Development and Implementation of a Continuous Strain Monitoring System on a Multi-Girder Composite Steel Bridge

This paper describes the implementation and evaluation of a long-term strain monitoring system on a three-span, multisteel girder composite bridge located on the interstate system. The bridge is part of a network of bridges that are currently being monitored in Connecticut. The three steel girders are simply supported, whereas the concrete slab is continuous over the interior supports. The bridge has been analyzed using the standard AASHTO Specifications and the analytical predictions have been compared with the field monitoring results. The study has included determination of the location of the neutral axes and the evaluation of the load distributions to the different girders when large trucks cross the bridge. A finite-element analysis of the bridge has been carried out to further study the distribution of live load stresses in the steel girders and to study how continuity of the slabs at the interior joints would influence the overall behavior. The results of the continuous data collection are being used to evaluate the influence of truck traffic on the bridge and to establish a baseline for long-term monitoring.     

AASHTO-LRFD Live Load Distribution for Beam-and-Slab Bridges: Limitations and Applicability

This paper presents a comparison between the live load distribution factors of simple span slab-on-girders concrete bridges based on the current AASHTO-LRFD and finite-element analysis. In this comparison, the range of applicability limits specified by the current AASHTO-LRFD is fully covered and investigated in terms of span length, slab thickness, girder spacing and longitudinal stiffness. All the AASHTO-PCI concrete girders (Types I–VI) are considered to cover the complete range of longitudinal stiffness specified in the AASHTO-LRFD. Several finite-elements linear elastic models were investigated to obtain the most accurate method to represent the bridge superstructure. The bridge deck was modeled as four-node quadrilateral shell elements, whereas the girders were modeled using two-node space frame elements. The live load used in the analysis is the vehicular load plus the standard lane load as specified by AASHTO-LRFD. The live load is positioned at the longitudinal location that produced the extreme effect, and then it is moved transversely across the bridge width in order to investigate all possibilities of one-lane, two-lane and three-lane design loads. A total of 886 bridge superstructure models were built and analyzed using the computer program SAP2000 to perform this comparison. The results of this study are presented in terms of figures to be practically useful to bridge engineers. This study showed that the AASHTO-LRFD may significantly overestimate the live load distribution factors compared to the finite-element analysis.     

Reliability-Based Load and Resistance Factor Rating Using In-Service Data

Traditional bridge evaluation techniques are based on design-based deterministic equations that use limited site-specific data. They do not necessarily conform to a quantifiable standard of safety and are often quite conservative. The newly emerging load and resistance factor rating (LRFR) method addresses some of these shortcomings and allows bridge rating in a manner consistent with load and resistance factor design (LRFD) but is not based on site-specific information. This paper presents a probability-based methodology for load-rating bridges by using site-specific in-service structural response data in an LRFR format. The use of a site-specific structural response allows the elimination of a substantial portion of modeling uncertainty in live load characterization (involving dynamic impact and girder distribution), which leads to more accurate bridge ratings. Rating at two different limit states, yield and plastic collapse, is proposed for specified service lives and target reliabilities. We consider a conditional Poisson occurrence of identically distributed and statistically independent (i.i.d.) loads, uncertainties in field measurement, modeling uncertainties, and Bayesian updating of the empirical distribution function to obtain an extreme-value distribution of the time-dependent maximum live load. An illustrative example uses in-service peak-strain data from ambient traffic collected on a high-volume bridge. Serial independence of the collected peak strains and of the counting process, as well as the asymptotic behavior of the extreme peak-strain values, are investigated. A set of in-service load and resistance factor rating (ISLRFR) equations optimized for a suite of bridges is developed. Results from the proposed methodology are compared with ratings derived from more traditional methods.     

Critical Evaluation of Strain Measurements in Glass Fiber-Reinforced Polymer Bridge Decks

An experimental study of principal strains and deflections of glass fiber-reinforced polymer (GFRP) composite bridge deck systems is presented. The experimental results are shown to correlate well with those of an analytical model. While transverse strains and vertical deflections are observed to be consistent, repeatable, and predictable, longitudinal strains exhibit exceptional sensitivity to both strain sensor and applied load location. Large, reversing strain gradients are observed in the longitudinal direction of the bridge deck. GFRP deck system geometry, connectivity, material properties, and manufacturing imperfections coupled with the observed strains suggest that the performance of these structures should be assessed under fatigue loading conditions. Recommendations for accurately assessing longitudinal strain in GFRP bridge decks are made, and a review of existing data is suggested.     

Experimental Study on Prefabricated Concrete Bridge Girder-to-Girder Intermittent Bolted Connections System

This paper reports on a new bridge deck slab flange-to-flange connection system for precast deck bulb tee (DBT) girders. In prefabricated bridge system made of DBT girders, the concrete deck slab is cast with the prestressed girder in a controlled environment at the fabrication facility and then shipped to the bridge site. This system requires that the individual prefabricated girders be connected through their flanges to make it continuous for live load distribution. The objectives of this study are to develop an intermittent bolted connection for DBT bridge girders and to provide experimental data on the ultimate strength of the connection system. This includes identifying the crack formation and propagation, failure mode, and ultimate load carrying capacity. In this study, three different types of intermittent bolted connection were developed. Four actual-size bridge panels were fabricated and then tested to collapse. The effects of the size and the level of the fixity of the connecting steel plates, as well as the location of the wheel load were examined. The developed joint was considered successful if the experimental wheel load satisfied the requirements specified in North American bridge codes. It was concluded that location of the wheel load at the deck slab joint affected the ultimate load carrying capacity of the connections developed. Failure of the joint was observed to be due to either excessive deformation and yielding of the connecting steel plates or debonding of the embedded studs in concrete.     

Fatigue Behavior and Retrofit Investigation of Distortion-Induced Web Gap Cracking

This paper studies a Kansas Department of Transportation welded plate girder bridge that developed fatigue cracks at small web gaps close to the girder top flange. Repair had been previously performed by softening the connection plate end with a slot retrofit, but cracks were recently found to have reinitiated at some of the repaired details and are again propagating. A comprehensive finite-element method study was performed to investigate the cracking behavior observed in the bridge and to recommend appropriate measures for future bridge retrofit. The analytical results show that stresses developed at the top flange web gaps could exceed yielding under the loading of an HS15 fatigue truck. The current slot repair used in the bridge was found to have introduced higher magnitude fatigue stresses in the web gap. To achieve a permanent repair of the bridge, it is recommended that a welded connection plate to flange attachment be used during future bridge retrofit. The web gap details should be able to withstand unlimited number of load cycles once this additional repair is performed.     

Implementation of a Long-Term Bridge Weigh-In-Motion System for a Steel Girder Bridge in the Interstate Highway System

This technical paper discusses the implementation of a long-term bridge weigh-in-motion system for use in determining gross vehicle weights of trucks crossing steel girder bridges. The system uses strain data to determine truck weights using an existing structural health monitoring system installed on a interstate highway bridge. The applied system has the advantage of not using any axle detectors in the roadway; and instead all analyses are performed using strain gauges attached directly to the steel girders, providing for a long-term monitoring system with minimal maintenance. Long-term data has been used to demonstrate that this method can be readily applied to gain important information on the quantity and weights of the trucks crossing the highway bridge.     

Northern Red Oak Glued-Laminated Timber Bridge

A 5-year program to monitor the performance of a red oak longitudinal girder, transverse deck glued-laminated (glulam) highway bridge is presented. The bridge design details, including preservative treatment results, are described. The live loading results indicate that the predicted and observed live load beam deflections agree to within 7% when the stiffness of the individual beam laminations is used as a predictor and a 10% increase in beam stiffness due to composite action between the deck panel and logitudinal girders is incorporated into the design. The dimensional stability of the deck panels over 3 years has been monitored and analyzed. Significant reflexive cracking of the asphaltic wearing surface has been observed at the interface between each red oak deck panel. This has been attributed to the gap provided between each panel during construction, to the placement of the waterproof membrane directly over the creosote-treated deck panels, and to improper mating of the deck panels to the beams during installation of the lag bolts. Long-term (3-year) dead load deflection measurements indicate that after approximately 1 year, dead load deflections remain nearly constant for the interior beams. Elevations of the lower surface of the two exterior beams fluctuate considerably and vary seasonally. There is no evidence of delamination of the girders or deck panels after 4 years. However, there is some evidence of delamination of the curbs and the tops of rail posts.     

Performance Evaluation through Field Testing of Century-Old Railroad Truss Bridge

Research has been carried out to evaluate the structural behavior and influence of aging on a century-old steel railroad truss bridge. The structure is located in Connecticut and services a large number of trains traveling into and out of New York City. The trusses are made of built-up members, with either multiple eyebars or laced channel sections. All panel point connections are joined with true pins and the interior panels are indeterminate. The bridge had experienced problems relating to the lateral shifting of some of the middepth pins. This study was carried out to evaluate the structural behavior and live load distribution throughout the bridge. A major component of the research involved extensive field monitoring. The results show that the actual live load distribution is significantly different than expected from conventional analytical approaches that were most likely used in the original design. The load distribution in multiple eyebar elements is far from uniform, and the distribution of shear through indeterminate panels is significantly different than expected from a normal truss analysis. Significant out-of-plane bending was found in the truss due to floorbeam end rotations, which is thought to be the major factor that is causing the pins to move. The study shows the necessity of using field monitoring to better understand the behavior of older bridges prior to the design of renovation approaches.     

Experimental Characterization and Optimization of Hybrid FRP/RC Bridge Superstructure System

An experimental investigation was performed to assess the performance of a hybrid fiber-reinforced polymer/reinforced concrete bridge system. The full-scale laboratory specimen was representative of an 813?mm (32?in.) wide strip of a completed bridge in San Patricio County, Tex. The specimen was first subjected to static loading prior to casting the reinforced concrete deck. Displacement, strain, and acoustic emission were recorded. After completion of the nondestructive static loading a reinforced concrete deck was cast in the laboratory to represent one unit of the completed bridge. Load was statically applied with several increased load cycles until failure occurred at a load level exceeding 18 times the calculated design load. The results of the static testing indicated that the original design of the hybrid bridge was very conservative. An optimized design of the hybrid bridge was then derived. The static load testing program and the resulting optimized design are described.     

Determination of Bridge Design Parameters through Field Evaluation of the Route 601 Bridge Utilizing Fiber-Reinforced Polymer Girders

The Route 601 Bridge in Sugar Grove, Virginia, spans 11.89?m (39?ft) over Dickey Creek. The bridge is the first to use the Strongwell 91.4?cm (36?in.) deep fiber-reinforced polymer double web beam in a vehicular bridge superstructure. Construction of the new bridge was completed in October 2001 and field testing was undertaken shortly thereafter, as well as in June of 2002, to assess any potential changes in structural performance. This paper details the field evaluation of the Route 601 Bridge. Using midspan deflection and strain data from the October 2001 and June 2002 field tests, AASHTO bridge design parameters were determined—namely, wheel load distribution factor g, dynamic load allowance IM, and maximum deflection. The wheel load distribution factor was determined to be S/4, a dynamic load allowance was determined to be 0.36, and the maximum deflection of the bridge was L/1,110. Deflection results were lower than the AASHTO L/800 limit. This discrepancy is attributed to partial composite action of the deck-to-girder connections, bearing restraint at the supports, and contribution of guardrail stiffness. It was found that diaphragm removal had a small effect on the wheel load distribution factor.     

Development and Evaluation of an Adhesively Bonded Panel-to-Panel Joint for a FRP Bridge Deck System

A fiber-reinforced polymer (FRP) composite cellular deck system was used to rehabilitate a historical cast iron thru-truss structure (Hawthorne St. Bridge in Covington, Va.). The most important characteristic of this application is reduction in self-weight, which raises the live load-carrying capacity of the bridge by replacing the existing concrete deck with a FRP deck. This bridge is designed to HL-93 load and has a 22.86?m clear span with a roadway width of 6.71?m. The panel-to-panel connections were accomplished using full width, adhesively (structural urethane adhesive) bonded tongue and groove splices with scarfed edges. To ensure proper construction, serviceability, and strength of the splice, a full-scale two-bay section of the bridge with three adhesively bonded panel-to-panel connections was constructed and tested in the Structures Laboratory at Virginia Tech. Test results showed that no crack initiated in the joints under service load and no significant change in stiffness or strength of the joint occurred after 3,000,000 cycles of fatigue loading. The proposed adhesive bonding technique was installed in the bridge in August 2006.     

Field Performance of the Fiber-Reinforced Polymer Deck of a Truss Bridge

The MD 24 Bridge over Deer Creek in Harford County, Md., was one of the projects chosen by the Federal Highway Administration’s Innovative Bridge Research and Construction Program for bridge deck replacement by fiber-reinforced polymer (FRP) composites. A thorough discussion is presented on Maryland State Highway Administration’s first bridge rehabilitation project utilizing a FRP deck. The discussion includes design details, installation procedure, construction methods and in situ load testing with a wireless monitoring system. The research team installed a monitoring system to record the effects of live loads on the bridge system, including truss members, steel stringers, and plate action of the FRP deck. Finite-element models were also used in this phase. Dynamic effects of the FRP system, composite action between steel stringers and the FRP deck as well as the effective width and distribution factors of stringers were obtained and compared with the AASHTO specifications. Recommendations are also offered on improving the design details based on this experience.     

Fatigue Behavior and Nondestructive Evaluation of Full-Scale FRP Honeycomb Bridge Specimen

An experimental investigation was performed to assess the projected fatigue performance of a fiber-reinforced polymer honeycomb bridge that has recently been completed in Troupsburg, N.Y. The laboratory specimen was representative of a 305-mm-wide strip of the completed bridge. The specimen was first subjected to fatigue loading. Load, displacement, and strain were measured every 25,000 cycles. The data indicated minimum signs of degradation after 2 million cycles of fatigue loading, as reflected in slightly increased values of vertical deflection and strain at midspan. After completion of the fatigue loading, the specimen was evaluated with acoustic emission. Load was statically applied and increased incrementally until failure occurred at a load level exceeding 16 times the fatigue level loading. The results of the static testing also indicated that only minor damage occurred due to fatigue. Field load testing of the actual bridge has been completed by the New York State Department of Transportation, and the results are discussed as they pertain to the fatigue and static load testing programs described.     

Distributed Strain Measurement in Steel Bridge with Fiber Optic Sensors: Validation through Diagnostic Load Test

Fiber optic sensing technologies are emerging as valid alternatives for the health monitoring of civil structures. Distributed sensors based on Brillouin scattering add the unique capability of measuring strain and temperature profiles along optical fibers. Measurement is performed by establishing the correlation between fiber strain and temperature, and the frequency shift of the Brillouin backscattered light induced by a monochromatic light pulse. The technology holds potential for use on large structures and integrated transportation infrastructure. Its effectiveness has been assessed through scaled laboratory experiments, whereas field validation is limited to very few demonstration projects conducted to date. This paper presents a pilot application of Brillouin optical time domain reflectometry to measure strain profiles along the steel girders of a continuous slab-on-girder bridge subjected to diagnostic load testing. One of the exterior continuous girders required heat-straightening after falling during construction due to wind. The significance of applying a distributed measurement technique lies in the potential to assess the global girder response, which would be impractical and uneconomical using discrete measurement techniques. A 1.16?km long sensing circuit was installed onto the web of four girders. The circuit comprises bare optical fiber sensors, and a novel adhesively bonded fiberglass tape with embedded sensing fibers for strain measurement and thermal compensation. The strain profiles were first converted into deflection profiles and validated against discrete deflection measurements performed with a high-precision total station system. Structural assessment based on comparison of the strain profiles with the results of three-dimensional finite-element analysis of the bridge superstructure, and with specification mandated criteria, indicated that the response of the girder under investigation was within the design limits, and did not pose serviceability concerns. Factors that may affect measurement accuracy are finally discussed on the basis of the experimental and numerical results.     

Testing, Analysis, and Evaluation of a GFRP Deck on Steel Girders

North Carolina has recently installed a fiber-reinforced polymer (FRP) deck on steel girders at a site in Union County. The bridge was instrumented with foil strain gauges, strain transducers, and displacement transducers. The bridge was then tested with a simulated MS-22.5 design load. Experimental data confirmed full composite interaction between the girders and the FRP deck panels. The neutral axis was measured to be 383?mm above the bottom flange of the 618-mm-deep girder. It was found that composite action could be estimated within 3% using a transformed section analysis of the deck panels. For two lanes loaded, the maximum live load distribution factor was computed to be 0.75. When looking at the overall performance of the structure, the deck deflected 5?mm, with the allowable stress at least 10 times over the maximum stress measured in the material. The girder deflection of 7?mm was well within the parameters set forth by AASHTO. Simple span deflection equations were found to conservatively model the anticipated deflection of the girders when using the transformed section properties.     

Study of Turkish Bridge Standards Involving a Reinforced Concrete Arch Bridge

Turkish bridge design standards were studied with a focus on the live load. Turkish design specifications were compared with American design specifications. Turkish bridge design specifications follow American Association of State Highway and Transportation Officials-Standard Specifications for Highway Bridges (AASHTO-SSHB), with the live load in Turkish standards given in tonnes, whereas in AASHTO-SSHB the live load is in tons. Turkish bridges are currently designed to either HS20 or HS30, the latter being 65% heavier than HS20-44. A reinforced concrete open spandrel arch bridge in Birecik, Turkey was analyzed using a service load approach according to AASHTO-SSHB with a heavy equipment transporter (HET), weighing 104,600?kg, as the live load. Dead load, live load, and impact were considered, and the analysis did not include any modification for possible deterioration, damage, or aging of the bridge. The bridge was not deemed adequate for passage of a HET using these assumptions.     

Fatigue of Diagonally Cracked RC Girders Repaired with CFRP

Fiber-reinforced polymers (FRP) are becoming more widely used for repair and strengthening of conventionally reinforced concrete (RC) bridge members. Once repaired, the member may be exposed to millions of load cycles during its service life. The anticipated life of FRP repairs for shear strengthening of bridge members under repeated service loads is uncertain. Field and laboratory tests of FRP-repaired RC deck girders were performed to evaluate high-cycle fatigue behavior. An in-service 1950s vintage RC deck-girder bridge repaired with externally bonded carbon fiber laminates for shear strengthening was inspected and instrumented, and FRP strain data were collected under ambient traffic conditions. In addition, three full-size girder specimens repaired with bonded carbon fiber laminate for shear strengthening were tested in the laboratory under repeated loads and compared with two unfatigued specimens. Results indicated relatively small in situ FRP strains, laboratory fatigue loading produced localized debonding along the FRP termination locations at the stem-deck interface, and the fatigue loading did not significantly alter the ultimate shear capacity of the specimens.     

Inclusion of Crawl Tests and Long-Term Health Monitoring in Bridge Serviceability Analysis

Due to limited resources, structural health monitoring (SHM) of highway bridges has to be integrated in structural performance assessment in a cost-effective manner. The instrumentation and the long-term SHM procedures are generally chosen with emphasis on most critical bridge components for a particular failure mode. However, global structural analysis is necessary to obtain useful structural performance information. It is then a major challenge to use monitoring data at some locations to perform a structural reliability analysis at other locations. In this paper, a methodology for lifetime serviceability analysis of existing steel girder bridges including crawl tests and long-term monitoring information is presented. The case where the initial goal of monitoring is to provide data for a fatigue analysis of some bridge components is considered. The monitoring results are used to perform a structural reliability analysis of different sections that are critical considering serviceability of the bridge. Limit state equations are used firstly by adhering to the load and strength formulas and requirements set forth in AASHTO specifications, and secondly by integrating monitoring information. Serviceability with respect to permanent deformation under overload is estimated for the girders with these two different methods and a time-dependent performance analysis is conducted by considering corrosion penetration. The proposed approach is applied to the I-39 Northbound Bridge over the Wisconsin River in Wisconsin. A monitoring program of that bridge was performed by the Advanced Technology for Large Structural Systems Center at Lehigh University.     

Live Load Distributions on an Impact-Damaged Prestressed Concrete Girder Bridge Repaired Using Prestressed CFRP Sheets

This paper describes detailed flexural behavior, including live load distributions, of a four-span prestressed concrete girder bridge supported by 14?m long C-shape girders (4 @ 14.05 = 56.2?m). The bridge has been damaged by frequent impact from heavy trucks, and repaired using prestressed carbon fiber-reinforced polymer sheets. A calibrated finite element analysis is conducted to investigate the flexural behavior (i.e., stress redistribution, deflection, live load distribution, and applied load effects) of the bridge in three different phases (i.e., undamaged, damaged, and repaired states) under various loading configurations. Strain localizations are noted at the damaged and repaired locations. Assessment of existing bridge codes such as the Association of State Highway and Transportation Officials Load Resistance Factor Design and Canadian Highway Bridge Design Code is conducted. The bridge codes predict well the nominal live load effect on the exterior girder, but underestimate the effect on the interior girders. A refined analysis may be recommended for this type of bridge.