Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
4CHAPTER 1 INTRODUCTION AND RESEARCH APPROACH 1.1 INTRODUCTION Conventional girder bridge superstructures usually are supported on bearings placed on the bridge substructure. The bearings are designed to support the vertical reaction of the bridge girders and may also be designed to restrain the hori- zontal movements of the bridge. The bearings are usually detailed to allow the superstructure to rotate at pier locations. The superstructures and substructures of conventional bridges are essentially designed as separate systems. Unlike conventional bridges, an integral connection between the superstructure and substructure provides some degree of continuity between the two systems. The elevation of the bot- tom of the integral pier cap may be the same as that of the bottom of girders. Moments, in addition to vertical and hori- zontal forces, are transferred from the superstructure to the substructure. In past applications, integral connections of steel bridge structures were typically used on bridges crossing over other highways or railway tracks at sharp skew angles. The use of integral connections allowed the pier cap bottom elevation to be the same as the elevation of the bottom of the girders. This allowed orienting the pier cap in the direction perpendicular to the girders without causing the pier cap to reduce the overhead clearance of the lower highway or railroad. This orientation of the pier cap eliminated the problems associated with orienting the pier cap at a sharp skew, which would be required if a con- ventional pier cap were used. The use of the integral pier cap also eliminated the need to raise the elevation of the bridge and bridge approaches to maintain underclearance while ori- enting conventional pier caps perpendicular to the girders. Integral connections were also used to enhance seismic per- formance of bridge structures. Figure 1 depicts a concrete bridge pier connected integrally to a steel superstructure. Past research on integral connections was essentially con- ducted on concrete structures. The use of integral connec- tions on concrete bridges helped in reducing the mass of con- crete bridges and, thus, improved their seismic performance. A composite steel girder bridge superstructure weighs sub- stantially less than a concrete superstructure. This reduction of mass in the superstructure reduces the seismic susceptibil- ity of bridge structures. Nevertheless, steel superstructures placed on top of large concrete drop bent caps or hammer- head piers can result in unnecessary mass, offsetting the ben- efits from the reduced weight of the steel. Integral construc- tion reduces this mass and, with close attention to detailing, provides improved aesthetics. This report presents the details and results of the work on the use of integral connections between steel I-girder bridge superstructures and concrete substructures. This work was conducted under NCHRP Project 12-54. Past use of integral pier connections for steel bridge struc- tures was reviewed to determine the best approach to this investigation. The reasons that led to using integral connec- tions in the past (type of construction, pier configurations, and the performance of integral pier bridges) were documented. In addition, research on integral connections for concrete structures was reviewed and summarized. Several integral pier cap connections were developed and reviewed by bridge designers and steel bridge fabricators. Concepts for the connections between the girders and the integral pier caps were also developed. The review of the expected performance of different concepts resulted in rec- ommending five connection concepts for further develop- ment and detailing. One concept representing I-girder super- structure supported on a box-beam integral pier cap and a reinforced-concrete single-column pier was recommended for detailed analytical and experimental validation. 1.2 PROBLEM STATEMENT AND RESEARCH OBJECTIVE The objective of was to develop recommended details, design methodologies, and specifications for integral con- nections of steel superstructures to concrete substructures. The recommended specifications were to be in a form suit- able for consideration by the AASHTO Highway Subcommit- tee on Bridges and Structures (HSCOBS). The stated objective was further narrowed to exclude integral abutments and only include integral connections between steel superstructures and concrete intermediate piers or bents. 1.3 SCOPE OF THE STUDY The scope of the study was generally determined by the tasks identified in the RFP as the tasks anticipated to be encompassed by the research. The task description, copied from the RFP, is provided below.
5Task 1. Review relevant domestic and foreign practice, per- formance data, research findings, cost comparisons, and other information related to integral connections. This information shall be assembled from technical literature and from unpub- lished experiences of engineers, bridge owners, steel suppli- ers, fabricators, and others. Information on field performance is of particular interest. Task 2. Identify integral connection concepts that enhance bridge performance and economy. Task 3. Based on review and assessment of the findings in Task 1 as well as new concepts generated by the research team in Task 2, recommend concepts to be fully developed, validated, and detailed. Task 4. Prepare a detailed work plan for the remainder of the project that includes a description of the proposed processes for validating the effectiveness of the integral connection concepts. Estimates of the cost to evaluate each concept shall be provided. Task 5. Submit an interim report to document Tasks 1 through 4 for review by the NCHRP panel. Task 6. In accordance with the approved work plan, con- duct analytical and/or experimental work to validate the inte- gral connections approved by the project panel for further development. Task 7. Based upon the results of Task 6, further develop and finalize integral connections that will enhance performance, economy, and construction ease. Task 8. Develop recommended methodologies, specifications, commentary, and design examples. Task 9. Submit a final report describing the entire research project. Include the recommended methodologies, specifica- tions with commentary, the design examples, and the recom- mended integral connection details as appendices. In addition to the inherent limitations of the study tasks, as the work progressed, other limitations to the scope were dictated by the type of connection and type of substructures selected for further study and by the characteristics of the structures analyzed. The system selected for detailed stud- ies and laboratory testing consists of a straight (non-curved) I-girder superstructure connected rigidly to a steel box-beam pier cap that is, in turn, connected integrally to a reinforced concrete single-column pier. All studies assumed that the column is at mid-width of the bridge and that the bridge is not skewed. Studying this system implies that the following types of structures and structural components were not directly studied: ⢠Curved structures, ⢠Multi-column substructures, ⢠Single-column piers with the column not at mid-width of the bridge, ⢠Superstructure girders other than I-girders, ⢠Pier caps other than steel box-beam pier caps, and ⢠Skewed superstructures. Throughout the work, engineering judgment was used, to the extent possible, to extrapolate the research results to cover these types of structures that were not directly covered in the study. 1.4 RESEARCH APPROACH To accomplish the stated objectives of the research and to cover the work on the tasks of the project, the following approach was followed: ⢠To gather information on past use and performance of integral pier caps, a questionnaire was prepared and was sent to all AASHTO voting and nonvoting members (i.e., DOTs in all states and Canadian provinces, as well as other quasi-governmental authorities such as turnpike authorities). A slightly modified questionnaire was sent to domestic researchers and bridge designers and inter- national bridge designers. The response to the question- naire was used to study the state of the practice of inte- gral connections. In addition to the questionnaire, an extensive litera- ture search was performed to identify and review rele- vant past research. ⢠To identify integral connection concepts that are potential candidates for this study, the research team studied the Figure 1. Concrete pier cap connected integrally to steel I-girder superstructure.
connection concepts used in the past and developed sev- eral other systems. In total, 14 systems were examined. ⢠Selection criteria were developed to assist in selecting a system for detailed studies. The selection criteria were based on the expected economy, constructability, and expected performance of the 14 systems considered. Several practicing engineers participated in this process and the average of the scores was recorded. The system with the highest score was selected for further study and was approved by the project Technical Panel. ⢠Analytical and experimental studies were conducted to validate the system approved by the project technical panel for further development. A two-span bridge was selected as the basis for these studies. Throughout this report, this bridge is referred to as âthe prototype bridge.â The experimental studies were accomplished by testing two, one-third-scale models of the pier region of the proto- type bridge. The specimens were tested under low-level loads 6 to simulate the performance under simulated service condi- tions and under cyclic loads to simulate performance during seismic events. Load, strain, displacement, and rotation mea- surements were recorded throughout the tests. The analytical studies were conducted on finite element models of both the prototype bridge and the test specimens. The analytical studies of the prototype bridge were used to validate the applicability of some of the existing design pro- visions, which were originally developed for girders sup- ported on conventional bearings, to girders of structures with integral connections. The analytical studies on the test spec- imen computer model were used to determine the anticipated forces acting on different components of the laboratory spec- imens and to validate the modeling technique by comparing the analytical results with the laboratory results. The results of the analytical and experimental studies were used to finalize the details of the integral connections and to develop design methodologies, specifications, commen- tary, and a detailed design example based on the proposed specifications.
