Flexible Ground Research Articles

Seismic performance of three-dimensional frame structures with underground stories

This paper investigates the seismic performance of moment-resisting frame steel buildings with multiple underground stories resting on shallow foundations. A parametric study that involved evaluating the nonlinear seismic response of five, ten and fifteen story moment-resisting frame steel buildings resting on flexible ground surface, and buildings having one, three and five underground stories was performed. The buildings were assumed to be founded on shallow foundations. Two site conditions were considered: soil class C and soil class E, corresponding to firm and soft soil deposits, respectively. Vancouver seismic hazard has been considered for this study. Synthetic earthquake records compatible with Vancouver uniform hazard spectrum (UHS), as specified by the National Building Code of Canada (NBCC) 2005, have been used as input motion. It was found that soil–structure interaction (SSI) can greatly affect the seismic performance of buildings in terms of the seismic storey shear and moment demand, and the deformations of their structural components. Although most building codes postulate that SSI effects generally decrease the force demand on buildings, but increase the deformation demand, it was found that, for some of the cases considered, SSI effects increased both the force and deformation demand on the buildings. The SSI effects generally depend on the stiffness of the foundation and the number of underground stories. SSI effects are significant for soft soil conditions and negligible for stiff soil conditions. It was also found that SSI effects are significant for buildings resting on flexible ground surface with no underground stories, and gradually decrease with the increase of the number of underground stories.

A Framework For Robust and Flexible Ground Station Networks

Space communication systems are plagued by complexity resulting in limited access to orbiting satellites and high mission operation costs that ultimately reduce mission yields and capabilities. In particular, ground stations, the access point between space and terrestrial networks, suffer from monolithic designs, narrow interfaces, and reliability issues that raise significant engineering barriers. We have reduced these barriers by decomposing monolithic ground station functions into basic component services and virtualizing their software and hardware interfaces. We captured these core services in a command and control protocol, called the Ground Station Markup Language. This modularization has also enabled us to apply recovery-oriented computing techniques, specifically recursive restarts, to quickly recover from software failures and improve overall ground station availability. Our testbed consists of globally distributed ground stations supporting university satellite missions. With case studies, we demonstrate how our work enables rapid prototyping and reconfiguration within a ground station and how it facilitates coordination of networked, heterogeneous ground stations by hiding low-level implementation details.

BeppoSAX: a historical perspective

Abstract Launched in April 1996, operational for six years, BeppoSAX has opened a new view on the X-ray universe, with its broad-band and wide angle instruments associated with flexible ground segment operations. In this paper we recall some of the history behind the development of the project, summarize some of its scientific achievements and comment on its heritage.

A recursive multibody model of a tracked vehicle and its interaction with flexible ground

A high-fidelity model of a tracked vehicle traversing a flexible ground terrain with a varying profile is presented here. In this work, we employed a recursive formulation to model the track subsystem. This method yields a minimal set of coordinates and hence, computationally more efficient than conventional approaches. Also, in the vehicle subsystem, the undercarriage frame is assumed to be connected to the chassis by a revolute joint and a spring-damper unit. This increase in system mobility makes the model more realistic. To capture the vehicle-ground interaction, a Winkler-type foundation with springs-dampers is used. Simulation runs of the integrated tracked vehicle system for vibrations for four varying ground profiles are provided.

Analysis of a Three-Dimensional Tank-Liquid-Soil Interaction Problem

A three-dimensional analysis of the dynamic behavior of liquid-filled elastic cylindrical tanks based on flexible grounds, undergoing horizontal and vertical earthquake excitation is presented. The interaction of the ideal fluid with the elastic shell and with the flexible ground yields a problem of linear potential theory which must be solved together with the equations of motion of the shell and of the ground. With the unknown modal shapes of vibration developed in Fourier and in Fourier-Bessel series, the partial differential equations are transformed into coupled generalized equations of vibration by a weighted residual approach. The results show the strong influence of the flexible ground characterized by a remarkable shifting of natural frequencies, by the existence of additional natural frequencies, and by high damping ratios.

An Analytical Model for Unanchored Fluid-Filled Tanks Under Base Excitation

A set of equations is derived describing the dynamic response of cylindrical liquid storage tanks under horizontal ground excitation. The structure consists of a flexible cylindrical tank with a roof and a bottom plate and rests on a flexible ground through a rigid foundation. Portion of the base of the tank may separate from and lift off the foundation during ground motion. The solution of the hydrodynamic problem is first found in closed form. Then, Hamilton’s principle is applied and the equations governing the behavior of the coupled fluid/structure/ground system are derived. During this procedure, the base uplifting is modeled by an appropriate rotational nonlinear spring placed between the base of the tank and the foundation.