National Academies Press: OpenBook

Connection of Simple-Span Precast Concrete Girders for Continuity (2004)

Chapter: Chapter 1 - Introduction and Research Approach

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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2004. Connection of Simple-Span Precast Concrete Girders for Continuity. Washington, DC: The National Academies Press. doi: 10.17226/13746.
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2004. Connection of Simple-Span Precast Concrete Girders for Continuity. Washington, DC: The National Academies Press. doi: 10.17226/13746.
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2004. Connection of Simple-Span Precast Concrete Girders for Continuity. Washington, DC: The National Academies Press. doi: 10.17226/13746.
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1CHAPTER 1 INTRODUCTION AND RESEARCH APPROACH PROBLEM STATEMENT AND RESEARCH OBJECTIVES Many states use continuous-for-live-load prestressed/ precast concrete bridges. These bridges are built by first plac- ing the precast/prestressed girders on the abutments and then casting a composite deck. Diaphragms are usually placed between the girder ends. Since the girders are not connected until the deck and diaphragms harden, the girders behave as simple spans for girder and slab dead load. After the concrete deck and diaphragms harden, they connect the girders together and make the entire structure continuous for any additional dead load and all live loads. Reinforcing bar placed in the deck over the connection between the girders provides the negative moment continuity. Early studies on this type of construction by the Portland Cement Association (PCA) showed that using a reinforced deck was an adequate connection for resisting negative moments over the piers and for shear (1–7). However, these studies also showed that cracking occurred in the diaphragm. The cause of this cracking was positive moment, which devel- oped from time-dependent deformations of the prestressed girders. It was recommended that a positive moment con- nection be made between the bottom of the girders and the diaphragms. Two basic types of positive moment connections were developed. The first was a bent bar connection, which was based on the PCA studies (1–7 ). In this connection, hooked, mild reinforcing bars are embedded in the end of the precast girder (see Figure 1). The hooks are then embedded in the diaphragm. The second type of connection is the bent-strand connection. In this type of connection, a predetermined length of prestressing strand is left protruding from the end of the girder when the girder is detensioned. This strand is then bent into a 90° hook, and the hook is embedded in the diaphragm (see Figure 2). The Missouri DOT published an investigation of this type of connection in the 1970s (8, 9, 10), the results of which are discussed in Chapter 2. In 1989, the National Cooperative Highway Research Program (NCHRP) published NCHRP Report 322 (11), which studied the forces that were likely to occur on posi- tive moment connections. As part of this study, the authors developed two computer programs to evaluate the positive moments caused by time-dependent effects and the moments caused by live loads. The authors of NCHRP Report 322 concluded that positive moment connections were costly and provided no structural benefit. The later conclusion is based on the fact that the positive moment connection restrains the girder ends. The designer must account for the effects of these restraint moments by adding them to the effects of the live-load moments. It was concluded that the maximum pos- itive moment in a span was virtually the same whether it was designed as a simple span or as a continuous span with both live-load and restraint moments. However, these conclusions were not universally accepted. Many engineers thought that there was still an advantage to making the bridge continuous and that positive moment con- nections were needed for continuity. These connections were assumed to control cracking at the diaphragms. There were also lingering questions about overall connec- tion. For the bent-strand connection, there was no accepted design method for determining the length of the bent strand and the number of strands needed. For both the bent-strand and bent-bar connections, there was concern over congestion in the diaphragm area. Many details called for several bars or strands extending from the ends of the girders and for the bars or strands of longitudinally adjacent girders to be meshed in the diaphragm area. This placed a large number of bars in a small area, often without adequate clearance between the bars or strands. There were concerns that this congestion would limit the capacity of the connections due to bar inter- actions and a possible inability to properly consolidate the concrete in the diaphragm. Some questioned whether cracking at the girder-diaphragm interface affected the continuity of the system. Some assumed that if the girder-diaphragm interface cracked, the joint would act like a hinge or a rotational spring. This would limit the system’s ability to transfer load across the joint and to reduce or eliminate continuity. The goals of this research are (1) to determine how continuous-for-live-load connections are used in the vari- ous states; (2) to experimentally determine capacities and behaviors of some typical connection details through test- ing; (3) to develop design methods and suggested changes to the AASHTO Load Resistance Factor Design (LRFD) Spec- ification (12) for making simple-span bridges continuous for live load.

2Figure 1. Bent-bar positive moment connection. Figure 2. Bent-strand positive moment connection.

OBJECTIVE OF THE STUDY The objective of the study, as stated by the project panel, was to recommend details and specifications for the design of durable and constructable connections that achieve structural continuity between simple-span precast/prestressed concrete girders. The specifications developed should be suitable for consideration by the AASHTO Highway Subcommittee on Bridges and Structures (HSCOBS). RESEARCH APPROACH This project was divided into eight tasks. Task 1: Review of Existing Data Task 1 consisted of reviewing the existing data on precast/ prestressed bridges made continuous for live load. Part of this task was accomplished with a literature search. The remainder was done using surveys of state DOTs, designers, fabricators, and contractors. The surveys focused on connection details, materials used, constructability issues, and reported problems. Task 2: Propose Connections To Be Tested Using the results of Task 1, it was determined that the typi- cal connection details could be divided into four broad cate- gories: (1) bent strand with the ends of the girders not embed- ded in the diaphragm, (2) bent bar with the ends of the girders not embedded in the diaphragm, (3) bent strand with the ends of the girders embedded into the diaphragm, and (4) bent bar with the ends of the girders embedded into the diaphragm. Some states used horizontal bars through the girder web for additional strength. There was also a suggestion that additional stirrups in the diaphragm, but outside of the girder bottom flange, would strengthen the connection. These data were used to propose six types of connection details for testing. Task 3: Prepare a Work Plan A work plan was developed which centered on testing six connection details for capacity and two full-size speci- mens. The capacity specimens consisted of two short (or stub) AASHTO Type II girders with a slab, connected to a diaphragm. The connections were loaded to the anticipated design load and then subjected to fatigue. The full-size spec- 3 imens simulated a single beam line in a two-span bridge. This specimen used Type III girders. In preparing this work plan, an analysis program, RESTRAINT, was developed. Task 4: Prepare an Interim Report Task 4 was a report summarizing Tasks 1–3. Task 5: Experimentally Validate the Connection Details In Task 5, the connection details were tested in accordance with the work plan. Task 6: Develop Connection Details, Recommended Specifications, Commentary, and Design Examples For Task 6, the pertinent sections of the AASHTO LRFD Specifications were reviewed and changes were recommended based on the results of Tasks 1–5. Task 7: Seismic Issues Using available literature and some limited data from the experiments, a discussion of seismic issues was prepared. Task 8: Prepare the Final Report The findings of Tasks 1–7 are summarized in this report. The results of the survey done under Task 1 are found in Chap- ter 2. Chapter 2 also contains the proposed connection details that were tested, details of the experimental work plan, and the experimental results (Tasks 2, 3, and 5). Chapter 3 discusses the results and interpretation of data and addressed the seismic issues (Task 7). Chapter 4 contains conclusions and recom- mendations for future research. The RESTRAINT spreadsheet program, developed under Task 3, is Appendix A and is avail- able upon request from NCHRP. Appendix B provides details of the experimental program (Task 5). The proposed changes to the AASHTO LRFD Specifications and Design Examples (Task 6) are contained in Appendixes C and D, respectively. Appendix E is the summary data from the project (in elec- tronic form) and is also available (as a diskette) upon request from NCHRP.

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 519: Connection of Simple-Span Precast Concrete Girders for Continuity includes recommended details and specifications for the design of continuity connections for precast concrete girders. Also included in the report are examples illustrating the design of four precast girder types made continuous for live load.

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