Assessment of Highway Bridge Upgrading by Dynamic Testing and Finite-Element Model Updating

The Land Transport Authority of Singapore has a continuing program of highway bridge upgrading for refurbishing and strengthening bridges to allow for increasing vehicle traffic and increasing axle loads. One subject of this program has been a short-span bridge taking a busy main road across a coastal inlet near a major port facility. Experiment-based structural assessments of the bridge were conducted before and after upgrading works including strengthening. Each assessment exercise comprised three separate components: (1) a strain and acceleration monitoring exercise lasting approximately one month; (2) a full-scale dynamic test carried out in a single day without closing the bridge; and (3) a finite-element model updating exercise to identify structural parameters and mechanisms. This paper presents the dynamic testing and the modal analysis used to identify the vibration properties and the quantification of the effectiveness of the upgrading through the subsequent model updating. Before and after upgrade, similar sets of vibration modes were identified, resembling those of an orthotropic plate with relatively weak transverse bending stiffness. Conversion of bearings from nominal simple supports to nominal full fixity was shown via model updating to be the principal cause of natural frequency increases of up to 50%. The utility of the combined experimental and analytical process in direct identification of structural properties has been proven, and the procedure can be applied to other structures and their capacity assessments.     

Development of FRP Short-Span Deployable Bridge—Experimental Results

For military and civilian applications, there exists a need for lightweight, inexpensive, short-span bridges that can be easily transported and erected with minimal equipment. Owing to its favorable properties, fiber-reinforced polymer (FRP) has been shown to be feasible for the construction of such bridges. Investigations into the behavior of a short-span bridge structural concept, adapted to the material properties of commercially available glass FRP (GFRP) pultruded products, are presented. A 4.8-m span prototype was built from GFRP sections, bonded throughout to form a tapered box beam, with a width of 1.2?m and a height at midspan of approximately 0.5?m. The box beam represents a single trackway of a double-trackway bridge, whose trackways could be connected by light structural elements. The quasi-static and dynamic behavior of the prototype box beam was investigated in ambient laboratory and field conditions to assess the design and construction techniques used, with a view to designing a full-scale 10-m GFRP bridge. Laboratory testing of the prototype box beam used single and pairs of patch loads to simulate wheel loading. These tests confirmed that the box beam had sufficient stiffness and strength to function effectively as a single trackway of a small span bridge. Field testing of the structure was undertaken using a Bison vehicle (13,000?kg), driven at varying speeds over the structure to establish its response to realistic vehicle loads and the effects of their movement across the span.     

Truck Models for Improved Fatigue Life Predictions of Steel Bridges

A new fatigue load model has been developed based on weigh-in-motion (WIM) data collected from three different sites in Indiana. The recorded truck traffic was simulated over analytical bridge models to investigate moment range responses of bridge structures under truck traffic loadings. The bridge models included simple and two?equally continuous spans. Based on Miner’s hypothesis, fatigue damage accumulations were computed for details at various locations on the bridge models and compared with the damage predicted for the 240-kN (54-kip) American Association of State Highway and Transportation Officials (AASHTO) fatigue truck, a modified AASHTO fatigue truck with an equivalent effective gross weight, and other fatigue truck models. The results indicate that fatigue damage can be notably overestimated in short-span girders. Accordingly, two new fatigue trucks are developed in the present study. A new three-axle fatigue truck can be used to represent truck traffic on typical highways, while a four-axle fatigue truck can better represent truck traffic on heavy duty highways with a significant percentage of the fatigue damage dominated by eight- to 11-axle trucks.     

Equivalent Modal Damping of Short-Span Bridges Subjected to Strong Motion

In this paper four different methods are investigated for estimating the equivalent modal damping ratios of a short-span bridge under strong ground motion by considering the energy dissipation at the boundary. The Painter Street Overcrossing (PSO) is investigated because of seismic data availability. Computed responses using the response-spectrum method with the equivalent damping ratios estimates are compared with the recorded responses. The results show that the four methods provide reasonable estimation of equivalent modal damping ratios and that neglecting off-diagonal elements in the damping matrix is the most efficient and practical method. The equivalent damping ratio of the PSO was nearly 25% under an earthquake with peak ground acceleration of 0.55g, which is much higher than the conventional assumption of 5%.     

Dynamic Performance Simulation of Long-Span Bridge under Combined Loads of Stochastic Traffic and Wind

Slender long-span bridges exhibit unique features which are not present in short and medium-span bridges such as higher traffic volume, simultaneous presence of multiple vehicles, and sensitivity to wind load. For typical buffeting studies of long-span bridges under wind turbulence, no traffic load was typically considered simultaneously with wind. Recent bridge/vehicle/wind interaction studies highlighted the importance of predicting the bridge dynamic behavior by considering the bridge, the actual traffic load, and wind as a whole coupled system. Existent studies of bridge/vehicle/wind interaction analysis, however, considered only one or several vehicles distributed in an assumed (usually uniform) pattern on the bridge. For long-span bridges which have a high probability of the presence of multiple vehicles including several heavy trucks at a time, such an assumption differs significantly from reality. A new “semideterministic” bridge dynamic analytical model is proposed which considers dynamic interactions between the bridge, wind, and stochastic “real” traffic by integrating the equivalent dynamic wheel load (EDWL) approach and the cellular automaton (CA) traffic flow simulation. As a result of adopting the new analytical model, the long-span bridge dynamic behavior can be statistically predicted with a more realistic and adaptive consideration of combined loads of traffic and wind. A prototype slender cable-stayed bridge is numerically studied with the proposed model. In addition to slender long-span bridges which are sensitive to wind, the proposed model also offers a general approach for other conventional long-span bridges as well as roadway pavements to achieve a more realistic understanding of the structural performance under probabilistic traffic and dynamic interactions.     

Probabilistic Fatigue Evaluation of Riveted Railway Bridges

A probabilistic fatigue assessment methodology for riveted railway bridges is presented. The methodology is applied to a typical, short-span, riveted U.K. railway bridge under historical and present day train loading. On the loading side, the problem is randomized through dynamic amplification and traffic volume; on the resistance side, the S-N curves and the cumulative damage model are treated probabilistically. Model uncertainty is represented by the ratio between actual and calculated stresses, the latter obtained through finite element analysis. Annual response spectra for a fatigue-critical connection are developed through Monte Carlo simulation, which show that there is a continual and accelerating increase in the mean stress range experienced by the connection with time. S-N curves proposed in United States and United Kingdom codes are used in combination with Miner’s rule, to estimate the remaining fatigue life of the connection for different target failure probabilities. Parametric studies revealed that fatigue life estimates exhibit the highest sensitivity to detail classification, to S-N predictions in the region of high endurances, and to model uncertainty. This highlights the importance of field monitoring for old bridges approaching the end of their useful life.     

Laboratory and Field Testing of Composite Bridge Superstructure

Bridge rehabilitation utilizing a hybrid fiber-reinforced polymeric composite has recently been completed in Blacksburg, Va. This project involved replacing the superstructure in the Tom's Creek Bridge, a rural short-span traffic bridge with a timber deck and corroded steel girders, with a glue-laminated timber deck on composite girders. To verify the bridge design and to address construction issues prior to the rehabilitation, a full-scale mock-up of the bridge was built and tested in the laboratory. This setup utilized the actual composite beams, glue-laminated timber deck panels, and the skewed geometry implemented in the rehabilitation. Following rehabilitation, the bridge was field tested under controlled conditions (vehicle load and position). Both tests examined service load deflections, girder strains, load distribution, degree of composite action, interpanel deck deflections, and impact factor. The field test results indicate a service load deflection of L∕400 under moving loads and a high factor of safety in the composite members against material failure. The data from the field test serve as a baseline reference for future field durability assessments as part of a long-term performance and durability study.     

Experimental Dynamic Response of a Short-Span Composite Bridge to Military Vehicles

A continued desire for increased mobility in the aftermath of natural disasters or on the battlefield has lead to the need for improved lightweight bridging solutions. Currently, within the U.S. military, there is a need for a lightweight bridging system for crossing short-span gaps up to 4 m (13.1 ft) in length. This paper describes the field testing of a newly developed lightweight fiber-reinforced polymer bridging system to meet the U.S. militaries needs. The study investigates dynamic impact loads of track and wheel vehicles at different crossing speeds to increase understanding of appropriate impact factors used in design. It was found that the impact loads for the bridge treadways were most sensitive to vehicle crossing speed and vehicle type (wheel versus track and axle spacing) with observed impact factors as high as 1.71.     

Thrust and Guide Devices for Launched Bridges

Incremental launching is a competitive construction method for medium-span (40–65 m) prestressed concrete bridges. It does not constrain the length and width of the superstructure, and bridges longer than 1 km and wider than 20 m have been successfully launched. This method is hardly constrained by the bridge layout, as varying plan curvatures can be solved by shifting launch supports and varying vertical curvatures by shimming the bottom edges of the superstructure. The launch of a prestressed concrete bridge involves enormous forces and requires the guide and control of big volumes. The devices used for this purpose are described along with their design criteria and optimum fields of utilization, and several suggestions derived from many years of launching practice are given.     

Vehicle Induced Dynamic Behavior of Short-Span Slab Bridges Considering Effect of Approach Slab Condition

In this paper the vehicle induced dynamic bridge responses are calculated by modeling the bridge and vehicle as one coupled system. The dynamic behavior of short slab bridges with different span lengths induced by the AASHTO HS20 truck is investigated. A parametric study is conducted to analyze the effects of different truck speeds and different road surface conditions. Critical truck speeds that result in peaks of dynamic response are found to follow the rule that describes the resonant vibration of bridges due to train loading. The approach slab condition that consists of faulting at the ends and deformation along the span is considered in the analysis. Although the effect of the along-span deformation on the dynamic response of bridges is trivial, the faulting condition of the approach slab is found to cause significantly large dynamic responses in short-span slab bridges. Impact factors obtained from numerical analyses are compared with those values specified in the AASHTO codes.     

Financial Viability of Fiber-Reinforced Polymer (FRP) Bridges

The application of fiber-reinforced polymer (FRP) technology to bridges can provide performance enhancements at a time when there is a large and growing need to replace aging bridges in the United States. However, construction costs are significantly higher than with traditional methods, and it is not clear if this technology can become competitive in the standard short-span bridge market. This study investigates current and future costs to determine how cost competitive this technology is likely to become, taking into account the expected improvements in manufacturing, transport, and installation, as well as life-cycle differences. Based on two demonstration FRP bridges and the learning curve approach, the results show that anticipated improvements would not be sufficient to compete on cost with reinforced-concrete bridges. Unless significant improvement also occurs in the cost of component material, this technology will not be cost competitive for the standard short-span bridge, and the application of FRP technology will be limited to other segments of the market, such as bridge deck construction and bridge repair.     

Behavior of Structural Composite Lumber T-Beam Bridge Girders after Fatigue Loading

Recent innovation in the engineered wood industry has produced structural composite lumber (SCL) that achieves excellent strength, stiffness, and efficient use of wood. Product variations of SCL, such as laminated veneer lumber (LVL) and parallel strand lumber (PSL), are currently being used in transportation to produce bridge girders and decks for rural and other low traffic volume roads. Although the elastic and shear properties of SCL are available, no attempt has been made to estimate the fatigue performance of bridge girders. This study tested 12 new and 2 old, weathered SCL T-beam bridge girders with material and preservative variations for AASHTO-specified flexural fatigue under a stress-controlled test setup simulating 60?years of service. Transverse posttension was applied to the girders simulating a real-life situation. Results from the study indicate that the girders are capable of withstanding the repetitive loads without much physical damage. A few of the LVL girders had severe delamination at the SCL-epoxy interface. The fatigued girders were loaded statically up to failure and compared with the ultimate flexural strength of fresh girders. The girders did not show any appreciable strength loss because of one million cycles of fatigue loading. There was no effect of SCL type and preservative treatment on fatigue strength.     

Monotonic and fatigue deformation of Ni-W directionally solidified eutectic

Unlike many eutectic composites, the Ni-W eutectic exhibits extensive ductility by slip. Furthermore, its properties may be greatly varied by proper heat treatments. Here results of studies of deformation in both monotonic and fatigue loading are reported. During monotonie deformation the fiber /matrix interface acts as a source of dislocations at low strains and an obstacle to matrix slip at higher strains. Deforming the quenched-plus-aged eutectic causes planar matrix slip, with the result that matrix slip bands create stress concentrations in the fibers at low strains. The aged eutectic reaches generally higher stress levels for comparable strains than does the as-quenched eutectic, and the failure strains decrease with increasing aging times. For the composites tested in fatigue, the aged eutectic has better high-stress fatigue resistance than the as-quenched material, but for low-stress, high-cycle fatigue their cycles to failure are nearly the same. However, both crack initiation and crack propagation are different in the two conditions, so the coincidence in high-cycle fatigue is probably fortuitous. The effect of matrix strength on composite performance is not simple, since changes in strength may be accompanied by alterations in slip modes and failure processes.     

Monotonic and fatigue deformation of Ni-W directionally solidified eutectic

Unlike many eutectic composites, the Ni-W eutectic exhibits extensive ductility by slip. Furthermore, its properties may be greatly varied by proper heat treatments. Here results of studies of deformation in both monotonic and fatigue loading are reported. During monotonie deformation the fiber /matrix interface acts as a source of dislocations at low strains and an obstacle to matrix slip at higher strains. Deforming the quenched-plus-aged eutectic causes planar matrix slip, with the result that matrix slip bands create stress concentrations in the fibers at low strains. The aged eutectic reaches generally higher stress levels for comparable strains than does the as-quenched eutectic, and the failure strains decrease with increasing aging times. For the composites tested in fatigue, the aged eutectic has better high-stress fatigue resistance than the as-quenched material, but for low-stress, high-cycle fatigue their cycles to failure are nearly the same. However, both crack initiation and crack propagation are different in the two conditions, so the coincidence in high-cycle fatigue is probably fortuitous. The effect of matrix strength on composite performance is not simple, since changes in strength may be accompanied by alterations in slip modes and failure processes.     

Fatigue Life Evaluation of Concrete Bridge Deck Slabs Reinforced with Glass FRP Composite Bars

Since bridge deck slabs directly sustain repeated moving wheel loads, they are one of the most bridge elements susceptible to fatigue failure. Recently, glass fiber-reinforced polymer (FRP) composites have been widely used as internal reinforcement for concrete bridge deck slabs as they are less expensive compared to the other kinds of FRPs (carbon and aramid). However, there is still a lack of information on the performance of FRP–reinforced concrete elements subjected to cyclic fatigue loading. This research is designed to investigate the fatigue behavior and fatigue life of concrete bridge deck slabs reinforced with glass FRP bars. A total of five full-scale deck slabs were constructed and tested under concentrated cyclic loading until failure. Different reinforcement types (steel and glass FRP), ratios, and configurations were used. Different schemes of cyclic loading (accelerated variable amplitude fatigue loading) were applied. Results are presented in terms of deflections, strains in concrete and FRP bars, and crack widths at different levels of cyclic loading. The results showed the superior fatigue performance and longer fatigue life of concrete bridge deck slabs reinforced with glass FRP composite bars.     

Reliability Analysis of Suspension Bridges against Fatigue Failure from the Gusting of Wind

A fatigue reliability analysis of suspension bridges due to the gustiness of the wind velocity is presented by combining overall concepts of bridge aerodynamics, fatigue analysis, and reliability analysis. For this purpose, the fluctuating response of the bridge deck is obtained for buffeting force using a finite-element method and a spectral analysis in frequency domain. Annual cumulative fatigue damage is calculated using Palmgren–Miner’s rule, stress-fatigue curve approach and different forms of distribution for stress range. In order to evaluate the reliability, both first-order second-moment (FOSM) method and full distribution procedure (assuming Weibull distribution for fatigue life) are used to evaluate the fatigue reliability. Probabilities of fatigue failure of the Thomas Bridge and the Golden Gate Bridge for a number of important parametric variations are obtained in order to make some general observations on the fatigue reliability of suspension bridges. The results of the study show that the FOSM method predicts a higher value of the probability of fatigue failure as compared to the full distribution method. Further, the distribution of stress range used in the analysis has a significant effect on the calculated probability of fatigue failure in suspension bridges.     

Cracking and Fracture of Suspension Bridge Wire

When water enters a suspension bridge cable, the wires that make up the cable start to deteriorate. The protective zinc coating is the first element that is damaged, followed by corrosion of the steel itself. The wires are subjected to high axial tensile stresses from the bridge loading, bending stresses caused by straightening the curved wires inside the cable, and residual stresses introduced in their manufacture. These stresses, along with the corrosive environment, can lead to stress corrosion cracking or hydrogen-assisted cracking, two processes that lead to eventual failure of the wires. A fracture analysis indicates that the wires in the cable may be subjected to slightly different forces than a wire tested in the laboratory, but that the results of laboratory tests will give conservative values of cable strength.     

Failure Diagram for Chemically Assisted Crack Growth

A failure diagram that combines the thresholds for failure of a smooth specimen to that of a fracture mechanics specimen, similar to the modified Kitagawa diagram in fatigue, is presented. For a given material/environment system, the diagram defines conditions under which a crack initiated at the threshold stress in a smooth specimen becomes a propagating crack, by satisfying the threshold stress intensity of a long crack. In analogy with fatigue, it is shown that internal stresses or local stress concentrations are required to provide the necessary mechanical crack tip driving forces, on one hand, and reaction/transportation kinetics to provide the chemical potential gradients, on the other. Together, they help in the initiation and propagation of the cracks. The chemical driving forces can be expressed as equivalent mechanical stresses using the failure diagram. Both internal stresses and their gradients, in conjunction with the chemical driving forces, have to meet the minimum magnitude and the minimum gradients to sustain the growth of a microcrack formed. Otherwise, nonpropagating conditions will prevail or a crack formed will remain dormant. It is shown that the processes underlying the crack nucleation in a smooth specimen and the crack growth of a fracture mechanics specimen are essentially the same. Both require building up of internal stresses by local plasticity. The process involves intermittent crack tip blunting and microcrack nucleation until the crack becomes unstable under the applied stress.     

Bridge Rating with Consideration for Fatigue Damage from Overloads

This paper presents a proposed rating model that incorporates the fatigue damaging effects of overloads. This is achieved by introducing a “fatigue index” in the rating equation. The index, which appears in the form of a correction factor in the rating equation, is intended as a means to reduce the rating value computed for a bridge in cases where the damage from overloads is expected to be significant. The use of this index by itself does not impose any upper limit on the total number of overloads that may annually be permitted on a bridge. However, because the use of the index will result in a lower rating value than those from current equations, it is expected that a certain number of overloads will ultimately be disallowed. This provides for a built-in mechanism that will eventually result in lower fatigue damage to highway bridges resulting from overloads. In developing the model, typical records of overloads were acquired and used in bridge structural analyses to determine the damaging effect of overloads. The study on five bridges showed that fatigue damage from overloads can use up about 3.5% of fatigue life over a 25-year period if the overload occurrences remain at the current level. The use of the proposed index is in line with this amount of fatigue damage. This percentage is rather low and may not, in fact, be critical for most bridges over a 25-year period. However for older bridges, this percentage of fatigue life consumption may become important. Many such bridges were designed for a lower gross truck weight than what is used today for bridge design. Some of these bridges are located along feeder ramps and must carry loads in excess of 356 kN (80 kip) in an overload situation. For this group of bridges, it may be important to consider imposing a limit on the amount of fatigue damage resulting from frequent overloads. However, additional studies on a larger pool of bridges will be needed to establish a baseline for a maximum percentage fatigue life that can be used for overload permits.