Journal of Bridge Engineering

The proposed assessment procedure presented in this paper may be used to establish inspection intervals for steel bridges with fracture critical members (FCMs) (if adopted by the Federal Highway Administration). The procedure proposed herein only applies to FCM inspections which are defined as follows: a hands-on inspection of a FCM or member components that may include visual and other nondestructive evaluations. The method is rather simple and provides an alternative procedure to the pure calendar based inspection methodology currently specified in the Code of Federal Regulations for all FCMs. There are only two requirements which must first be satisfied in order to use the assessment. The first is that the routine 24-month inspection must continue to be performed on the bridge under evaluation. The second is that a FCM inspection must be performed on the bridge or the FCM under evaluation prior to the implementation of the resulting inspection intervals given from this assessment. The “initial inspection” should be performed as a typical FCM inspection. The assessment procedure is intended to be applied to individual FCMs and not the entire bridge itself. However, on bridges with multiple FCMs, the “worst case” FCM may be evaluated using the assessment and the resulting inspection interval may be applied to all FCMs on the bridge (for simplicity and bookkeeping purposes). The decision of whether to apply the assessment to all FCMs or the bridge’s worst case FCM is left up to the owner or engineer. The approach is not based on more rigorous damage tolerant design concepts or other fracture mechanics based methods. Nevertheless, the method is a first step in providing a more rational alternative to calendar based inspection and allows owners to prioritize structures and resource allocation. Using engineering experience and judgment, the interval for the hands-on inspection of steel bridges with FCMs can then be selected based on upper limits established through a consensus. Although the focus is on fracture critical bridges and members, the writers believe that the concept could be extended to a variety of structure types and configurations.

The Hale Boggs Bridge opened to traffic on October 5, 1983. At the time, it was the first U.S. cable-stayed crossing over the Mississippi River. The PE (polyethylene) protective sheathing was damaged in many of the cables before and during installation, and after the opening of the bridge to traffic. Repairs were attempted to correct the defects in cable sheathing. Many of the repairs performed poorly and failed to protect the main tension element. The condition of 39 out of 72 cables indicated a critical need for repair and timely action was recommended. To address these damages, and to assure the structural integrity of the bridge structure, several strategies involving a range of repair and replacement options were evaluated using life cycle cost analysis. It was concluded that the strategy to replace all cables presents the best value among evaluated alternatives. The design of the complete 72 cable array replacement is the first occasion on which this process is attempted in North America. The final design of the replacement cables is heavily influenced by the geometric restrictions of the existing anchorage locations. The replacement cables are being designed for a 75-year design life and incorporated with the latest advancements in corrosion protection and vibration control. Maintenance of traffic design is an essential part of the project. The bridge is a critical regional link and constitutes a hurricane evacuation route. Traffic maintenance during cable replacement was designed to be as unobtrusive to the public and commerce as practical. This paper describes efforts associated with cable condition assessment, rehabilitation strategy, and design considerations and concepts, undertaken by the writers since 2002 to improve the condition of this major river crossing.

Constructed in 1972 with ASTM A36 (250 MPa) steel, a highway bridge in Maryland is comprised of seven welded steel plate girders of a constant web depth of 2,286 mm (90 in.). In March 2003, the web fractures of two steel girders were discovered in a three-span continuous superstructure unit. A full-height web fracture occurred in an interior girder at a cross frame connection plate; and a partial-height web fracture occurred in an exterior girder at an intermediate transverse stiffener next to a cross frame. The investigation of the girder fractures involved fracture surface examination, material testing, fracture mechanics analysis, and comprehensive finite-element modeling for fracture driving forces. The fracture mechanics analysis indicated that a brittle web fracture could occur at a high stress level with either a surface crack or a through-thickness crack of certain dimensions. Finite-element analysis using a global model and submodels investigated three possible causes: (1) localized distortion of the unsupported web gap due to the lateral forces of cross frame members; (2) fabrication induced out-of-flatness of the web plate under in-plane loading; and (3) residual stresses at the fracture origin area due to the stiffener-to-web welds. The investigation concluded that one or a combination of these can result in the high local tensile stresses triggering a brittle web fracture with certain crack dimensions at the fracture origin area. Several retrofit concepts were investigated for their effectiveness in reducing stresses in the fracture origin area. Bridge inspections in the subsequent 6 years after the web fractures have not reported any other cracks in the bridge.

This paper addresses the effect of solar loading on thermal imaging for the detection of subsurface deterioration in concrete bridge components. The deterioration of the concrete resulting from corrosion of embedded mild reinforcing and prestressing steel results in delamination and spalling that can affect the strength and serviceability of a concrete structure. The ability to detect this deterioration during its early stages, when mitigation efforts can be employed, can provide a useful tool for maintenance and inspection personnel. Infrared thermography is a tool that has the potential to detect these subsurface defects to compliment inspection efforts and provide an additional means of condition assessment for a bridge. However, the technique relies on thermal gradients developing in the concrete such that a temperature contrast exists between damaged concrete and sound concrete. The environmental conditions at the bridge, such as direct solar loading, ambient temperature variation, and wind affect the thermal gradient in the concrete, and hence, the ability to image subsurface features. The effects of direct solar loading on the detection of subsurface targets in a concrete test block have been studied. Quantitative measurements of the thermal contrast that appear in thermal images of the test block are reported and analyzed. The effect of the depth of the embedded target is discussed, as well as the timing of inspection (relative to sunrise) that resulted in maximum contrast in thermal images.

As glass fiber-reinforced polymer (GFRP) bridge decks are becoming a feasible alternative to the traditional concrete bridge decks, an innovative methodology to evaluate the in situ conditions are vital to GFRP bridge decks’ full implementation. Ground penetrating radar (GPR) typically performs well in detecting subsurface condition of a structural component with moisture pockets trapped within the material. On the other hand, infrared thermography (IRT) is traditionally known for its ability to detect air pockets within the material. In order to evaluate both nondestructive testing methods’ effectiveness for subsurface condition assessment of GFRP bridge deck, debonds of various sizes were embedded into a GFRP bridge deck module. A 1.5 GHz ground-coupled GPR system and a radiometric infrared camera were used to scan the deck module for condition assessment. Test results showed that both GPR and IRT retained their respective effectiveness in detecting subsurface anomalies. GPR was found to be capable of detecting water-filled defects as small as $5\times 5{\text{cm}}^2$ in plan size, and as thin as 0.15 cm. Furthermore, tests on additional specimens showed that the GPR system offers some promise in detecting bottom flange defects as far down as 10 cm deep. IRT, on the other hand, showed that it is capable of finding both water-filled and air-filled defects within the top layers of the deck with solar heating as main source of heat flux. While test results showed IRT is more sensitive to air-filled defects, water-filled defects can still be detected with a large enough heating mechanism. The experiments showed that a more detailed and accurate assessment can be achieved by combining both GPR and IRT.

This paper presents a probabilistic model of the load rating process incorporated with field inspection observations applied to a prestressed concrete bridge to capture deterioration characteristics. The main computational tool used is a Bayesian network. The model is developed around the main load-carrying member, an interior beam, and the effects of corrosion of its interior steel. Bridge load ratings are calculated as variables based on the following design methodologies: allowable stress; load factor; and load and resistance factor design. Two investigations on an actual bridge are conducted that demonstrate the predictive and diagnostic capabilities of the model. The results show the usefulness of this model in bridge management.

On the evening of December 27, 2005 the fascia beam supporting the east side parapet wall of the third span of the Lake View Drive Bridge failed under the action of dead load. To gain insight into the potential causes of the failure a series of forensic analyses were conducted on the beams decommissioned from the bridge. The study correlates external observations of surface condition with internal chloride profile, depth of carbonation, and existing corrosion. The forensic investigation indicated that strand cover was reduced due to the construction methods of the time. The chloride level in the concrete at the lower layer of strands was high enough that corrosion would be expected. Chloride attack was identified to have come from the leakage of water between beams from the bridge deck surface above. Based on the research findings recommendations are made for visual inspection, and guidelines are provided for condition rating of noncomposite prestressed concrete box beam bridges.

The New York State Department of Transportation (NYSDOT) maintains an inventory of over 17,000 highway bridges across the state. These bridges are inspected biennially or more often as necessary. Bridge inspectors are required to assign a condition rating for up to 47 structural elements of each bridge, including 25 components of each span of a bridge, in addition to the general components common to all bridges. The bridge condition rating scale ranges from 7 to 1; 7 being new and 1 being in failed condition. These condition ratings may be used to calculate the deterioration rates for each bridge element, while considering the effects of key factors, such as the bridge material type, on the deterioration rates. This paper describes an approach based on the Weibull distribution to calculate the deterioration rates of typical bridge elements in New York State using historical bridge inspection data and compares the results with those using the traditionally used Markov chains approach. It is observed that the Weibull-based approach performs better in terms of the observed conditions than the traditionally used Markov chains approach for developing deterioration curves for different bridge elements. Both Markov chains and Weibull-based approaches have been incorporated into a computer program that generates the deterioration curves for specific bridge elements based on historical NYSDOT bridge inspection data dating back to 1981. Case studies on the deterioration rates of various bridge elements in New York State are presented to demonstrate the two approaches. The case studies show that the element deterioration rate information can be used to determine the expected service life of different bridge elements under a variety of external factors. This information is extremely valuable for making bridge management decisions. Based on the Weibull-based approach, the deterioration rates for typical bridge elements in New York State have been presented.

The national bridge inventory (NBI) system, a database of visual inspection records that tallies the condition of bridge elements, is used by transportation agencies to manage the rehabilitation of the aging U.S. highway infrastructure. However, further use of the database to forecast degradation, and thus improve maintenance strategies, is limited due to its complexity, nonlinear relationship, unbalanced inspection records, subjectivity, and missing data. In this study, soft computing methods were applied to develop damage prediction models for bridge abutment walls using the NBI database. The methods were multilayer perceptron network, radial basis function network, support vector machine, supervised self-organizing map, fuzzy neural network, and ensembles of neural networks. An ensemble of neural networks with a novel data organization scheme and voting process was the most efficient model, identifying damage with an accuracy of 86%. Bridge deterioration curves were derived using the prediction models and compared with inspection data. The results show that well developed damage prediction models can be an asset for efficient rehabilitation management of existing bridges as well as for the design of new ones.

The purpose of this paper is to illustrate the bridge inspection philosophy of two different large railways. North American railways are primarily public investor owned businesses and must show adequate financial returns to survive. Also, without adequate financial returns it is very difficult to secure the funding necessary to adequately maintain structures and provide the service to customers that generates sufficient profit. The bridge inspection criteria are based primarily on ensuring a safe and reliable operation, an appropriate response, and the information with which to provide adequate funding for maintenance and replacement. Two different railway company philosophies are described. Of interest to the highway bridge community is how traffic volumes and structure load capacity influence inspection frequency and level of inspection. Items similar to Federal Highway Administration bridge inspection criteria are not discussed.