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

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

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

Dynamic behaviour and seismic response of structures isolated with low shape factor bearings

This study investigates the mechanical behaviour of laminated elastomeric bearings with a low shape factor (LSF) and the dynamic response of structures mounted on them. Axial loads have a significant influence on the mechanical behaviour of the LSF bearings. Most of the existing theories and mechanical models for laminated bearings cannot be employed for LSF bearings because they disregard the important effects of axial shortening and bulging of the rubber layers on the horizontal bearing stiffness. In this study, a simplified model originally developed for slender rubber blocks is employed for describing the mechanical behaviour of LSF bearings, and validated against the experimental results on low-damping LSF bearings manufactured and tested at Tun Abdul Razak Research Center (TARRC). The proposed model is then used to simulate the seismic response of a structural prototype mounted on the low-damping LSF bearings and tested at University of Naples Federico II on a shaking table under horizontal seismic input. Further analyses are carried out to evaluate how the bearing shape factor affects the dynamic and seismic response of the prototype. The study provides some useful insight into the complex mechanical behaviour of LSF bearings and of structures mounted on them

Influence of ground motion characteristics on the optimal single concave sliding bearing properties for base-isolated structures

This study examines the influence of ground motion characteristics on the optimal friction properties of single concave sliding bearings employed for the seismic isolation of structural systems. The evaluation of the optimal properties is carried out by considering a non-dimensional formulation which employs the peak ground acceleration (PGA) and the peak ground acceleration-to-velocity (PGA/PGV) ratio as ground motion parameters. A two-degree-of-freedom (2dof) model is employed to describe the isolated system and two different families of records representative respectively of near fault and far field seismic inputs are considered. Following the nondimensionalization of the equation of motion for the proposed ground motion parameters, it is shown that the non-dimensional responses obtained for the two types of seismic inputs are similar. This result confirms that PGA/PGV is a good indicator of the frequency content and of other characteristics of ground motion records, helping to reduce the scatter in the response. Regression expressions are also obtained for the optimal values of the friction coefficient that minimizes the superstructure displacements relative to the base as a function of the abovementioned ground motion parameter and of the dimensionless system parameters. These expressions can be used for the preliminary estimation of the optimal properties of isolation bearings with a single concave sliding surface or double concave sliding surfaces with equal friction coefficient

Assessment of seismic-induced pounding risk based on probabilistic demand models

The seismic-induced pounding between adjacent buildings is an undesirable event that can cause major damage and even structural collapse for structures with inadequate separation distance. This issue is particularly important in metropolitan areas, where the land space is limited and expensive. In order to minimize the pounding risk, existing design codes provide simplified numerical procedures and analytical rules for estimating the minimum separation distance that is needed to avoid pounding under a target seismic hazard scenario. However, these code procedures are characterized by unknown safety levels and, thus, do not permit to control explicitly the risk of pounding or the consequences of the impact. Previous research by two of the authors developed a reliability-based design methodology for the separation distance that corresponds to a target probability of pounding during the design life of adjacent buildings. This methodology was successfully applied to linear elastic structures. Further studies are required to make reliability-based methodologies applicable in an efficient way to more complex nonlinear building models, which require the use of computationally expensive numerical simulations to accurately predict the structural response. This paper illustrates an efficient probabilistic seismic demand model (PSDM) for pounding risk assessment consistent with modern performance-based design frameworks. A PSDM consists in the analytical representation of the relation between a seismic intensity measure (IM) and an engineering demand parameter (EDP). In this specific problem, the EDP of interest is the peak relative displacement between the adjacent buildings at the most likely impact location. The PSDM can be used to estimate the seismic vulnerability and the mean annual frequency of pounding between adjacent buildings via convolution with the site’s hazard curve. First, an extensive parametric study is performed by considering the case of two adjacent buildings modeled as linear singledegree-of-freedom (SDOF) systems. Different IMs are proposed for the problem at hand, whose choice is motivated mainly by efficiency criteria. The parametric study results are utilized to evaluate the efficiency and sufficiency of the proposed IMs employed in conjunction with a PSDM based on the linear regression of the seismic demand variation with respect to the IM in the log-log space. Successively, the case study of two realistic steel buildings modeled as nonlinear hysteretic multi-degree-of-freedom sheartype systems is considered to evaluate the effectiveness and accuracy of the IMs and PSDM introduced for the buildings described as SDOF systems. A bilinear PSDM is proposed to achieve a better fit of the seismic median demand and dispersion over the entire range of seismic excitation levels. Finally, comparisons are made between the risk estimates obtained by using the linear and bilinear PSDMs and the corresponding estimates obtained via incremental dynamic analysis (IDA) in order to evaluate and compare the accuracy of the proposed regression models. It is found that the use of a bilinear PSDM in conjunction with cloud analysis provides seismic pounding risk estimates that are very close to those obtained through IDA at a small fraction of the computational cost and without scaling the records

Probabilistic seismic demand analysis for pounding risk assessment

This study aims to develop a Probabilistic Seismic Demand Model (PSDM) for pounding risk assessment suitable for use within modern performance-based design frameworks. In developing a PSDM, different choices can be made regarding the intensity measures (IMs) to be used, the record selection, the analysis technique applied for estimating the system response for different IM levels, and the model to be employed for describing the response statistics given the IM. In the present paper, some of these choices are analyzed and discussed by considering the case of two adjacent buildings modeled as single-degree-of- freedom systems with linear and nonlinear hysteretic behavior. Based on the comparison, an optimal demand model is sought as the one that permits to achieve confident estimates of the response parameter of interest, i.e., the relative displacement demand, with few time-history analyses. This property allows reducing the complexity and computational cost associated with the pounding risk assessment

Rapid earthquake loss updating of spatially distributed systems via sampling-based bayesian inference

Within moments following an earthquake event, observations collected from the affected area can be used to define a picture of expected losses and to provide emergency services with accurate information. A Bayesian Network framework could be used to update the prior loss estimates based on ground-motion prediction equations and fragility curves, considering various field observations (i.e., evidence). While very appealing in theory, Bayesian Networks pose many challenges when applied to real-world infrastructure systems, especially in terms of scalability. The present study explores the applicability of approximate Bayesian inference, based on Monte-Carlo Markov-Chain sampling algorithms, to a real-world network of roads and built areas where expected loss metrics pertain to the accessibility between damaged areas and hospitals in the region. Observations are gathered either from free-field stations (for updating the ground-motion field) or from structure-mounted stations (for the updating of the damage states of infrastructure components). It is found that the proposed Bayesian approach is able to process a system comprising hundreds of components with reasonable accuracy, time and computation cost. Emergency managers may readily use the updated loss distributions to make informed decisions

Seismic performance of structural systems equipped with buckling-restrained braces

Buckling-restrained braces (BRBs) are often employed for the seismic retrofit of existing buildings and the design of new structures, given their significant contribution in terms of stiffness and added damping. However, BRBs are characterized by a low lateral post-elastic stiffness, leading to excessive residual deformations that may compromise reparability. Moreover, accumulation of plastic deformations in the BRBs may compromise the capability of withstanding multiple earthquakes and aftershocks. The objective of this paper is to provide insight into the performance and residual capacity of dual systems made of BRB frames coupled with moment-resisting frames, through a simplified single-degree-of-freedom model. A non-dimensional formulation of the equation of motion is introduced, the statistic of the normalized peak, residual displacements and cumulated ductility of the system is evaluated for a set of ground motion records. Different values\ud of the BRB target maximum ductility and coupled frame properties are considere

Comparison of different models for high damping rubber bearings in seismically isolated bridges

Steel-reinforced high damping natural rubber (HDNR) bearings are widely employed in seismic isolation applications to protect structures from earthquake excitations. In multi-span simply supported bridges, the HDNR bearings are typically placed in two lines of support, eccentric with respect to the pier axis. This configuration induces a coupled horizontal-vertical response of the bearings, mainly due to the rotation of the pier caps. Although simplified and computationally efficient models are available, which neglect the coupling between the horizontal and vertical response, their accuracy has not been investigated to date. In this paper, the dynamic behaviour and seismic response of a benchmark three-span bridge are analysed by using an advanced HDNR bearing model recently developed and capable of accounting for the coupled horizontal and vertical responses, as well as for significant features of the hysteretic shear response of these isolation devices. The results of the analyses shed light on the importance of the bearing vertical stiffness and how it modifies the seismic performance of isolated bridges. Successively, the seismic response estimates obtained by using simplified bearing models, whose use is well established and also suggested by design codes, are compared against the corresponding estimates obtained by using the advanced bearing model, to evaluate their accuracy for the current design practice