Paolo Castaldo

Seismic reliability analysis of base-isolated structures with friction pendulum system

The friction pendulum system (FPS) is becoming a widely used technique for seismic protection and retrofit of buildings, bridges, and industrial structures due to its remarkable features. Experimental data also showed that the coefficient of friction depends on several effects (i.e., sliding velocity, cycling effect) so that it can be assumed as a random variable. The aim of the study consists in evaluating the seismic reliability of a base-isolated structure with FP isolators considering both isolator properties (i.e., coefficient of friction) and earthquake main characteristics as random variables. Assuming appropriate density probability functions for each random variable and adopting the LHS method for random sampling, the input data set has been defined. Several 3D nonlinear dynamic analyses have been performed considering both the vertical and horizontal components of each seismic excitation in order to evaluate the system response. In particular, monovariate and multivariate probability density and cumulative distribution functions have been computed and, considering the limit state thresholds and domains (performance objectives) defined respectively on mono/bi-directional displacements, assumed as earthquake damage parameter (EDP) according to performance-based seismic design, the exceeding probabilities (structural performances) have been evaluated. Estimating the reliability of the superstructure, substructure and isolation level led to define reliability-based abacus to design the FP system.

Structural safety of existing buildings near deep excavations

Structural safety of existing buildings near deep excavations is evaluated by computing exceeding probabilities of different damage criteria within a simplified probabilistic methodology based on monovariate or multivariate probabilistic analyses employing the results of a numerical model of the boundary value problem. Different limit domains, defined on one or more deformation parameters and associated to limit states, are used to contemplate: the type of the structural system (i.e., reinforced concrete or masonry buildings); the foundation typology (i.e., strip/raft or pad foundations). The sensitivity analysis is developed considering the design of a new underground station in Naples (Italy).

Evaluation of Rotation Capacity of RHS Aluminium Alloy Beams by FEM Simulation: Temper T4

The aim of this work is the numerical assessment of the ultimate behaviour of aluminium alloy beams subjected to non-uniform bending. An extensive numerical analysis has been performed by means of FE code ABAQUS with reference to RHS sections considering different values of the main geometrical and mechanical parameters. In particular, regarding the geometrical parameters the flange slenderness, the flange-to-web slenderness ratio and the moment gradient parameter have been considered. In particular, their influence on the ultimate behaviour of such beams has been investigated by adopting the material constitutive law proposed by Eurocode 9 based on the Ramberg-Osgood model. The investigations concern these parameters considered separately as well as their interaction. The results are herein reported with reference to temper T6 and show the importance of the investigated parameters on the buckling strength and the rotational capacity of aluminium alloy beams. Temper T6 gives rise to a quite low hardening compared to temper T4, which is analysed in a companion paper.

Proposal for an empirical evaluation of rotation capacity of RHS aluminium alloy beams based on fem simulations

The aim of this work is the development of an empirical relationship for evaluating the rotation capacity of RHS aluminium alloy beams, for temper T4 and T6. The proposed relationships are based on the numerical results coming from an extensive parametric analysis performed by means of FE code ABAQUS for different materials, which gain insight into the influence of all the geometrical and mechanical parameters affecting the ultimate behaviour. In particular, the influence of the materials strain hardening, flange slenderness, web stiffness, shape factor and moment gradient the on the plastic behaviour of such beams has been investigated. Successively, by means of monovariate and multivariate non linear regression analyses, empirical relationships are provided in order to predict the rotation capacity of RHS aluminium alloy beams starting from their geometrical and mechanical properties. This paper is focused on this issue.

SEISMIC RETROFIT OF EXISTING BUILDINGS THROUGH THE DISSIPATIVE COLUMNS

A new replaceable hysteretic damper to better control seismic building damage, consisting of two or more adjacent steel vertical elements (columns) connected to each other with continuous mild/low strength steel shear links, is investigated in this study. New Dam-pers, called Dissipative Columns (DC), are continuously linked with X-shaped steel plates and provide additional stiffness and damping to a lateral system. The Dissipative Column has been conceived as a device installed within a frame or as an external damper to provide ma-cro-dissipation. In fact, considering different configurations, a parametric analysis is devel-oped, through non-linear pushover and cyclic analyses carried out in ABAQUS, in order both to evaluate the effect of the main geometrical and structural parameters and provide the de-sign capacity curves of this new damper. Moreover, non linear dynamic analyses of an exist-ing building without and with the Dissipative Columns have been performed in SAP2000 in order to evaluate the supplemental damping provided by the new damper. The DC can be considered a new damping device, easy to install in new as well as existing buildings in order to protect them from seismic damage.

SEISMIC RELIABILITY-BASED DESIGN OF STRUCTURES ISOLATED BY FPS

The paper deals with the seismic reliability of base-isolated structural systems equipped with friction pendulum isolators (FPS) in order to provide useful design recommen-dations. A two-degree-of-freedom model is adopted by accounting for the superstructure flex-ibility, whereas the FPS isolator behaviour is described by adopting a widespread model which considers the variation of the friction coefficient with the velocity. The spectral dis-placement corresponding to the isolated period has been chosen as intensity measure (IM). The uncertainty in the seismic inputs as well as the friction coefficient at large velocity are considered as random variables modeled through appropriate probability density functions. Monte Carlo simulations are developed in order to evaluate the probabilities exceeding dif-ferent limit states related to both superstructure and isolation level defining the seismic fra-gility curves. Finally, considering the seismic hazard curve related to an Italian site, closed-form expressions are derived with the aim to design the radius in plan of the friction pendu-lum isolators in function of the expected reliability level.

Passive Energy Dissipation Devices

In this chapter, the supplemental passive energy dissipation devices, metallic or hysteretic dampers, frictional, viscoelastic and viscous (linear and non-linear) dampers, developed and studied over the years, are briefly described. For each type of device, the physical, mechanical and technological aspects are analysed by describing the construction, hysteretic behavior, physical models, advantages, and disadvantages. Then, the more appropriate mathematical laws to model their dynamic behavior, with particular reference to the viscous and viscoelastic, are described. Finally, a comparison between all the different types of device is reviewed and the main recommendations, reported in the international codes with specific reference to the viscous and viscoelastic, are explained.

Lifetime Axial-Bending Capacity of a R.C. Bridge Pier Cross-Section Subjected to Corrosion

Reinforced concrete structures in service may be affected by aging, which may include changes in strength and stiffness assumed in structural design, in particular when the concrete is exposed to an aggressive environment. In this context, this paper provides a computational probabilistic approach to predict the time-evolution of the mechanical and geometrical properties of a statically determinate r.c. structural system (i.e. bridge pier) subjected to corrosion-induced deterioration, due to diffusive attack of chlorides, in order to evaluate its service life. Adopting appropriate degradation models of the material properties, concrete and reinforcing steel, as well as assuming appropriate probability density functions related to mechanical and deterioration parameters, the proposed model is based on Monte Carlo simulations in order to evaluate time variant axial force-bending moment resistance domains, with the aim to estimate the time-variant reliability index. Finally, an application to estimate the expected lifetime of a r.c. bridge pier is described.

Time-Variant Reliability of RC Structures

Reinforced concrete structures are generally affected by degradation phenomena, which results in a time variability in strength and stiffness beyond the baseline conditions which are assumed in structural design, in particular when the concrete is exposed to an aggressive environment. Therefore, structural safety should realistically be considered time-variant. This paper provides a probabilistic approach to predict the time-evolution of the mechanical and geometrical properties of a reinforced concrete structural element (i.e., bridge pier) subjected to corrosion-induced deterioration, due to diffusive attack of chlorides, in order to evaluate its service life. The proposed model is based on Monte Carlo simulations in order to evaluate time variant axial force-bending moment resistance domains, with the aim to estimate the time-variant reliability index. Finally, an application to estimate the expected lifetime of a deteriorating reinforced concrete bridge pile is proposed.

Modeling of Viscoelastic Dissipative Bracing Systems

This chapter deals with the theory of viscoelasticity and the discrete models such as, for example, the Kelvin and Maxwell models. The aim of this chapter is to assess the dynamic behavior of viscoelastic dissipative bracing systems taking into account the presence of the brace. In fact, the viscoelastic damper is modeled as the Kelvin model, whose behavior is dependent, in itself, on frequency; the viscoelastic damper-brace component can be studied through the Poynting-Thomson model which presents even more dependence on the frequency. Similarly, the viscous (linear or non-linear) damper-brace component can be studied through the Maxwell model, characterized by a frequency dependent dynamic response. In both cases, because of the frequency dependence, in the dynamic field, dynamic “reduced” magnitudes correspond to the static magnitudes of the viscoelastic dissipative bracing system, in other terms, between the static and dynamic behavior, there is a reduction in the effectiveness of the viscoelastic dissipative bracing system.