Browsing by Author "Almazan, Jose L."
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- ItemA bidirectional and homogeneous tuned mass damper: A new device for passive control of vibrations(Elsevier Sci. Ltd., 2007) Almazan, Jose L.; Llera Martin, Juan Carlos de la; Inaudi, Jose A.; Lopez Garcia, Diego; Izquierdo, Luis E.Passive tuned-mass dampers (TMDs) are a very efficient solution for the control of vibrations in structures subjected to long-duration, narrow-band excitations. In this study, a Bidirectional and Homogeneous Tuned Mass Damper (BH-TMD) is proposed. The pendular mass is supported by cables and linked to a unidirectional friction damper with its axis perpendicular to the direction of motion. Some advantages of the proposed BH-TMD are: (1) its bidirectional nature that allows control of vibrations in both principal directions; (2) the capacity to tune the device in each principal direction independently; (3) its energy dissipation capacity that is proportional to the square of the displacement amplitude, (4) its low maintenance cost. Numerical results show that, under either unidirectional or bidirectional seismic excitations, the level of response reduction achieved by the proposed BH-TMD is similar to that obtained from an "ideal" linear viscous device. Moreover, experimental shaking table tests performed using a scaled BH-TMD model confirm that the proposed device is homogeneous, and, hence, its equivalent oscillation period and damping ratio are independent of the motion amplitude. (C) 2006 Elsevier Ltd. All rights reserved.
- ItemBase-structure interaction of linearly isolated structures with lateral-torsional coupling(ELSEVIER SCI LTD, 2008) Seguin, Carlos E.; de la Llera, Juan C.; Almazan, Jose L.The linear earthquake response of seismically isolated structures with lateral-torsional coupling is investigated. Emphasis is placed on developing simplified procedures for estimating the amplification of edge displacements of the superstructure and isolated base. The three-dimensional response of asymmetric buildings is cast under a dynamic base-superstructure interaction formulation. Among the wide range of possibilities to represent this interaction, two simplified models were selected. The first model accounts for the base-superstructure interaction through a correction of the mass matrix of the superstructure, while the second assumes a pseudo-static response of the superstructure subject to three lateral inertial force distributions. Symbolic expressions are derived to compute the edge response by using the pseudo-static method. Such expressions are simple to use and show better accuracy than the one implicit in current isolation codes. Finally, the response of a six-story asymmetric building example demonstrates the application of the proposed procedure, and results are compared with the true peak response at both edges computed from integration of the equations of motion of the isolated structure. (c) 2007 Published by Elsevier Ltd.
- ItemLinear isolation of stainless steel legged thin-walled tanks(ELSEVIER SCI LTD, 2007) Almazan, Jose L.; Cerda, Fernndo A.; De la Llera, Juan C.; Lopez Garcia, DiegoBecause of the booming wine industry in some seismic countries such as the US, Chile and Argentina, seismic protection of wine storage systems is of practical importance. In this study, the dynamic response of thin-walled legged wine tanks with seismic isolation is investigated. Linear fluid-structure interaction has been considered in the analyses performed with SAT-LAB and ADINA and two choices for the seismic isolation system have been evaluated: (i) a traditional lateral isolation system (LAI), and (ii) a vertical-rocking isolation system (VRI). As far as the authors know, the latter has not been considered in previous studies, and results show that it is a viable alternative for legged tanks. Moreover, all isolation devices have been designed considering the space limitations that are inherent to the implementation of these systems in practice. Results obtained for a suite of records and different structures have shown that both types of isolation lead to response reductions of the tank stresses. The reduction ranges from 20% to 79% for the LAI models and 31%-91% for the VRI, relative to the case without isolation. These promising results have implications in the selection of the thickness of the wall and the overall dimensions of the tank. (C) 2006 Elsevier Ltd. All rights reserved.
- ItemThree-dimensional behavior of a spherical self-centering precast prestressed pile isolator(WILEY, 2009) Junemann, Rosita; de la Llera, Juan C.; Besa, Jaime; Almazan, Jose L.A 3D analytical formulation of a precast prestressed pile (PPP) seismic isolator with top and bottom spherical rolling kinematic constraints is proposed. The PPP isolator was initially conceived its a low-cost seismic isolation (and foundation) system for housing units of low-income people. Since these Structures are usually located at sites with poor soil conditions, the PPP isolator also works its a foundation pile by connecting the superstructure with more competent soil layers. The non-holonomic nature of the rolling constraint is dealt with by a structural formulation. The proposed 3D formulation is validated by numerical results obtained from a previously proposed formulation for the 2D problem, and a contact finite element model in ANSYS (www.ansys.com). Other issues associated with the dynamic response of isolated structures with the PPP are also examined. such as expected response reductions, variation in the axial force of the central prestressed cable, and torsional response amplifications. Finally, guidelines to estimate the actual 3D response using 2D analysis results are investigated. Copyright (C) 2009 John Wiley & Sons, Ltd.
- ItemTorsional balance as new design criterion for asymmetric structures with energy dissipation devices(WILEY, 2009) Almazan, Jose L.; de la Llera, Juan C.Lateral-torsional coupling in asymmetric-plan buildings leads to correlated translations and rotations of the building plan, which generate uneven distributions of deformation demand among resisting planes. The deformation demand of a resisting plane depends on the relative magnitude of the plan translation and rotation and on the correlation between the two signals. Thus, small rotations highly correlated with building translation may lead to significantly different deformations of the resisting planes at the building edges. Consequently, the use of supplemental dampers is intended not only to reduce the magnitude of the plan translation and rotation, but also the correlation between these motions. For the sake of simplicity, linear viscous dampers are used in this investigation, which properly located in plan lead to a minimum response of the geometric center, thus achieving the same mean-square value of the displacements at the building edges. Mathematically, this condition may be understood as creating zero correlation between the translations and rotation at the geometric center of the plan, which represents an uncoupling in the mean-square sense. Results show that the optimal damper location depends on the static eccentricity and frequency ratio of the bare structure, the total amount of supplemental damping considered, and the frequency content of the excitation. Through a final 6-story model example, the torsional balance concept is demonstrated to work on multistory buildings subjected to bidirectional ground motions. Copyright (C) 2009 John Wiley & Sons, Ltd.
- ItemTorsional balance of plan asymmetric structures with viscoelastic dampers(ELSEVIER SCI LTD, 2007) Garcia, Marcos; de la Llera, Juan C.; Almazan, Jose L.This investigation deals with the torsional balance of elastic asymmetric structures with viscoelastic dampers. Plan asymmetry leads to an uneven lateral deformation demand among structural members and to unbalanced designs with larger capacities in some resisting planes. The analytical and experimental response of stiffness and mass-eccentric structures subjected to different ground motions is investigated herein. Viscoelastic dampers are capable of controlling the lateral-torsional coupling of a structure by placing the so-called Empirical Center of Balance (ECB) of the plan equidistant from all building edges. To improve the damper efficiency, a mechanical deformation amplifier was included in the one-story experimental building model. Results show that the displacement demand at the building edges and that of resisting planes at equal distance from the ECB may be similar if the damper is optimally placed. It was observed that optimal damper eccentricity values tend to increase linearly as the stiffness or mass eccentricities increase, and that response reduction factors ranging from 1.5 to 3 are possible with a small capacity damper. Moreover, viscoelastic dampers are equally effective in controlling lateral-torsional coupling of torsionally flexible as well as stiff structures. (C) 2006 Elsevier Ltd. All rights reserved.
- ItemTorsional balance of plan-asymmetric structures with frictional dampers: Experimental results(WILEY, 2006) Vial, Ignacio J.; de la Llera, Juan C.; Almazan, Jose L.; Ceballos, VictorThis investigation deals with the measured seismic response of a six-storey asymmetric structural model with frictional dampers. Its main objective is to experimentally prove the concept of weak torsional balance for mass- and stiffness-eccentric model configurations. The goal is to control the torsional response of these asymmetric structures and to achieve, if possible, a weak form of torsional balance by placing the so-called empirical centre of balance (ECB) of the structure at equal distance from the edges of the building plan. The control of the dynamic response of asymmetric structures is investigated herein by using steel-teflon frictional dampers. As expected from theory, experimental results show that the mean-square and peak displacement demand at the flexible and stiff edges of the plan may be similar in magnitude if the dampers are optimally placed. Frictional dampers have proven equally effective in controlling lateral-torsional coupling of torsionally flexible as well as stiff structures. On the other hand, it is shown that impulsive ground motions require larger frictional capacities to achieve weak torsional balance. Copyright (C) 2006 John Wiley & Sons, Ltd.