Tuning of record-based stochastic ground motion models for hazard-compatibility and applications to seismic risk assessment

This paper discusses the tuning of stochastic ground motion models so that compatibility of the resultant hazard with Ground Motion Prediction Equations (GMPEs) is directly established. This is facilitated by proper optimization for the predictive relationships involved in such models that relate seismicity characteristics to the model parameters. A computationally efficient approach is discussed that can seamlessly provide ground motions that match any chosen GMPE for any desired set of seismicity characteristics or structural periods. Foundation of the methodology is the development of a metamodel that facilitates a simplified relationship between the ground motion model parameters and its median predictions. This metamodel is subsequently used to efficiently select the predictive relationships that optimize the match to the chosen GMPE with the numerical accuracy of the metamodel predictions also explicitly incorporated in this optimization. The implementation of the tuned model within seismic risk assessment is also demonstrated.

Design of flow isolation systems through multi-objective criteria for the seismic-risk performance

This paper discusses a probabilistic framework for performance assessment and optimal design of floor isolation systems for the protection of acceleration sensitive contents. A multi-objective formulation is considered for the optimization problem with the two competing objectives corresponding to (i) maximization of the level of protection offered to the sensitive content (acceleration reduction) and (ii) minimization of the demand for appropriate clearance to avoid collision to surrounding objects (floor displacement reduction). Uncertainties are addressed by characterizing these objectives in terms of the associated seismic risk, whereas a surrogate modeling approach is developed to evaluate this risk and support the design optimization. As an illustrative example, the design of a polynomial friction pendulum isolator system is presented. The formulation is demonstrated to efficiently provide design solutions with different performance levels across the considered competing objectives, offering a range of options for selecting the final protection system.

The Response of the Seismically Isolated Bolu Viaduct Subjected to Fault Crossing

The effect of fault crossing on the response of the seismically isolated Bolu Viaduct is investigated. The seismic ground motions at the site of the Viaduct are generated following a rigorous methodology, which is based on seismological data available in the literature and finite-fault modelling techniques of the extended source. The generated motions are used to study the effect of fault crossing and fault crossing angle on the nonlinear behaviour of the seismically isolated Viaduct. It is shown that fault crossing is an important factor in the earthquake response of seismically isolated bridges, and that this effect needs to be considered in the design and detailing of the isolation system.