The design of prestressed concrete bridge girders has changed significantly over the past several decades. Specifically, the design procedure to calculate the shear capacity of bridge girders that was used forty years ago is very different than those procedures that are recommended in the current AASHTO LRFD Specifications. As a result, many bridge girders that were built forty years ago do not meet current design standards, and in some cases warrant replacement due to insufficient calculated shear capacity. However, despite this insufficient calculated capacity, these bridge girders have been found to function adequately in service with minimal signs of distress. The objective of this research was to investigate the actual in service capacity of prestressed concrete girders that have been in service over an extended period of time. The actual capacity was compared with calculated values using the AASHTO LRFD Specifications.
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Author: Morcous, George | Size: 3.37 MB | Format:PDF | Quality:Original preprint | Publisher: University of Nebraska, Omaha | Year: 2010 | pages: 54
The National Bridge Inspection Standards require highway departments to inspect, evaluate, and determine load ratings for structures defined as bridges located on all public roads. Load rating of bridges is performed to determine the live load that structures can safely carry at a given structural condition. Bridges are rated for three types of loads, design loads, legal loads, and permit loads, which is a laborious and time-consuming task as it requires the analysis of the structure under different load patterns. Several tools are currently available to assist bridge engineers to perform bridge rating in a consistent and timely manner. However, these tools support the rating of conventional bridge systems, such as slab, I-girder, box girder and truss bridges. In the last decade, NDOR has developed innovative bridge systems through research projects with the University of Nebraska - Lincoln. An example of these systems is tied-arch bridge system adopted in Ravenna Viaduct and Columbus Viaduct projects. The research projects dealt mainly with the design and construction of the new system, while overlooking the load rating. Therefore, there is a great need for procedures and models that assist in the load rating of these new and complex bridge systems. The objective of this project is to develop the procedures and models necessary for the load rating of tied-arch bridges, namely Ravenna and Columbus Viaducts. This includes developing refined analytical models of these structures and performing rating factor (RF) calculations in accordance to the latest Load and Resistance Factored Rating (LRFR) specifications. Two-dimensional and three-dimensional computer models were developed for each structure and RF calculations were performed for the primary structural components (i.e. arch, tie, hanger, and floor beam). RFs were calculated assuming various percentages of section loss and using the most common legal and permit loads in the state of Nebraska in addition to AASHTO LRFD live loads. In addition, the two structures were analyzed and RFs were calculated for an extreme event where one of the hangers is fully damaged.
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The Load and Resistant Factor Design (LRFD) approach is based on the concept of structural reliability. The approach is more rational than the former design approaches such as Load Factor Design or Allowable Stress Design. The LRFD Specification for Bridge Design has been developed through the 1990s and 2000s. In the development process, many factors were carefully calibrated such that a structure designed with LRFD can achieve a reliability index of 3.5 for a single bridge girder (probability of failure of about 2 in 10,000). As the initial development of the factors in the LRFD Specification was intended to be applied to the entire nation, state-specific traffic conditions or bridge configuration were not considered in the development process. In addition, due to lack of reliable truck weigh data in the early 1990s in the U.S., the truck weights from Ontario, Canada measured in the 1970s were used for the calibration. Hence, the reliability of bridges designed with the current LRFD specification needs to be evaluated based on the Missouri-specific data and the load factor needs to be re-calibrated for optimal design of bridges. The objective of the study presented in this report is to calibrate the live load factor in the Strength I Limit State in the AASHTO LRFD Bridge Design Specification. The calibration is based on the Missouri-specific data such as typical bridge configurations, traffic volume, and truck weights. The typical bridge configurations and the average daily truck traffic of the bridges in Missouri are identified from statistical analyses of 2007 National Bridge Inventory. The Weigh-In-Motion (WIM) data from 24 WIM stations in Missouri are used to simulate realistic truck loads. Updated material and geometric parameters are also used to update the resistance distributions. From this study, it was found that most representative bridges in Missouri have reliability indices higher than 3.5. For many bridges in rural areas with Average Daily Truck Traffic (ADTT) of 1,000 or less, the average reliability indices are higher than 5.0. This study proposes a table of calibration factors which can be applied to the current live load factor of 1.75. The calibration factor is developed as a function of ADTT such that bridge design practitioners can select a calibration factor considering the expected ADTTs of a bridge throughout its life span. Impact of the calibration factor on the up-front bridge construction cost is also presented.
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Monitoring and Load Distribution Study for the Land Bridge
Author: Ghorbanpoor, Al | Size: 8.73 MB | Format:PDF | Quality:Original preprint | Publisher: University of Wisconsin, Milwaukee | Year: 2010 | pages: 179
A monitoring program and a live load distribution study were conducted for the Land Bridge, located on State Highway 131 between Ontario and LaFarge in southwest Wisconsin. The bridge is a 275-ft long curved double trapezoidal steel box girder construction. Hybrid HPS70W and A588 weathering steels were used for the construction of the bridge. The monitoring program included measurements of live load and thermal strains as well as displacements for the girders over a four-year period. The effects of the in-service live load, in terms of both the applied stresses and the number of load cycles, were found to be insignificant. The thermal stress levels were found to be more significant but with only a limited number of load cycles. It was also found that there was no significant change in the load pattern, for both the stress level and number of load cycles, over the four years of the monitoring program for the bridge. The observed stress levels in the bridge were found to be below the fatigue stress threshold prescribed by AASHTO. This indicated that an infinite life could be expected for the bridge when fatigue is a consideration for the steel box girders. The live load distribution study for the Land Bridge included a field testing, a 3-D numerical simulation, and a comparative study of the results with those determined by the provisions of the AASHTO standard and LRFD specifications. Good agreement was achieved between the load distribution factor values that were obtained from the field testing and the numerical simulation. The comparison of the results with the values obtained from the AASHTO specifications indicated that over-conservative results yielded from the standard specifications while the results from the LRFD specifications were under-conservative. It is recommended that an additional study be performed to overcome this shortcoming of the current design specifications.
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Calibration of Resistance Factors Needed in the LRFD Design of Drilled Shafts
Author: Abu-Farsakh, Murad Y | Size: 911 KB | Format:PDF | Quality:Original preprint | Publisher: Louisiana Transportation Research Center | Year: 2010 | pages: 110
The first report on Load and Resistance Factor Design (LRFD) calibration of driven piles in Louisiana (LTRC Final Report 449) was completed in May 2009. As a continuing effort to implement the LRFD design methodology for deep foundations in Louisiana, this report will present the reliability based analyses for the calibration of the resistance factor for LRFD design of axially loaded drilled shafts. A total of 16 cases of drilled shaft load tests were available to authors from Louisiana Department of Transportation and Development (LADOTD) archives. Out of those, only 11 met the Federal Highway Administration (FHWA) "5%B" settlement criterion. Due to the limited number of available drilled shaft cases in Louisiana, additional drilled shaft cases were collected from state of Mississippi that has subsurface soil conditions similar to Louisiana soils. A total of 15 drilled shafts from Mississippi were finally selected from 50 available cases, based on selection criteria of subsurface soil conditions and final settlement. As a result, a database of 26 drilled shaft tests representing the typical design practice in Louisiana was created for statistical reliability analyses. The predictions of total, side, and tip resistance versus settlement behavior of drilled shafts were established from soil borings using the FHWA O’Neill and Reese design method via the SHAFT computer program. The measured drilled shaft axial nominal resistance was determined from either the Osterberg cell (O-cell) test or the conventional top-down static load test. For the 22 drilled shafts that were tested using O-cells, the tip and side resistances were deduced separately from test results. Statistical analyses were performed to compare the predicted total, tip, and side drilled shaft nominal axial resistance with the corresponding measured nominal resistance. Results of this showed that the selected FHWA design method significantly underestimates measured drilled shaft resistance. The Monte Carlo simulation method was selected to perform the LRFD calibration of resistance factors of drilled shaft under strength I limit state. The total resistance factors obtained at different reliability index were determined and compared with those available in literature. Results of reliability analysis, corresponding to a target reliability index of 3.0, reveals resistance factors for side, tip, and total resistance factor are 0.20, 0.75, and 0.5, respectively.
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Development of Load and Resistance Factor Design for Ultimate and Serviceability Limit States of Transportation Structure Foundations
Author: Salgado, Rodrigo Purdue University Woo, San Inn Kim, Dongwook | Size: 3.57 MB | Format:PDF | Quality:Original preprint | Publisher: Joint Transportation Research Program | Year: 2011 | pages: 76
Most foundation solutions for transportation structures rely on deep foundations, often on pile foundations configured in a way most suitable to the problem at hand. Design of pile foundation solutions can best be pursued by clearly defining limit states and then configuring the piles in such a way as to prevent the attainment of these limit states. The present report develops methods for load and resistance factor design (LRFD) of piles, both nondisplacement and displacement piles, in sand and clay. With the exception of the method for design of displacement piles in sand, all the methods are based on rigorous theoretical mechanics solutions of the pile loading problem. In all cases, the uncertainty of the variables appearing in the problem and of the relationships linking these variables to the resistance calculated using these relationships are carefully assessed. Monte Carlo simulations using these relationships and the associated variabilities allow simulation of resistance minus load distributions and therefore probability of failure. The mean (or nominal) values of the variables can be adjusted so that the probability of failure can be made to match a target probability of failure. Since an infinite number of combinations of these means can be made to lead to the same target probability of failure, the authors have developed a way to determine the most likely ultimate limit state for a given probability of failure. Once the most likely ultimate limit state is determined, the values of loads and resistances for this limit state can be used, together with the values of the mean (or nominal) loads and resistances to calculate load and resistance factors. The last step in the process involves adjusting the resistance factors so that they are consistent with the load factors specified by American Association of State Highway and Transportation Officials (AASHTO). Recommended resistance factors are then given together with the design methods for which they were developed.
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Author: Adams, Michael Nicks, Jennifer Stabile, Tom Wu, Jonathan Schlatter, Warren Hartmann, Joseph | Size: 8.67 MB | Format:PDF | Quality:Original preprint | Publisher: Federal Highway Administration | Year: 2011 | pages: 174
This manual outlines the state-of-the-art and recommended practice for designing and constructing Geosynthetic Reinforced Soil (GRS) technology for the application of the Integrated Bridge System (IBS). The procedures presented in this manual are based on 40 years of State and Federal research focused on GRS technology as applied to abutments and walls. This manual was developed to serve as the first in a two-part series aimed at providing engineers with the necessary background knowledge of GRS technology and its fundamental characteristics as an alternative to other construction methods. The manual presents step-by-step guidance on the design of GRS-IBS. Analytical and empirical design methodologies in both the Allowable Stress Design (ASD) and Load and Resistance Factor Design (LRFD) formats are provided. Material specifications for standard GRS-IBS are also provided. Detailed construction guidance is presented along with methods for the inspection, performance monitoring, maintenance, and repair of GRS-IBS. Quality assurance and quality control procedures are also covered in this manual.
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Calibration of Load and Resistance Factors in LRFD Foundation Design Specifications
Author: Wang, Zuocai | Size: 5.27 MB | Format:PDF | Quality:Original preprint | Publisher: Missouri University of Science and Technology, Rolla | Year: 2011 | pages: 146
This report summarizes the findings and recommendations on the impact of foundation settlements on the reliability of bridge superstructures. As a collaborative effort of an overall initiative for the development of load and resistance factor design (LRFD) foundation design specifications, this study is focused on the investigation of pros and cons for including foundation settlements in bridge designs under gravity loads. Settlement was modeled both probabilistically and deterministically. In the case of a random settlement variable, a lognormal distribution was used in reliability analysis with a fixed coefficient of variation of 0.25. Dead and live loads were modeled as random variables with normal and Gumbel Type I distributions, respectively. Considering the regional traffic condition on Missouri roadways, the effect of a live load reduction factor on bridge reliability was also investigated. Therefore, a total of eight cases were discussed with a complete combination of settlement modeling (mean and extreme values), design consideration (settlements included and excluded), and live load reduction (unreduced and reduced live loads). Based on extensive simulations on multi-span bridges, bridges designed without due consideration on settlements can tolerate an extreme settlement of L/3500 - L/450 under unreduced live loads and up to L/3500 under reduced live loads without resulting in a reliability index below 3.5 (L=span length). Depending upon span lengths and their ratio, the reliability of existing steel-girder bridges is consistently higher than prestressed concrete and solid slab bridges. The shorter and stiffer the spans, the more significant the settlement’s effect on the reliability of bridge superstructures. As the span length ratio becomes less than 0.75, the girder and solid slab bridges’ reliability drops significantly at small settlements. A concrete diaphragm is very susceptible to the differential settlement of bridges, particularly for moment effects. Two recommendations were made to address settlement effects in bridge design: (1) settlement is considered in structural design and no special requirement is needed for foundation designs unless settlement exceeds the AASHTO recommended settlement limit of L/250, and (2) settlement is not considered in structural design as in the current Missouri Department of Transportation (MoDOT) practice but ensured below the tolerable settlement (e.g., L/450 for steel girders, L/2500 for slabs, and L/3500 for prestressed concrete girders). The first method provides a direct approach to deal with settlements and has potential to reduce overall costs in bridge design. The second method may result in oversized foundations.
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Author: Pantelides, Chris P University of Utah, Salt Lake City Liu, Ruifen University of Utah, Salt Lake City | Size: 2.65 MB | Format:PDF | Quality:Original preprint | Publisher: University of Utah, Salt Lake City | Year: 2011 | pages: 69
The present research project investigates lightweight and normal weight concrete precast panels for highway bridge decks. The deck panels are reinforced with Glass Fiber Reinforced Polymer (GFRP) bars. Due to the lack of research on lightweight concrete members reinforced with GFRP bars, the AASHTO LRFD Bridge Design Guide Specifications for GFRP Reinforced Concrete Decks do not permit the use of lightweight concrete when GFRP bars are used as flexural reinforcement. The ACI 440.1R-06 design guidelines do not provide any design information regarding the use of lightweight concrete reinforced with GFRP bars. In this research, the experimental performance of lightweight concrete versus normal weight concrete precast GFRP reinforced deck panels is investigated in terms of flexural capacity, panel deflections, and shear capacity.
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Evaluation of Long-Term Prestress Losses in Post-Tensioned Box-Girder Bridges
Author: Shing, P Benson Kottari, Alexandra | Size: 1.37 MB | Format:PDF | Quality:Original preprint | Publisher: University of California, San Diego | Year: 2011 | pages: 90
This study assessed the accuracy of the long-term prestress-loss estimation methods given in the current AASHTO LRFD Specifications for post-tensioned bridge girders, and developed more suitable analysis methods for these members. A refined analysis method and a simplified method were proven to be accurate using field data collected from two bridge structures that were monitored for prestress losses over a period of more than four years. Formulas for calculating the creep and shrinkage of concrete have been evaluated with the material data obtained from concrete cylinders cast with the same batches of concrete used for the monitored bridge girders, and have been used for the loss calculations. The formulas in AASHTO 2004 provide a much better correlation with the creep and shrinkage data obtained from the concrete cylinders than AASHTO 2007. In general, the AASHTO 2007 formulas significantly under-estimate the creep and shrinkage of the concrete cylinders, and result in calculated long-term losses that are much lower than the measured values. The creep and shrinkage formulas provided in AASHTO 2004 therefore are recommended for the prestress-loss calculations using the proposed refined analysis method. Suggestions are provided for possible implementation of the proposed analysis methods in the AASHTO LRFD Specifications for calculating long-term prestress losses in post-tensioned bridge girders. Both methods are expressed in forms that can be readily implemented in the AASHTO LRFD Specifications.
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