Seismic Response Analysis of Continuous Multispan Bridges with Partial Isolation

Partially isolated bridges are a particular class of bridges in which isolation bearings are placed only between the piers top and the deck whereas seismic stoppers restrain the transverse motion of the deck at the abutments. This paper proposes an analytical formulation for the seismic analysis of these bridges, modelled as beams with intermediate viscoelastic restraints whose properties describe the pier-isolator behaviour. Different techniques are developed for solving the seismic problem. The first technique employs the complex mode superposition method and provides an exact benchmark solution to the problem at hand. The two other simplified techniques are based on an approximation of the displacement field and are useful for preliminary assessment and design purposes. A realistic bridge is considered as case study and its seismic response under a set of ground motion records is analyzed. First, the complex mode superposition method is applied to study the characteristic features of the dynamic and seismic response of the system. A parametric analysis is carried out to evaluate the influence of support stiffness and damping on the seismic performance. Then, a comparison is made between the exact solution and the approximate solutions in order to evaluate the accuracy and suitability of the simplified analysis techniques for evaluating the seismic response of partially isolated bridges

Scavi e ricerche nell'antica Plestia

in particolare pp. 213-231, 252-27

Assessment of optimal design methods of viscous dampers

Viscous dampers are often used for seismic protection and performance enhancement of building frames. The optimal design of such devices requires the modelling and propagation of the uncertainties related to the earthquake hazard. Different approaches are available for the seismic input characterisation and for the probabilistic response evaluation. This work analyzes the effect of different characterizations of the seismic input and of the response evaluation on the design of dampers for building frames. The seismic input is represented as a stochastic process and the optimal damper properties are found via a reliability-based design procedure aiming at controlling the frame performance while limiting the damper cost. Two simplified approaches are used to design the viscous damper of a multi-storey steel frame and the design results are compared with those obtained by considering a rigorous design approach resorting to advanced simulations for the response assessment. The first methodology evaluates the response through a prefixed probabilistic demand model, while the second approach considers the average response for a given hazard level only. The comparison allows to evaluate and quantify the effect of the seismic input uncertainty treatment on the system and damper performances

Analysis and comparison of two different configurations of external dissipative systems

This paper deals with the seismic protection of existing buildings, especially r.c. frame ones, by means of external passive dissipative systems. These type of systems provide larger flexibility in controlling the structural behavior, and some feasibility advantages, but their efficiency in terms of performance still need to be proven. In particular, this study analyzes and compares the performance of two external solutions using linear fluid viscous dampers (FVDs) for the seismic upgrading of an existing benchmark structure, the Van Nuys building. The first arrangement is a recent solution, known as "Dissipative Tower", which exploits the rocking motion of a steel truss hinged at the foundation level for the dampers activation; the second one consists in coupling the building with an external stiff contrasting structure, where the dampers are located horizontally at the storey level. First, a state space formulation of the problem, based on the assumption of linear elastic behavior for both the existing frame and the external dissipative structures, is presented in general terms. The proposed formulation, suitable for both the external arrangements, allows to evaluate the influence of the dissipative solutions on the system modal properties. Successively, the performance of the two proposed external passive structures, is evaluated and compared with that of the bare existing frame, by considering important engineering demand parameters (EDPs) such as interstorey drifts, absolute accelerations and shear actions resisted by the frame and by external systems

A parametric study on the axial behaviour of elastomeric isolators in multi-span bridges subjected to horizontal seismic excitations

This paper investigates the potential tensile loads and buckling effects on rubber-steel laminated bearings on bridges. These isolation bearings are typically used to support the deck on the piers and the abutments and reduce the effects of seismic loads and thermal effects on bridges. When positive means of fixing of the bearings to the deck and substructures are provided using bolts, the isolators are exposed to the possibility of tensile loads that may not meet the code limits. The uplift potential is increased when the bearings are placed eccentrically with respect to the pier axis such as in multi-span simply supported bridge decks. This particular isolator configuration may also result in excessive compressive loads, leading to bearing buckling or in the attainment of other unfavourable limit states for the bearings. In this paper, an extended computer-aided study is conducted on typical isolated bridge systems with multi-span simply-supported deck spans, showing that elastomeric bearings might undergo tensile stresses or exhibit buckling effects under certain design situations. It is shown that these unfavourable conditions can be avoided with the rational design of the bearing properties and in particular of the shape factor, which is the geometrical parameter controlling the axial bearing stiffness and capacity for a given shear stiffness. Alternatively, the unfavourable conditions could be reduced by reducing the flexural stiffness of the continuity slab

Reliability-based optimal design of nonlinear viscous dampers for the seismic protection of structural systems

Viscous dampers are widely employed for enhancing the seismic performance of structural systems, and their design is often carried out using simplified approaches to account for the uncertainty in the seismic input. This paper introduces a novel and rigorous approach that allows to explicitly consider the variability of the intensity and characteristics of the seismic input in designing the optimal viscous constant and velocity exponent of the dampers based on performance-based criteria. The optimal solution permits controlling the probability of structural failure, while minimizing the damper cost, related to the sum of the damper forces. The solution to the optimization problem is efficiently sought via the constrained optimization by linear approximation (COBYLA) method, while Subset simulation together with auxiliary response method are employed for the performance assessment at each iteration of the optimization process. A 3-storey steel moment-resisting building frame is considered to illustrate the application of the proposed design methodology and to evaluate and compare the performances that can be achieved with different damper nonlinearity levels. Comparisons are also made with the results obtained by applying simplifying approaches, often employed in design practice, as those aiming to minimize the sum of the viscous damping constant and/or considering a single hazard level for the performance assessment

Multi-EDP Performance Assessment of a Steel BRBF Under Ground Motion Sequences

The behaviour of Buckling-Restrained Braces (BRBs) is characterised by steady and nearly symmetrical hysteretic loops that provide large energy dissipation capacity. However, their low post-yielding stiffness may result in large residual deformations at the end of the earthquake motion. Moreover, the cumulative ductility demand due to repeated plastic excursions can lead to low-cycle fatigue failure. These two unfavourable conditions could be exacerbated by the occurrence of subsequent earthquakes (e.g., mainshock-aftershock sequences or multiple earthquakes during the design lifetime). Therefore, their assessment requires a framework that considers ground motion (GM) sequences. In this paper, a case study structure subjected to GM sequences is analysed, considering three Engineering Demand Parameters (EDPs) relevant to the performance evaluation of the BRBs and of the frame, namely the maximum ductility demand in the BRBs, the cumulative ductility demand in the BRBs, and the residual inter-storey-drift in the frame. These EDPs are assessed both independently and simultaneously, in order to establish the risk of overpassing any capacity limit that may lead to the collapse or demolition of the structure

Identification of critical mechanical parameters for advanced analysis of masonry arch bridges

The response up to collapse of masonry arch bridges is very complex and affected by many uncertainties. In general, accurate response predictions can be achieved using sophisticated numerical descriptions, requiring a significant number of parameters that need to be properly characterised. This study focuses on the sensitivity of the behaviour of masonry arch bridges with respect to a wide range of mechanical parameters considered within a detailed modelling approach. The aim is to investigate the effect of constitutive parameters variations on the stiffness and ultimate load capacity under vertical loading. First, advanced numerical models of masonry arches and of a masonry arch bridge are developed, where a mesoscale approach describes the actual texture of masonry. Subsequently, a surrogate kriging metamodel is constructed to replace the accurate but computationally expensive numerical descriptions, and global sensitivity analysis is performed to identify the mechanical parameters affecting the most the stiffness and load capacity. Uncertainty propagation is then performed on the surrogate models to estimate the probabilistic distribution of the response parameters of interest. The results provide useful information for risk assessment and management purposes, and shed light on the parameters that control the bridge behaviour and require an accurate characterisation in terms of uncertainty