PhD in Civil Engineering
Structural Mechanics

Current Research

• Investigation of Strain Localization in Geomaterials (Bardet). Discontinuities in the form of cracks, fissures, and inclusions are often present in soils and soft rocks. They concentrate stresses within the materials subjected to loads, initiate and propagate shear bands, and ultimately lead to global failures of soil masses. New analytical models are being created, based on recent developments in continuum mechanics and finite element methods. The continuum mechanics description includes higher-order continua (e.g., nonlocal, higher- strain gradient, and micropolar theories). The finite element methods enhance the spatial discretization and attempts to get mesh insensitivity (e.g., strain projection approach and adaptive remeshing).
• Active Control Methods for Flexible Structures. Active control methods suitable for on-line application to reduce the oscillations of nonlinear flexible structures (such as buildings and cable-supported bridges) are being explored. Control action is being furnished by an internal mechanism of momentum transfer between the primary structure and several nonlinear auxiliary mass dampers.
• Earthquake Response Characteristics of Bridges under Multiple-Support Excitations. Out-of-phase displacements of bridge columns and pier foundations may cause significant forces and/or displacements in structural components, and the magnitude of this effect is not well understood. Efforts in this project are directed toward the utilization of USC's two shake tables system to investigate the effects of differential (or phased) ground motion from multiple- support excitations on bridge structure response.
• Identification of Building Structural Dynamic Parameters. Use of accelerographic records obtained during earthquake ground shaking for the estimation of stiffness and damping parameters in building structures is being studied. The use of the data obtained for artificial excitations and ambient vibrations is also under investigation. Emphasis is placed on development of efficient algorithms for identification. Accuracy of the parameter estimates is investigated. The problem of uniqueness of the identified parameters is investigated analytically.
• Testing, Analysis and Design of Reinforced Concrete Structures Subject to Strong Ground Motion. Tests are conducted on beam-column connections utilizing the USC multiple shake table system, and data obtained from instrumented buildings, are analyzed to calibrate global models for nonlinear transient analysis of multistory buildings. The applications of high strength concrete, non- destructive evaluation techniques, and repairing and strengthening of structures are considered in this comprehensive research program.
• Semiactive Damping and Structural Control (Johnson). One of the most promising technologies for mitigating vibration, induced by seismic, wind, and human excitations, is semiactive damping technology. This method of structural control uses "smart" controllable dampers that retain the stability and reliability of passive damping devices but are controllable, so they can often attain performance comparable to fully active devices but with low-power requirements. This technology is in development for a wide range of structural applications, including "smart" base isolation and "smart" damping of stay cables.
• Controlled Monte Carlo Simulation (Johnson). Standard Monte Carlo Simulation (MCS) is the most versatile and general tool for studying stochastic dynamical systems. These systems appear in a wide range of fields, from economic modeling to the response of structures to earthquake and wind loads. Yet standard MCS can be extremely computationally intensive. This research is developing new "controlled" MCS methods, primarily based on genetic algorithms, that direct more computational effort toward events critical to component and system failures, thereby reducing the computational intensity.
• Deformations Near Underground Infrastructure (V. Lee). Los Angeles began the construction of the L.A. subway system in the early 1990's. Such construction in recent years has experienced some difficulties. It is important to understand the causes of this structural damage. The County of Los Angeles is in a seismic region. We need to study the structural deformations resulting from the strong earthquake ground motions in the vicinity of the Los Angeles subway. The research started with the analysis of the deformations of surface canyons above an underground tunnel. This case involves the diffraction of seismic waves in the presence of both, surface and subsurface topographies. The analysis resulted in a much larger than expected (as much as three times) amplifications of surface displacements. The same methodology has also been applied to a practical problem from the 1994 Northridge earthquake. Case studies were made on large flexible corrugated metal pipes (CMP) that were shaken and damaged during the quake in the vicinity of the Van Norman Complex, in the San Fernando Valley.
• Adaptive Finite Element Analysis of Problems in Structural Mechanics (Wellford). As many numerical algorithms for solving structural mechanics problems are approximate, there is a need for techniques which adapt the mathematical models so as to minimize the inherent error. In finite element procedures, this is often achieved by introducing smaller elements (h-methods), increasing the polynomial degree of the existing elements (p-methods) or repositioning the nodes (r-method). Adaptive methods, of various types, are being studied to define optimum finite element solution algorithms.
• Modeling of Reinforced Concrete Structures using the Finite Element Method (Wellford). The performance of reinforced concrete structures, under static and dynamic loads, can be quite complex, involving highly nonlinear behavior. This behavior is characterized by nonlinear material behavior in the concrete, including cracking, loss of confinement, and softening behavior under cyclic load. In addition, reinforcing steel bars may have a strain hardening behavior after yield and the bond between the steel and concrete may be broken, resulting in slip and pullout. Complicated localization phenomena may result at concentrated plastic hinges. Computational mechanics and finite element methods are being developed to simulate this complex phenomenon.
• Stability of Stochastic Systems (M. Shinozuka). A methodology for generating sample functions for colored noise that has some relative advantages over existing ones is being studied. This involves simulating a stochastic process in terms of harmonic functions, the amplitude of which is related to the power spectral density of the noise. The computational advantages of the fast Fourier transform are utilized and the numerical integration of the noise equation is avoided. In light of this methodology, existing algorithms for colored-noise simulation are critiqued and compared with the present one. Numerical results for bi-stable systems and self-excited systems are derived that are significantly different from the solutions obtained under white noise assumptions.
• Development of System Identification Techniques (M. Shinozuka). This research looks into the application of a number of system identification techniques to the problems of earthquake engineering. A number of techniques for structural system identification have been developed over the past few years. Many of these techniques have been successful at identifying properties of linearized and time-invariant equivalent structural systems. Most of these techniques were verified using mathematical models simulated on the computer. In this study, a number of structural identification algorithms are reviewed and applied to the identification of structural systems subjected to earthquake excitations. The algorithms are applied to experimental data obtained in controlled laboratory conditions. The data pertain to acceleration records from two building models subjected to various loading conditions. The performance of the various identification algorithms is critically assessed, and guidelines are obtained regarding their suitability to various engineering applications.
• Non-Periodic Inspections of Fatigue-Sensitive Structures using Bayesian Analysis (M. Shinozuka). A Bayesian analysis methodology is introduced to determine appropriate non-periodic inspection intervals of fatigue-sensitive structures so that their reliability remains above a pre-specified minimum level throughout their service life. Fatigue damage is considered to initiate in a structural element when cracks develop, whether or not they are detected. The fatigue process then continues by crack propagation resulting in strength degradation. If a fatigue crack is detected during an inspection, the cracked component is assumed to be repaired or replaced with a new one resulting in the renewal of its residual strength and fatigue characteristics and thus increasing the overall reliability of the structure. This Bayesian approach is unique and novel in that it allows one to utilize judiciously the results of earlier inspections for the purpose of determining the time of the next inspection and estimating the values of several parameters involved in the problem that can be treated as uncertain. Applications can be found in the areas of aerospace engineering, offshore engineering, and structural engineering.
• Elastic Stability of Imperfection-Sensitive Structures (M. Shinozuka). The study of the effect of structural imperfections on the bifurcation buckling of structures has both great theoretical and practical interest. The theoretical interest stems from the sensitive dependence of the response (buckling strength of the structure) on the initial data (loads and material and geometric properties) so that the response exhibits certain characteristics of chaotic systems. In fact, the buckling response of imperfection-sensitive structures is considered as the archetype for a large class of similar problems, covering the full spectrum of the engineering sciences. The practical interest stems from the primary importance in advanced engineering applications of the structures whose bifurcation buckling response is sensitive to structural imperfections. Such structures include thin shells, space structures, beams with thin-walled sections, arches, trusses, and frames. In certain cases, the detrimental effect of the structural imperfections on the buckling strength is so strong, that it can decrease the buckling strength of the structure even below 50 percent of its nominal value.
• Remote Sensing Techniques in Damage and Loss Estimation (M. Shinozuka). Structural damage assessment, utilizing remote sensing capabilities in near real-time, is of major practical interest. This is referred to as global damage assessment, where a structure is looked at as a whole as opposed to the case of local damage assessment in which a particular structural component is of interest. The study is of a multidisciplinary nature and involves fields of physics, optics, electromagnetics, signal processing, image processing, computer vision, imaging, etc. Both Optical and Synthetic Aperture Radar (SAR) images from both airborne and satellite platforms are being studied. Image simulation is carried out for proof of concept studies and to develop a knowledge base for image interpretation. The study utilizes remotely sensed images and applies statistical methods for damage identification by comparing the images before and after the event of interest such as a disastrous earthquake. This is useful in order to extract useful information from the scene of interest for damage assessment, loss estimation and emergency recovery efforts.
  
• Application of GIS System in Lifeline System Performance (M. Shinozuka). This research looks into the application of the Geographic Information System (GIS) for the assessment of existing lifeline systems, such as transportation networks, electrical power systems and water systems. With the aid of damage data, collected from past earthquakes, the fragility curves of the components of the system are developed by using the maximum likelihood method. Monte Carlo simulation is performed based on these fragility curves. These techniques are applied to Los Angeles and Orange County transportation networks, LADWP (Los Angeles Department of Water and Power) electric power systems and MLGW (Memphis Light, Gas and Water Systems) electric power and water systems. The results show that these techniques are very effective in the seismic analysis of lifeline systems.
• Development of Bridge Fragility Curves (M. Shinozuka). Bridges are potentially one of the most seismically vulnerable structures in the highway system. While performing a seismic risk analysis of a highway system, it is imperative to identify seismic vulnerability of bridges associated with various states of damage. The vulnerability information must account for a multitude of uncertain sources involved, for example, in estimation of seismic hazard, structural characteristics, soil-structure interaction, and site conditions. The current research is focused on the following methods: (1) quasi-static and design code consistent analysis, (2) utilization of damage data associated with past earthquakes, and (3) numerical simulation of bridge seismic response based on structural dynamics.
• Experimental and Analytical Studies of Structures and Structural Elements Subjected to Variable Axial Loads (Xiao). Problems in analysis and design of structures and components subjected to variable axial loads are addressed in this project. Model columns are tested under cyclic lateral forces and variable axial loads to investigate the effect of axial load variation on seismic performance of columns. Analytical methods are developed and calibrated using the test results.
• Experimental and Analytical Studies of Mechanical Behavior of Concrete under Triaxial Stresses (Xiao). Research is in progress to investigate the mechanical behavior of concrete under a triaxial stress state. The focus is on the study of the interaction mechanisms between the confined concrete and confining materials such as composite or steel jackets, tubes and transverse reinforcing bars.
  
  

The Faculty

Selected Publications