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142 4.1 Conclusions A full-depth precast concrete deck panel system was developed. The system includes an innovative girderâdeck connection spaced up to 6 ft, compared to the current limit of 4 ft. It also includes other innovative features, such as the use of UHPC for the joint between the girder and the deck. UHPC is used for the transverse joint between panels if no longitudinal post-tensioning is provided. If post-tensioning is provided, a simplified duct-in-duct post- tensioning is proposed to eliminate the difficult field-splicing of the ducts. The shear pockets exist only in the transverse joints between the panels if the panels are made 6-ft long. If the panels are 12-ft long, only one intermediate pocket would be required at each girder line. The deck slab can be solid, or it can be ribbed to reduce its weight and, thus, increase allowance for additional loads in situations where such an upgrade is desirable. The interface shear joints (i.e., pockets) are filled with UHPC grout, while the haunches between the shear joints in the space between the beam top face and the deck soffit may be left ungrouted. This is called a âdiscrete jointâ system, which would reduce the labor and materials needed to fill the haunch. Also, blind grouting of the haunchâand associated questionable qualityâwould be eliminated. All analysis and testing are performed on the option with discrete joints. Owners who choose to grout the haunches would thus have a more conservative system, as filled haunches increase the systemâs stiffness. Because the haunches are not counted on for structural loading, a secondary but important conclusion is to convince designers that a simple, relatively low-strength, flowable fill would be adequate for the haunches. UHPC was selected as the recommended material for filling the shear connection joints and the transverse joints if no post-tensioning is used. The size of the shear pockets, as well as the shear key at the transverse joints between panels, were optimized to minimize the volume of the UHPC used to fill these spaces. The analytical investigation conducted in the project has demonstrated the viability of increasing the shear connection spacing up to 6 ft without significant change in behavior. For two-way decks that are only supported at discrete joints along the girder lines, design aids are developed in this report to facilitate design of the reinforcementâespecially in the longitudinal direction. In addition, the analytical investigation has shown that behavior of the beam in the longitudinal direction can be reasonably predicted using the conventional EulerâBernoulli Beam Theory. Three-dimensional nonlinear finite element analysis has proved to be a valuable tool for investigating the stress concentration around the shear connectors. In addition, it provides insights into how forces are distributed among the shear connectors in the joint. In this project, the finite element analysis indicates that the quality of the grouting material of the shear C H A P T E R 4 Conclusions and Recommendations for Future Research
Conclusions and Recommendations for Future Research 143 connection joints is the most critical factor in ensuring adequate structural capacity. Therefore, the research team concluded that UHPC is the best available joint material. The experimental investigation conducted with concrete girders in the project has shown that full composite action is expected in flexure design at all Service and Strength Limit States. In addition, full composite action is expected in vertical shear design even when the spacing between the shear connectors exceeds the girder depth. The horizontal shear nominal capacity of the proposed novel connection for concrete girder to concrete deck, shown in Figure 4.1, at a spacing up to 6 ft is 310 kip. This capacity is higher than the demand for most prestressed concrete girder bridges, as shown in the design examples. This capacity is proposed for the unique connection being proposed in this research, as demonstrated in the examples. The experimental investigation conducted with steel girders in the project has shown that fatigue load has no detrimental effect on the composite action of the slabâbeam system made with joints spaced at 6 ft. Therefore, no changes are proposed for the fatigue design of the shear connectors given in Article 188.8.131.52 of the AASHTO LRFD Bridge Design Specifications. Full composite action is expected at all Service and Strength Limit States. No reduction of the full composite beam stiffness is warranted, except for deflection calculation where a 75% reduction factor should be applied to the full composite beam stiffness. The strength tests on the push-off specimens and large-scale beam have shown that a 0.72 group reduction factor is proposed to be applied to the stud strength (i.e., Equation 184.108.40.206.3-1 of the AASHTO LRFD Bridge Design Specifications). This group reduction factor is considered conservative because failure in these tests was observed in the deck panels, rather than in the UHPC joints. Future testing may provide a more accurate estimate of this factor. The research team has determined that construction can be significantly accelerated when all grouting of pockets, as well as transverse joints, is done in a single operation. This would imply that both the deck and the beam share in the post-tensioning force applied after grouting. Figure 4.1. Novel connection for concrete girder to concrete deck.
144 Simplified Full-Depth Precast Concrete Deck Panel Systems Analysis has indicated that the average loss of prestress (from the deck to the beam) is in the range of 2% to 20% for simple span bridges. However, this significant improvement was not found to be true for continuous spans, as the post-tensioning secondary moments may be largeâor even largerâthan the primary moments. It is possible to have solutions that allow post-tensioning after grouting, even for continuous spans. However, the solutions are not direct and cannot be generalized at this time for all practical cases. Guidelines are provided to help engineers in implementing this system for highway bridges. A draft of proposed revisions to the AASHTO LRFD Bridge Design Specifications provisions is also presented. 4.2 Recommendations for Future Research The recommendation of introducing a 0.72 group reduction factor to Equation 220.127.116.11.3-1 of the AASHTO LRFD Bridge Design Specifications is conservative and can be used in practice at this time until further research justifies raising this factor. The 0.72 proposed factor was influenced by the relatively low capacity observed in the push-off specimens. However, it is well known that push-off specimens almost always yield low capacity, compared to full beams. It is recommended that this factor be reevaluated using large-scale composite beam specimens. The research team recommends that a parametric study be performed to find practical solutions for application of post-tensioning in continuous span bridges. Such study could propose a new way of determining tendon profiles that pass through the beams, instead of the conventional method of placing the post-tensioned tendons at mid thickness of the deck. These design complications may be justified by the significant shortening of construction time when one stage of grouting in the field is eliminated.