Browsing by Author "de la Llera, Juan C."

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    A numerical model for non-linear shear behavior of high damping rubber bearings
    (2023) Gallardo, Jose A.; de la Llera, Juan C.; Restrepo, Jose I.; Chen, Michelle
    The dynamic behavior of isolated structures is strongly controlled by the force-deformation constitutive behavior of the isolators. Among the different types of existing isolation devices, High Damping Rubber Bearings (HDRBs) are commonly used in practice, which behavior is highly non-linear and difficult to model analytically. Consequently, this article proposes a simple, but sufficiently accurate, mathematical model for simulating the non-linear shear behavior of HDRBs under large deformations, and an estimation procedure for its parameter values using the geometrical features and mechanical characteristics of the device. First, we briefly describe the phenomena observed in the experimental test data, as well as other phenomena not observed within the range of experimental deformations. Then, the mathematical formulation is presented, which is based on the consideration of two components connected in parallel, a hyperelastic spring and a dissipative component. The governing equation for the former is derived from the expanded formulation of the Mooney-Rivlin model for isotropic hyperelastic materials, and the latter from a Bouc-Wen model with hardening. A novel model is included to account for stiffness degradation, including scragging and Mullins effects, which is developed from experimental data of 924 tested devices. The proposed model fits well the experimental test results of HDRBs with different geometric features and material properties. Based on the evolution laws for the different variables, the model can be successfully used in structural dynamic analysis. To facilitate model calibration, a statistical estimation procedure is proposed to reduce the 17 force- deformation constitutive model parameters of the isolator to 9 unknown parameters, which are computed from the geometric features of the device and mechanical characteristics of the rubber material. This makes the calibration of the force-deformation constitutive model parameters feasible. The estimation procedure successfully predicts the behavior of an average device within a batch of HDRBs, showing good agreement with two different experimental datasets.
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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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    Modeling hospital resources based on global epidemiology after earthquake-related disasters
    (2024) Merino, Yvonne; Ceferino, Luis; Pizarro, Sebastian; de la Llera, Juan C.
    Injured people require hospital emergency services and timely medical treatment after extreme earthquakes. Earthquake-related patients often have trauma injuries and stress-linked (ischemic) ailments that require multiple healthcare procedures, such as minor orthopedic treatment, surgical treatment of fractures, and thrombolysis or thrombectomy. Hospital operation models have been proposed to examine these healthcare procedures; however, they exhibit two fundamental gaps that hinder their ability to assess critical service areas after earthquakes. First, these models rest heavily on emergency procedures based on injury severity rather than type. Second, healthcare demands are often modeled from injury profiles after moderate earthquakes in the United States without including epidemiology data after large earthquakes globally. This approach has led to oversimplified hospital emergency services and resource utilization representation. This research presents a new hospital operations model based on patient injury type and worldwide earthquake epidemiology to fill these gaps. We build the model using discrete-event simulations to capture dynamic metrics on hospital operational outcomes after the earthquake, such as patient time-to-treatment and unassisted patient ratio. We then studied how these metrics vary with different levels of functional capacity in the specific hospital resources. Our results showed that waiting times for emergency department (ED)-level patients vary non-linearly with changes in the number of functional service areas. Also, significant reduction in the waiting time for hospital-level procedures was found for relatively small decrease in the bed occupancy rate, for example, if reverse triage procedures are activated (i.e. a discharge of non-critical patients admitted before the earthquake). Our findings provide a valuable tool for decision-making in hospital preparedness as they explicitly measure the impacts of functional capacity on key healthcare metrics for specific earthquake-related patients.
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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.