Michel Ghosn

Cost and safety optimization of structural design specifications

Abstract The design of buildings, bridges, offshore platforms and other civil infrastructure systems is controlled by specifications whose purpose is to provide the engineering principles and procedures required for evaluating the safety of structural systems. The calibration of these codes and specifications is a continuous process necessary to maintain a safe national and global infrastructure system while keeping abreast of new developments in engineering principles, and data on new materials, and applied loads. The common approach to specification calibration is to use probabilistic tools to deal with the random behavior of materials and to account for the uncertainties associated with determining environmental and other load effects. This paper presents a procedure to calibrate load factors for a structural design specification based on cost and safety optimization. The procedure is illustrated by determining load factors that may be applicable for incorporation in a bridge design specification. Traditional code calibration procedures require a set of pre-determined safety levels that should be used as target values that each load combination case should satisfy. The procedure in this paper deduces the failure cost implied in present designs, and provides consistent safety levels for all load combination cases. For greater accuracy, load effects showing variance in time have been modeled by separating them into two random variables; time dependent r.v. (wind speed, vehicular loads, etc.) and time independent r.v. (modeling uncertainties). The total expected lifetime cost is used in the optimization to account for both initial construction cost and future equivalent failure costs.

Seismic Fragility of Multispan Simply Supported Steel Highway Bridges in New York State. II: Fragility Analysis, Fragility Curves, and Fragility Surfaces

This paper presents the seismic fragility analysis of a typical multispan simply supported steel bridge in New York State. A detailed description of the bridge model including an analysis of parameter uncertainties was provided in the companion paper. The companion paper also describes a sensitivity analysis that was performed to determine the most critical parameters that control the seismic response of the bridge. A set of statistically independent bridge samples and earthquake samples were specified for the fragility analysis. Two alternative seismic retrofit designs were also presented in the companion paper. The results of the seismic fragility analysis performed in this paper based on the data assembled in the companion paper show that typical multispan simply supported steel bridges in New York State have more than 50% probability of exhibiting slight damage when subjected to earthquakes with peak ground accelerations (PGAs) of 0.51 g. A 50% probability of incurring moderate damage is observed for ...

Seismic Fragility of Multispan Simply Supported Steel Highway Bridges in New York State. I: Bridge Modeling, Parametric Analysis, and Retrofit Design

This paper studies the dynamic seismic behavior of a typical highway bridge in New York State. The topological layout and structural details of this multispan simply supported steel-girder bridge are identified as the most typical of the New York State Department of Transportation bridge inventory database. Three-dimensional finite-element models of the bridge are established considering the nonlinear behavior of critical bridge components. An in-depth parametric study is carried out to evaluate the sensitivity of the bridge’s seismic response to variations in its structural parameters. The parametric analysis determined that uncertainties associated with the steel reinforcement’s yield strength, the superstructure’s weight, the expansion joints’ gap size, the friction coefficient of expansion bearings, and the concrete compressive strength should be considered during the fragility analysis of the bridge system. The Latin hypercube sampling (LHS) approach is used to obtain representative samples for the f...

Load capacity evaluation of existing railway bridges based on robustness quantification

This paper presents a methodology for the load capacity evaluation of existing railway bridges based on the quantification of their robustness. The proposed technique uses the results of a non-linear structural analysis of a bridge system and couples the analysis with modified member and system reliability evaluations in order to assess the bridges robustness and its overall system capacity. Bridge robustness is measured by the so-called redundancy factors, which serve to compare the systems capacity to member capacity. The application of the proposed approach to the safety assessment of an existing reinforced concrete railway bridge demonstrates its efficiency. The example shows that the proposed methodology is sufficiently accurate and simple for it to be used in the bridge assessment process on a routine basis. Also, the example indicates that the proposed redundancy factors provide an adequate and objective measure of robustness.

Performance Indicators for Structural Systems and Infrastructure Networks

AbstractEstablishing consistent criteria for assessing the performance of structural systems and infrastructure networks is a critical component of communities’ efforts to optimize investment decisions for the upkeep and renewal of the built environment. Although member-level performance and reliability assessment procedures are currently well-established, it is widely recognized that a member-oriented approach does not necessarily lead to an efficient utilization of limited resources when making decisions related to the management of existing deteriorating structures or lifeline systems, especially those that may be exposed to extreme events. For this reason, researchers have renewed their interests in developing system-level assessment methods as a basis to modern structural and infrastructure performance evaluation and design processes. Specifically, system-level performance metrics and characteristics such as reliability, redundancy, robustness, resilience, and risk continue to be refined. The objecti...

Seismic Fragility of Retrofitted Multispan Continuous Steel Bridges in New York

Various retrofit measures, such as elastomeric bearings, lead-rubber bearings, viscous dampers, and jacketing with carbon fibers, are commonly used to improve the seismic performance of multispan continuous steel highway bridges. In this paper, we have investigated the effectiveness of these retrofit measures through comparisons of seismic fragility of as-built and retrofitted multispan continuous steel bridges. Both elastomeric and lead-rubber bearings reduce the fragility of bridge piers significantly through isolation effects. Wrapping of piers with fiber-reinforced polymer (FRP) increases the effective ductility of piers through confinement and shifts the failure mode of a FRP wrapped pier to rupture of the FRP at much higher peak ground acceleration. The use of viscous dampers in combination with elastomeric bearings is effective in reducing fragilities because of both pier ductilities and bearing displacements. Hence, all four seismic retrofit strategies are effective in improving the safety of bridge components during earthquakes.

Procedure for Statistical Categorization of Overweight Vehicles in a WIM Database

AbstractRecently observed increases in the numbers of permitted and illegal overweight trucks travelling over U.S. highways have raised concerns over their contributions to the reduction in the service lives of pavements and bridges and the costs of maintaining, upgrading and replacing the highway infrastructure system. Cost allocation studies are used by transportation officials to help their asset management processes and to establish truck traffic and permitting policies taking into consideration information on the composition of overweight trucks and their permit categorization. In recent years, cost allocation studies have heavily relied on data assembled by weigh-in-motion (WIM) systems which provide information on traffic counts, truck axle configurations and weights for various highway classes and economic regions. However, WIM data by themselves do not provide information on the numbers of illegal overweight trucks because many of the overweight trucks may have been issued permits that allow them...

Response Functions and System Reliability of Bridges

Redundancy of a structural system is defined as the capability of the system to carry additional loads after the failure of one or more of its components. Research studies to include redundancy in the U.S. bridge design and evaluation codes are underway. These efforts are based on calibrating system factors that could be added to the checking equations. The purpose of these system factors is to “reward” redundant designs by allowing savings in required member capacities, and “penalize” nonredundant designs by requiring their members to be more conservative. The calibration process uses a target redundancy index as the safety criterion that bridges should satisfy. The redundancy index is defined as the difference between the reliability index of the bridge system and the reliability index of the members. This paper illustrates the method proposed to calculate the redundancy indices of typical bridges. The member safety index is calculated, as traditionally done, using the results of an elastic analysis of the bridge. The system reliability index accounts for the nonlinear behavior of the bridge system and the redistribution of the applied loads after the failure of a member. In this paper, the response function (response surface) method is used to calculate the system safety indices of typical multi-beam steel and prestressed concrete bridges. System factors are then calibrated to reflect the level of redundancy that different bridge configurations possess.

Simplified probabilistic model for maximum traffic load from weigh-in-motion data

Abstract This paper reviews the simplified procedure proposed by Ghosn and Sivakumar to model the maximum expected traffic load effect on highway bridges and illustrates the methodology using a set of Weigh-In-Motion (WIM) data collected on one site in the U.S.A. The paper compares different approaches for implementing the procedure and explores the effects of limitations in the site-specific data on the projected maximum live load effect for different bridge service lives. A sensitivity analysis is carried out to study changes in the final results due to variations in the parameters that define the characteristics of the WIM data and those used in the calculation of the maximum load effect. The procedure is also implemented on a set of WIM data collected in Slovenia to study the maximum load effect on existing Slovenian highway bridges and how the projected results compare to the values obtained using advanced simulation algorithms and those specified in the Eurocode of actions.

Simplified probabilistic non-linear assessment of existing railway bridges

An objective of the European Commissions Sixth Framework Research Project, Sustainable Bridges, is to advance understanding of the behaviour of existing railway bridges and develop tools to assess their ability to safely handle future traffic demands and extend their service lives. This paper presents the findings of a study that reviews structural safety models applicable to the assessment of existing bridges. The study proposes the use of simplified probabilistic non-linear structural analysis methods to provide more accurate assessments of the load capacity of bridge systems than traditional methods. The simplified methods use the results of a limited number of deterministic non-linear structural analyses and apply these results into a reliability framework. The application of the proposed methods is illustrated by assessing the safety of an existing bridge. The accuracy and efficiency of the simplified methods are verified by comparing the results of the simplified methods to those obtained from full probabilistic non-linear analysis procedures.