Development Length for Headed Bars in Slab-Column Joints of RC Slab Bridges
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Development Length for Headed Bars in Slab-Column Joints of RC Slab Bridges

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    Final Report
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    In accordance with the Caltrans Seismic Design Criteria, the superstructure in a slab bridge should remain essentially elastic and only the pile extensions/columns are permitted to develop inelastic deformations during a seismic event. Hence, the longitudinal reinforcement extending from a pile extension must have sufficient embedment length in the slab to develop the full tensile strength of the bars. The use of headed deformed bars can significantly reduce the required embedment length and also avoid congestion that could be introduced by hooked bars. According to ACI 318, for 5,000-psi concrete and Grade-60 bars, a minimum development length of 14 times the bar diameter ( b d ) is required for headed bars. This may increase the cost of slab bridges because it may call for a thicker slab to accommodate the development length. However, the ACI specification does not take into account of the benefits of vertical stirrups present in the slab-column joint region of a slab bridge. The main objective of the research presented in this report is to determine the minimum development length required for headed bars in the slab-column joint region of a slab bridge designed according to MDT 20-7 (October 2014) of Caltrans, and to investigate if the development length can be reduced to 10 b d . To this end, three full-scale slab-column assemblies were tested. The specimens had a 2-ft. diameter, 12-ft. tall cast-in-place RC column and a 16-in. thick slab. The columns were connected to the slabs with headed bars. Specimen #1 had an embedment length of 9.8 b d for the headed bars, while Specimens #2 and #3 had embedment lengths of 8.7 b d and 11 b d , respectively. Specimen #3 had a drop cap in the slab to accommodate the longer development length. Grade-60 steel was used and the concrete had a target compressive strength of 5,000 psi. The study also includes nonlinear finite element analyses of slab-column assemblies and a numerical parametric study to evaluate design variables not covered in the experimental program. This study has shown that for slab concrete with an expected compressive strength of 4.5 ksi and Grade-60 steel, an embedment length of 11 b d is adequate for headed bars in slab-column joints designed according to MTD 20-7 (Caltrans, October 2014). Based on the test data, it is recommended that MTD 20-7 be modified to include four additional stirrups adjacent to the column cage, as it was done for Specimens #2 and #3. Furthermore, the amount of vertical stirrups in the 2nd row and farther away from the column cage can be reduced. The bar heads should be below the top mat of reinforcement in the deck slab. Embedment lengths of 8.7 b d and 9.8 b d were able to develop the moment capacity of the columns but resulted in moderate to severe punching cracks in the cover concrete of the slabs and more pinched lateral column force-vs.-column displacement hysteresis curves. However, a finite element analysis has shown that if there is not enough room to provide an embedment length of 11 b d in a 16-in. slab, it may be possible to reduce the embedment length and increase the distance of the bar head from the slab surface to reduce or avoid punching damage. This has to be confirmed by an experimental study.
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