A new adjustable damper is evaluated in this study. The damper is capable of providing different levels of damping based on the fluid pressure differential across the piston and fluid flowing through an adjustable orifice. Such dampers can be used for seismic protection of structures. This study is aimed at evaluating the effectiveness of adjustable dampers for mitigation of earthquake excitations. The new damper is tested under cyclic and ramp loadings. From the tests, the damping properties are established. Analytical studies are performed based on the test results. A two-story base isolated structure with adjustable damper is studied analytically. The analytical model is subjected to several near-fault earthquakes. It is shown by means of experimental and analytical studies that the adjustable damper is effective in reducing the seismic response of base isolated buildings subjected to near-fault earthquakes.

Thorough understanding of dynamic system behavior is of major importance in many fields in science and engineering. When the spectral content of a process is changing with time, neither the time nor the frequency domain alone is sufficient to describe the process accurately. It is clearly recognized that a method to analyze excitation and response processes in both time and frequency domains is needed. This dissertation introduces a method to estimate the response of nonlinear systems to non-stationary excitations described by their wavelet coefficients. Time-frequency localization properties of wavelets are utilized to capture the evolutionary behavior of the spectral characteristics of the non-stationary processes and to describe the time dependent behavior of a nonlinear system. The wavelets-based approach developed in this thesis employs harmonic wavelets due to their concise form in the frequency domain, where each point in the frequency domain belongs to one particular scale only, thereby allowing one to use the terms "scale" and "frequency band" interchangeably. Utilizing this attractive property of the harmonic wavelets, an explicit relationship between the harmonic wavelet coefficients of a process and its time dependent spectral content is derived. To estimate the response of a nonlinear system, a wavelets-based equivalent linearization method is introduced, where the nonlinear system is replaced by its linear equivalent with time varying stiffness and damping coefficients. The applicability and the physical soundness of the procedure developed in this dissertation are demonstrated with examples from systems with elastic and hysteretic nonlinearity using data pertaining to simulated and recorded earthquake accelerograms. The applications presented in this dissertation show that the wavelet based linearization method is a quite promising tool for "real world" vibration problems.

Real-time structural health monitoring technique based on the interaction matrix formulation is developed in this study. New observer based methods such as ARMarkov observer and Input Error Function (IRF) observer are proposed to estimate extent of damage in real-time. Mathematical formulation for IRF based on interaction matrix formulation is developed in this study. Internal relative virtual force and equivalent system concepts are developed to establish IRF for real-time structural damage detection and localization. Input error function based structural damage detection method is applied to a three-dimensional truss structure, in which nonlinear and time-varying damage in members is detected and localized in real-time. IRF observers are proposed to estimate the extent of damage in real-time. Extent of damage is determined by minimizing the IRF. Optimization procedure proposed is robust with respect to system uncertainty and noise levels in the input-output measurements. When compared to state space observer based structural damage detection methods, in the design of ARMarkov observers, system and noise statistics are not necessary. Additionally, the method does not require initial conditions and is also robust to noisy output measurements. Furthermore, Sensitivity Enhancing Control (SEC) algorithms can be incorporated into ARMarkov observers to detect small structural damages. Proposed ARMarkov observers based on interaction matrix formulation are applied to track progressive stiffness deterioration in real-time in a two-dimensional truss structure. Experimental verification of an input failure detection algorithm is performed on a NASA 8-Bay truss structure, where, failure among the actuators is detected and distinguished in real-time. Structural damage detection methods based on ARMarkov observers and IRF observers are verified experimentally on a time varying two-degree of freedom system. Stiffness variation in this system is tracked in real-time using the proposed structural damage detection methods. IRF based structural damage detection is evaluated for structural health monitoring study of IASC-ASCE structural health monitoring benchmark problem. It is shown that the proposed formulation performs well for different damage cases established by IASC-ASCE structural health monitoring task group.

Immersive Virtual Environment (VE) technology has found varying applications in different fields of science and engineering. While there exist several areas in structural engineering that can benefit from the use of Immersive VE technology there is a general lack of these applications in the structural engineering domain. One of the main reasons for this is the high degree of familiarity with a host of low level hardware and software technology required on the part of the designer. In this work we have developed a conceptual framework for developing tools that present Immersive VE designers in the structural engineering domain with a high level interface that hides the low level details. A sample implementation of this framework designed to use the CAVE/ImmersaDesk systems was also developed. Finally the effectiveness and applicability of the implementation was tested by designing two trial implementations.

The elastic and inelastic responses of the force-excited single-degree-of-freedom (SDOF) systems are investigated from the gross characteristics of forcing functions such as the frequency content, the integrals and the amplitudes. The studies are carried out initially with simple version of force-excitation and then, using progressively complicated version of forcing functions having more than one frequency. The forces are chosen in such a way that they resemble the simple version of wave-induced forces, thus, paying a special attention to compliant systems. The responses due to actual wave-induced forces are also studied considering the surface record from the Gulf of Mexico during Camille hurricane (August, 1969). The responses are displayed in tripartite response spectra and explained from the characteristics of the forces. The effect of the frequency content in the forces and their integrals are addressed while interpreting the results. The studies are also made based on the parameters such as damping of the systems and the periodicity of forcing functions. An attempt is made to approximate the elastic responses from the characteristics of the integrals of forcing functions in low and low-medium frequency region. The inelastic responses are studied based on the gross characteristics of forcing function and they are compared with the elastic responses. Low-frequency systems of high ductility and low ductility are given a special attention due to current interest in compliant systems.

A deformation-based method that can be used for the analysis of the seismic response and for the design of waterfront retaining structures is developed. The study is focused on the behavior of gravity caisson-type quay walls. The key element for the success of the method lies on the selection of an appropriate constitutive model that can describe adequately the monotonic and cyclic undrained behavior of soil subjected to arbitrary stress paths. To this end, an existing generalized plasticity model, developed by Pastor et al. (1990), was examined and modified in order to improve its performance. Comparisons with a centrifuge model study and a case history from the recent 1995 Kobe earthquake on gravity quay walls are used for the evaluation of the new method of analysis, based on the modified generalized plasticity model. Particular emphasis is given to the undrained behavior of the foundation and backfill sand, and its effect on the deformational mechanism and damage of the quay walls.

Many existing flat-plate buildings are seismically deficient and pose a threat to life safety if subjected to ground motions of even low to moderate intensity. Failure in such structures is typically the result of punching failure at the slab-column connection. Because of this, performance-based retrofit procedures are needed to upgrade these non-ductile buildings. This investigation evaluated the use of lightweight pumice stone concrete (LWPSC) infill panels as a retrofit alternative for flat-plate buildings. Six four-tenth-scale slab-column subassemblies were designed and detailed based on ACI 318-63 and current performance-based testing requirements. Except for one bare frame specimen, all the subassemblies were retrofitted with prefabricated LWPSC infill panels and subjected to quasi-static loading conforming to FEMA 273. The geometry of the individual units was governed by weight limitations for handling and erection. Among the variables studied were connections between the slabs and the infill wall and the addition of uniformly distributed perforations (circular and rectangular openings). All of the retrofitted specimens had significant increases in both strength and stiffness over that of the bare frame. The behavior of the specimen with the infill panels not attached to the slabs was similar to that of a masonry wall without any connections to the frame. Although diagonal tension cracks formed within the recessed region, ultimate failure of the infill did not occur. Instead, frame-wall interaction transmitted large concentrated shear forces into the column that eventually contributed to failure of the longitudinal tension splice. The remaining subassemblies all had connections to the slabs and perforations within the wall. Specimens with circular holes experienced uniformly distributed cracking throughout the entire area of the infill wall. The chosen configuration, however, did not sufficiently weaken the wall, and shear failure of the column stopped the test. Because of its ductility and energy dissipation mechanism, the most promising infill panel configuration contained rectangular perforations. Two different reinforcement patterns were tested using rectangular openings. The addition of diagonal reinforcement between openings had the effect of increasing the yield strength of the wall as well as better maintaining post-yield deterioration.

Sliding isolation systems are effective in strong earthquakes; however, base displacements can be excessive in near source ground motions. Although passive nonlinear dampers can limit base displacements, the isolation forces, superstructure interstory drifts, and superstructure accelerations will be higher. Use of smart devices such as Magneto-rheological (MR) dampers, and semi active independently variable stiffness (SAIVS) devices in the isolation system may provide significant advantages. Independently varying damping and stiffness systems, used separately and together in sliding isolated buildings and bridges is investigated, analytically and experimentally, in this study. New analytical models of the sliding isolated structure with MR dampers and SAIVS devices are developed. New nonlinear control algorithms are developed. Extensive numerical simulations are performed with several near fault ground motions. Structural models and devices are tested on a shaking table in real time and the performance is evaluated analytically and experimentally. It is shown that (1) the newly developed SAIVS device is capable of varying the stiffness continuously and smoothly and the proposed analytical model captures the behavior of the device satisfactorily, (2) the newly developed analytical model for the MR damper predicts the behavior of the damper satisfactorily, (3) the MR damper along with the newly developed Luapunov controller, when introduced at the isolation level, reduces the isolation displacements further than the passive cases, while maintaining the forces, drifts, and accelerations within bounds in both buildings and bridges, (4) the SAIVS incorporated at the isolation level, in the controlled mode switching based on the newly developed control algorithm, reduces the sliding bearing displacements further than the passive open and closed cases, while the total forces, drifts, and accelerations are comparable to the least of the passive cases in both buildings and bridges, and (5) the SAIVS and MR damper in the controlled mode, when incorporated at the isolation level, reduce the displacements further than the passive, variable stiffness, and variable damping cases, while maintaining forces at the isolation level, drifts, and accelerations within bounds. The analytical and experimental study prove that the independently varying stiffness and damping systems and the developed controllers reduce the response of sliding isolated buildings and bridges significantly in near fault earthquakes.

This research focuses on isolation and quantification aspects of structural damage detection using global damage detection methods. It includes two parts: (1) isolation and detection of failure of the actuator/sensor/structural elements in real-time using interaction matrix formulation and parity based method, and (2) isolation and quantification of the structural damage using a novel physical parameter realization method. The failure of actuator/sensor/structural element considered in this thesis can be any type of deviation from the normal operating conditions. First a novel algorithm is developed to isolate and detect the time instants of actuator failure based on the interaction matrix formulation, leading to one error function for each actuator. The existence of interaction matrix is extended to eliminate the state and the unexamined inputs from the error function. For measurement noise free case, the error function will have non-zero values when the examined actuator fails to follow the commanded input, regardless of the status of other actuators. Further the coefficients of the error function are directly calculated from the healthy input data, from the examined actuator, and all measured outputs. Thus the need to know the state-space model of the system is bypassed and it also avoids the errors incurred in the system identification step. Combining the interaction matrix formulation with inverse model, novel sensor failure detection algorithm is developed. The error function, one for each uncertain sensor, can isolate and detect the real-time failure of the examined uncertain sensor. Parity based method is extended for structural damage detection. By utilizing the geometric properties and frequency information incurred by the damaged elements, a robust and sensitive structural damage detection filter is developed. The effectiveness of the developed algorithms is demonstrated using numerical simulations and verified using experimental results. A novel off-line physical parameter realization method is developed in this thesis. By assuming that the geometric connections of the stiffness and damping matrices are known, the stiffness and damping matrices can be fully realized with limited number of inputs and outputs. Then the developed algorithm is applied for structural damage detection. Using numerical simulations it is shown that the method can detect, isolate, and quantify the severity of average in the stiffness and damping of structural elements.

A model of a marine riser system which is appropriate for the study of its dynamic behavior in deep water conditions is developed. This model is used to estimate the steady-state riser response to harmonic excitation. In this regard, the finite element method yields a linear discrete multi-degree-of-freedom structural model; its stiffness matrix varies with time due to riser top tension fluctuations. Further, Morison's equation is used for estimating the hydrodynamic load on the riser. Due to the nonlinearity of the drag term appearing in this equation, the riser equation of motion becomes nonlinear. An approximate analysis procedure based on the concepts of equivalent linearization and of time averaging leads to efficient determination of the riser maximum stress. A continuous beam model under constant tension which is equal to the average of the top and the bottom riser tension is used to provide an estimate of its natural frequencies. Numerical results from a variety of parameter studies are reported.