Prefabricated Steel Bridge Systems: Final Report
This chapter introduces the problem statement and the background related to the research study. It also presents the objectives of the study and outlines the organization of the report.
1.1 State of the Problem and Background
The nation's infrastructure of highway bridges is plagued with two major problems: premature deterioration and structural deficiency, both of which were underscored as strategic research issues in a recent NSF study ("Civil" 1998). At the national level, over 28% of all bridges are classified as structurally deficient or functionally obsolete ("The Status" 1999). Even newer bridges have shown a growing rate of premature decay. A major effort is now underway to rebuild the nation's civil infrastructure. In order to simply maintain the current conditions of highway bridges (with no improvement), an average annual cost of $5.2 billion is needed through the year 2011 ("The Status" 1999) for rehabilitation and replacement of existing bridges. Hence, it is vital to the U.S. economy that cost-effective structural systems and materials be explored in order to extend service life and to improve performance of highway transportation infrastructure facilities.
Many of the bridges requiring replacement are classified as short span bridges. Most of these bridges are located on busy highways and in congested areas. The direct and indirect costs due to traffic detours, loss of use for the extended construction time, and disruption to the local economy can, in many cases, exceed the raw cost of the bridge structure. Therefore, increased emphasis is being placed on improving work-zone safety and minimizing traffic disruption associated with bridge construction projects, while maintaining construction quality. For example, in Florida, the Department of Transportation estimates that for recent bridge replacement projects around the city of Tampa, the costs of detours and maintenance of traffic during construction were about 60% of the total construction costs. In discussions with State transportation officials of Florida, Texas, and Georgia it has been determined that 50% of the budget for bridge construction over the next ten years will be directed toward bridge replacement in congested areas. They further indicate that such high projected costs point to an urgent need for development of new bridge technologies aimed at significant reduction of construction time and costs associated with life cycle and maintenance of bridges. The aggressive use of prefabricated structural components and systems in bridge design and construction considerably minimize the cost of traffic maintenance and detours.
The current approach to bridge construction relies mainly on cast-in-place substructures and the use of prefabricated beams and cast-in-place concrete for the superstructures. Generally, this approach requires a long construction time that can extend more than six months for a short span bridge (up to 80 feet). While this extended construction time might not be an issue in the construction of new bridges, it has a significant economical impact on bridge replacement, especially if located in a congested area.
Rapid construction concepts have long been used by the railroad industry to avoid service interruption, however, such innovations in highway bridge construction have been limited. The main reason for this limitation is that the expansion of the country's infrastructure system over the past 50 years (after World War II) involved the building of new bridges and roads where construction time did not represent any significant problems. However, with a significant number of bridges approaching or surpassing their design life, bridge replacement is becoming a major focus of the bridge construction industry. Many of these bridges are located in congested areas where traditional methods of bridge construction are no longer suitable or economical.
While many states have developed standard bridge elements, minimal attempt has been made to develop a standardized, modular bridge system. With the present and increasing demand for bridge replacement standardization, modular bridge systems present the best option for rapid and economical construction.
Mass-produced elements can be quickly assembled and could reduce design time and cost, minimize forming and labor costs, and minimize lane closure time. Even at a higher initial cost, the use of prefabricated systems on bridges subjected to a high volume of traffic may be justified due to excessive lane closure times being avoided.
It is possible to replace almost any portion of a bridge with a prefabricated element/system, and to complete the installation during off-peak traffic periods with minimum traffic disruption. In addition, utilizing a new generation of high performance materials could help achieve enhanced durability and performance.
1.2 Objectives of the Study
The objectives of the study are:
- Assess the use of new and innovative prefabricated steel bridge systems/elements and methods in bridge construction, rehabilitation and replacement. This assessment is predicated on design effort for a system, on-site construction time, minimum travel lane closure time and minimum environmental impact.
- Identify the present most suitable prefabricated steel systems for bridge construction, rehabilitation, and replacement.
- Identify any existing problems hindering the wide spread use of these systems and develop possible solutions. The criteria used to evaluate the suitability of systems include; minimal traffic disruptions, life-cycle cost, ease of construction, quality assurance, and durability.
- Develop promising steel bridge concepts, ideas and approaches using innovative technologies and techniques that will accelerate the construction of bridges, extend service life and reduce traffic disruptions with improved work zone safety and minimal adverse effect on environment and community.
- Develop the application limits for at least two of the innovative concepts studied under "4" and identify all issues related to constructability and economical erection procedures for further development and refinements. The steel bridge concepts may be applicable to a full range of structure sizes and applications in conjunction with innovative joining processes and known high performance materials such as high performance concrete and/or high performance steel. The effort will concentrate on concepts that provide for the "Get In, Get Out and Stay Out" philosophy.
These objectives are achieved through the following work program:
Task 1: Perform a literature search of published research and articles including but not limited to publications of FHWA, NCHRP, Transportation Research Information Services (TRIS), AASHTO Technology Implementation Group, Departments of Transportation (DOT), U.S. Patent and Trademark Office, and Internet sources. A main objective is to summarize information and to document the use of innovative prefabricated steel systems for bridge construction, rehabilitation and replacement. The extent and use of such systems by US states and Canadian provincial DOTs is reported. The study evaluates commonly used systems as well as new systems not in common use that will minimize traffic disruption and result in reduced construction time and traffic lane closures.
Task 2**: Develop a data collection Survey for the purpose of canvassing US state and Canadian transportation agencies, steel fabricators, bridge erectors, contractors and designers. The objective is to seek information regarding practices and views on innovative prefabricated steel bridge components and systems.
** A survey questionnaire was developed at the early stages of the project, however, in presenting and discussing the prepared document with the FHWA Technical Advisory Group, it was decided to not disseminate the questionnaire. The consensus of opinion was that sufficient detailed information currently exists from previous similar research and that the knowledge and experience of the Technical Advisory Group members and of the authors of this report is substantial and sufficient.
Task 3: Prepare and submit for approval a Draft Synthesis Report. This report addresses prefabricated components and systems for steel bridge rehabilitation and replacement. The most suitable prefabricated steel systems are identified. Any problems hindering the wide spread use of these systems are assessed and possible solutions are recommended.
Task 4: Upon completion of reviews by the FHWA Technical Advisory Group and subsequent meetings, detailed analyses and further refinements are performed for the final bridge concepts. Two systems are recommended for further development. The criteria used to evaluate suitability of the systems include; minimal traffic disruptions, life-cycle costs, ease of construction, quality assurance, and durability.
Task 5: Subsequent to completion of final development of the bridge concepts chosen, the concepts are presented to experienced members of the construction industry, FHWA and State DOTs in order to solicit their input. The concepts are then modified to include this feedback and detailed drawings and specifications are developed.
Task 6: Perform optimization analysis of the proposed systems in order to evaluate potential structural performance and maximum possible limits. The optimization is conducted using specialized software and account for all important details including material, the nature of vehicular loading, fatigue limits, and the latest AASHTO LRFD Specifications design requirements. These optimization analyses lead to a better understanding of the overall structural performance of the proposed systems and help to identify any weaknesses that should be addressed. The simulations are also beneficial for optimizing the systems and evaluating the potential benefits and risks of various types of details.
Task 7: Develop recommendations for future research, testing and monitoring. In developing details for the bridge systems selected, consideration is given to the possibility of developing comprehensive built-in monitoring capabilities. These options are available to owners to allow monitoring of crucial components of the developed systems. The ability to install a monitored prefabricated system is another significant advantage over conventional construction technology, whereby monitoring systems are tailored for in-place conditions and are installed only after the bridge is built. Recommendations for future field-testing may include limited proof testing of connections or components and applying the refined concepts into future field applications.
Task 8: The Final Report includes a technical paper and a Microsoft PowerPoint presentation with narrative. The report is modified to incorporate review and approval comments received from the FHWA Technical Advisory Group.
1.3 Organization of the Report
This report documents the entire work program and presents the findings, conclusions, and recommendations. The remaining chapters are organized as follows:
Chapter 2, Findings from the Literature Review, presents the summary findings of the literature review on the history of the two main designs deemed practical in today's industry: the truss panel/floor beam/deck system and the longitudinal beam/deck system.
Chapter 3, Important Elements to Meet Current Needs, presents the important elements required to meet the current needs of designers rebuilding the nation's infrastructure. Issues discussed include system adaptability, connection details between components, use of innovative materials, standardization of components, design and construction specifications, transportation weight and size limits, and limits to standard erection cranes and equipment. Discusses current State of the Practice and strengths and weaknesses of current approach.
Chapter 4, The chapter summarizes the evaluation process and the development steps for two new bridge concepts for accelerated steel bridge construction. Factors discussed include the effects of bridge span lengths, various component configurations, ease of transportation, ease and speed of construction, and improvements in bridge service life and durability.
Chapter 5, A system optimization analysis is carried out to establish the maximum span lengths for the two modular systems presented in Chapter 4 and the most economical sectional design. The steel member design is optimized using global optimization methods. This chapter presents an overview of the different optimization methods and review the works reported in literature and related to steel member optimization.
Chapter 6, Provides presentation and discussion of optimization results.
Chapter 7, Conclusions and Recommendations, provides concluding remarks, discusses the applicability of current research to practice and provides recommendations for future research.
Also presented are a Reference Section and a Bibliography at the conclusion of each chapter if applicable. In addition, Appendix A and B include sample output files for the optimizations analysis.