Elevators serve a critical function in essential facilities such as hospitals and need to remain operational during and after earthquakes. However, they are still known to malfunction during earthquakes even after several design and sensing improvements required by the current code have been incorporated. Most of the damages were experienced or caused by the rail-counterweight system. Being the heaviest component of an elevator, the counterweight induced strong dynamic effects to the guiding system sometimes even collided and damaged the passenger car. A realistic analytical model of rail-counterweight system of an elevator that includes details of the supporting system is developed in this study. The nonlinearities caused by closing of the code specified clearances play an important role in determining the dynamic behavior of the system, and are thus included in this study. Also included are the acceleration inputs from different floor of building and the effect of different location of the counterweight along the guide rail. Parametric study is carried out to investigate the effect of different parameters on the seismic responses of the rail-counterweight system. In order to improve the seismic performance of the rail-counterweight system, several protective schemes are investigated. One simple approach is to increase the damping of the system using additional discrete viscous dampers. However, there is not much space available for installing the devices, and placement parallel to the spring at the roller guide assemblies is not quite effective due to contact between the restraining plate at the roller guide assemblies and the rail that makes the roller guides ineffective. Another method is to convert the top part of the weights into a tuned mass damper. This method can reduce the maximum stress in the rail if designed properly. The effectiveness of the passive tuned mass damper can be improved further by using it in an active mode by installing an actuator between the mass damper and the counterweight frame. The numerical results that confirm the effectiveness of such an active tuned mass damper are presented. As an alternative to the fully active control scheme, a semi-active control scheme using a magnetorheological damper device between the mass damper and the frame is also studied. This control approach is found to be as effective in reducing the seismic response as a fully active scheme. Since this MR damper can be operated using a simple battery, the external power requirements for implementation of this approach are quite minimal. / Ph. D.

No description available.

A number of methods have been proposed that utilize the time domain transformations of the frequency dependent impedance functions to perform time-history analysis of structures accounting for soil-structure interaction (SSI). Though these methods have been available in literature for a number of years, this study is the first to rigorously examine the limitations and advantages of these methods in comparison to one another. These methods contain certain stability issues that required investigating which lead to the formation of an analysis procedure that assesses a transform method’s stability. The general applicability of these methods was demonstrated by utilizing them to model increasingly sophisticated reference problems. Additionally the suitability of these methods to being incorporated into hybrid simulations of nonlinear inelastic structures considering soil-structure interaction was confirmed. The modelling of a nonlinear structure considering soil-structure interaction is an improvement over the most common modelling strategies that model solely linear-elastic behaviour.

A number of methods have been proposed that utilize the time domain transformations of the frequency dependent impedance functions to perform time-history analysis of structures accounting for soil-structure interaction (SSI). Though these methods have been available in literature for a number of years, this study is the first to rigorously examine the limitations and advantages of these methods in comparison to one another. These methods contain certain stability issues that required investigating which lead to the formation of an analysis procedure that assesses a transform method’s stability. The general applicability of these methods was demonstrated by utilizing them to model increasingly sophisticated reference problems. Additionally the suitability of these methods to being incorporated into hybrid simulations of nonlinear inelastic structures considering soil-structure interaction was confirmed. The modelling of a nonlinear structure considering soil-structure interaction is an improvement over the most common modelling strategies that model solely linear-elastic behaviour.

In order to control the vibration response of high rise buildings during seismic events, energy absorbing passive damping devices are most commonly used for energy absorption. Today there are a number of types of manufactured dampers available in the market, which use a variety of materials and designs to obtain various levels of stiffness and damping. Some of these include friction, yielding, viscoelastic and viscous dampers. These dampers are usually installed between two load bearing elements (walls or columns) in new buildings. In existing buildings, which require retrofitting, they could be installed in cut-outs of shear walls, as evidenced from recent investigations. An effective damping system can result in higher levels of safety and comfort, and can also lead to considerable savings in the total cost of a building. This thesis treats seismic mitigation of multistorey buildings using embedded dampers. Three types of damping mechanisms, viz, friction, viscoelastic, and combined friction-viscoelastic were investigated. Finite element methods were employed in the analysis using the program ABAQUS version 6.3. A direct integration dynamic analysis was carried out to obtain the damped and undamped responses of the structure in terms of deflections and accelerations at all storeys in order to evaluate the effectiveness of the damping system in mitigating the seismic response. The damping mechanisms have been modelled as (i) a linear spring and dash-pot in parallel for the viscoelastic damper, (ii) a contact pair with friction parameter for a friction damper and (iii) a hybrid damper consisting of both a viscoelastic and a friction damper. The earthquake events used in this study have been applied as acceleration time-histories at the base of the structure in the horizontal plane. Concrete material properties were chosen to represent the model as many high-rise buildings are constructed by using reinforced concrete. Several medium and high-rise building structures with embedded dampers in different configurations and placed in various locations throughout the structure were subjected to different earthquake loadings. Influence of damper type and properties, configuration and location were investigated. Results for the reduction in tip deflection and acceleration for a number of cases demonstrate the feasibility of the technique for seismic mitigation of these structures for a range of excitations, even when the dominant seismic frequencies match the natural frequency of the structure. Results also provide information which can be used for optimal damper placement for seismic mitigation.

This research consists of full-scale cyclic shake table tests to investigate liquefied residual strength of #2/16 Monterey Sand. A simple shear testing apparatus was mounted to a full-scale one-dimensional shake table to mimic a confined layer of saturated sand subjected to strong ground motions. Testing was performed at the Parson’s Geotechnical and Earthquake Laboratory at California Polytechnic State University, San Luis Obispo. T-bar penetrometer pullout tests were used to measure residual strength of the liquefied soil during cyclic testing. Cone Penetration Testing (CPT) was performed on the soil specimen throughout testing to relate the laboratory specimen to field index test data and to compare CPT results of the #2/16 Monterey sand before and after liquefaction. The generation and dissipation of excess pore pressures during cyclic motion are measured and discussed. The effects of liquefied soil on seismic ground motion are investigated. Measured residual strengths are compared to previous correlations comparing liquefied residual strength ratios and CPT tip resistance.

Several passive energy dissipation devices have been implemented in practice as the seismic protective systems to mitigate structural damage caused by earthquakes. The solid viscoelastic dampers are among such passive energy dissipation systems. To examine the response reducing effectiveness of these dampers, it is necessary that engineers are able to conduct response analysis of structures installed with added dampers accurately and efficiently. The main objective of this work, therefore, is to develop formulations that can be effectively used with various models of the viscoelastic dampers to calculate the seismic response of a structure-damper system. To incorporate the mechanical effect from VE dampers in the structural dynamic design, it is important to use a proper force-deformation model to correctly describe the frequency dependence of the damper. The fractional derivative model and the general linear model are capable of capturing the frequency dependence of viscoelastic materials accurately. In our research, therefore, we have focused on the development of systematic procedures for calculating the seismic response for these models. For the fractional derivative model, we use the G1 and L1 algorithms to derive various numerical schemes for solving the fractional differential equations for earthquake motions described by acceleration time histories at discrete time points. For linear systems, we also develop a modal superposition method for this model of the damper. This superposition approach can be implemented to obtain the response time history for seismic input defined by the ground acceleration time history. For random ground motion that is described stochastically by the spectral density function, we derive an expression based on random vibration analysis to compute the mean square response of the system. It is noted that the numerical computations involved with the fractional derivative model can be complicated and cumbersome. To alleviate computation difficulty, we explore the use of a general linear model with Kelvin chain analog as a physical representation of the damper properties. The parameters in the model are determined through a curve fitting optimization process. To simplify the analytical work, a self-adjoint system of state equations are formulated by introducing auxiliary displacements for the internal elements in the Kelvin chain. This self-adjoint system can then be solved by using the modal superposition method, which can be extended to develop a response spectrum approach to calculate the seismic design response for the structural system for seismic inputs defined by design ground response spectra. Numerical studies are carried out to demonstrate the applicability of these formulations. Results show that all the proposed approaches provide accurate response values, and the response reduction effects of the viscoelastic dampers can be evaluated to assess their performance using these models and methods. However, the use of a general linear model of the damper is the most efficient. It can capture frequency dependence of the storage and loss moduli as well as the fractional derivative model. The calculation of the response by direct numerical integration of the equations of motion or through the use of the modal superposition approach is significantly simplified, and response spectrum formulation for the calculation of seismic response of design interest can be conveniently formulated. / Ph. D.

The flexibility of the floor slabs is quite often ignored in the seismic analysis of structures. In general, the rigid behavior assumption is appropriate to describe the in-plane response of floors. For seismic excitations with vertical components, however, the flexibility of the floor slabs in the out-of-plane direction may play a significant role and it can result in an increase in the seismic response. The simplified procedures used in the current practice to include the floor flexibility can lead to highly conservative estimates of the slab and supported equipment response. To include floor flexibility, a detailed finite element model of the structure can be constructed, but this procedure leads to a system with large degrees of freedom the solution of which can be time consuming and impractical. In this study, a new dynamic condensation approach is developed and proposed to reduce the size of the problem and to calculate the seismic response of structures with flexible floor slabs. Unlike other currently available dynamic condensation techniques, this approach is applicable to classically as well as nonclassically damped structures. The approach is also applicable to structures divided into substructures. The approach can be used to calculate as many lower eigenproperties as one desires. The remaining higher modal properties can also be obtained, if desired, by solving a complementary eigenvalue problem associated with the higher modes. The accuracy of the calculated eigenproperties can be increased to any desired level by iteratively solving a condensed and improved eigenvalue problem. Almost exact eigenproperties can be obtained in just a few iterative cycles. Numerical examples demonstrating the effectiveness of the proposed approach for calculating eigenproperties are presented. To calculate the seismic response, first the proposed dynamic condensation approach is utilized to calculate the eigenproperties of the structure accurately. These eigenproperties are then used to calculate the seismic response for random inputs such as a spectral density function or inputs defined in terms of design response spectra. Herein, this method is used to investigate the influence of the out-of-plane flexibility of the floor slabs on the response of primary and secondary systems subjected to vertical ground motions. The calculated results clearly show that inclusion of the floor flexibility in the analytical model increases the design response significantly, especially when computing acceleration floor response spectra. This has special relevance for secondary systems and equipment the design of which are based on the floor response spectra. The accuracy of the results predicted by two of the most popular methods used in practice to consider the floor flexibility effects, namely the cascade approach and the modified lumped mass method, is also investigated. The numerical results show that the cascade approach overestimates the seismic response, whereas the modified lumped mass method underestimates the response. Both methods can introduce significant errors in the response especially when computing accelerations and floor response spectra. For seismic design of secondary systems supported on flexible slabs, the use of the proposed condensation approach is thus advocated. / Ph. D.

The Upper Devonian–Lower Mississippian Bakken Formation in the Williston Basin is estimated to have significant amount of technically recoverable oil and gas. The objective of this study is to identify differences in the character of the seismic response to Bakken interval between locations with high and poor production rates. The predicted seismic responses of the Bakken Formation will hopefully help achieve such discrimination from surface seismic recordings. In this study, borehole data of Bakken wells from both the Cottonwood and the Sanish Field were analyzed, including density information and seismic P and S wave velocities from Sonic Scanner logs. The Bakken Formation is deeper and thicker (and somewhat more productive) in the Sanish Field and is shallower and thinner in the Cottonwood Field. The Upper and Lower Bakken shale units are similar and can be characterized by low density, low P and S wave velocities and low Vp/Vs ratios. The Sonic Scanner data suggest that the Upper and Lower Bakken shales can be treated as VTI media while the Middle Bakken may be considered as seismically isotropic. Models of seismic response for both fields were constructed, including isotropic models and models with variations in VTI, HTI, and the combination of VTI and HTI in the Bakken intervals. Full offset elastic synthetic seismograms with a vertical point source were generated to simulate the seismic responses of the various models of Bakken Formation. This sensitivity study shows pronounced differences in the seismic reflection response between isotropic and anisotropic models. P-P, P-SV and SV-SV respond differently to anisotropy. VTI anisotropy and HTI anisotropy of the Bakken have different character. In particular, types of seismic data (P-P, P-SV, and SV-SV) and the range of source-receiver offsets that are most sensitive to variations in anisotropic parameters and fluid saturation were identified. Results suggest that bed thickness, anisotropy of the Upper and Lower Bakken shales, fractures/cracks and fluid fill in the fracture/cracks all influence the seismic responses of the Bakken Formation. The paucity of data available for “poorly” producing wells limited the evaluation of the direct seismic response to productivity, but sensitivity to potentially useful parameters was established. / text

Characterizing natural and man-made fracture networks is fundamental to predicting the storage capacity and pathways for flow of both carbonate and shale reservoirs. The goal of this study is to determine the seismic response specifically to networks of fractures clustered closely together through the analysis of seismic wavefield scatter, directional phase velocities, and amplitude attenuation. To achieve this goal, finite element modeling techniques are implemented to allow for the meshing of discontinuous fracture interfaces and, therefore, provide the most accurate calculation of seismic events from these irregular surfaces. The work presented here focuses on the center layer of an isotropic model that is populated with two main phases of fracture network alteration: a single large-scale cluster and multiple smaller-scale clusters. Phase 1 first confirms that the seismic response of a single idealized vertically fractured cluster is distinct crosscutting energy within a seismogram. Further investigation shows that, as fracture spacing within the cluster decreases, the depth at which crosscutting energy appears exponentially increases, placing it well below the true location of the cluster. This relationship holds until 28% of the fractures are moved from their uniformly spaced locations to random locations within the cluster. The vertical thickness of the cluster has little effect on the location or strength or the crosscutting signature. Phase 2 shows that, although clusters of more randomly spaced fractures mask crosscutting energy, a marked decrease in amplitude coinciding with a bend in the wavefront produces a heterogeneous anisotropic seismic response. This amplitude decay and heterogeneous anisotropy is visible until cluster spacing drops below one half of the wavelength or the ratio of fractured material to matrix material within a cluster drops below 37%. Therefore, the location of an individual fracture cluster can be determined from the location of amplitude decay, heterogeneous anisotropy, and crosscutting energy. Furthermore, the density of the cluster can be determined from the degree of amplitude decay, the angle of heterogeneous anisotropy, and the depth of cross-cutting energy. These relationships, constrained by limits on their detectability, can aid fracture network interpretation of real seismic data. / text