Terry J Wipf

Evaluation and Field Load Testing of Timber Railroad Bridge

Several spans of a 60-year-old open-deck timber railroad bridge on the Southern Pacific Railroad Line (now the Union Pacific) in Southwest Texas were field tested. The tests were conducted with the sponsorship and cooperation of the Association of American Railroads to determine the vertical live load distribution characteristics of the superstructure. The bridge was originally constructed with Douglas-fir larch solid sawn stringers but was rehabilitated on several occasions to allow comparisons to be made with respect to different rehabilitation options, including the use of a helper stringer and the use of glued laminated timber (glulam) stringers. The test spans measured approximately 4.1 m (13.5 ft) center-to-center of supports and included two closely “packed” chords, each consisting of four timber stringers (one test span included an additional helper stringer added to one chord). One chord was made up of glulam timber and the other was made up of solid sawn timber. The bridge superstructure was generally in satisfactory condition, with some stringer horizontal splitting noted over the bents. The bents were in reasonably good condition, but chord bearing was uneven on bent caps. Static and dynamic deflection load test data were obtained using a special test train. The test results indicate that the glulam chord performed better than the older sawn stringer chord, even when a helper stringer was added. Individual stringers within a chord did not always share the load equally.

DIAGNOSTIC LOAD TESTS OF A PRESTRESSED CONCRETE BRIDGE DAMAGED BY OVERHEIGHT VEHICLE IMPACT

A series of diagnostic load tests performed on two prestressed concrete bridges located in western Iowa are discussed. The bridges are dual prestressed concrete I-beam structures. In June 1996, an overheight vehicle struck the westbound structure and caused significant loss of section and cracking. As a result of the severity of the damage and because of concerns about the remaining capacity and long-term durability of the damaged beams, the Iowa Department of Transportation decided to remove the two most severely damaged beams. The diagnostic load-testing portion of the research program consisted of positioning test vehicles of known weight at predetermined locations along the deck of the damaged westbound and undamaged eastbound bridge. Single-and dual-truck tests were conducted on each bridge. Following replacement of the damaged beams in the westbound structure, additional tests were conducted. The results of these three load tests are compared to determine the effect of the localized beam damage on the overall live load distribution pattern in the bridge. The objective of this research is to determine the effects of damage on the load distribution and the remaining strength of damaged prestressed concrete bridges. Noticeable differences in response were detected in the westbound and eastbound bridges before beam replacement, with the difference essentially disappearing after the repair of the westbound bridge. The research project also involved model bridge testing, along with the repair of the beams that were removed from service and those that were intentionally damaged in the laboratory. The project is now complete.

Lateral Live-Load Distribution Characteristics of Simply Supported Steel Girder Bridges Loaded with Implements of Husbandry

This paper discusses the effect of agricultural live-loads on lateral load distribution characteristics of girder bridges on rural roadways in the United States. In this study, load distribution factors for bridges subjected to agricultural vehicles frequently used on rural roads are calculated based upon codified processes, field test results, and simulations. As part of this work, five simply supported steel girder bridges in Iowa were selected for field tests with four agricultural vehicles and a highway-type truck. Strain sensors were mounted on the bottom flanges of girders at midspan of all five bridges. Strain data resulting from the test vehicles were measured and used to determine girder distribution factors for each bridge. These strain data were also used to calibrate analytical models of the bridges. Over 120 agricultural vehicles were identified and used to analytically load the models. Girder distribution factors were then computed using responses from the vehicle-induced model simulations. Findings revealed that the analytical and field distribution factors were in most cases smaller than code-specified values, as has been observed by others. In some cases, however, these factors exceeded code values. Furthermore, the variability in agricultural vehicles’ characteristics had a significant impact on the live-load distribution factors for each bridge.

Field Validation of a Statistical-Based Bridge Damage-Detection Algorithm

This paper describes a field validation of a second-generation, statistical-based damage-detection algorithm and its ability to detect actual damage in bridges accurately. The algorithm had been theoretically validated previously. For the field tests, in lieu of introducing damage to a public bridge, two sacrificial specimens that simulated damage-sensitive locations of the bridge were mounted on the bridge, and different types and levels of damage in the form of cracks and simulated corrosion were induced in the specimens. Using strain data collected from sensors on the sacrificial specimens and on the bridge, the algorithm correctly identified the damage. Analysis of data from sensors far away from the damaged area revealed a relatively high false-positive rate.

Structural Behavior of Waffle Bridge Deck Panels and Connections of Precast Ultra-High-Performance Concrete: Experimental Evaluation

The AASHTO strategic plan in 2005 identified extending the service life of bridges and accelerating bridge construction as two of the grand challenges in bridge engineering. Previous studies have shown that using a prefabricated full-depth precast concrete deck system not only accelerated the bridge deck rehabilitation process but also extended its service life with reduced user delays and lower life-cycle costs. The recent use of ultra-high-performance concrete (UHPC) in the United States for bridge applications has proved efficient and economical because of its superior structural and durability characteristics. On the basis of the advantages of UHPC and precast systems, a design for a full-depth UHPC waffle deck panel system was developed. A full-scale, single-span, 60-ft long by 33-ft wide prototype bridge with full-depth prefabricated UHPC waffle deck panels has been planned as a replacement bridge in Wapello County, Iowa. In support of this project, structural performance and constructability of the UHPC waffle deck system and its critical connections were studied through an experimental program at Iowa State University. Two prefabricated, full-depth, UHPC waffle deck panels were connected to two 24-ft long precast prestressed girders, and the system was tested under service, fatigue, and ultimate loads. On the basis of the test observations and results and the experience gained from fabrication of deck panels and casting of UHPC infill joints (transverse and longitudinal), the prefabricated UHPC waffle deck system concept was found to be a viable option to achieve the goals of the AASHTO strategic plan.

Development of a Procedure for Fatigue Design of Slender Support Structures Subjected to Wind-Induced Vibration

Cantilevered signal, sign, and light support structures are used nationwide on major Interstate highways, national highways, local highways, and at local intersections for traffic control. Recently, a number of failures of these structures have been characterized as wind-induced fatigue failures. It is widely accepted that there is considerable lack of accuracy in the calculation of wind-induced loads on high mast light poles (HMLPs) in both the AASHTO and the Canadian Highway Bridge Design Code provisions. A coupled model for predicting buffeting- and vortex shedding–induced response for slender support structures was developed. To accomplish this, monitoring of long-term response behavior of an HMLP subjected to wind-induced vibration and wind tunnel experiments was used to study global behavior and to extract important parameters. From the long-term field monitoring and wind tunnel experiments, the two critical types of wind vibration (natural wind gusts or buffeting and vortex shedding) were individually identified for in-depth analysis. Finally, a coupled dynamic model in time domain was developed for predicting the wind-excited response and was validated by comparing the simulation results with the field-collected data. The fatigue life of a specific HMLP was also estimated with the stress amplitudes predicted by the time-domain model and was validated with statistical extrapolation of the field data.

Development of Live-Load Distribution Factors for Glued-Laminated Timber Girder Bridges

This paper presents simple relationships for calculating live-load distribution factors for glued-laminated timber girder bridges with glued-laminated timber deck panels. Analytical models were developed using the Ansys 11 finite-element program, and the results were validated using recorded data from four in-service timber bridges. The effects of the bridge span length, the spacing between girders, and the bridge width on the distribution of the live load were investigated by using the validated models. The live-load distribution factors obtained from the field test and the analytical models were compared with those obtained using the AASHTO LRFD Bridge Design Specifications live-load distribution relations. The comparison showed that the live-load distribution factors obtained by using the AASHTO LRFD Bridge Design Specifications were conservative. For this reason, statistical methods were used to develop accurate relationships that can be used to calculate the live-load distribution factors in the desi...

Bridge Structural Health–Monitoring System Using Statistical Control Chart Analysis

Technological advancements in sensing, computing, and networking have facilitated the application of long-term structural health-monitoring (SHM) and made it a more widely accepted tool to improve bridge management. A fiber-optic strain-based SHM system developed in cooperation with the Iowa Department of Transportation is described; the system continuously monitors bridge performance under ambient traffic loads to detect damage. Although this SHM system has broad potential application to numerous types of structural deterioration or damage, the system is presented here by using a case study associated with fatigue cracking in the web gap region. The described SHM system autonomously collects, reduces, and analyzes strain data and determines the damage occurrence in a near-real-time fashion. In this system, statistical control chart analysis is applied over the strategically defined damage indicator to determine the damage mathematically. Data preprocessing strategies were also studied to limit the impact of major non-structural factors causing strain variations. The effectiveness of the system was demonstrated with field-collected predamage data and synthetic postdamage data. Experimental research is being conducted to validate the system at the field-testing level.

Preservation Treatment for Wood Bridge Application

Timber can often be a cost-effective construction material for new bridges. The durability of the bridge greatly depends on proper attention to construction details and fabrication, as well as proper preservative treatment before, during, and after construction. Material repair and replacement costs for bridges are a considerable expense for highway agencies. To address these needs, the objectives of an investigation were to determine the field effectiveness of various treatment alternatives used on Iowa roadway projects and to provide information on preservative treatments, inspection techniques, and current specifications for bridge owners. Special emphasis was placed on providing up-to-date synthesized information for county engineers to maintain their timber bridge inventory more effectively. The project scope included a literature review, identification of testing techniques, on-site inspections of bridges in Iowa, and a review of current specifications and testing procedures. On the basis of information evaluated, these general conclusions were made: copper naphthenate was recommended as the plant-applied preservative treatment for timber bridges, American Wood Protection Association Standards and Best Management Practices should be followed to ensure high-quality treatment of timber materials, and bridge maintenance programs would be enhanced by the development of an effective construction and remedial treatment process to improve bridge durability.

Damage detection in bridges through fiber optic structural health monitoring

A fiber optic structural health monitoring (SHM) system was developed and deployed by the Iowa State University (ISU) Bridge Engineering Center (BEC) to detect gradual or sudden damage in fracture-critical bridges (FCBs). The SHM system is trained with measured performance data, which are collected by fiber optic strain sensors to identify typical bridge behavior when subjected to ambient traffic loads. Structural responses deviating from the trained behavior are considered to be signs of structural damage or degradation and are identified through analytical procedures similar to control chart analyses used in statistical process control (SPC). The demonstration FCB SHM system was installed on the US Highway 30 bridge near Ames, IA, and utilizes 40 fiber bragg grating (FBG) sensors to continuously monitor the bridge response when subjected to ambient traffic loads. After the data is collected and processed, weekly evaluation reports are developed that summarize the continuous monitoring results. Through use of the evaluation reports, the bridge owner is able to identify and estimate the location and severity of the damage. The information presented herein includes an overview of the SHM components, results from laboratory and field validation testing on the system components, and samples of the reduced and analyzed data.