Friction Pendulum Devices Research Articles

Seismic reliability analysis of isolated deck bridges using friction pendulum devices

In this study, the seismic reliability of multi-span continuous deck bridges equipped with isolation friction pendulum (FP) devices is investigated. The relevant aleatory uncertainties associated to the sliding friction coefficient of the FP isolators and to the seismic inputs are considered. A six-degree-of-freedom model is established to reproduce the elastic behavior of the reinforced concrete (RC) pier, the stiff response of the deck supported by the isolation devices and the non-linear response of the FPS bearings which depends on the sliding velocity. Moreover, the RC abutment is assumed as infinitely rigid. For what concerns the seismic inputs, a group of natural seismic records having various characteristics is adopted and properly scaled to increasing levels of intensity. The random variability of the friction coefficient is modelled by suitable probabilistic distribution. Then, considering several bridges and isolator configurations, the fragility curves of the RC pier and of the isolator devices (FP) are determined. Finally, in agreement with the hazard curve of the specific site, the convolution integral is adopted to determine the seismic reliability curves in the performance domain.

Sliding pendulum isolators without secretes

Over the last decades, anti-seismic devices have gained increasing interest in the civil engineering field. The introduction of the base isolation system has led to a new concept in the construction panorama in terms of human life safety, a new way of thinking on new constructions, improvement and retrofitting on existent structures. Therefore, rubber and friction isolators have been deeply investigated to hence performances and predict dynamic behaviour during an earthquake. While the response of the former is characterised by the composition of the elastomeric compound, the latter features special materials able to dissipate energy by moving on smooth surfaces. This paper focuses on friction pendulum devices and addresses its attention on the behaviour of sliding materials. It is well-known that stick-slip phenomenon occurs when friction excitation is present and, in the anti-seismic field is important to reduce it and have a well-representative mathematical law able to describe it. Therefore, Hirun International after performing several treatments of the sliding materials has set up a special processing to guarantee a stable response of the HI-M material used on pendulum devices. The paper, after a brief presentation of the special sliding material, shows a comparison between the material with and without the treatment in terms of the force-displacement law. The paper also analyses in detail the cinematic behaviour of the sliding pendulum with one or two main sliding surfaces, with and without central articulation and determines the stress distribution in the sliding surfaces for the different cases.

Bridges seismically isolated by DCFP devices: a study on how the main parameters of the problem affect the seismic performance of the system

This paper deals with the evaluation of the main relevant properties of the double concave friction pendulum devices, namely DCFP, able to influence the seismic behaviour of infrastructures such as isolated multi-span continuous deck bridges. With this aim, a numerical model has been proposed to simulate the dynamic behaviour of seismically isolated bridges, and so an eight-degree-of-freedom system adopted in order to model the elastic reinforced concrete pier and the isolators devices. Furthermore, the interaction pier-abutment has been taken into account and the former modelled as a fully fixed restrain. The deck has been assumed rigid in its plane and a non-linear velocity dependent law adopted to model the friction along the two surfaces of the pendulum isolators. The equations of motion have been proposed in a non-dimensional form by means of a preliminar dymensional analysis. Finally, a widely parametric analysis has been conducted and record-to-record variability taken into account by using a set of natural records with different characteristics either in terms of epicentral distance and magnitude. The response of the system in terms of pier and deck displacements, is presented in a probabilistic fashion and as independent from the seismic intensity thanks to the abovementioned dimensional analysis. The main evidence of the study is how the main structural parameters involved in the analysis are able to control and influence the seismic response, such that the outcomes of this research could be used to better design or retrofit multispan coninuous deck bridges isolated with DCFP.

Influence of the Pier-abutment-deck interaction on the Seismic Response of Bridges Equipped with FPS

This work regards the analysis of the seismic response of bridges isolated with single concave friction pendulum devices, including or neglecting the presence of the rigid abutment. Two six degree-of-freedom models are considered for the two cases, in order to represent the response of the elastic reinforced concrete pier and of the infinitely rigid reinforced concrete deck. The behavior of the friction pendulum isolators accounts for the non-linear dependency of the friction coefficient on the sliding velocity. The comparison is carried out by including the aleatory uncertainty in the seismic input (i.e., accounting for different natural records with different characteristics) and by varying the modelling properties within a parametric analysis. Finally, by solving the equations of motion in nondimensional form, the difference between the two models are deepened in terms of influence of the seismic isolation.

Seismic Upgrading of Existing Reinforced Concrete Buildings Using Friction Pendulum Devices: A Probabilistic Evaluation

In many countries around the world a huge number of existing reinforced concrete (RC) structures have been realized without account for seismic detailing, even if they are located in areas subjected to high seismicity. In this context, several passive seismic protection techniques have been developed and applied to retrofit these structures such as, for an example, seismic isolation. The aim of this work is to characterize in probabilistic terms the seismic performance of a framed RC building retrofitted by means of sliding friction pendulum (FPS) devices. Specifically, the response of an existing RC building located in a high seismicity area in Italy is investigated. After the description of the main available information about the structure, a non-linear numerical model has been defined by means of fiber-elements approach. Then, non-linear dynamic analyses considering multiple recorded ground motions with the three accelerometric components have been carried out to assess the seismic response of the building with and without the retrofitting intervention composed of FPS isolators. Finally, the results are processed to achieve a probabilistic assessment of the seismic performance of the retrofitting intervention.

FINE-TUNING OF MODELLING STRATEGY TO SIMULATE THERMO-MECHANICAL BEHAVIOUR OF DOUBLE FRICTION PENDULUM SEISMIC ISOLATORS UST ESTIMATOR

The use of friction pendulum devices has recently attracted the attention of both academic and professional engineers for the protection of structures in seismic areas. Although the effectiveness of these has been shown by the experimental testing carried out worldwide, many aspects still need to be investigated for further improvement and optimisation. A thermo-mechanical model of a double friction pendulum device (based on the most recent modelling techniques adopted in multibody dynamics) is presented in this paper. The proposed model is based on the observation that sliding may not take place as ideally as is indicated in the literature. On the contrary, the fulfilment of geometrical compatibility between the constitutive bodies (during an earthquake) suggests a very peculiar dynamic behaviour composed of a continuous alternation of sticking and slipping phases. The thermo-mechanical model of a double friction pendulum device (based on the most recent modelling techniques adopted in multibody dynamics) is presented. The process of fine-tuning of the selected modelling strategy (available to date) is also described.

Modeling of the temperature rises in multiple friction pendulum bearings by means of thermomechanical rheological elements

Even if the effectiveness of friction pendulum bearings has been extensively proven by means of numerous experimental programs carried out worldwide, many aspects concerning their behavior under seismic action still need to be clarified. One of these is related to the temperature rises induced by the heat generated by friction during the dynamic sliding of the surfaces in contact, which may significantly affect the superficial frictional properties of the sliding surfaces involved, thus reducing the overall performance of the isolating system, up to re-coupling the structure with the ground shaking, in a limit scenario. With the aim to contribute to a better understanding of this aspect, and to develop a simplified tool capable to reproduce the hysteretic force–displacement loops together with the corresponding temperature variations, a thermo-mechanical model for the multiple friction pendulum devices is proposed. The model is based on the combination of simple thermomechanical rheological elements and does not require the evaluation of any convolution integral arising from the solution of the heat conduction problem as it happens with many existing models. The model is numerically implemented under displacement-control and its effectiveness is validated through the numerical simulation of some recent experimental results that shows a good agreement with the observed behavior.

Seismic reliability-based ductility demand for hardening and softening structures isolated by friction pendulum bearings

This study aims at proposing seismic reliability-based relationships between the behavior factors and the displacement demand for nonlinear hardening and softening structures isolated by friction pendulum system devices considering several structural properties. An equivalent 2dof model having both a hardening and softening postyield slope is used to describe the superstructure behavior, whereas a velocity-dependent model is adopted for the friction pendulum system response. The yielding characteristics of the superstructures, related to life safety limit state, are designed according to the seismic hazard of L'Aquila site (Italy) for increasing behavior factors, as provided from NTC08. Considering natural seismic records and several elastic and inelastic building properties, different postyield hardening and softening stiffness values, different seismic intensity levels, and modeling the friction coefficient as a random variable, incremental dynamic analyses are performed to evaluate the seismic fragility of these structural systems. By means of the convolution integral between the fragility curves and the seismic hazard curves corresponding to L'Aquila site (Italy), the reliability curves of the equivalent hardening and softening base-isolated structural systems, with a lifetime of 50 years, are defined. Specifically, seismic reliability-based linear and multilinear regression expressions between the displacement ductility demand and the behavior factors for the superstructure as well as seismic reliability-based design abacuses for the friction pendulum devices are proposed.

Seismic Retrofit of a Multispan Prestressed Concrete Girder Bridge with Friction Pendulum Devices

The paper deals with the proposal and application of a procedure for the seismic retrofit of an existing multispan prestressed concrete girder bridge defined explicitly for the use of friction pendulum devices as an isolation system placed between piers top and deck. First, the outcomes of the seismic risk assessment of the existing bridge, performed using an incremental noniterative Nonlinear Static Procedure, based on the Capacity Spectrum Method as well as the Inelastic Demand Response Spectra, are described and discussed. Then, a specific multilevel design process, based on a proper application of the hierarchy of strength considerations and the Direct Displacement‐Based Design approach, is adopted to dimension the FPD devices. Furthermore, to assess the impact of the FPD nonlinear behaviour on the bridge seismic response, a device model that reproduces the variation of the normal force and friction coefficient, the bidirectional coupling, and the large deformation effects during nonlinear dynamic analyses was used. Finally, the paper examines the effects of the FPD modelling parameters on the behaviour of the retrofitted bridge and assesses its seismic response with the results pointing out the efficiency of the adopted seismic retrofit solution.

Seismic reliability‐based ductility demand evaluation for inelastic base‐isolated structures with friction pendulum devices

SummaryThe aim of this work is to propose seismic reliability‐based relationships between the strength reduction factors and the displacement ductility demand of nonlinear structural systems equipped with friction pendulum isolators (FPS) depending on the structural properties. The isolated structures are described by employing an equivalent 2dof model characterized by a perfectly elastoplastic rule to account for the inelastic response of the superstructure, whereas, the FPS behavior is described by a velocity‐dependent model. An extensive parametric study is carried out encompassing a wide range of elastic and inelastic building properties, different seismic intensity levels and considering the friction coefficient as a random variable. Defined a set of natural seismic records and scaled to the seismic intensity corresponding to life safety limit state for L'Aquila site (Italy) according to NTC08, the inelastic characteristics of the superstructures are designed as the ratio between the average elastic responses and increasing strength reduction factors. Incremental dynamic analyses (IDAs) are developed to evaluate the seismic fragility curves of both the inelastic superstructure and the isolation level assuming different values of the corresponding limit states. Integrating the fragility curves with the seismic hazard curves related to L'Aquila site (Italy), the reliability curves of the equivalent inelastic base‐isolated structural systems, with a design life of 50 years, are derived proposing seismic reliability‐based regression expressions between the displacement ductility demand and the strength reduction factors for the superstructure as well as seismic reliability‐based design (SRBD) abacuses useful to define the FPS properties. Copyright © 2016 John Wiley & Sons, Ltd.

Seismic fragility and reliability of structures isolated by friction pendulum devices: seismic reliability‐based design (SRBD)

SummaryThe paper deals with the seismic reliability of elastic structural systems equipped with friction pendulum isolators (friction pendulum system). The behavior of these systems is analyzed by employing a two‐degree‐of‐freedom model accounting for the superstructure flexibility, whereas the friction pendulum system device behavior is described by adopting a widespread model that considers the variation of the friction coefficient with the velocity. With reference to medium soil condition, the uncertainty in the seismic inputs is taken into account by considering a set of artificial records, obtained through Monte Carlo simulations within the power spectral density method, with different frequency contents and characteristics depending on the soil dynamic parameters and scaled to increasing intensity levels. The sliding friction coefficient at large velocity is also considered as random variable modeled through a uniform probability density function. Incremental dynamic analyses are developed in order to evaluate the probabilities exceeding different limit states related to both r.c. superstructure and isolation level defining the seismic fragility curves through an extensive parametric study carried out for different structural system properties.Finally, considering the seismic hazard curves related to a site near L'Aquila (Italy), the seismic reliability of the r.c. superstructure systems is evaluated, and seismic reliability‐based design abacuses are derived with the aim to define the radius in plan of the friction pendulum devices in function of the structural properties and reliability level expected. Copyright © 2016 John Wiley & Sons, Ltd.

Numerical modelling of variable friction sliding base isolators

As the desire for high performance buildings increases, a number of solutions to enhance the dynamic response of structures have been explored and implemented. In recent years, base isolation has become a practical and effective strategy for earthquake resistant design. To this end, a number of isolation solutions are currently available. Among these, laminated lead rubber bearings and friction pendulum devices have been most widely used in the last 25 years. Recent studies have explored the possibility of employing flat or curved sliding-surface base isolators with variable friction coefficients. These new types of devices have been shown to be capable of performing theoretically better than traditional sliding isolators in light of their higher energy-absorption capacity. This paper provides insight into the mechanics of variable friction sliding isolators, exploring viable ways of dealing with these types of elements from a numerical point of view. A number of 2D non-linear time history analyses, in which the new elements are treated as non-linear translational spring elements, are conducted and the results are discussed in detail.

Approximate Method for Transverse Response Analysis of Partially Isolated Bridges

Current analysis procedures for seismically isolated bridges frequently use an equivalent (approximate) linearization approach to represent the response of nonlinear isolation/energy dissipation devices. Linearization allows standard linear elastic analysis methods, e.g., the response spectrum method, to be conveniently used for design purposes. The linearization approach is by nature an iterative method implying the need to repeatedly correct and analyze a numerical finite-element model. A further simplification could be achieved using closed form equations to represent (1) the structure displacement patterns and (2) the restoring forces from structural elements. The paper explores such a possibility with reference to partially isolated continuous bridges, i.e., bridges with isolation devices at piers and pinned supports at abutments. The role and effect of higher modes of vibration on the system response are discussed, and an approximate method is proposed to account for such effects. An improvement of the classical Jacobsen’s approximation for the effective viscous damping ratio is also proposed using the results of response history analyses. The latter are carried out on two-dimensional numerical models of five case studies, generated from a real existing bridge supposed to be isolated with friction pendulum devices. Comparison of approximate predictions with response history analysis results is presented and discussed. Nonlinear dynamic analyses of a three-dimensional numerical model of the existing bridge were also carried out for comparison purposes.

Damage identification procedure for seismically isolated bridges

SUMMARY This paper deals with a procedure for the identification of the damage in bridge structures equipped with isolators and/or energy dissipating devices. The procedure is based on the availability of accelerometric records from any simple sensor network installed on existing bridges. The proposed algorithm provides an assessment of the performance degradation of conventional structural components as well as installed isolators and energy dissipators, obtained from changes in modal characteristic of the structural response. A new index localization and severity index is introduced to be used for ordinary structural elements and anti-seismic devices. For the validation of the procedure, the algorithm has been applied to a continuous beam as well as to a bridge structure equipped with Friction Pendulum devices. The proposed procedure shows a high level of accuracy in the damage localization and severity assessment also in a complex scenario of damage. The severity index is also interpreted in terms of physical quantities (e.g. friction coefficient) representative of the specific device performance. For this reason the procedure appears feasible for implementation on real structures with the advantage of providing direct indicators of the early stages of degradation of performance parameters. This information can be used to design inspection and maintenance plans. Copyright © 2011 John Wiley & Sons, Ltd.