Effects of Edge-Stiffening Elements and Diaphragms on Bridge Resistance and Load Distribution

This research investigates the effects of barriers, sidewalks, and diaphragms (secondary elements) on bridge structure ultimate capacity and load distribution. Simple-span, two-lane highway girder bridges with composite steel and prestressed concrete girders are considered. The finite-element method is used for structural analysis. For the elastic range, typical secondary elements can reduce girder distribution factors (GDF) between 10 and 40%, depending on stiffness and bridge geometry. For the inelastic response, steel is modeled using von Mises yield criterion and isotropic (work) hardening. Concrete is modeled with a softening curve in compression with the ability to crack in tension. At ultimate capacity, typical secondary elements can reduce GDF an additional 5 to 20%, and bridge system ultimate capacity can be increased from 1.1 to 2.2 times that of the base bridge without secondary elements, depending on bridge geometry and secondary-element dimensions.     

Effect of Intermediate Diaphragms on Decked Bulb-Tee Bridge System for Accelerated Construction

One of the promising systems for accelerated bridge construction is the use of the decked precast prestressed concrete girders or decked bulb-tee girders for the bridge superstructure. Using the calibrated three-dimensional finite-element models through field tests, a parametric study was conducted to determine the effect of intermediate diaphragms on the deflections and flexural strains of girders at the midspan as well as the live load forces in the longitudinal joint. The following diaphragm details were considered: different diaphragm types (steel and concrete), different diaphragm numbers between two adjacent girders, and different cross-sectional areas for steel diaphragms. Five bridge models with different diaphragm details were developed, and the short span length effect on the bridge behavior was also studied. It was found that as long as one intermediate diaphragm was provided between two adjacent girders at midspan, changing the diaphragm details did not affect the girder deflection, the girder strain, and the live load forces in the longitudinal joint significantly. The effect of diaphragms on the midspan deflection was more prominent in the short span bridge; however, the reduction in the maximum bending moment by the diaphragms was more significant in the long span bridge than in the short span bridge. Specific design recommendation is provided in this paper.     

Field Evaluation of Decommissioned Noncomposite Steel Girder Bridge

Field tests conducted on a noncomposite steel girder bridge are described. Two separate 36.6 m (120 ft) units, each three-span continuous, were subjected to increasing static loads by means of a trailer and concrete barriers. Results show that the girders acted as partially composite sections in the positive moment region up to the onset of yield. Due to curb participation and the transverse location of the applied load, exterior girders exhibited a higher degree of partially composite action. In the negative moment region, partially composite action was evident only in the exterior girders. As a result of partially composite action and curb participation, the yield load was about 7% higher than predicted. Bearing restraint is shown not to have a significant impact on the behavior of the tested bridges. In addition, the stiffness of the interior girders, as measured under the constant weight of a dump truck, are shown to be virtually unaffected by the heavy trailer loads. More significant changes in girder stiffness were observed between different transverse load positions of the dump truck.     

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.     

The beta-adrenomimetic effect of human and animal blood serum

The conventional analysis and design of highway bridges ignore the contribution of sidewalks and∕or railings in a bridge deck when calculating the flexural strength of superstructures. The presence of sidewalks and railings or parapets acting integrally with the bridge deck have the effect of stiffening the outside girders and attracting more load while reducing the load effects in the interior girders. This paper presents the results of a parametric study showing the influence of typical sidewalks and railings on wheel load distribution as well as on the load-carrying capacity of highway bridges. A typical one-span, two-lane, simply supported, composite steel girder bridge was selected in order to investigate the influence of various parameters such as: span length, girder spacing, sidewalks, and railings. A total of 120 bridges were analyzed using three-dimensional finite-element analysis. American Association of State Highway and Transportation Officials (AASHTO) HS20 design trucks were positioned in both lanes to produce the maximum moments. The finite-element analysis results were also compared with AASHTO wheel load distribution factors. The AASHTO load and resistance factor design (LRFD) wheel load distribution formula correlated conservatively with the finite-element results and all were less than the typical empirical formula (S∕5.5). The presence of sidewalks and railings were shown to increase the load-carrying capacity by as much as 30% if they were included in the strength evaluation of highway bridges.     

Girder Differential Deflection and Distortion-Induced Fatigue in Skewed Steel Bridges

The prevalence of fatigue cracking in steel bridge girders due to out-of-plane web distortion motivates development of procedures to evaluate the effects of distortional fatigue. In a previous study sponsored by the Minnesota Department of Transportation (Mn/DOT) the frequency and magnitude of distortional stresses on a typical skewed, steel bridge with staggered, bent-plate diaphragms were assessed. The results revealed a diaphragm deformation mechanism that causes distortional fatigue in the girder web gap, leading to simple, accurate estimates of fatigue stress if bridge properties and differential vertical deflection between girders are known. In the present study, linear finite element models are used to represent composite steel bridges and identify bridge parameters that influence relative deflection of adjacent girders. Parameters found to have a significant effect on differential deflection include girder spacing, angle of skew, span length, and deck thickness. These results are incorporated in a simple procedure that is intended for use in management schemes for skewed, steel-girder bridges, with staggered, bent-plate diaphragms, susceptible to web gap distortional fatigue.     

Stiffness and Strength of Metal Bridge Deck Forms

Light gauge metal sheeting is often utilized in the building and bridge industries for concrete formwork. Although the in-plane stiffness and strength of the metal forms are commonly relied upon for stability bracing in buildings, the forms are generally not considered for bracing in steel bridge construction. The primary difference between the forming systems in the two industries is the method of connection between the forms and girders. In bridge construction, an eccentric support angle is incorporated into the connection details to achieve a uniform slab thickness along the girder length. While the eccentric connection is a benefit for slab construction, the flexible connection limits the amount of bracing provided by the forms. This paper presents results from the first phase of a research study investigating the bracing behavior of metal bridge deck forms. Shear diaphragm tests were conducted to determine the shear stiffness and strength of bridge deck forms, and modified connection details were developed that substantially improve the bracing behavior of the forms. The measured stiffness and strength of diaphragms with the modified connection often met or exceeded the values of diaphragms with conventional noneccentric connections. The experimental results for the diaphragms with the modified connection details dramatically improve the potential for bracing of steel bridge girders by metal deck forms.     

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.     

Load and Resistance Factor Calibration For Wood Bridges

The paper presents the calibration procedure and background data for the development of design code provisions for wood bridges. The structural types considered include sawn lumber stringers, glued-laminated girders, and various wood deck types. Load and resistance parameters are treated as random variables, and therefore, the structural performance is measured in terms of the reliability index. The statistical parameters of dead load and live (traffic) load, are based on the results of previous studies. Material resistance is taken from the available test data, which includes consideration of the post-elastic response. The resistance of components and structural systems is based on the available experimental data and finite element analysis results. Statistical parameters of resistance are computed for deck and girder subsystems as well as individual components. The reliability analysis was performed for wood bridges designed according to the AASHTO Standard Specifications and a significant variation in reliability indices was observed. The recommended load and resistance factors are provided that result in consistent levels of reliability at the target levels.     

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.     

Influence of Secondary Elements and Deck Cracking on the Lateral Load Distribution of Steel Girder Bridges

The AASHTO LRFD load distribution factor equation was developed based on elastic finite element analysis considering only primary members, i.e., the effects of secondary elements such as lateral bracing and parapets were not considered. Meanwhile, many bridges have been identified as having significant cracking in the concrete deck. Even though deck cracking is a well-known phenomenon, the significance of pre-existing cracks on the live load distribution has not yet been assessed. The purpose of this research is to investigate the effect of secondary elements and deck cracking on the lateral load distribution of girder bridges. First, secondary elements such as diaphragms and parapets were modeled using the finite element method, and the calculated load distribution factors were compared with the code-specified values. Second, the effects of typical deck cracking and crack types that have a major effect on load distribution were identified through a number of nonlinear finite element analyses. It was established that the presence of secondary elements may produce load distribution factors up to 40% lower than the AASHTO LRFD values. Longitudinal cracking was found to increase the load distribution factor by up to 17% when compared to the LRFD value while the transverse cracking was found to not significantly influence the transverse distribution of moment.     

Full-Scale Test of Continuity Diaphragms in Skewed Concrete Bridge Girders

Continuity diaphragms used in prestressed girder bridges on skewed bents have caused difficulties in detailing and construction. The results of the field verification for the effectiveness of continuity diaphragms for skewed, continuous, and prestressed concrete girder bridges are presented. The current design concept and bridge parameters that were considered include skew angle and the ratio of beam spacing to span (aspect ratio). A prestressed concrete bridge with continuity diaphragms and a skewed angle of 48° was selected for full-scale test by a team of engineers from Louisiana Department of Transportation and Development and the Federal Highway Administration. The live load tests performed with a comprehensive instrumentation plan provided a fundamental understanding of the load transfer mechanism through these diaphragms. The findings indicated that the effects of the continuity diaphragms were negligible and they can be eliminated. The superstructure of the bridge could be designed with link slab. Thus, the bridge deck would provide the continuity over the support, improve the riding quality, enhance the structural redundancy, and reduce the expansion joint installation and maintenance costs.     

Comparison of Measured and Computed Stresses in a Steel Curved Girder Bridge

Steel curved I-girder bridge systems may be more susceptible to instability during construction than bridges constructed of straight I-girders. The primary goal of this research is to study the behavior of the steel superstructure of a curved steel I-girder bridge system during all phases of construction and to ascertain whether the actual stresses in the bridge are represented well by linear elastic analysis software developed for this project and typical of that used for design. Sixty vibrating wire strain gauges were applied to a two-span, four-girder bridge, and elevation measurements were taken by a surveyor's level. The resulting stresses and deflections were compared to computed results for the full construction sequence of the bridge as well as for live loading from up to nine 50-kip trucks. The analyses correlated well with the field measurements, especially for the primary flexural stresses. Stresses due to lateral bending and restraint of warping induced in the girders and the stresses in the cross frames were more erratic but generally showed reasonable correlation. In addition, it is shown that, for the magnitude of live load applied to the bridge, analyses in which composite behavior is assumed in the negative moment region yield better correlation than analyses in which just the bare steel girders are used (no shear connectors were used on the bridge in the negative moment region). It is concluded that the curved girder analysis software captures the general behavior well for these types of curved girder bridge systems at or below the service load level, and that the stresses in these bridges may be relatively low if their design is controlled largely by stiffness.     

Load Distribution for a Highly Skewed Bridge: Testing and Analysis

The American Association of State Highway and Transportation Officials (AASHTO) specifications provide formulas for determining live load distribution factors for bridges. For load distribution factors to be accurate, the behavior of the bridge must be understood. While the behavior of right-angle bridges and bridges with limited skews is relatively well understood, that of highly skewed bridges is not. This paper presents a study aimed at developing a better understanding of the transverse load distribution for highly skewed slab-on-steel girder bridges. The study involved both a diagnostic field test of a recently constructed bridge and an extensive numerical analysis. The bridge tested and analyzed is a two-span, continuous, slab-on-steel composite highway bridge with a skew angle of 60°. The bridge behavior is defined based on the field test data. Finite-element analyses of the bridge were conducted to investigate the influence of model mesh, transverse stiffness, diaphragms, and modeling of the supports. The resulting test and analytical results are compared with AASHTO’s Load and Resistance Factor Design formulas for live load distribution to assess the accuracy of the current empirical formulas.     

Field Testing and Analysis of CRC Deck Girder Bridges

This paper presents findings of field tests and analysis of two conventionally reinforced concrete (CRC) deck girder bridges designed in the 1950s. The bridges are in-service and exhibit diagonal cracks. Stirrup strains in the bridge girders at high shear regions were used to estimate distribution factors for shear. Impact factors based on the field tests are reported. Comparison of field measured responses with AASHTO factors was performed. Three-dimensional elastic finite-element analysis was employed to model the tested bridges and determine distribution factors specifically for shear. Eight-node shell elements were used to model the decks, diaphragms, bent caps, and girders. Beam elements were used to model columns under the bent caps. The analytically predicted distribution factors were compared with the field test data. Finally, the bridge finite-element models were employed to compare load distribution factors for shear computed using procedures in the AASHTO LRFD and Standard Specifications.     

Live Load Distribution Formulas for Single-Span Prestressed Concrete Integral Abutment Bridge Girders

In this study, live load distribution formulas for the girders of single-span integral abutment bridges (IABs) are developed. For this purpose, two and three dimensional finite-element models (FEMs) of several IABs are built and analyzed. In the analyses, the effects of various superstructure properties such as span length, number of design lanes, prestressed concrete girder size, and spacing as well as slab thickness are considered. The results from the analyses of two and three dimensional FEMs are then used to calculate the live load distribution factors (LLDFs) for the girders of IABs as a function of the above mentioned parameters. The LLDFs for the girders are also calculated using the AASHTO formulas developed for simply supported bridges (SSBs). The comparison of the analyses results revealed that LLDFs for girder moments and exterior girder shear of IABs are generally smaller than those calculated for SSBs using AASHTO formulas especially for short spans. However, AASHTO LLDFs for interior girder shear are found to be in good agreement with those obtained for IABs. Consequently, direct live load distribution formulas and correction factors to the current AASHTO live load distribution equations are developed to estimate the girder live load moments and exterior girder live load shear for IABs with prestressed concrete girders. It is observed that the developed formulas yield a reasonably good estimate of live load effects in prestressed concrete IAB girders.     

Live-Load Distribution Factors in Prestressed Concrete Girder Bridges

This paper presents an evaluation of flexural live-load distribution factors for a series of three-span prestressed concrete girder bridges. The response of one bridge, measured during a static live-load test, was used to evaluate the reliability of a finite-element model scheme. Twenty-four variations of this model were then used to evaluate the procedures for computing flexural live-load distribution factors that are embodied in three bridge design codes. The finite-element models were also used to investigate the effects that lifts, intermediate diaphragms, end diaphragms, continuity, skew angle, and load type have on distribution factors. For geometries similar to those considered in the development of the American Association of State Highway and Transportation Officials Load and Resistance Factor Design Specifications, the distribution factors computed with the finite-element models were within 6% of the code values. However, for the geometry of the bridge that was tested, the discrepancy was 28%. Lifts, end diaphragms, skew angle, and load type significantly decreased the distribution factors, while continuity and intermediate diaphragms had the least effect. If the bridge had been designed using the distribution factors calculated with the finite-element model rather than the code values, the required concrete release strength could have been reduced by 6.9 MPa (1,000 psi) or the live load could have been increased by 39%.     

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.     

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.