Journal of Bridge Engineering

In the field of nondestructive testing of structural components, active thermography methods are increasingly in demand. The most experience is available in testing of carbon-fiber-reinforced plastic (CFRP) or similar types of composite material. One of the emerging techniques is ultrasound excited thermography, which has not been fully transferred to massive steel members used in constructional steelwork thus far. The idea of ultrasound excitation is to generate elastic waves that propagate inside the investigated structure. In case of internal flaws, such as cracks, the boundary faces move relative to each other. The resulting rubbing and clapping of crack faces generate frictional heat, which is detected by means of an infrared camera. This paper demonstrates the usage of high-frequency mechanical excitation to detect cracks in hot-rolled girders and reduced plate specimens. The ultrasonic lock-in and the ultrasonic sweep thermography approaches are presented. Localized heating of the crack regions and distributed heating patterns caused by material damping can be observed. The influences of the tuned frequency and the crack depth as well as effects of prestressing and repeated excitation are discussed. In addition to experimental results, a finite-element simulation of the thermostructural problem is conducted. The model can be easily adjusted to match the experimental results of a performed ultrasonic sweep thermography.

The potential for structural capability degrading effects caused by corrosion is of profound importance and must be both fully understood and reflected in bridge inspection and maintenance programs. As the number of steel bridge infrastructures increases throughout the world, it is an exigent task to conduct regular and detailed corrosion surface investigations to evaluate their residual strength capacities and to develop analytical models to understand their current conditions and critical locations, as well as yield and ultimate behaviors. This paper presents a simple, accurate, and rapid assessment method and an effective maintenance management strategy developed by using the results of tensile coupon tests conducted on numerous corroded plates obtained from a steel plate girder used for about 100 years with severe corrosion and an finite element method (FEM) analytical approach proposed by measuring only the maximum corroded depth, which can be used to make reliable decisions affecting cost and safety.

The I-35W St. Anthony Falls Bridge was constructed to replace the steel truss bridge that collapsed on August 1, 2007. The design of the replacement bridge featured a smart-bridge system. The smart-bridge concept included instrumentation for long-term monitoring of the structural behavior of the bridge. Truck-load tests were conducted prior to opening the bridge and 26 months later to measure the response of the structure under controlled loading. The measurements were used to validate a FEM model of the bridge constructed to further investigate the behavior of the structure. The correlation between computed and measured results was found to be good. This paper describes the bridge, the instrumentation installed within the bridge, the FEM model, and validation of the model with respect to the truck-load tests. Recommendations are provided for static instrumentation plans of concrete box-girder structures.

The seismic design and response prediction of irregular bridges supported on piers of unequal heights—a commonly adopted solution when crossing steep-sided valleys—represent a particularly challenging problem that is yet to be effectively addressed by seismic design code provisions worldwide. From a force-based design perspective, shorter piers are subjected to increased ductility demand, and consequently damage tends to localize in these relatively stiff piers at increasing seismic hazard levels. This paper presents an investigation of the seismic response of a few schemes of a three-span case-study continuous bridge (commonly encountered in practice), featuring two unequal piers with relative heights of 0.5, 0.64, 0.75, and 0.86, respectively. Static pushover and time history (under incrementally scaled-up actual records) nonlinear inelastic analyses are performed using OpenSees to check the validity of Eurocode 8 (EC8) and recently proposed AASHTO-LRFD provisions for regular seismic behavior of ductile bridges dimensioned per a force-based design procedure. It has been demonstrated that satisfying the EC8 regularity condition does not necessarily result in a near-simultaneous failure of unequal piers at the extreme hazard level, especially for a low ratio between pier heights ($\le 0.5$). It has been further concluded that regularity criteria in both provisions fall short of providing the absolute regular seismic behavior of the bridge under investigation. Finally, a new design criterion is introduced (and verified) herein and is promoted for bridges with unequal height piers but with the same cross-section dimensions as may be typically dictated by some compelling aesthetical and practical considerations. However, it is worth mentioning that pier behavior (and, hence, failure) is assumed to be governed by flexure, and no shear failure and buckling are considered.

This paper uses the fragility function method to study the effects of pounding and skewness on the seismic behaviors of typical multispan RC highway bridges. Three-dimensional numerical models are built for typical three-span bridges in California that include nonlinear pier columns, distributed gap elements simulating pounding at both expansion joints and end abutments, and soil-structure interaction effects. The spatial differences of seismic excitations at various supports caused by varied soil profile properties and wave propagation effects, etc., are also included. As indicated by the nonlinear time history analyses using a large set of three-component earthquake ground motion records, fragility functions are derived and compared for bridges with various structural details, including the location of gaps, deck skew angle, and gap size. Although pounding causes significant local damage, the fragility functions show relatively moderate change in damage probability of the entire bridge. When pounding does not happen, skewed bridges outperform straight ones because of their coupled responses. For straight bridges, the pounding has a limited effect on bridge level damage and can be negligible. However, pounding results in increased damages of skewed bridges which aggravate with large skew angles. Using fragility functions, this paper clarifies the interactive roles of skewness and pounding for seismic damage probability of multispan highway bridges. These findings can provide valuable guidance for future bridge design.

A regular inspection of a bridge carrying U.S. Route 422 over the Schuylkill River in Berks County, Pennsylvania, revealed cracks in the floor beam–to–cantilever bracket tie plates. Similar fatigue damage of this type has been observed over two decades ago in other bridges. During this period, much advancement has taken place in structural evaluation through finite-element analysis. This paper presents a comprehensive procedure for the examination and retrofitting of tie plates using analysis and field measurement. A field and forensic investigation was conducted to examine the cause of the crack formation and the behavior of the bridge. The results of the field study and a fractographic examination confirmed that the cracks had occurred because of cyclic horizontal bending of the tie plates, which was generated by secondary out-of-plane bending of the top of the floor beam and cantilever bracket. Field measurements and evaluation indicated that some of the as-built tie plate details had exceeded their estimated fatigue life as computed according to specifications. An analysis by the finite-element method predicted locations of likely future fatigue damage. A retrofit consisting of removal of the restraint between the tie plate and the girder by unbolting the connection or through complete replacement of the tie plate resulted in a significantly lower stress in the tie plate. Measurements show that the unbolted condition will allow an infinite fatigue life. The removal of the tie plate restraint, however, elevated stresses in the web region and could produce fatigue damage in the distant future if the magnitudes of these stresses are high. This was examined through monitoring. The comprehensive procedure for investigation of tie plate cracks is recommended for handling possible future cases of similar damage.

This article presents the results of a numerical investigation of the flexural behavior of continuous externally prestressed concrete beams. Aspects of behavior studied include the increase in stress in external tendons, moment redistribution in the postelastic range, and secondary moments as a result of prestressing. A finite-element model for the full-range analysis of continuous externally prestressed concrete beams is introduced. The model predictions agree well with the experimental results. The analysis shows that the ultimate stress increase in external tendons of continuous beams is dependent on both the number and rotation of plastic hinges that can be developed at failure load. The degree of moment redistribution is significantly influenced by the nonprestressed tension steel and the pattern of loading. An approach based on the linear transformation concept is designed to examine the secondary moments over the entire loading up to the ultimate. The results indicate that the secondary moments increase linearly with the prestressing force and can be conveniently calculated by an elastic analysis.

A dynamic amplification factor (DAF) or a dynamic load allowance (IM) is typically used in bridge design specifications to include dynamic effects from vehicles on bridges. The calculated live-load stress ranges may not be correct because of varied dynamic amplification effects in different regions along the bridge, different road roughness conditions, and multiple stress-range cycles generated for one vehicle passage on the bridge. In this study, a reliability-based dynamic amplification factor on stress ranges (DAFS) for fatigue design is proposed to include the fatigue damage from multiple stress-range cycles resulting from each vehicle passage at varied speeds under various road conditions on the bridge’s life cycle. A numerical simulation toward solving a coupled vehicle-bridge system, including a three-dimensional (3D) suspension-vehicle model and a 3D dynamic bridge model, is used to obtain the revised equivalent stress range. The revised equivalent stress range is defined on an equivalent fatigue-damage basis; namely, the fatigue damage from multiple stress ranges with varied amplitudes is equivalent to the fatigue damage from one stress cycle of the revised equivalent stress range. Dynamic amplification factor on stress ranges is then defined as the ratio of the nominal live-load stress range and the maximum static stress range. A parametric study on DAFS was carried out to analyze the effect from multiple variables on the bridge’s life cycle, for instance, faulting days in each year, vehicle speed limit and its coefficient of variance, vehicle type distribution, and annual traffic increase. To appreciate the difference between the proposed DAFS and the traditional DAF, the calculated fatigue lives from the six approaches related to DAFS or DAF are compared with each other. Similar to DAF for dynamic response on displacements, DAFS is proposed to obtain dynamic stress ranges for fatigue design. As a result, once the DAFS values are available, the dynamic stress ranges for fatigue design can be easily obtained by multiplying the maximum static stress range and the DAFS, which helps to preserve both the accuracy and simplicity of bridge fatigue design.

Previous experimental studies have shown superior performance of concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) under static or pseudostatic loading. This study has focused on the effects of fiber type and architecture and the combined shear and flexure on the cyclic behavior of CFFT columns. One control RC and five CFFTs with different fiber types and architecture and shear span-to-depth ratios were tested under a constant axial load and reverse cyclic lateral loads. One of the tubes was off-the-shelf filament-wound product, whereas the other four were made using hand layup in the laboratory. The flexural strength and initial stiffness of the CFFT columns were shown to be dominated by the longitudinal tensile strength and stiffness of the FRP tube, respectively. On the other hand, the modulus of elasticity of the FRP tube in both the longitudinal and hoop directions was shown to be a dominant factor in the ductility of CFFT columns. The CFFT column with the combination of carbon fibers in the longitudinal direction and glass fibers in the hoop direction showed the highest energy dissipation, and all nonslender and slender CFFT columns showed a similar mode of flexural failure. An analytical study was carried out to comprehensively investigate the effects of fiber architecture and shear span-to-depth ratio on the cyclic behavior of CFFT columns.

A destructive field testing of a simply supported channel-beam bridge was carried out to gain insight into the ultimate load-carrying capacity of existing RC bridges. Based on a field inspection and assessment of the material strength, structural damage, and structural degradation before testing, a detailed test program was developed. It was observed from the testing that the field bridge exhibited local nonlinearities owing to its beam-to-beam bolted connections and initial damages. The failure mode of the tested bridge was the typical flexural failure characterized by the yielding of rebars and concrete crushing. Some concrete failures were also observed at the location of bridge bearings. The observed load-carrying capacity was much higher than that of the original design of the tested bridge and many more studies are needed to understand the real performance of field bridges.

Probabilistic evaluation on aeroinstability of long-span bridges is a practical requirement because the aeroinstability behaviors of a bridge are seriously affected by structural uncertainties and wind fluctuations. In this paper, a practical framework is proposed to perform reliability analyses on flutter and aerostatic instability of a long-span bridge, in which uncertainties from structural properties and the wind environment are considered. Modified limit state functions for aeroinstability reliability analyses are first established. Then the critical wind speeds in the functions are evaluated by empirical formulas. The structural dynamic characteristics involved in the empirical formulas are expressed by structural random variables with approximate functions based on the idea of response surface method, and a Monte Carlo simulation is further conducted to obtain the statistical properties of the critical wind speeds. Finally, the failure probability of aeroinstability is obtained by the conventional reliability analytical method. The present approach is applied to the reliability analysis of a long-span cable-stayed bridge, indicating the effectiveness of this approach.

Assessment of lateral excess pressure attributable to train live load on the backfill embankment is one of the main subjects in railway bridge abutment and retaining wall design. Relevant design codes usually define a uniform infinite surcharge on the surface of the embankment to evaluate the lateral live load–induced pressure on the bridge abutments. This simplified method does not consider the actual pattern of operational live load and its induced pressure distribution on the width and height of the wall. In the current study, the operational railway live load pattern LM71 was applied to accurately calculate the laterally induced pressure on the abutment. In this regard, the elasticity-based equations of the laterally induced pressure caused by rectangular area and strip loads were utilized to evaluate the lateral pressure distribution on the abutment attributable to the railway live load. For evaluating the resultant lateral force and moment on the wall, a numerical integration was conducted on the lateral pressure values in the width and height of the wall. Because the laterally induced pressure is related to Poisson’s ratio, a sensitivity analysis was done on this parameter to show its effect on the resultant lateral force on the wall. As an applicable result, it was shown that, in the case in which Poisson’s ratio is equal to 0.3, the induced resultant horizontal forces on the walls with heights of 1.5–7 m are considerably underestimated compared with the values obtained in the current study. Consequently, an analytical equation was proposed for surcharge estimation, which led to a safer structural design of the railway bridge abutments.