Load and resistance

The AASHTO load and resistance factor Highway Bridge Design Specifications were developed with the intent of implementing a more rational approach for the design of highway structures. As opposed to allowable stress design, wherein all uncertainty is embedded within a factor of safety, the LRFD approach applies separate factors to account for uncertainty in load and material resistance. The load and resistance factors developed for NCHRP Project 12-33 were calibrated using a combination of reliability theory, fitting to ASD, and engineering judgment. Calibration using reliability theory is preferred because the approach permits selection of a target reliability or safety index that reflects the probability of failure of a structure component. However, reliability-based calibration requires access to sufficient data to statistically define the variation and distribution of load and resistance using mathematical relationships. Calibration by fitting and judgment is used in conjunction with or in lieu of reliability-based calibration when sufficient data are not available in order to ensure that designs are comparable to accepted engineering practice.

Loads are either transient or permanent. The load factors in the LRFD Specifications for the majority of transient load types were developed primarily using reliability theory and load test data. Except for the dead weight of structural components and attachments and the weight of wearing courses for which load factors could be developed using reliability theory, the load factors for permanent loads used for substructure design were developed subjectively using engineering judgment, and the judged relative reliability as compared to the range of Pmax and Pmin established for other permanent loads for which load statistics were better known.

Another concern with load factors is their applicability to short-term loading or temporary structures. The load factors in the LRFD Specifications were developed for a design life of 75 years. As a result, resistance factors calibrated for temporary design result in unusually high values because all of the uncertainty regarding the temporary loading must be incorporated.

The objective of this research is to develop recommended revisions to load factors needed for substructure element design that are consistent with the calibration work for related studies. This objective will be met through the following tasks: (1) Literature Search — A literature search will be conducted to gather information from related work on calibration of load factors and specifications for foundations of other structures such as retaining walls and culverts. (2) Development — Compile and evaluate data on the variation and distribution of permanent loads for vertical and horizontal earth pressure, earth surcharge, and downdrag. (3) Testing and Analysis — Using reliability-based methods where practical, perform calibration analyses to develop recommended values of Pmax and Pmin for permanent loads needed for substructure design. Develop recommended revisions to load factors needed for substructure design. (4) Reporting — Prepare a report that summarizes the research and recommendations to the AASHTO Bridge Committee.

High-strength concrete

When LRFD Specifications were written, there was a lack of data to demonstrate that the provisions were applicable when the concrete compressive strengths were above 10.0 ksi. However, recent research has started to address design issues with higher-strength concretes. In addition, the FHWA Showcase Projects encourage the use of high-strength concrete in bridge structures. There is, therefore, a need to expand the LRFD Specifications to allow for greater use of high-strength concrete. This project will support the necessary research required to remove the barriers in the LRFD Specifications related to shear in high-strength concrete. The specific topics to be addressed include the concrete contribution to shear resistance in high-strength concrete, maximum and minimum transverse reinforcement limits, and bond issues related to shear.

The objective of the project is to conduct the necessary research so that the implied strength limitation of 10.0 ksi and other barriers to high-strength concrete in the LRFD Specifications can be removed in relation to the shear provisions. Expected tasks include the following: (1) Review research and practice regarding the behavior and design of reinforced and prestressed high-strength concrete bridge girders subjected to shear. (2) Determine factors (such as concrete material properties, debonded strand, draped strand, section shape, amount of prestress, and transverse reinforcement details) that affect the shear resistance of high-strength concrete bridge girders, and identify tests to determine the effects of these factors. (3) Develop a work plan for experimental and analytical investigation of shear resistance and factors affecting shear resistance in reinforced and prestressed high-strength concrete bridge girders. (4) Submit an Interim Report that documents the research performed in Phase I and includes an updated work plan for Phase II of the project. Following review of the interim report by the NCHRP, the research team will make the required presentation to the project panel. Work on Phase II of the project will not begin until the interim report is approved and the Phase II work plan is authorized by NCHRP. (5) Execute the work plan. The experimental work shall involve large-size members with representative cross sections because of the unknown size effects with small-size specimens. (6) Prepare a final report that includes proposed revisions to the AASHTO LRFD Bridge Design Specifications to eliminate the barriers regarding the use of high-strength concrete related to shear issues.

Results of the research will be directly applicable to the specifications and will benefit the whole bridge community. The anticipated product from this research will include proposed revisions to the AASHTO LRFD Bridge Design Specifications that eliminate the barriers regarding the use of high-strength concrete related to shear issues. The long-term benefits of this research include lower initial costs for bridges, lower maintenance costs, and longer life for bridges. These result in lower cost and less inconvenience.

Prestressed girders

Bridge spans exceeding 160 ft. are generally not designed with prestressed concrete girders because of handling and hauling limitations due to size and weight. Designers most often use steel plate girders for the 200 to 300 bridges built in this span range each year. The problem created by this limited choice is that it leads to decreased competition between steel and concrete, which results in higher costs to owners. Further, it does not provide designers with the option of using a material that has been proven to exhibit excellent durability and low life-cycle costs.

Proposed solutions for increasing spans include splicing individual girder segments together to achieve longer simple or multiple spans, connecting simple spans together to achieve continuity, and producing large haunched pier sections in segments to be field spliced. The ability to fabricate prestressed concrete girders in transportable lengths and with manageable weights and then splice them together using post-tensioning has been demonstrated successfully on numerous bridges in the United States. However, this experience and the technology are fragmented and job-specific. Designers hesitate to attempt these designs because of lack of experience and the limited information available. The size and weight of many haunched pier sections precludes land transportation.

By incorporating recent developments such as high-performance concrete, improved post-tensioning solutions, and the experiences of others, a recommended design practice together with fabrication and construction details could be developed. These practices and standard details would reduce owner concerns and provide designers with guidance to enable more structures of this type to be proposed. Standardization of designs and details is the key to reducing costs and to engaging the confidence of designers.

The most significant aspect of this research is to develop and test new techniques to eliminate the hauling and handling limitations on the large, haunched pier girders. Successful methods to reduce size and weight to permit easier transportation will enable these solutions to be applied readily throughout the country. This research will provide owner agencies and designers with a recommended design practice, recommended standard details, a flow chart of options, and design examples for using the technology. Laboratory research will provide confidence that methods proposed to segment heavy sections will be workable and reliable.

The objective of this research is to develop a recommended practice, recommended standard details, and solutions for achieving longer spans using precast, prestressed concrete sections. The use of HPC will be incorporated into all options to extend strength and service characteristics.

Preliminary work on the use of spliced pier girders has resulted in several possible solutions. Options for the pier girder, in order of increasing span length, are: (a) Use a standard girder cross-section without haunches. (b) Use a haunched section that meets hauling limitations determined in Task 1. (c) Use a standard girder upper-segment together with a separate tapered haunch segment. This option will require mating the two segments and developing a connection across the horizontal joint. (d) Use match-casting techniques to create vertical joints in the large haunched girder.

It is envisioned that at least seven tasks will be necessary: Survey the literature, bridge designers, and bridge owners. The purpose of this task is to establish the present state of the art of prestressed spliced girder bridges. The weight, height, and length limitations that are enforced in the United States would be determined. Develop a detailed design and conceptual details of an interior girder in a three-span bridge, choosing span lengths that encompass solutions a and b. Develop conceptual details and the design of a horizontally segmented, haunched section in solution c. Develop conceptual details and the design of a vertically segmented, haunched section in solution d. Select the most promising solutions and conduct laboratory tests to provide confidence in the solutions. Based on the results of tasks, prepare recommended practices and details for use by owners and designers. Sample cost estimates will be prepared for varying spans and haunch types that compare spliced concrete girder costs to steel plate girder costs for the benefit of owner and designer comparisons. Ongoing projects will be studied for incorporation and testing of applicable concepts developed. A final report and visual materials will be prepared to provide the technology transfer to owners and designers.

The results of this research will be put into practice immediately upon completion. The need for these solutions has been steadily increasing. The impact of project delays on work-zone safety and speed of construction require an immediate alternative solution for the designer. Lower first costs, competitive impacts, and maintenance advantages will pay off in large savings system-wide.

Slab width

Effective slab width is used to compute the stiffness of composite steel bridge members and to analyze section properties for design checks. As such, the effective width directly affects the computed moments, shears, torques, and deflections for the composite section and also affects the proportions of the steel section and the number of shear connectors that are required. The effective slab width is particularly important for serviceability checks (fatigue, overload, and deflection), which can often govern the design.

In current U.S. steel-bridge design specifications, the effective slab width for all types of composite steel bridge members (interior girders) is specified to be the least of 12 times the least thickness of the deck, one-fourth the span length of the girder, and the girder spacing. When girder spacings were typically 8 ft. or less, the effective width computed according to this requirement almost always included all of the deck. With the ever-increasing use of wider girder spacings, the contribution of the additional width of deck is not currently recognized. Field measurements of modern composite steel bridges indicate that recognition of more of the concrete deck is often necessary to better predict composite dead- and live-load deflections. The AASHTO Guide Specifications for Segmental Concrete Bridges recognize the entire deck width to be effective unless shear lag adjustments become necessary.

These criteria are applied to all types of composite steel bridge members with either conventionally reinforced or prestressed decks. Steel box-girder bridges, two-girder structures, and composite deck systems that participate structurally with tied arches or cable-stayed bridges represent a few examples of cases for which the effective width of the slab is likely to be different than for more conventional multi-stringer I-girder bridges. Special provisions may also be necessary for determining the effective slab width for girders that are composite with vaulted or constant-depth precast post-tensioned decks on bridges with unusually wide girder spacings. The effective width of decks using high-strength concrete may also be affected by the larger elastic and shear moduli of the concrete.

The objective of this research is to determine an improved and more rational approach for the computation of effective slab width for different types of composite steel bridge members in both typical and atypical modern bridge structures for incorporation into the AASHTO specifications. The approach that is developed must be suitable for design office use (that is, presented in the form of equations, tables, charts, or some other format deemed acceptable by the AASHTO Highway Subcommittee of Bridges and Structures).

The use of more realistic and rational effective slab width criteria should lead to significant economies in composite steel bridge design by allowing the use of a larger composite cross section to resist the stresses and by ensuring that steel and concrete designs are on a more equal footing in the computation of effective slab width. The results of this study may also lead to additional consideration of more unique and cost-effective structural systems for steel bridges. It also may be possible to upgrade the load ratings of some composite steel structures.

Concrete decks are one of the most vulnerable structural components of the bridge. An improved understanding of the transfer of forces through the deck leading to more accurate computations of the effective slab width should improve the overall durability and performance of concrete bridge decks. Better performance and safer composite steel structures are also likely to result from improved predictions of actual behavior, greater consistency between analysis and design assumptions, and the introduction of new criteria to better handle more complex situations for which the original criteria were not intended.

The anticipated products from this research will include recommended revised specification language for inclusion in all applicable AASHTO design and rating specifications in a format suitable for consideration by the AASHTO Subcommittee on Bridges and Structures.

Abutments and piers

Full-scale tests conducted by the FHWA and by Colorado DOT in Denver on GRS bridge abutments and piers with segmental modular block facing have demonstrated excellent performance characteristics and very high load-carrying capacity. GRS bridge abutments and piers are more tolerable to differential settlement, more adaptable to low quality backfill, easier to construct, and more economical than their conventional counterparts with reinforced concrete. Moreover, GRS bridge abutments have shown good promise to eliminate the bumps that often occur at the end of bridge structures. The GRS bridge abutment and pier system can be put into service within two weeks and can be built by maintenance personnel.

This system may have considerable advantages for pedestrian structures, especially where access by heavy equipment is not available. It is economical for temporary pier use due to its easy demolition and the recyclable nature of its components; for emergency work due to reduced lead time and lower equipment and skills demands; and for massive looking piers that are desired for aesthetic reasons. Colorado DOT recently designed and constructed a GRS abutment to support both the bridge and approach roadway structures. The project includes the construction of a shallow strip spread foundation that supports the bridge abutment and is placed directly on the geogrid-reinforced earth wall. Abutments directly supported on the reinforced soil mass are more economical and are considered to eliminate the bridge bump problem in this case because the projected settlements are small.

The design of this bridge is overly conservative because it is the first of its kind in standard highway practice and because of the critical nature of this structure (it will support six lanes over Interstate 25). The early-measured results for this structure suggest that the performance of the reinforced abutment walls under service load is very satisfactory, showing small movements and indicating that the design was overly conservative.

The technology of segmental GRS bridge abutments and piers has not been adopted in highway bridge construction. The primary obstacles are threefold. First is the lack of a rational and reliable design method for such bridge-supporting structures. For example, although the vertical reinforcement spacing has been found to have a very strong effect on the performance of structures, current design methods fail to reflect this important fact. Also, field-measured strains are known to be drastically smaller that those predicted by the current design methods. Clearly, the current design methods are deficient. The second obstacle is the lack of well-developed guidelines and specifications for construction of the structures. Proper construction guidelines and specifications are critical to successful application of this technology. The third obstacle is a perceived suspicion that polymeric geosynthetics may not be sufficiently strong and stable during the design life of the bridge structures because of their relatively high service loads.

The objectives of this research are to develop a rational and reliable design method for segmental GRS bridge abutments and piers; to develop construction guidelines and specifications for these bridge-supporting structures; and to investigate the load-carrying capacity of these structures, including the long-term creep behavior under design loads.

These objectives can be accomplished through five tasks: Conduct a literature search to examine the long-term performance of GRS bridge abutments and piers that have been constructed in the United States and around the world. The search should also include design methods and construction guidelines and specifications. Preliminary construction guidelines and specifications should be established based on the documented information. Conduct laboratory tests to investigate long-term soil-reinforcement interactive creep behavior with different types of soils and reinforcements. The preliminary construction specifications should also be evaluated through these tests. Develop a numerical model for analysis of GRS abutments and piers. The numerical model should be verified with available full-scale test results. The analytical model should be employed to establish a preliminary design method for GRS bridge abutments and piers. Construct and test full-scale abutments and piers under well-controlled conditions to evaluate their performance. The tests should be performed to verify the preliminary design method and to establish a rational and reliable design method. Prepare a report to summarize the results of this study. The proposed design method and construction guidelines and specifications of GRS abutments and piers should be documented in detail.