Diagnostic Load Testing Research Articles

Experimental Behavior of a Steel Girder–Stringer–Floorbeam Bridge

The presented case study summarizes a quick-response project for the Tennessee Department of Transportation (TDOT), which focused on the steel-stringer behavior for a girder–stringer–floorbeam bridge. TDOT was confronted with relatively low stringer load ratings for many of their 31 similar structures. As a result, TDOT initiated this field study to ultimately produce refined load ratings. This information was then to be utilized for deciding whether an intervention (e.g., retrofit) was necessary. Diagnostic load testing was the approach selected and a representative bridge was chosen. Then load testing was comprehensively designed and executed. This included the installation of 42 strain gauges, data-acquisition quality-control testing, weighing and measuring test trucks, load testing of the structure, and then removal of the system. The load-test data provided insight into the true behavior of the bridge. The stringers were subjected to a combined interaction of the global and local response. Overall, the local demands were the largest component, but the magnitudes were well below the predicted values. This was partly because of the lateral distribution of the load. Additionally, partial composite action was identified between the steel stringers and concrete slab. Refined load ratings were calculated using the field-measured data and following the AASHTO Manual for Bridge Evaluation. The comparison of predicted to measured responses clearly indicated the conservative nature of the prior analytical calculations. The stringer load ratings were increased by 68% based on the field testing. In the end, the steel-stringer load ratings were sufficient, with no intervention necessary.

Diagnostic load testing and assessment of a corroded corrugated metal pipe culvert before rehabilitation

Buried culverts are important structures within the civil infrastructure that convey water beneath roadways, bridges, and other systems while also being load bearing. However, many culverts are deteriorating and reaching the end of their design life span, where assessment methods to determine whether replacement or rehabilitation techniques such as spray applied pipe liners (SAPLs) are needed to extend their service life. In this study, mounted sensors and vision-based measurement techniques are used to assess the in situ conditions of a culvert consisting of corrugated metal pipes before being rehabilitated with geopolymer (cementitious-based) SAPLs. Results from diagnostic load tests (variating from static to dynamic) are presented to evaluate maximum deformations, verify the load-carrying effectiveness of the SAPLs, and compare results to design calculations due to the presence of a live load acting on the culvert.

Assessment of existing concrete bridges by load testing: barriers to code implementation and proposed solutions

As the existing bridge stock is aging, the task of assessing these bridges becomes increasingly important. One of the assessment methods for existing bridges is load testing. Improvements in the field of diagnostic load testing are related to the use of numerical models. Improvements in the field of proof load testing focus on the safety of the execution of the test as well as the required load in the test. What is still lacking is a reflection of these recent advances in the codes and guidelines used for load testing of bridges. Two approaches are proposed to address this lack. The first approach attempts to answer fundamental questions with regard to bridge load testing through research. The second approach is to coordinate efforts and facilitate collaboration and exchange of ideas internationally through the IABMAS Technical Committee on Bridge Load Testing. In conclusion, it is expected that these efforts will form the basis of improved recommendations for the assessment of concrete bridges by load testing to be included in codes and guidelines and to serve the community of engineers faced with the task of assessing ageing infrastructure.

Finite Element Model Updating of RC Bridge Structure with Static Load Testing: A Case Study of Vietnamese ThiThac Bridge in Coastal and Marine Environment.

Diagnostic load testing refers to the use of the measured historical responses of the structure in the field data to better understand its dynamic and static structural behaviours. It is important and necessary to predict the health state, load capacity, and aging of the structure by updating the finite element (FE) model, which can give useful information to aid the design of retrofits and the maintenance of the existing bridge in the future. The paper presents an update of the full-scale FE model for the reinforced concrete (RC) bridge structure over the seawater river based on the experimental strains under the static load testing in which the representative FE model of the actual structure is determined from the optimisation procedures. The optimisation variables are applied, including the cross-sectional properties and concrete material calibrated through the genetic algorithm (GA) optimisation in the MATLAB software, which interfaces with the FE modelling in the scripting of the SOFISTIK TEDDY software automatically. The bending moments at the mid-span of the RC girders are determined in the FE modelling to compute stresses, which are compared with the measured stresses through optimisation scenarios with a percentage error of the objective function less than 10%. The measured data of concrete strains are recorded from reusable strain transducers installed on the mid-span girders for every bridge span, which are used to calibrate the bridge model in static load testing. The novelty of the solution is to implement innovative techniques using field data as an improved approach for calibrating automatically the analytical FE model parameters of all RC spans of the bridge until its static behaviours are very similar to those of the actual bridge. The final updated FE modelling is used to apply truck load configurations according to bridge design standards such as the AASHTO specifications, which can predict the load limits of the existing bridge structure more accurately and reliably. These proposed approaches can be applied to large bridges as well as complex structures with supporting FE analysis software and data processing software.

Monitoring structural responses during load testing of reinforced concrete bridges: a review

Nowadays, with the aging of the bridges and the advancements in technology, load testing has emerged as an effective method to assess existing concrete bridges with missing information, or where analytical methods do not provide an accurate assessment. Two types of load tests are identified: diagnostic load tests and proof load tests. Both rely on field measurements of parameters or structural responses of the bridge during the test. A diagnostic load test measures the response of the bridge so that analytical models can be calibrated and evaluated. In a proof load test, the bridge directly demonstrates that it can carry a certain load. Since large loads are applied, the bridge needs to be carefully monitored. In this case, monitoring the measurements provide a warning to avoid damage. This paper reviews the literature on reported load tests and the measurement techniques used during these tests. It also includes a review of traditional and recently developed sensing technologies. Finally, the measurement requirements for diagnostic and proof load tests are given as well as a flow chart to guide engineers in the selection process of appropriate monitoring and measurement techniques during load tests. This paper can serve engineers during the preparation of a load test.

Structural and load parameter estimation of a real‐world reinforced concrete slab bridge using measurements and Bayesian statistics

AbstractThis paper describes a static diagnostic load testing and measurement campaign of a reinforced concrete road bridge in Amsterdam. We consider 29 vertical translation sensors and 37 strain sensors. Multiple Bayesian parameter estimations are performed to estimate two structural and two load parameters of a three‐dimensional finite element (FE) model. The structural parameters are the concrete elastic modulus of the deck and the rotational spring stiffness at the piers. The load (truck) parameters are the load mass and position, which are estimated from separate measurements. We found that estimating (updating) the two selected structural parameters considerably improves the vertical translation model prediction accuracy over the FE model that was built before the measurement campaign, that is, the R2 score increases from 0.90 to 0.98. Moreover, the load (truck) mass and position parameters can be estimated with high accuracy and high precision. When all measurements are used, the load mass and position are estimated with an error, respectively, less than 1.5 tonnes and 0.5 m with respect to the ground truth. On the negative side: when only certain strain measurements are used, the 90% posterior credible interval is about 3 m and 10 tonnes off the ground truth. Although the results are promising, they are restricted to the considered case and, more importantly, to operational loading conditions where the loading is well controlled. All data, models, and code are freely available upon request.

Modeling and analysis of a prestressed girder bridge prior to diagnostic load testing

Progressive deterioration is a problem that affects road infrastructure, especially bridges. This requires the development of methods for its adequate detection and revision, one of them being load testing. Within load testing, finite element analysis (FEA) models provide initial information to understand the behavior of a structure and plan accordingly, which represents a fundamental step towards a precise structural evaluation of a bridge. This study focused on the modeling and analysis of the static response of the bridge over the river Lili in Cali, Colombia, a prestressed girder bridge programmed to undergo a diagnostic load test. A linear FEA model was created with information from a manual survey and from other bridges’ plans designed and built under the regulations in force at the time. Due to the absence of plans and design specifications for the bridge, variations were applied to certain model parameters (stiffness of diaphragms and elastomeric bearings), to quantify their effect on the overall behavior of the bridge. The analysis included obtaining the critical position for the design vehicles, the transversal distribution of stresses and determining the influence of the variation parameters in the response of the structure. Results showed that the critical combinations for bending moment and shear were when the loads were the closest to the exterior girders, being these elements the most affected. The variation on the modulus of elasticity for the diaphragms and the stiffness of the elastomeric bearings did not significantly influence the results for bending moment and shear, nor the critical position. Girder distribution factors (GDF) from the model were compared to previous research, finding similarities in shape and value with other FEA models and experimental results. Finally, an instrumentation plan focused on the girders of the bridge was proposed based on the zones where the maximum effects are expected. The findings in this study show how linear FEA models provide initial but relevant information regarding the critical position of design vehicles, the distribution of stresses and the expected values for bending moment and shear under design loads.

3-D frame behaviour and load rating of truss bridges in railway infrastructure

Railway truss bridge load rating is studied herein using full scale diagnostic load test and 3-D finite element analysis. which has not been typically practiced. Forty-eight bridges of Canadian National Railway are included, with eight load tested. One of the focuses is the effect of a collision strut member (CS) to the behaviour of main truss members, especially the end post L0U1. For simplicity, CS was ignored in design and past load rating using 2-D truss analysis. Its effect is identified herein for increased live load stress by inducing bending, leading to unconservative rating. In addition, these spans are found to behave more closely to frames. Out-of-plane bending of main truss members is also studied as part of 3-D behaviour. A multiplicative adjusting factor (AF) is accordingly developed and recommended as the realistic solution to enhance load rating by addressing bending previously ignored in truss models, for a large number of bridges in the infrastructure network. The findings of this study have been implemented in practice and can be useful for design and evaluation of truss bridges in North America and other continents for stress calculation in both strength and fatigue limit states, while localised stress analysis with fine mesh for connections was out of this study’s scope.

Bridge Load Testing: State-of-the-Practice

Bridge load testing can answer a variety of questions about bridge behavior that cannot be answered otherwise. The current governing codes and guidelines for bridge load testing in the United States are the 1998 NCHRP Manual for Bridge Rating through Load Testing and Chapter 8 of the AASHTO Manual for Bridge Evaluation. Over the last two decades, the practice of load testing has evolved, and its intersections with other fields have expanded. The outcomes of load tests have been used to keep bridges open cost-effectively without unnecessarily restricting legal loads, when theoretical analyses cannot yield insights representative of in-service performance. Load testing data can be further used to develop field-verified finite-element models of tested bridges to understand these structures better. In addition, structural reliability concepts can be used to estimate the probability of failure based on the results of load tests, and noncontact measurement techniques capturing large surfaces of bridges allow for better monitoring of structural responses. Given these developments, a new Transportation Research Board (TRB) Circular, Primer on Bridge Load Testing, has been developed. This document contains new proposals for interpreting the results of diagnostic load tests, loading protocols, and the determination of bridge load ratings based on the results of proof load tests. In addition, included provisions provide an estimation of the resulting reliability index and the remaining service life of a bridge based on load testing results. The benefit of load testing is illustrated based on a cost-benefit analysis. The current state-of-The-practice has demonstrated that load testing is an effective means for answering many important questions regarding bridge behavior that are critical to decisions on bridge maintenance or replacement. Load testing has evolved over its history, and the newly developed TRB Circular reflects this evolution in a practical way.

Editorial: Diagnostic and Proof Load Tests on Bridges

Editorial: Diagnostic and Proof Load Tests on Bridges

Performance assessment of a concrete railway bridge by diagnostic load testing

AbstractDiagnostic load testing can be a suitable method for condition assessment of existing bridges. This paper presents the evaluation of a concrete railway bridge built in the 1940s based on diagnostic load testing. The test was the outcome of a visual inspection and preliminary theoretical analysis that found the bridge to be in unsatisfactory condition. Unlike the other possibilities that infrastructure management bodies have in such situations, for example, structure replacement, traffic reorganization, closure of the bridge, etc., diagnostic load testing is a low cost and quick method that indicates the bridge response to real traffic load. The paper describes the test plan and implementation covering the static and dynamic load tests. The experimental methods comprise superstructure displacements measurement, strain measurement on concrete and reinforcement and determination of bridge dynamic parameters. The obtained test results analysis enabled more accurate diagnosis of the actual bridge condition. The bridge demonstrated satisfactory load bearing capacity and response to real service load in terms of registered levels of displacement and strain and proper dynamic performance.

Bridge Load Rating Through Proof Load Testing for Shear at Dapped Ends of Prestressed Concrete Girders

Load ratings of the 1967 built I-195 westbound bridge over Seekonk River in Rhode Island are governed by shear at the dapped ends of prestressed concrete (PSC) girders. Insufficient analytical load ratings, in combination with visible shear cracks at the dapped ends, prompted the need for a refined method of load rating through load testing. The proof load testing method was chosen over diagnostic testing for a higher level of reliability due to uncertainties involved in calculating shear capacities of the dapped ends. A proof load test was successfully completed with a maximum proof load of two test trucks of approximately 100,000 lbs each crossing the bridge side-by-side without any signs of distress or nonlinear behavior observed. Vehicle loading dynamic impact was also assessed during the load test. Test measurements indicated full composite action between the bridge deck and the PSC drop-in girders, which was not accounted for in the analytical rating of the dapped ends. Test results suggested that the strut-and-tie and shear-friction analysis methods underestimate the shear resistance of the dapped ends by considering only the stirrup reinforcement and draped prestressing strands within the PSC girder. This paper provides detailed information on bridge proof load testing in general concept and procedure, comparisons with the diagnostic load testing method, field operation, examination of test measurements for linear elastic structural behavior, and determining bridge load ratings based on test results per The Manual for Bridge Evaluation of AASHTO.

Diagnostic Load Testing for Improved Accuracy of Bridge Load Rating

Bridge load rating provides a standardized procedure to determine the safe load-carrying capacity of a bridge, thereby allowing engineers to establish posting and permitting requirements. The structural condition of components, material properties, loads, and traffic conditions all contribute to the load rating, which describes the capacity of the controlling component of the structure. The primary purpose of bridge load rating is to ensure public safety. The bridge industry has permitted nondestructive load testing, at the discretion of the owner, to establish the safe load-carrying capacity of a given bridge when coupled with analysis and sound engineering judgment. The direct measure of structural response through diagnostic live load testing is viewed as a more accurate method of determining capacity and requires minimal assumptions regarding load distribution. Diagnostic live load testing often results in higher loading postings when compared to traditional analysis because of the elimination of conservative assumptions. A number of data acquisition (DAQ) systems are available for implementation in the load rating process. When correctly implemented, the systems provide an improved understanding of the live load response. Furthermore, deferring repair or replacement through diagnostic load testing can deliver immediate and long-term cost savings. As such, implementation of a diagnostic load testing program may have significant short- and long-term paybacks for both bridge owners and users. The current study employed three DAQ systems to demonstrate how diagnostic load testing can be used to augment traditional load rating procedures. The study revealed that diagnostic load testing of a rural, simple-span steel girder bridge resulted in an improved load rating of 273%, refining the test truck inventory rating from 1.5 to 4.1. Similarly, diagnostic load testing of a rural, simple-span steel pony truss bridge resulted in an improved load rating of 170%, going from a test truck inventory rating of 4.5 to 7.6. Analysis of the field-collected data demonstrated the improved ratings were attributable to a number of factors not considered during traditional load rating, such as site-specific factors and contribution of nonstructural components as well as a true measure of the dynamic amplification, load distribution, and composite action. The positive results showcase the possible improvement to load rating by using diagnostic load testing as compared to traditional load rating procedures, leading to a safer bridge inventory.

Monitoring and Diagnostic Load Testing of a Damaged Railway Bridge

This study presents a complex experimental research of a damaged steel railway bridge. Before the reconstruction, the elastic behavior of the material was evaluated using the hole-drilling strain gauge method of determining residual stresses at the relevant cross-sections. During the reconstruction project (lifting of the structure), a short-term monitoring system was installed at the critical cross-sections for continuous recording of relative strain. The aim was to evaluate the quality of the reconstruction intervention and prevent further damages. Following a successful reconstruction, a diagnostic load testing was performed according to Croatian standards. The purpose of the load testing (static and dynamic) was to evaluate the ability of the bridge to carry the design loads and calibrate the finite element models. During static load testing loading vertical displacement was measured as well as relative strain. Dynamic load testing of the bridge was performed in order to determine the main dynamic parameters of the structure and to calculate the dynamic factor. In order to select the appropriate measurement parameters and methods used during this experimental research it was necessary to consider the bridge type, materials and reconstruction or strengthening interventions. Especially, since this bridge was an example of insufficient inspection and maintenance during service. A well-designed monitoring and diagnostic load testing needed to be performed in order to obtain useful results for the decision makers involved.

Load Distribution of a Prestressed Self-Consolidating Concrete Bridge

Bridge A7957 is the first Missouri Department of Transportation (MoDOT) large-scale project using self-consolidating concrete (SCC) and high-strength self-consolidating concrete (HS-SCC). The objective of this research was to monitor the initial in-service behavior of the precast-prestressed concrete primary elements of Bridge A7957 and to obtain the load distribution of the bridge using field and finite element models (FEM) data. An initial series of diagnostic load tests was conducted on the bridge superstructure. Embedded sensors recorded strain variations at different section of the instrumented girders for different load configurations. An automated total station (ATS) collected the vertical deflection of the girders at several locations during the application of different test loads. The load distribution for moment was obtained experimentally (using deflection and strain data), FEMs, and using the AASHTO LRFD Bridge Design Specifications. The distribution factors for moment estimated with the AASHTO LRFD equations resulted in larger values compared to field test and FEM results. No difference was observed between the response of the SCC and conventional concrete members during the first series of field load tests.

Nonlinear flexural distribution behavior and ultimate system capacity of skewed steel girder bridges

Bridge designs are routinely developed using simplified 1D approximations of structural behavior and assume linear elastic behavior throughout the structure. More rigorous methods can reveal significant unquantified reserve capacity with 3D system behavior as ductile members experience yielding, but reserve capacity can be overestimated if cracking in a concrete deck is neglected. The primary objective of this study is to characterize the influence of concrete deck nonlinearity on 3D system behavior and capacity of skewed steel girder bridges. Structural behavior is examined throughout the range of potential applied loads, from initial linear elastic behavior to initial yielding and ultimate conditions. Additionally, this study provides a secondary benefit by comparing the applicability of two general load distribution characterization methods: response fractions (displacement, strain, or curvature ratios) versus extracted and integrated results from a rigorous analytical model. Analytical models, accounting for material inelasticity in the steel girders, are constructed for each of two experimentally tested bridges. Skew is varied parametrically from 0° to 60°. Concrete deck is modeled with a tension cracking stress limit consistent with typical design assumptions in United States practice. Results are also presented for the 60° case with linear elastic deck to directly compare the influence of cracking versus non-cracking deck. The analyses indicated that neglecting concrete cracking noticably affected capacity by overestimating slab plate flexure between abutments, resulting in increasingly unconservative reserve capacity evaluations with increasing skew. Bridges with small skews evaluated using only elastic analysis were found to be reasonably well characterized using response fractions feasibly obtained from diagnostic load tests. However, bridges with high skews and bridges evaluated using ultimate capacity from detailed finite element analysis, regardless of skew, will receive unnecessarily conservative flexural capacity ratings if response fractions are relied upon to represent analogous internal load effects.

Evolution of Bridge Diagnostic Load Testing in the USA

Driven by advancements in technology, along with the ever-growing demands of our aging infrastructure, structural evaluation through testing has progressed from a number of research endeavors to a full-fledged industry. Guidelines for Non-Destructive Evaluation (NDE) methods, diagnostic load testing, and proof load testing were published by the American Association of State Highway and Transportation Officials (AASHTO) in 2003 and were largely shaped by the technologies that were available in the 1990s. While specifications in subsequent AASHTO bridge evaluation manuals have remained relatively constant over the last 15 years, commercial applications have sprung forth in all areas of field-based bridge assessment. Much of the development has revolved around inexpensive high-speed data acquisition, high-speed wireless communications, and the surge of cloud-based data management. The market of equipment and services has developed around three main camps; NDE, Controlled Load Testing, and Structural Health Monitoring (SHM), with each field having a variety of subcategories. For example, Diagnostic Load Tests and Proof Load Tests are the two primary forms of controlled load testing. Selection of the appropriate tools and methods to solve any bridge assessment is a function of what information is available and the required functional serviceability of the structure. In many cases, a combination of methods must be employed to achieve a complete assessment. As such, the lines between NDE, Testing, and SHM have blurred and have become increasingly integral gears of a larger machine. This article provides a history of the primary field bridge evaluation techniques and includes the driving forces in commercial development. Focus is placed primarily on diagnostic load test methodology; however, a variety of testing methods are outlined to illustrate the best tool for the job, along with the several pros and cons of various testing and monitoring methods. Finally, a glimpse into the future of bridge evaluation is provided based on current trends and emerging technologies.

Experimental Determination of the Longitudinal Pier Stiffness of a Long Railway Viaduct

Track–bridge interaction plays a decisive role in the design of long railway bridges due to the high braking and acceleration forces that occur and the fact that the continuous rail is attached to the superstructure. A fundamental parameter for the calculation of the effects of track–bridge interaction is the equivalent longitudinal stiffness of piers and abutments with fixed bearings. The equivalent horizontal stiffness is commonly calculated using a pile group model. The static and ‘dynamic’ stiffnesses of the Itz valley railway viaduct were determined experimentally by using a static diagnostic load test and a braking test, which allowed for the verification of the additional rail stresses and the bearing forces with realistic input parameters. Furthermore, numerical 3D FE analyses of the deep foundation system were carried out to provide class-A predictions of the experimental results. In this article, the experimental setup and the execution and evaluation of the two tests are presented. A comparison of the experimental results and the numerical predictions is also carried out

Structural Health Monitoring of a Cable-Stayed Bridge Using Regularly Conducted Diagnostic Load Tests

The management and maintenance of cable-stayed bridges represents a major investment of human and financial capital. One possible approach to reducing the cost while simultaneously improving the process is by utilizing structural health monitoring (SHM) systems to enable diagnostic load tests to be regularly and efficiently conducted. The Indian River Inlet Bridge (IRIB), a 533-meter long cable stayed bridge, was opened for traffic in 2012. From the very early stages of the design process, the Center for Innovative Bridge Engineering (CIBrE) at the University of Delaware (UD) worked with the Delaware Department of Transportation (DelDOT) and their design-build team of Skanska and AECOM to plan and install a comprehensive structural health monitoring (SHM) system. The SHM system is a fiber-optic based design with more than 120 sensors of varying type distributed throughout the bridge. The system, which not only collects data continuously during normal operation, has also been utilized during regularly scheduled controlled diagnostic load tests being used to monitor ongoing bridge performance. This paper will present results from a series of six diagnostic load tests which have been performed over the first six years of the bridge’s service life (just prior to the bridge’s opening, and then again at six months, one year, two years, four years, and six years). The results of these six load tests have enabled the bridge’s baseline performance to be established. They have also provided an opportunity to develop a process for comparing future biennial tests to the evolving database of results, thereby enhancing DelDOT’s ability to operate and maintain the bridge.

Towards standardisation of proof load testing: pilot test on viaduct Zijlweg

Proof load tests of bridges can be very useful for structures with a lack of information, or for structures of which the effect of material degradation is difficult to assess. Contrary to diagnostic load testing, proof load testing is not well-defined in current standards in terms of required load and analysis of measurements. The risk related to the high loads used in proof load testing requires standardisation for these tests. The paper highlights important considerations for proof load testing that may lead to the development of guidelines in the Netherlands, by illustrating a pilot study on the viaduct Zijlweg in the Netherlands. This reinforced concrete bridge rates too low in shear. Topics of interest are the required load that the bridge has to withstand to be approved by the load test and the interpretation of the measurements during the test to avoid permanent damage to the structure. These measurements were compared to the stop criteria from existing codes for buildings, to examine if recommendations for the use with bridges can be formulated. The final result of the test on this case study is that the capacity of the viaduct is proven to be sufficient for shear and bending moment.