Harry W. Shenton

Bridge-Condition Assessment and Load Rating Using Nondestructive Evaluation Methods

In most cases, bridge-condition assessment is made according to visual inspections, and bridge-load ratings are determined with fairly simple analytical methods and without site-specific, live-load, bridge-response data. As a result, estimates of bridge load-carrying capacity are often quite conservative. The increased weight of today’s trucks compared with design loads that are used for older bridges, combined with the continued aging and deterioration of our nation’s bridges, has resulted in a significant number of them being classified as structurally deficient. Reliable condition assessments are essential to ensure the safety of the traveling public. Furthermore, because load-carrying capacity is often used to prioritize bridges for repair, rehabilitation, and replacement, and because funds for these actions are limited, it is more important than ever that these estimates be as accurate as possible. To achieve this goal, researchers at the University of Delaware have been working with engineers at the Delaware Department of Transportation to develop methods for improving the accuracy of bridge-capacity evaluation through use of nondestructive evaluation techniques. Among the methods currently used are diagnostic load testing and in-service monitoring. These methods are described, and a detailed case study that illustrates the applied methodologies is discussed.

Bridge 1-351 over muddy run : Design, testing, and erection of an all-composite bridge

Bridge 1.351 on Business Route 896 in Glasgow, Delaware, was replaced with one of the first state-owned all-composite bridges in the nation. Composites are lightweight construction materials that do not corrode, which results in benefits such as ease of construction and reduced maintenance costs. A summary of the design, large-scale testing, fabrication, erection, and monitoring of this bridge is presented. The bridge was designed to AASHTO load and resistance factor design specifications. A methodology was developed to incorporate the engineering properties of these unique composite materials into the design. The bridge consists of two 13 × 32 ft (3.96 × 9.75 m) sections joined by a unique longitudinal joint. The sections have sandwich construction consisting of a core [28 in. (71.12 cm) deep] and facesheets [0.4 to 0.6 in. (10.16 to 15.24 mm) thick] that provide shear and flexural rigidity, respectively. The composite bridge was fabricated with E-glass preforms and vinyl-ester resin, which offers excellent structural performance and long-term durability. Each of the sections was fabricated to near-net shape in a single step by a vacuum-assisted resin transfer molding process. The overall structural behavior has been accurately predicted with simple design equations based on sandwich theory for anisotropic materials. Large-scale testing of full-sized subcomponents was conducted to prove that the design satisfied deflection, fatigue, and strength limit states. A redundant longitudinal joint was designed that consisted of both an adhesively bonded vertical joint between sections and splice plates. Assembly procedures were developed, and transverse testing of the full-sized joint was conducted. Final bridge sections were proof-tested to the strength limit state. The construction phase included section positioning, joint assembly, and application of a latex-modified concrete wear surface. The bridge was reopened to traffic on November 20, 1998. Results from the long-term monitoring effort will be documented.

Using diagnostic load tests for accurate load rating of typical bridges

Historically, bridge load ratings based solely on theoretical calculations tend to be overly conservative, due to the many assumptions made in the modelling of a bridge. A controlled load test can be used to determine a more accurate load rating of the bridge. An overview of typical controlled load tests is presented in this paper. Specific attention is given to determining the bridges actual live load distribution, assessing support fixity, unintended composite action, and contributions from non-structural components. The manner in which these factors are computed and applied directly to standard beam models currently used to load rate bridges is also discussed.

Behavior of open steel grid decks for bridges

Abstract The life cycle of grid decks has come full circle from their introduction in the 1920s and 1930s, through their maturity in 1950s and 1960s, to their reintroduction in the 1980s. Many of these decks have been performing satisfactorily for 50 or more years of service. Open grid decks offer a lightweight deck alternative to reinforced concrete decks. Despite the good performance history of grid decks, some bridge owners are hesitant to utilize them, even in situations where weight savings is at a high premium. With a better understanding of grid deck behavior, the manufacturing process can be optimized, and design methods improved. Hence, poor details that may lead to fatigue problems can be avoided and design efficiency can be achieved. This paper presents results of research conducted with the goal of providing a better understanding of open steel grid deck behavior through experimental testing and numerical and analytical analyses. Four full-scale open grid decks were tested to experimentally quantify their structural behavior. Three-dimensional finite element models were developed for the grid decks and calibrated using the experimental results. Classic orthotropic thin plate theory and the theory for beams on elastic foundation were applied to the open decks and compared with the finite element (FE) results. Finally, parametric studies were conducted and used to quantify the effect of variations in the significant design parameters. The results of the parametric studies can be applied to optimize future grid deck designs.

Dead Load Based Damage Identification Method for Long-term Structural Health Monitoring

A novel damage identification procedure is presented that is ideally suited for long-term structural health monitoring of large civil structures. The procedure is based on the redistribution of dead load stress that occurs in a structure when damage takes place. The damaged structure is modeled using finite elements, in which the damage is represented by a section of reduced flexural rigidity. Forward analyses are first presented to show how the dead load is redistributed for different damage scenarios. The inverse damage identification problem is set up as a constrained optimization problem and solved using a real coded genetic algorithm. The proposed method correctly identified damage for a wide range of locations and damage severities. Results show that damage is difficult to identify when it is close to the inflection points of the structure, where the dead load strain is zero, and when the damage is not located between two sensors. The effect of measurement error is investigated.

Effect of stiffness variability on the response of isolated structures

Results are presented of an investigation, the objective of which was to determine the relationship between the stiffness variability of the bearings of an isolation system and the response variability of the structure. The system is modeled as a rigid, rectangular structure that is free to translate and rotate. The isolation system consists of N isolation bearings arranged in a rectangular pattern, each with a stiffness ki that is an independent, normally distributed, random variable. Response spectrum analysis is used to obtain the analytical solution for the structure response. Approximate closed-form expressions are obtained for the variance of the centreline displacement, rotation, corner displacement and base shear, that are in terms of the variability of the isolator stiffness, aspect ratio of the structure, and the number and layout of isolation bearings. Results show that the standard deviation of the centreline displacement and base shear decrease with increasing number of isolation bearings, and are independent of the aspect ratio and layout of isolators, and in all cases are less than 1/4 the standard deviation of the isolator stiffness. The standard deviation of the corner displacement is a function of all of the system parameters, and is bounded below by the standard deviation of the centreline displacement and above by the standard deviation of a bar aligned perpendicular to the direction of ground motion with m isolation bearings distributed along the length. The approximate expressions are shown to be in good agreement with the results of Monte Carlo simulations. The results should be of use to designers of isolated structures and manufacturers of isolation systems, in assessing the significance of stiffness variability on the response of the isolated structure. Copyright

Diagnostic and In-Service Testing of Transit Railway Bridge

Results of a diagnostic load test and in-service monitoring of a steel through-girder bridge are presented. The bridge currently has a low load rating that is controlled by the flexural capacity of the steel deck trough. The diagnostic load test was conducted using the regularly scheduled transit trains, with no interruption to the daily service. A total of 16 strain transducers were mounted to the bridge girders and trough; five dynamic train passes were recorded. The largest tensile and compressive stresses recorded in the edge girder were 33.5 MPa and 23.4 MPa, respectively; the largest tensile and compressive stresses in the deck trough were 25.9 MPa and 27.9 MPa, respectively. The test results showed the effective width of the deck trough to be 2.89 m for a single axle and 5.19 m for a 2.74-m spaced axle pair. An in-service monitoring system was also placed on the bridge to record the stress cycles experienced by the bridge over a 1-week period. During the in-service monitoring period, 163 trains crossed the bridge with an associated 1,456 axles. The mean peak tensile stress in the deck trough caused by locomotives crossing the bridge was 26 MPa with a standard deviation of 1.8 MPa. The test results showed that the peak deck trough strains due to live load were only 15 percent of the computed values used in the rating. As a result, the bridge load rating can safely be increased, and the current speed restrictions can be lifted.

PERFORMANCE OF GLASS FIBER-REINFORCED POLYMER DECK ON STEEL GIRDER BRIDGE

Because of the continued deterioration of U.S. bridges combined with the increasing cost of bridge maintenance, the U.S. bridge inventory continues to experience a backlog of structurally deficient bridges. One potential solution is the implementation of new high-performance materials. Because of their many beneficial characteristics, including being lightweight, having high strength- and stiffness-to-weight ratios, and being corrosion resistant, advanced polymer composites represent one such alternative. In 1999 the Delaware Department of Transportation rehabilitated a deteriorated concrete slab-on-steel girder bridge by removing the concrete slab and replacing it with a lightweight glass fiber-reinforced polymer (GFRP) deck. In the rehabilitation, the existing abutments were slightly modified and the original steel girders were retained. On July 28, 1999, a diagnostic load test of the bridge was performed using a fully loaded 10-wheel dump truck. The test included stationary load cases, semistatic load passes, and dynamic load passes. The details of the GFRP slabon-steel girder bridge as well as the bridge performance as determined from the field load test are presented.

Full-Scale Destructive Bridge Test Allows Prediction of Ultimate Capacity

Contemporary bridge design is generally based on designing individual members for the maximum force effect that each member may experience. This approach ignores the system-level interactions of these individual members, which may greatly increase the ultimate strength of the complete structure. In an attempt to understand better the load redistribution mechanisms that lead to this increase in ultimate strength, the destructive testing of a skewed steel I-girder bridge was planned and executed. However, because of the enormous system-level reserve capacity of the structure, the structure generally responded elastically, despite being loaded with the equivalent of 17 HS-20 design vehicles. Thus, a validated finite element analysis (FEA) approach was used to predict the ultimate capacity of this structure and to analyze the force redistribution as the bridges elastic limit was exceeded. The FEA results predicted that the equivalent of 41 HS-20 vehicles was needed to plastify fully the four girders of the subject bridge and show the significant amount of yielding that occurs in the cross-frames when the ultimate capacity of the girders is achieved.

Using Diagnostic Load Tests for Accurate Load Rating of Typical Bridges

Historically, bridge load ratings based solely on theoretical calculations tend to be overly conservative, due to the many assumptions made in modeling of the bridge. A controlled load test can be used to determine a more accurate load rating of the bridge. Presented in the paper is an overview of a typical controlled load test. Specific attention is given to determining the actual load distribution in the bridge, assessing support fixity, unintended composite action, and contributions from nonstructural components. The manner in which these factors are adjusted and applied to a simple beam model of the bridge for load rating is also discussed.