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51 CHAPTER 4 CONCLUSIONS AND SUGGESTED RESEARCH CONCLUSIONS equations developed by Salmons and others (810, 13). Con- nections designed using these equations were found to have This research detailed studies on connections for precast/ adequate strength. The connections fail by the strands pulling prestressed concrete bridge girders made continuous for out of the concrete. In embedded connections, there is also a live load. From the research, the following conclusions can pull-out failure when the girder pulls out of the diaphragm. be drawn. Since the equations developed by Salmons and his coauthors A survey was conducted of state DOTs, fabricators, and appear to be adequate for design of the bent-strand connec- designers concerning connections for precast/prestressed con- tion, they have been placed in the commentary of the pro- crete bridge girders made continuous for live load. The results posed revisions to the AASHTO LRFD Bridge Design Spec- of the survey show the following: ifications, which is found in Appendix C. Bent-bar connections designed such that the embedment Of 51 respondents, 35 had used, fabricated, or designed of the bar into the girder and the embedment of the hooks into continuous-for-live-load bridges. the diaphragm meet the provisions of the AASHTO LRFD In all but one case, negative moment continuity was Specifications (12) have adequate strength. There was a con- established by the use of a reinforced concrete deck slab. cern that congestion in the diaphragm area might reduce the Positive moment connections are used with the capacity because of bar interactions, difficulty in consolidat- continuous-for-live-load bridges to control positive ing the concrete, or both. It was noted in the experiments that moments caused by creep and shrinkage of the precast bent-bar-type connections must have the bars placed asym- girders. In almost all cases, the positive moment connec- metrically with respect to the cross section to allow meshing tion is made by using bent-strand or bent-bar connections. of the bars. This asymmetry causes asymmetrical responses in Of those who used positive moment connections, 80% the connection. The connection fails by yield of the steel fol- also embedded the ends of the girders into the diaphragm. lowed by a pull-out failure in the diaphragm. The pull-out fail- While the respondents reported some problems with con- ure occurred in both embedded and nonembedded specimens. structability, all indicated that the problems were minor. Embedding the girder ends in the diaphragm appears to Limited cost data was obtained, and it showed that the reduce the stresses in the positive moment area; but, because costs of providing continuous-for-live-load connections this effect relies on the bond of a cold (or construction) joint, were insignificant. The highest cost was $200 per girder. the effect is difficult to quantify. Additional stirrups placed in the diaphragm, just outside Positive moment connections should be proportioned to of the girders, do not increase the strength of the connection. resist the moments caused by creep and shrinkage of the gird- However, these stirrups cross the diagonal cracks that form ers and deck slab, live load, and temperature effects. How- in the diaphragm when the final pull-out failure occurs, and ever, the results of an analytical study show that providing the stirrups increase the ductility of the connection after the positive moment connections with a capacity above 1.2 Mcr connection fails. (where Mcr is the positive cracking moment of the composite Bars placed horizontally through the webs of the girders cross section) is not efficient. It is suggested that if analysis increase both the strength and the ductility of the connection. shows the positive moment connection needs a capacity However, there is significant cracking in the girders at fail- above 1.2 Mcr, steps be taken to reduce the formation of pos- ure, and this may be undesirable. itive moments. The easiest way to do this is to specify a min- To limit tensile stresses in the diaphragm, some state DOTs imum age of the girders at the formation of continuity to pour part of the diaphragm before the deck slab is cast. It is allow some of the girder creep and shrinkage to occur before thought that the weight of the deck slab will cause the girder continuity is created. ends to rotate into the diaphragm and compress it. This method At present, there is no accepted method for designing the was found to be only marginally effective. However, it could bent-strand connection. In this study, the number of strands possibly be improved if a tension tie was provided at the top and the extended length of strand needed were found from of the girders.