Roberto Villaverde

Aseismic roof isolation system: analytic and shake table studies

Presented are the features of a roof isolation system that is proposed as a device to reduce the seismic response of buildings. Presented also are the details of and results from analytical and experimental studies conducted with a small-scale laboratory model to assess the feasibility and effectiveness of such a device. The roof isolation system entails the insertion of flexible laminated rubber bearings between a buildings roof and the columns that support this roof, and the installation of viscous dampers that are connected to the roof and a structural element below the roof. It is based on the concept of a damped vibration absorber and on the idea of making the roof, rubber bearings, and viscous dampers respectively constitute the mass, spring, and dashpot of such an absorber. The model considered in the analytical and experimental studies is a 2.44-m high, five-storey, moment-resisting steel frame, with a fundamental natural frequency of 2.0 Hz. In the experimental study the frame is tested with and without the proposed roof isolation system on a pair of shaking tables under a truncated version of one of the accelerograms from the 1985 Mexico City earthquake. In the analytical study, the frame is also analysed with and without such a system and under the same ground motion except that the ground motion accelerations are properly magnified to study the effectiveness of the roof isolation system when the frame is stressed beyond its linear range of behavior. It is found that the suggested device effectively reduces the seismic response of the frame, although the extent of this reduction depends on how large its non-linear deformations are. Based on these findings, it is concluded that the proposed roof isolation system has the potential to become a practical and effective way to reduce earthquake damage in low- and medium-rise buildings.

Seismic reliability of electrical power transmission systems

Abstract The reliability of electric power transmission systems is important for the probabilistic safety assessment of nuclear power plants under a given earthquake loading as it relates to the loss of off site power to the nuclear power plants. Here, a comprehensive model to evaluate the seismic reliability of electric power transmission systems is presented. The model provides probabilistic assessments of structural damage and abnormal power flow that can lead to power interruption in a transmission system under a given earthquake. With the proposed methodology seismic capacities of electrical. equipment are determined on the basis of available test data and simple modeling from which fragility functions of specific substations are developed. Earthquake ground motions are defined as stochastic processes. Probabilities of network disconnectivity and abnormal power flow are assessed through Monte Carlo simulations. The proposed model is applied to the electric power network in San Francisco and vicinity under the 1989 Loma Prieta earthquake, and the probabilities of power interruption are contrasted with the actual power failures observed during that earthquake.

Approximate formulas to calculate the seismic response of light attachments to buildings

Abstract Simple approximate formulas are proposed to compute the maximum response of equipment or any other light secondary system attached to buildings subjected to earthquake ground motions. The formulas are derived on the basis of a modified version of the conventional response spectrum method and the consideration of building and attachment as one unit. Notwithstanding, they are expressed in terms of the independent dynamic properties of the two components and ordinates from the response spectrum of a specified ground motion. Secondary systems with multiple degrees of freedom attached to one or two arbitrary points of a supporting multistory structure may be considered. As presented, however, the formulas are restricted to cases in which the independent primary and secondary systems are linear elastic with classical modes of vibration, and the masses of the secondary system are small in comparison with those of the primary one. Their accuracy is verified by means of a comparative study with time-history solutions. In this comparative study, the approximate formulas yield an average error of about 4% and a maximum of about 22%.

Simplified approach for the seismic analysis of equipment attached to elastoplastic structures

Abstract A simple approximate procedure is presented to estimate the maximum response of equipment, piping, or any other light secondary system mounted on nonlinear structures subjected to earthquake ground motions. The procedure is based on the consideration of structure and equipment as an integrated combined system, and on a response spectrum method for the analysis of nonlinear multistory structures. It is formulated in terms of the initial dynamic properties of the independent structure and equipment components, and the nonlinear response spectrum of a specified earthquake ground motion. It may be applied to any linear multiple-degree-of-freedom secondary system connected at one or two arbitrary points of a multistory structure. It fully takes into account the interaction between primary and secondary systems and the nonclassical damping character of structure-equipment systems. It is restricted, however, to structures with elastoplastic load-deformation behavior and to those cases in which the mass of the secondary system is small in comparison with the mass of the structure. Its accuracy is evaluated by means of a comparative study with the numerical integration solutions of a number of idealized systems. In this comparative study, the proposed procedure estimates the numerical integration solutions with an average error of about 2%.

Simplified seismic analysis of piping or equipment mounted on two points of a multistory structure

Abstract A simplified procedure is presented to calculate the maximum earthquake response of light mechanical or electrical equipment supported at two arbitrary points of a building structure. The procedure is derived on the basis of the application of the response spectrum method to a combined structure-equipment system, but, in an effort to avoid the complications of such an approach, it is formulated in terms of the dynamic characteristics of the two independent components. It fully takes into account the interaction between the two subsystems, and avoids the generation of floor response spectra and the need to consider two different support excitations. The formulation is attained by using Hurtys component mode synthesis technique and Rayleighs principle. Linear systems with classical modes of vibration and small structure to equipment mass ratios are considered. The simplicity of the method is demonstrated by a numerical example, and its accuracy verified by a comparative study. In this comparative study, the suggested procedure estimates correct solutions with an average error of about 2%.

Base isolation with sliding hydromagnetic bearings: concept and feasibility study

Abstract A description is made of a newly developed base isolation system that in a simple and cost-effective way may significantly reduce the lateral forces transmitted to buildings, bridges and other structures during earthquakes. The system operates on the basis of (a) sliding bearings that incorporate a hydrostatic scheme to minimise the friction between bearings and base plates, and (b) the generation through the use of modern rare earth permanent magnets of damping forces to reduce the bearings’ displacements to practical levels and counteracting forces to push the bearings back toward their initial positions if displaced. The sliding bearings are fabricated with steel tubes, steel cap plates, a low-viscosity fluid, sealing elastomeric O-rings and rare earth permanent magnets attached to the steel tubes. The isolation system is comprised of these bearings, aluminium base plates and rare earth permanent magnets connected to the periphery of these plates. Presented also is a study conducted with a six-storey building to examine the feasibility and practicality of the proposed system. As conceptualised and based on the results from the feasibility study, it is concluded that the suggested isolation scheme is feasible, effective and easy to build and install.

Improved Modelling of Electrical Substation Equipment for Seismic Loads

Publisher Summary This chapter discusses the improvement in modeling of electrical substation equipment for seismic loads. Improvements in the methods used for modeling electric substation equipment through a combination of experimental and analytical studies are explored. The experimental studies were a combination of in-situ field measurements of this equipment at four sites selected from Pacific Gas & Electric facilities and laboratory experimental modal analyses. The in-situ tests considered the transmissibility of the ambient and forced vibration ground motion to the support locations of the equipment, as well as the low level response of the equipment itself due to force-calibrated hammer excitation. The experimental modal analyses considered the linear response of the equipment under low level, force-calibrated hammer excitation. The analytical studies interpreted the vibration data using commercially available software in order to define the frequency, mode shape and damping characteristics of the equipment. Multiple degree-of-freedom, lumped parameter modal models were developed for the equipment. Simple analytical models were developed that closely matched the experimental data in order to predict the transformers dynamic response under dynamic loading. These modal models can be integrated with standard finite elements to model structural response modifications due to mass or stiffness changes resulting from support structures. The analytical studies recommend improved methods for modeling equipment and major components.