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.     

Dynamic Response of a Highway Bridge Subjected to Moving Vehicles

Several full-scale load tests were performed on a selected Florida highway bridge. The bridge was dynamically excited by two fully loaded trucks, and the strain, acceleration, and displacement at selected points were recorded for the investigation of the bridge’s dynamic response. Experimental data were compared with simplified vehicle and bridge finite-element models. The vehicle was represented as a three-dimensional mass–spring–damper system with 11?degrees of freedom, and the bridge was modeled as a combination of plate and beam elements that characterize the slab and girders, respectively. The equations of motion were formulated with physical components for the vehicle and modal components for the bridge. The coupled equations were solved using a central difference method. It was found that the numerical analysis matched well with the experimental data and was used to successfully explain critical dynamic phenomena observed during the testing. Impact factors for this tested bridge were thoroughly investigated by using these models.     

Model Validation for Bridge-Road-Vehicle Dynamic Interaction System

The dynamic response of highway bridges subjected to moving truckloads has been observed to be dependent on (1) dynamic characteristics of the bridge; (2) truck configuration, speed, and lane position on the bridge; and (3) road surface roughness profile of the bridge and its approach. Historically, truckloads were measured to determine the load spectra for girder bridges. However, truckload measurements are either made for a short period of time [for example, weigh-in-motion (WIM) data] or are statistically biased (for example, weigh stations) and cost prohibitive. The objective of this paper is to present results of a 3D computer-based model for the simulation of multiple trucks on girder bridges. The model is based on the grillage approach and is applied to four steel girder bridges tested under normal truck traffic. Actual truckload data collected using a discrete bridge WIM system are used in the model. The data include axle loads, truck gross weight, axle configuration, and statistical data on multiple presence (side by side or following). The results are presented as a function of the static and dynamic stresses in each girder and compared with code provisions for dynamic load factor. The study provides an alternate method for the development of live-load models for bridge design and evaluation.     

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.     

Predicting Truck Load Spectra under Weight Limit Changes and Its Application to Steel Bridge Fatigue Assessment

Truck weight-limit regulations have significant influence on truck operating weights. These regulations directly influence loads applied to highway facilities, such as bridges and pavements. “Truck weight” herein collectively refers to a vehicle’s gross weight, axle weights, and axle configuration. Truck load spectra as a result of truck weight limits are important to bridge engineering in many respects, such as that of determining requirements for evaluation and design of bridges for both strength and fatigue. This paper’s objective is to present a new method for predicting truck weight spectra resulting from a change in truck weight limits. This method is needed to estimate impacts of the change on highway bridges such as accelerated fatigue accumulation. Historical and recent truck weight data are used to test and illustrate the proposed method, and the results show its good prediction capability. This method is also applied here to an example of estimating the impact on steel bridge fatigue due to a possible increase in the gross-vehicle-weight limit from 356 kN (80 kips) on five axles to 431 kN (97 kips) on six axles. Also included is an investigation of the AASHTO fatigue truck model for steel bridge evaluation. Results show that the current fatigue truck model may become invalid under the studied scenario of truck weight-limit increase.     

Life-Cycle Cost of All-Composite Suspension Bridge

This paper reports on the design of two highway suspension bridges made of conventional steel and advanced all-composite carbon fiber reinforced polymer (CFRP), and analyzed their life-cycle costs. The writers assumed that the pultrusion molding method would mainly be used for all composite highway bridges, because of its relatively high quality control performance and mass-production capability. First, the writers obtained the steel and composite highway bridge design in the same dimensional specification. Second, they acquired the future cost of the CFRP pultrusion product through hearing research from a fiber reinforced polymer manufacturer. Third, they calculated the initial costs of the steel bridge and CFRP bridge based on the design specification and the future cost of CFRP. Fourth, they compared the life-cycle cost of the steel and CFRP bridges under several conditions of discount rate, repair cost, and cycle. Finally, they found the critical condition where the CFRP bridge becomes more life-cycle cost-effective than the conventional steel bridge, if they could have expected the drastic cost reduction of the CFRP product.     

System for In-Service Strain Monitoring of Ordinary Bridges

An in-service bridge monitoring system (ISBMS) has been developed to provide near real-time web-based monitoring of live load strains in a bridge. The monitoring system is small, battery operated, can be rapidly deployed, and is programmed and interrogated via a user-friendly web interface. The ISBMS has been designed to be portable and used on an “as-needed” basis as a diagnostic tool or for health monitoring of ordinary bridges. The system is based on a small single-board computer with analog inputs; it also includes a cellular digital packet data modem for communication via the existing cellular network. Strains are measured using either a full bridge strain transducer or a quarter bridge foil strain gauge. The system has three modes of operation; The peaks program records peak live load strains that exceed a specified threshold, the time history program captures dynamic waveforms that exceed a specified threshold, and the rainflow program counts varying amplitude strain cycles. The selection and setup of the program, and retrieval of data is handled through a custom designed web interface. The system has been tested in the laboratory and in the field on a heavily traveled steel girder bridge. The data obtained from the ISBMS can be used for load rating using site specific data, fatigue investigations, monitoring bridge performance under permit loads, and as part of the biannual inspection of ordinary bridge.     

Experimental Evaluation of Dynamic Effects for a Selected Highway Bridge

The paper presents an experimental study of the actual dynamic effects for a preselected typical highway bridge. Knowledge of the dynamic impact factors is important for accurate determination of the ultimate load capacity and performance assessment of constructed bridges. Static and dynamic field tests were performed on a two-lane concrete highway bridge built in 1999 on U.S. 90 in northwest Florida. During the tests, one or two fully loaded trucks crossed over the bridge, which was instrumented with strain gauges, accelerometers, and displacement transducers. A wooden plank was placed across the lanes for some runs to trigger extensive dynamic vibration and to simulate poor road surface conditions. Data collected from the tests were used for comprehensive assessment of the bridge under dynamic loading. Impact factors obtained from the tests with higher speeds were found larger than corresponding values recommended by bridge codes. Analysis revealed that stiff vehicle suspension, road surface imperfection, and “bouncing” of the truck loading contributed to the high impact factors. Experimental data were also used for validation of the finite-element models developed for the vehicle–bridge system.     

Temporal Nature of Fatigue Damage in Highway Bridges

To develop a valid method of estimating fatigue life of a highway bridge, it is first necessary to have a reasonable understanding of the manner in which fatigue damage occurs over time. This paper presents the findings from an extensive highway bridge monitoring program focused on monitoring vehicle-induced strain cycles; the cause of fatigue damage. The purpose of the study was to identify the temporal character (if any) of fatigue damage accumulation. A sample of 24 bridges in the State of Ohio was monitored; each bridge in the sample for 1?year. The data collected during the study captured hourly, daily, and monthly variation in fatigue damage accumulation. The raw data were resolved to a fatigue damage metric representation. The damage metric was analyzed with the intent of determining the temporal nature of fatigue damage accumulation. The analyses led to the development of postulates regarding fatigue damage in highway bridges. The postulates stated in this study should provide a sound basis for development of a fatigue-life estimation procedure using site-specific strain cycle histograms collected over an abbreviated time window.     

In-Service Diagnostics of a Highway Bridge from a Progressive Damage Case Study

Development of diagnostic and prognostic routines for application to in-service measurements from highway bridges necessitates analysis of experimental measurements from in-service highway bridges under natural or prescribed induced damage. This is generally limited to the unique opportunity of investigating end-of-service life bridges prior to reconstruction and consequently only a limited library of such case studies exist. This paper documents a field test of an end-of-service bridge span with prescribed progressive damage to a bearing as well as several diaphragm connections. Thirty dual-axis accelerometers were distributed across the bridge span with data acquisition and transmission facilitated by a real-time lossless wireless sensor network. A highway department service truck applied traffic excitation to the structure through routine passes on a consistent lane of traffic. Output-only system identification was applied to the baseline time history response to develop a state-space model of the bridge dynamics used for forward prediction in the form of a Kalman filter. Simple statistical evaluation of the prediction error in the model demonstrates the variance can be used to localize and generally quantify the degree of damage in the structure. The case study additionally illustrates the potential importance of monitoring lateral acceleration along the girders to permit identification of damage to elements, such as the diaphragms, that contributing primarily to the lateral and torsional response of primary structural members.     

Truck Loads and Bridge Capacity Evaluation in China

Over the past 2–3 decades, the economic development in China has natured the establishment of a highway network with a large number of bridges. However, there are still no nationwide specification provisions for assessing their safe load carrying capacity. The research work reported herein focuses on developing reliability based requirements for this purpose. In this study, weigh-in-motion data for more than 7.3 million trucks were gathered from highways in three provinces of China, continuously over 1–16 months in 2006 and 2007. The data were processed and projected to model the live-load spectrum over 3-year and 100-year periods, respectively. The former is the required bridge inspection interval and the latter the bridge design life span, according to current Chinese maintenance and design specifications. The proposed projection method is shown to be more reliable compared with those reported. The resulting load spectra are used to assess the structural reliability of typical Chinese highway bridges at the component level. Based on the accordingly selected target reliability index, the live-load factors for bridge evaluation are developed in this study, proposed to be included in the Chinese national specifications.     

Ambient Vibration Monitoring of a Highway Bridge Undergoing a Destructive Test

Developing a technique to continuously monitor in-service highway bridges is one of the major research focuses at the Connecticut Department of Transportation and the University of Connecticut. The goal has been to use ambient traffic loading as the force to excite a measurable parameter that is sensitive to overall structural integrity. In this study, the dynamic responses of a full-scale steel-girder highway bridge during the passage of a small truck were measured using a number of sensors that could be reasonably implemented on a network of in-service bridges. Measurements were taken before and during the staged introduction of a simulated crack in one of the main supporting girders. The crack was introduced in five stages until it extended through two-thirds of the depth of the girder. Accelerometers were placed at various locations on each girder. Frequency spectra for each stage of the testing were compared to those recorded before the introduction of the crack to determine which aspects of the spectra were sensitive to the change in stiffness. The results indicate that monitoring the amplitudes at the natural frequencies and the frequency response spectrum using the cross signature assurance criterion can be used as an indicator that significant cracks have developed in a multigirder highway bridge.     

Effect of Vehicle Velocity on the Dynamic Amplification of a Vehicle Crossing a Simply Supported Bridge

Many writers, using both experimental tests and complex numerical models, have examined the effect of vehicle velocity on a highway bridge’s dynamic amplification. Although these tests and models give valuable quantitative information on dynamic amplification, they give little insight into how amplification is affected by individual vehicle/bridge parameters. This paper uses relatively simple numerical models to investigate the effect of vehicle velocity on a bridge’s dynamic amplification. A single vehicle crossing a simply supported bridge is modeled as a constant point force. A set of critical velocities are determined associated with peaks of dynamic amplification for all beams. The reasons for these large amplifications are discussed. A more complex finite element model, validated with field tests, is used to test the applicability of the conclusions obtained from the simple models to a realistic bridge/vehicle system.     

Full-Scale Tests of Bridge Steel Pedestals

Overheight vehicle collisions can cause major damage to bridges. To address the issue of limited vertical clearance heights and reduce the likelihood of impact damage, the Georgia Department of Transportation has implemented a program to elevate major highway bridges using very short columns referred to as steel pedestals. The process to elevate the bridges and install the steel pedestals is cost effective and efficient, resulting in minimum disruption to highway traffic. However, in practice, these pedestals are not detailed to provide end fixity, so they add considerable flexibility to the superstructure supports and potentially make the bridge more susceptible to instability and damage from seismic loads. Therefore, there is a need to evaluate how these steel pedestals will perform under the low-to-moderate earthquakes expected in this region. A full-scale 12.2?m (40?ft) dual steel girder simply supported bridge elevated with 500?mm (19?in.) and 850?mm (33?1/2?in.) steel pedestals is constructed based on typical field procedures. The full-scale bridge specimen is subjected to quasistatic unidirectional reversed cyclic loads to determine the strength and deformation capacity of the steel pedestals and overall system performance. The kinematics, mechanisms, and load–displacement hysteretic relationships of the bridge steel pedestals and its components are presented. Results show that the steel pedestals undergo kinematic rigid body motion, dissipate energy, and demonstrate reasonable deformation and strength capacities when subjected to quasistatic, reversed cyclic loads.     

Construction and Testing of an Innovative Concrete Bridge Deck Totally Reinforced with Glass FRP Bars: Val-Alain Bridge on Highway 20 East

The Val-Alain Bridge, located in the Municipality of Val-Alain on Highway 20 East, crosses over Henri River in Québec, Canada. The bridge is a slab-on-girder type with a skew angle of 20° over a single span of 49.89?m and a total width of 12.57?m. The bridge has four simply supported steel girders spaced at 3,145?mm. The deck slab is a 225-mm-thick concrete slab, with semi-integral abutments, continuous over the steel girders with an overhang of 1,570?mm on each side. The concrete deck slab and the bridge barriers were reinforced with glass fiber reinforced polymer (GFRP) reinforcing bars utilizing high-performance concrete. The Val-Alain Bridge is the Canada’s first concrete bridge deck totally reinforced with GFRP reinforcing bars. Using such nonmetallic reinforcement in combination with high-performance concrete leads to an expected service life of more than 75?years. The bridge is well instrumented with electrical resistance strain gauges and fiber-optic sensors at critical locations to record internal strain data. Also, the bridge was tested for service performance using calibrated truckloads. Design concepts, construction details, and results of the first series of live load field tests are presented.     

Long-Term Durability of State Street Bridge on Interstate 80

Destructive and nondestructive techniques were employed to evaluate the long-term durability of the carbon fiber reinforced polymer (CFRP) composite and externally CFRP-reinforced concrete of the State Street Bridge. Nondestructive evaluation was conducted through strain gauges, tiltmeters, thermocouples, and humidity sensors installed on the bridge bents for real-time health monitoring. Destructive tests were performed to determine the ultimate tensile strength, hoop strength, concrete confinement enhancement, and bond-to-concrete capacity of the CFRP composite for 3 years of exposure. Thermographic imaging was used for detection of voids between CFRP composite and concrete. Although environmental conditions were found to have an effect on the durability of the CFRP composite and CFRP-reinforced concrete substrate, no evidence of steel reinforcement corrosion was observed, and the CFRP composite retrofit is still effective after 3 years.     

Baseline Models for Bridge Performance Monitoring

A baseline model is essential for long-term structural performance monitoring and evaluation. This study represents the first effort in applying a neural network-based system identification technique to establish and update a baseline finite element model of an instrumented highway bridge based on the measurement of its traffic-induced vibrations. The neural network approach is particularly effective in dealing with measurement of a large-scale structure by a limited number of sensors. In this study, sensor systems were installed on two highway bridges and extensive vibration data were collected, based on which modal parameters including natural frequencies and mode shapes of the bridges were extracted using the frequency domain decomposition method as well as the conventional peak picking method. Then an innovative neural network is designed with the input being the modal parameters and the output being the structural parameters of a three-dimensional finite element model of the bridge such as the mass and stiffness elements. After extensively training and testing through finite element analysis, the neural network became capable to identify, with a high level of accuracy, the structural parameter values based on the measured modal parameters, and thus the finite element model of the bridge was successfully updated to a baseline. The neural network developed in this study can be used for future baseline updates as the bridge being monitored periodically over its lifetime.     

Differences between Calculated and Measured Long-Term Deflections in a Prestressed Concrete Girder Bridge

The monitoring of five precast, prestressed bridge girders during fabrication and service provided the opportunity to observe changes in camber over time. These camber variations were compared with corresponding strain and temperature measurements. Each of the girders was cast outside during the winter. As a result, the cold ground acted as a heat sink, and a significant temperature gradient existed during curing of each of the instrumented girders. These temperature gradients are believed to have caused the wide range in the short- and long-term cambers. A procedure to calculate the effect that curing temperatures have on girder camber is presented. In addition, the measured camber values are compared with predicted values using the multiplier method, improved multiplier method, and a detailed time-step method. It was found for the long-span girders that the measured camber values were on average within 10% of the predicted values using the detailed time-step method, but ranged from 22% lower to 27% higher for the simpler methods.     

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.