Browsing by Author "Almazan, Jose L."

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    A 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.
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    Base-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.
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    Comparative analysis of the dynamic behavior of structures equipped with cylindrical and spherical frictional bearings subjected to near-fault earthquakes
    (2024) Auad, Gaspar; Almazan, Jose L.; Vilca, Federico
    The effectiveness of seismic isolation in slender structures subjected to intense ground motion can be compromised by unwanted uplifting, a phenomenon often observed in frictional bearings featuring spherical sliding surfaces, such as the Friction Pendulum System (FPS) device. One solution to this issue involves incorporating a mechanism within the isolation devices to prevent uplifting. This study introduces an innovative numerical formulation for the tension -restraint Friction Pendulum isolator, referred to as the XY-FP bearing, which integrates complete frictional coupling and accounts for large displacements. Unlike conventional designs, the XYFP bearing employs cylindrical sliding surfaces to achieve its isolation mechanism. Two structural models were created to investigate the dynamic behavior disparities between isolation systems utilizing cylindrical versus spherical frictional isolators. The models represent a single -story base -isolated building and a slender structure. Comparative analysis reveals that cylindrical frictional isolators exhibit reduced maximum displacements and increased sensitivity to ground motion directionality. Notably, using cylindrical frictional isolators can substantially increase maximum inter -story drifts and maximum absolute accelerations. Evaluation of the dynamic response of the slender building demonstrates the capacity of XY-FP bearings to mitigate undesired bearing uplifting effectively. However, the tensile forces generated by the isolators may surpass the magnitude of the static vertical load due to the superstructure's self -weight. Extending the isolated period is one potential strategy for alleviating the uplift magnitude or maximum tensile force.
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    Development of a pressure-, velocity-, and acceleration-dependent phenomenological friction model using experimental data of sliding tests between 11 polymers and stainless steel
    (2024) Tapia, Nicolas F.; Reyes, Sergio I.; Vassiliou, Michalis F.; Almazan, Jose L.
    This paper experimentally investigates the frictional behavior between stainless steel and 11 polymers. Particularly, the dependence of the friction coefficient on the sliding velocity, pressure, and acceleration is quantified. The novelty of this work lies in quantifying the acceleration-dependent nature of friction, correlating it to the well-documented Stick-Slip effect. The experimental setup consisted of two parallel stiff steel beams, one above the other, with a separation of 95 mm, and steel surfaces welded at the inner sides for sliding the polymers. Cylindrical polymer pads were placed between the stainless-steel surfaces and connected to a dynamic actuator to apply the displacement protocol. The protocol consisted of consecutive nominally constant-velocity ramp cycles covering velocities from 1 mm/s to 300 mm/s (with 20 mm/s increments). An additional vertical force was applied with a hydraulic actuator to reach nominal pressures in the polymers between 5 and 80 MPa. The results showed that the friction coefficient depends on the velocity, pressure, and acceleration of the motion, and a phenomenological model on these three variables is proposed. The velocity dependence can be represented through a logarithmic relationship, while the pressure dependence is through an exponential decay relationship. The acceleration dependence was represented through a linear relationship, which could capture the stick-slip effect. Overall, this work contributes to a better understanding of friction for seismic isolation systems. Since friction is the main source of energy dissipation in such structures, the proposed model will allow a higher accuracy in predicting variables of interest during the dynamic analyses of seismically isolated structures with frictional systems.
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    Evaluating the use of variable curvature frictional isolators to mitigate the adverse effects of internal lateral impacts
    (2022) Auad, Gaspar; Almazan, Jose L.
    Under high magnitude ground motions, excessive base displacement can generate internal lateral impacts between inner sliders and restraining rims of frictional isolators. This study aims to evaluate the use of variable curvature frictional isolators to mitigate the adverse effects of these internal impacts. One specific geometry of the sliding surface was studied, obtained by revolving an ellipse around a vertical axis. Due to the shape of the sliding surface, the pendular force transmitted by the bearing exhibits a smooth hardening behavior. A physical model for dynamic analysis of base-isolated structures equipped with variable curvature frictional bearings is presented. This numerical model accounts for effects such as large deformation, P - Delta, sticking, uplift, and lateral and vertical impact behavior. The impact parameters of the physical model were calibrated using a Finite Element Model subjected to dynamic loads. A three-dimensional model of a base-isolated structure was developed to characterize the dynamic behavior of an adaptive passive isolation system under different base displacement demands. Finally, a nonlinear reinforced concrete frame equipped with isolators with spherical or elliptical sliding surfaces was examined to determine the average reduction in the Engineering Demand Parameters affected by internal impacts. On average, reductions of 44%, 11%, 8%, and 6% in the maximum response of the base shear, first inter-story drift, second inter-story drift, and third inter-story drift are obtained by replacing classical Friction Pendulum System (FPS) devices with variable curvature isolators.
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    Experimental validation of an energy-dissipating anchoring system for continuously-supported storage tanks
    (2023) Tapia, Nicolas F.; Almazan, Jose L.; Valdebenito, Nathaly; Reyes, Sergio I.
    This paper presents a novel energy dissipation device called NWS-TADAS and its experimental validation. The device was inspired by the flexural failure of the top plate of anchor bolt chairs in wine storage tanks observed after the 2015 Mw 8.3 Illapel, Chile Earthquake. The device is proposed to replace traditional anchors in continuously-supported storage tanks. This device provides advantages compared to other steel dampers in terms of constructability and force-deformation relationship. TADAS and similar steel energy-dissipation devices have a relatively high constructive difficulty derived from their welding process. The proposed device has no welds on the dissipation plate, hence its nomenclature (Non-Welded Symmetrical TADAS). In addition, design deformations are not chosen consistently with the total failure energy for the expected number of cycles during its lifetime, resulting in non-optimal devices (underutilized). Full-scale specimens were manufactured in AISI-304 stainless steel and ASTM-A36 steel and tested under two types of displacement protocols. Both materials were chosen because they are the most used for tanks in the food industry and industrial storage. The results showed that A36 carbon steel presented a smaller hardening. In all cases, the plastic deformations on the surface of the dampers were not uniform as expected in triangle-shape devices, being concentrated at the center and close to the embedment zone and causing a premature fatigue failure. The dissipated energy for consecutive cycles was stable, and using additional stacked dampers in the system had no significant effects on the hysteretic behavior. The system showed no rate dependence. The dampers presented almost identical force-deformation relationships for two consecutive earthquakes; the dissipated energy was less than 20% of their total capacity. The latter result validates their capacity to withstand several earthquakes without being replaced, further concluding that the NWS-TADAS is a viable alternative to be implemented as an anchoring system on storage tanks and other types of structures. Finally, it is concluded that performing testing protocols defined with a statistical approach may be more meaningful for dampers subject to earthquake demands.
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    Lateral Impact Resilient double concave Friction Pendulum (LIR-DCFP) bearing: Formulation, parametric study of the slider and three-dimensional numerical example
    (2021) Auad, Gaspar; Almazan, Jose L.
    Seismic isolation has been one of the best alternatives in protecting structures, being frictional isolators one of the most used devices in constructing isolation systems. Under extreme seismic inputs, the impact between the inner slider and the restraining rims of the plates could occur. This internal impact has been indicated as one of the main contributors to the failure of frictional pendulum bearings. This paper presents a new type of frictional device, the Lateral Impact Resilient double concave Friction Pendulum (LIR-DCFP) bearing. This device has an improved inner slider that enhances the dynamic behavior if the lateral capacity of the isolator is overcome. The three-dimensional formulation of the LIR-DCFP bearing is presented and proposed to perform dynamics analyzes. A parametric analysis was performed to evaluate the influence of the geometry of the inner slider. Finally, a comparative three-dimensional analysis between the dynamic response of a structure isolated using LIR-DCFP bearings and double concave Friction Pendulum (DCFP) bearings is presented. Important reductions in the base shear, inter-story drifts, and absolute accelerations are achieved using LIR-DCFP bearings.
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    Linear 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, Diego
    Because 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.
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    Seismic reliability of structures equipped with LIR-DCFP bearings in terms of superstructure ductility and isolator displacement
    (2022) Auad, Gaspar; Castaldo, Paolo; Almazan, Jose L.
    This research deals with the seismic reliability of non-linear base-isolated structures equipped with Lateral Impact Resilient Double Concave Friction Pendulum (LIR-DCFP) devices. Specifically, exceeding probabilities within the reference lifetime are assessed with respect to both superstructure ductility and isolator displacement demand. The innovative LIR-DCFP bearing has an improved inner slider with an internal gap and is capable to reduce adverse effects of the lateral impact between the inner slider and the restraining rims. The dynamic behavior of the superstructure is represented by a simplified one-degree-of-freedom model describing its lateral response. The isolation system is characterized by a model based on rigid body dynamics also including the lateral impact behavior. A wide parametric analysis is developed for several system properties considering the friction coefficients as relevant random variables. Different sets of natural seismic records able to match conditional spectra for a site in Riverside (California) were selected to consider the aleatory uncertainties of the seismic input. Incremental dynamic analyses were performed to determine the statistics of significant engineering demand parameters and compute probabilities exceeding specific limit states to define fragility curves. Finally, employing seismic hazard curves, the seismic reliability of isolated structures was evaluated. For increasing values of the internal gap, structures equipped with LIR-DCFP devices exhibit better seismic performance with respect to classical DCFP bearings with same size, especially, if the superstructure is designed to behave essentially elastic when the lateral capacity of the isolation level is not reached, or the hardening post-yield stiffness of the superstructure is relatively high. Reductions up to 20% in the exceeding probabilities within 50 years related to the ductility demand are achievable using the suggested LIR-DCFP isolator.
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    Static and dynamic experimental validations of a frictional isolator with improved internal lateral impact behavior
    (2024) Auad, Gaspar; Almazan, Jose L.; Tapia, Nicolas; Quizanga, Diego
    Under extreme seismic conditions, large displacements in frictional isolators may trigger internal impacts between sliders and restraining rims, jeopardizing the effectiveness of using seismic isolation interfaces. These impacts may produce the failure of the bearing or a dramatic increase in the displacement demand of the superstructure. A novel configuration of a frictional device has been suggested to improve the seismic performance of base-isolated structures. The Lateral Impact Resilient Double Concave Frictional Pendulum (LIR-DCFP) bearing has an enhanced inner slider with a plane high-friction interface and an internal gap. The internal lateral impact causes a high-friction slide that limits the magnitude of the impact force and dissipates an additional amount of energy. This work presents and discusses the main results of static and dynamics tests concerning the LIR-DCFP bearing ' s lateral behavior. The data obtained was employed to calibrate a numerical modal aiming to represent the response of a timber structure equipped with LIR-DCFP bearings. Static tests were conducted to confirm the expected lateral behavior of the novel device. Special attention was paid to constructing a proper slider with a high-friction interface, the main feature of the isolator. The results of the static tests indicate that the construction of this improved slider, which exhibits a high-friction phase only when the impact is produced, is feasible. Finally, dynamic tests of a timber superstructure equipped with four LIR-DCFP bearings subjected to unidirectional inputs were conducted. A proper response of the isolation system was obtained, even when internal lateral impacts were observed. The results of these experimental tests suggest that using LIR-DCFP isolators represents an excellent alternative to mitigate the adverse effects of lateral internal impacts.
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    Three-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.
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    Torsional 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.
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    Torsional 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.
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    Torsional balance of plan-asymmetric structures with frictional dampers: Experimental results
    (WILEY, 2006) Vial, Ignacio J.; de la Llera, Juan C.; Almazan, Jose L.; Ceballos, Victor
    This 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.