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46 CHAPTER 3 INTERPRETATION, APPRAISAL, AND APPLICATION CAPACITY OF CONNECTION DETAILS fere with the formwork. The bars are installed straight and then bent later. This is a labor-intensive operation, and the The results of the experimental work answered several resulting hooks are usually not uniform, making it difficult to questions about details for positive moment connections. install anchor bars in the corners of the hooks. The uneven This work examined both bent-strand and bent-bar type con- hooks may also cause uneven stresses in the bars. One possi- nections. Connections using embedded girder ends, addi- ble solution is to make 180 bends in the bars (see Figure 70). tional stirrups, and horizontal web bars were also examined. Another concern with bent-bar connections is that the bar The survey results showed that a bent-strand type of con- pattern must usually be asymmetrical to allow for the mesh- nection was used in many states, but the AASHTO LRFD ing of the bars. This results in asymmetrical behavior of the Specifications and Standard Specifications do not provide a connection, asymmetrical stresses in the bars, and asymmet- means of determining the strand length needed for this con- rical crack openings. Some states avoid this problem by using nection (12, 15). A set of equations from research done for a wide diaphragm and not meshing the bars. the Missouri DOT was found (810, 13) and was used to Assembly of the bent-bar connection was slightly more dif- detail the bent-strand connections in this study. The bent- ficult than for bent-strand connection. The strands are flexible strand connection, designed using the proposed equations, and can be easily moved by hand from side to side during had adequate capacity to resist positive moment. The con- assembly. This is not true of the bars. However, neither type nections were easy to fabricate and to erect. of connection required extraordinary effort to assemble. Using the bent strand connection may also help to reduce Embedment of the end of the girders into the diaphragm did congestion at the end of the girder. The AASHTO LRFD seem to reduce the stress in the connection and, in general, the Specifications require a check on the amount of longitudinal embedded connections had a higher number of cycles to fail- reinforcement at the end of concrete beams (12). The contri- ure. However, this effect may be difficult to quantify. The bution of the prestressing strand is reduced if there is an inad- reduction in stress is due to the weak chemical bond between equate development length. As a result, there is often a need the girder and diaphragm concrete and/or frictional effects as for additional steel in the already congested end region of the the girder tries to pull out. The magnitudes of these types of girder. The extended bent strand clearly provides anchorage stresses vary widely and may be difficult to assess. It is prob- into the diaphragm. For the extended bent strand, the pull-out ably best to ignore any embedment effects in design and to stress should be the sum of the pull-out stress from the devel- allow the embedment to provide an additional, although vari- opment length at the end of the girder plus the pull-out stress able, factor of safety. from the bent strand as calculated by the given equations. The placement of additional stirrups in the diaphragm just The bent-bar connections were designed using the provi- outside of the girder bottom flange (see Figure 22) does not sions of the AASHTO Standard Specifications for hooked increase strength. In fact, prior to pull-out of the connection, a bars (15). These provisions are nearly identical to the AASHTO vibrating wire strain gage placed in the diaphragm but outside LRFD Specification provisions for hooked bars (12), so the of the area between the girder ends (see Figure 22) showed results are applicable to either specification. The connections almost no response. This indicates that there is no stress in the were capable of developing the nominal moment capacity. In diaphragm outside of the cross section of the girder; therefore, the case of the bent bars, there was some concern that con- it is not surprising that steel placed in the diaphragm out- gestion in the diaphragm region might reduce the capacity side of the area between the girder ends does not affect behav- since it is well known that there are interaction effects when ior before pull-out. However, after the connection pulls out, bars are in proximity to each other; however, this was not the additional stirrups arrest the diagonal cracks that form in found to be the case. the diaphragm and provide additional ductility (see Figure 23). The bent-bar connections were found to be more difficult The specimen using the additional stirrups had the girder ends to construct. In flanged members such as bulb-T or I sections, embedded in the diaphragm, and the experimental evidence the tails of the hooked bars extend beyond the top of the bot- suggests that embedded ends are needed for the additional stir- tom flange. Pre-bent bars cannot be used because they inter- rups to provide the ductility. The large diagonal cracks the