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1 CHAPTER 1 INTRODUCTION AND RESEARCH APPROACH PROBLEM STATEMENT caused by live loads. The authors of NCHRP Report 322 AND RESEARCH OBJECTIVES concluded that positive moment connections were costly and provided no structural benefit. The later conclusion is based Many states use continuous-for-live-load prestressed/ on the fact that the positive moment connection restrains the precast concrete bridges. These bridges are built by first plac- girder ends. The designer must account for the effects of ing the precast/prestressed girders on the abutments and then these restraint moments by adding them to the effects of the casting a composite deck. Diaphragms are usually placed live-load moments. It was concluded that the maximum pos- between the girder ends. Since the girders are not connected itive moment in a span was virtually the same whether it was until the deck and diaphragms harden, the girders behave as designed as a simple span or as a continuous span with both simple spans for girder and slab dead load. After the concrete live-load and restraint moments. deck and diaphragms harden, they connect the girders together However, these conclusions were not universally accepted. and make the entire structure continuous for any additional Many engineers thought that there was still an advantage to dead load and all live loads. Reinforcing bar placed in the making the bridge continuous and that positive moment con- deck over the connection between the girders provides the negative moment continuity. nections were needed for continuity. These connections were Early studies on this type of construction by the Portland assumed to control cracking at the diaphragms. Cement Association (PCA) showed that using a reinforced There were also lingering questions about overall connec- deck was an adequate connection for resisting negative tion. For the bent-strand connection, there was no accepted moments over the piers and for shear (17 ). However, these design method for determining the length of the bent strand studies also showed that cracking occurred in the diaphragm. and the number of strands needed. For both the bent-strand The cause of this cracking was positive moment, which devel- and bent-bar connections, there was concern over congestion oped from time-dependent deformations of the prestressed in the diaphragm area. Many details called for several bars or girders. It was recommended that a positive moment con- strands extending from the ends of the girders and for the nection be made between the bottom of the girders and the bars or strands of longitudinally adjacent girders to be meshed diaphragms. in the diaphragm area. This placed a large number of bars in Two basic types of positive moment connections were a small area, often without adequate clearance between the developed. The first was a bent bar connection, which was bars or strands. There were concerns that this congestion based on the PCA studies (17 ). In this connection, hooked, would limit the capacity of the connections due to bar inter- mild reinforcing bars are embedded in the end of the precast actions and a possible inability to properly consolidate the girder (see Figure 1). The hooks are then embedded in the concrete in the diaphragm. diaphragm. The second type of connection is the bent-strand Some questioned whether cracking at the girder-diaphragm connection. In this type of connection, a predetermined interface affected the continuity of the system. Some assumed length of prestressing strand is left protruding from the end that if the girder-diaphragm interface cracked, the joint would of the girder when the girder is detensioned. This strand is act like a hinge or a rotational spring. This would limit the then bent into a 90° hook, and the hook is embedded in the system's ability to transfer load across the joint and to reduce diaphragm (see Figure 2). The Missouri DOT published an or eliminate continuity. investigation of this type of connection in the 1970s (8, 9, The goals of this research are (1) to determine how 10), the results of which are discussed in Chapter 2. continuous-for-live-load connections are used in the vari- In 1989, the National Cooperative Highway Research ous states; (2) to experimentally determine capacities and Program (NCHRP) published NCHRP Report 322 (11), behaviors of some typical connection details through test- which studied the forces that were likely to occur on posi- ing; (3) to develop design methods and suggested changes to tive moment connections. As part of this study, the authors the AASHTO Load Resistance Factor Design (LRFD) Spec- developed two computer programs to evaluate the positive ification (12) for making simple-span bridges continuous for moments caused by time-dependent effects and the moments live load.
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2 Figure 1. Bent-bar positive moment connection. Figure 2. Bent-strand positive moment connection.