Emergency vehicles such as fire apparatus are often heavier than typical commercial vehicles. The Fixing America's Surface Transportation Act (FAST Act), signed into law in 2015 includes new truck size and weight provisions that exempt emergency vehicles from meeting the nationwide Interstate truck weight limits on the Interstate System and routes within reasonable access to the Interstate. The emergency vehicles exempted from these weight limits by the FAST Act can create greater load effects in bridges than previously recognized legal loads. NCHRP Project 20-07 Task 410 was initiated in March 2018 by the Transportation Research Board (TRB) with the objective of proposing modifications to the AASHTO Manual for Bridge Evaluation (MBE) to provide guidance for the load rating of bridges for the FAST Act Emergency Vehicles (EVs). In that study, research was undertaken utilizing recent WIM data to establish live load factors and multiple presence factors that are appropriate for emergency vehicles based on likely traffic situations and exposure intervals consistent with those specified in the AASHTO MBE. Load and Resistance Factor Rating (LRFR) EV load factors were then calibrated based on a reliability analysis methodology which is the basis for the current LRFR criteria in AASHTO MBE. The proposed live load factors were calibrated to achieve an average reliability index beta=2.50 for simple span and continuous bridges with spans up to 300-ft in length.

This paper presents an improved reinforced concrete (RC) steel bar deterioration model that incorporates pitting corrosion and considers the change in after-cracking corrosion rate to assess the time-dependent seismic fragility of RC bridge substructures in marine environments. The proposed model is implemented to conduct a probabilistic seismic fragility analysis of a three-span continuous box girder bridge accounting for uncertainties in bridge geometry, material properties, ground motion and corrosion parameters. The results show that the effect of chloride-induced corrosion cannot be neglected when performing the seismic fragility analysis of RC bridge substructures in marine environments. Additionally, the calculated time-dependent fragility curves indicate that there is a nonlinear accelerated growth of RC column vulnerability during the service life of highway bridges, especially after twenty-five years of exposure to chlorides.

Based on current rating methods, about one in nine of the 607,380 US bridges are considered to be structurally deficient. However, not all these bridges are at risk of collapse as current code-specified analytical methods are generally conservative and may underestimate the true safety levels of existing bridges. For these reasons, there has been considerable interest in developing methods that combine field measured data with analytical models to obtain more accurate assessments of existing bridges. This paper presents a Response-Based Load and Resistance Factor Rating (RB-LRFR) method that utilizes strain data to evaluate the safety of existing bridge members. Appropriate live load factors are calibrated to reflect the uncertainties associated with estimating the parameters and random variables needed to rate a bridge component using field data. The implementation of the proposed methodology is illustrated using a composite steel girder bridge as an example.

Traditional condition rating procedures concentrate on member condition and give little consideration to global structural behavior. In this paper a methodology is proposed to modify the correction factor applied during the assignment of bridge condition rating to account for bridge system behavior. Robustness and redundancy concepts, that have been used in the past for structural design of new bridges and for assessment of existing bridges, are in this work extended to condition rating. In this paper, corrosion of steel girders is considered as the main cause of material deterioration. By including the proposed correction factors, bridge inspectors will decrease condition ratings of less robust bridges that exhibit low levels of system capacity in their damaged state and increase the rating of robust bridges. Different system-based correction factors are assigned based on structural types and configurations, as well as the type, location, and extent of damage. The system-based correction factors proposed in this research are calibrated based on reliability index measures. The aim of condition ratings is to assign a numerical value or a grammatical concept (good, bad, fair, …) that reflects the state of structural components of a bridge as well as non-structural parts. A correction factor, ϕs is exclusively applied to the strength term to define a corrected condition rating, I*: (1) I * = I s t r e n g t h ϕ s + I n o n − s t r e n g t h https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/eq141.tif"/> where I* is the corrected condition rating, Istrength is the original condition rating related to the structural members contributing to system strength, Inon−strength is the original condition rating term related to the elements not contributing to system strength, ϕs is the system correction factor that accounts for the redundancy of a specific bridge type and configuration. The Istrength term can itself be generalized through the following equation: (2) I s t r e n g t h = ∑ I d e t e r i o r a t e d ϕ r https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/eq142.tif"/> where Idegteriorated is the term related to a specific level of deterioration detected in a structural member under inspection, ϕr is the robustness factor accounting for the consequences of that particular damage on the entire structural system. The structures analyzed are simply supported multibeam bridges (Figure 1). Figure 1 Typical superstructure cross section configuration of multi-girder bridge steel bridge. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig33_1.tif"/> The corrective factors found as result of the calibration process lead to values higher than 1.0 whenever the redundancy of a given bridge leads to a structural performance higher than the target. In other words, the structural consequences of member failure are less severe than the consequences of member failure in the bridge used as a base for the calibration. On the other hand, when a bridge presents a system factor smaller than 1.0, it means that the consequence of a member failure in a certain damage state is more severe than in the target bridge. This implementation of system correction factors in bridge condition rating procedures can be of particular interest when trying to manage the rehabilitation of a group of bridges with similar damages but of different structural configurations. In those cases, typical condition rating would rate all bridges with a similar mark. By applying the corrective factors, bridge agencies will be able to give priority to those bridges that present weaker redundancy and robustness characteristics.

Current bridge design and assessment practices remain primarily focused on evaluating the strength and serviceability of individual structural members and components. While this traditional member oriented approach has led to the design of safe bridge infrastructure networks, it is widely recognized that the approach does not necessarily lead to an accurate evaluation of the actual structural safety levels nor to the efficient utilization of resources when making decisions related to the management of existing deteriorating structures given the limited funds currently available for rehabilitating and replacing our ageing bridge stock. For this reason, there is renewed interest in developing system-level assessment methods as a basis for modern bridge safety evaluation and design processes. This paper reviews recent proposals for developing and implementing system performance criteria in bridge engineering. The paper addresses the establishment of performance-based design methods and structural redundancy and robustness metrics as well as network based ranking criteria. Insights from these reviews outline the benefits of transitioning bridge design and safety assessment processes from the traditional component-level approach to one that seeks uniform levels of risk for structural systems and infrastructure networks. Examples are presented to illustrate the implementation of these concepts in bridge engineering practice.

The aim of this study is to find system factors to implement in the evaluation/design equation of highway bridge decks to consider redistribution of stresses after the failure of one main member of the deck. The method is applied to a set of highway bridges representative of the precast prestressed box-beam typology. A probabilistic study is necessary to characterize the strength of the system and therefore evaluate the safety of the bridges by means of reliability analysis. An iterative calibration process is then implemented on each selected bridge to assess the fulfillment of the reliability requirements considering each bridge designed with a set of trial system factor. The final result is the proposal of a set of system factors that penalizes the design/assessment of non redundant bridges when compared to more redundant configurations.

A numerical method based on Ghosn (1998) is used to evaluate structural redundancy of an integral example bridge under lateral loads. This method requires the implementation of a non-linear static analysis by means of a 3D spatial frame model. System capacity is evaluated in its original configuration, in a deteriorated configuration due to corrosion of the rebar steel in the columns, and in a strengthened situation in which the deteriorated columns are wrapped with FRP obtaining a confinement effect. The variation of redundancy is monitored along the life-steps of the example structure.

A methodology is proposed to analyze the structural redundancy of bridge systems under lateral loads. The paper compares the results obtained from a probabilistic approach to those of a simplified deterministic analysis. A non-linear finite element analysis based on a 3-D frame model is performed for an example bridge considered to be representative of the behavior of structures with integral column-superstructure connections. The analysis accounts for material non-linearity using realistic models for the stress-strain relationships of the different constituents. The quantification of the available redundancy levels is evaluated using reliability as well as deterministic criteria previously proposed in NCHRP 458.

A methodology is proposed to analyze the structural redundancy of bridge systems under lateral loads. The paper compares the results obtained from a probabilistic approach to those of a simplified deterministic analysis. A non-linear finite element analysis based on a 3-D frame model is performed for an example bridge considered to be representative of the behavior of structures with integral column-superstructure connections. The analysis accounts for material non-linearity using realistic models for the stress-strain relationships of the different constituents. The quantification of the available redundancy levels is evaluated using reliability as well as deterministic criteria previously proposed in NCHRP 458.

This paper presents several procedures for estimating the expected maximum load effect on a highway bridge. These procedures, of various levels of complexity, explain how site-specific truck weight and traffic data collected using Weigh-In-Motion systems (WIM) can be used to obtain estimates of the maximum live load for the design life of a bridge, specified to be 75 years as per the AASHTO LRFD code, or the two-year return period to be used for the load capacity evaluation of existing bridges. The models require as input the WIM data collected at a site after being "scrubbed" from statistical outliers and filtered to remove inherent WIM equipment and measurement errors. The application of the proposed procedures is described using data collected at several sites throughout the U.S. A sensitivity analysis is also performed to identify the most critical parameters that control the projections of the expected maximum load.

The US Federal Bridge Formula was produced as a result of analysis completed over 35 years ago and enacted into US federal law 25 years ago. The bridge formula, "Federal Bridge Formula-B", remains in operation in the US although alternatives to the formula in its current form have periodically been proposed. At this time, there is no limit on commercial vehicles in the form of a bridge formula in the European Union. This paper presents background information on the US Bridge Formula, describes a few alternatives to it that have been proposed in the past and presents observations on its effects if formula would apply to commercial vehicles not only in the EU, but globally.

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 earthquakes with PGA=0.63 g and 50% probabilities of extensive damage and collapse are obtained for earthquakes with PGAs equal to 1.02 and 1.50 g, respectively. The detailed fragility analysis of the as-built bridge shows that the fixed steel bearings in the bridge are the most vulnerable components. Hence, the two most appropriate seismic retrofit measures consist of (i) steel bearing replacement by elastomeric bearings and (ii) deck/girder-splicing (continuity) with steel bearing replacement by elastomeric bearings. The seismic fragility analysis shows that although both retrofit strategies reduce the fragility of bridge piers drastically as compared to the as-built condition, the second retrofit strategy (i.e., the combination of steel bearing replacement and superstructure continuity) is overall more effective in reducing the seismic fragility of both piers and bearings. The results of the fragility analysis developed in this paper for both as-built and retrofitted bridges would help state engineers develop effective strategies for seismic retrofit prioritization and network seismic vulnerability assessment.

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 fragility analysis based on the mean values and probability distributions of each critical random variable. The LHS is thus used to create a set of nominally identical but statistically different bridge samples for performing the fragility analysis. The individual bridges from this statistically representative set are matched with earthquake samples of various intensities for the nonlinear seismic demand analysis. The seismic capacity of critical bridge components are estimated for each bridge sample from published experimental data. Through extensive numerical simulations, the sensitivity analysis identified the most vulnerable bridge components. Two seismic retrofit strategies for reducing the seismic risk of multispan simply supported steel bridges are studied: (i) steel bearing replacement by elastomeric bearings and (ii) deck/girder-splicing (continuity) with steel bearing replacement by elastomeric bearings. The analysis verified that retrofit option ii is the most effective. The finite-element model of the bridge samples, along with the assembled data on parameter uncertainties and member capacities, as well as a simulated set of ground motion records are suitable for use during the development of fragility curves for bridges with and without retrofit. Detailed description of the fragility analysis is presented in the companion paper.

The debonding of FRP from concrete substrate is studied using a stochastic finite element simulation of the direct shear test. The instability at final failure in the debonding of FRP from concrete is shown to be the result of snapback. The local variations in the interface fracture properties are shown not to significantly influence the severity of snapback.

A common mode of failure in fiber reinforced concrete beam structures is progressive debonding which takes in the form of interfacial crack propagation. Instability of the structure occurs When the length of debonding becomes sufficiently large. Failure accompanied by snapback has been observed owing to elastic unloading. The results from a numerical analysis using a cohesive law that exhibit softening are obtained. They are discussed and compared with test data based specimens involving concrete beam clamped to I-beam. Data acquisition was made by computer. A trade off is found between the load plateau during debonding and stability of final failure due-to snap-back.

A method to model the nonlinear behavior of bridges using a grillage discretization is proposed. A modified stiffness matrix is used to account for material nonlinearity. The method accounts for material nonlinearities due to bending and shearing stresses. A program is developed to perform an incremental analysis of bridges subjected to vehicular loads. The validity of the model and program is verified by comparing to the results of full-scale field tests and published analytical results.