Jorge Márquez Balderrama
Published: 2015
Total Pages: 378
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The 2011 PCI Bridge Design Manual provides preliminary design charts for selecting the girder size and number of prestressing strands for a given span length and beam spacing but only for [small leter f with hook]ʹ[subscript c] = 8,000 psi (55.2 MPa). This single strength limits the use of the charts, particularly for states considering ultra-high performance concrete (UHPC). Accordingly this dissertation presents a simplified procedure to develop preliminary design charts for prestressed concrete bulb-tee girders considering service load stress limits, flexural strength and stresses at release. The results for a BT-72 beam are first compared with the 2003 PCI design charts originally developed based on the AASHTO Standard Specifications. The procedure is then adapted to the AASHTO LRFD Bridge Design Specifications and verified with the prevailing 2011 PCI design charts. Finally, new LRFD charts are generated for NSC, HPC, and UHPC with 0.5, 0.6, and 0.7-in. (13, 15 and 18 mm) strands for simple and two-span continuous bridges to illustrate the simplified procedure and potential impact of UHPC, larger strand size, and continuity on bridge girders. The new LRFD charts are shown to be accurate for the design assumptions made since an excellent agreement (within 2% and 4%) resulted between the preliminary design charts developed in this study and those given in the 2003 and 2011-PCI Bridge Design Manuals. The "transition point" is identified which provides the information needed for a designer to distinguish the zones between fully prestressed (uncracked), partially prestressed, and non-prestressed (cracked) members. The preliminary design charts demonstrate the effect of using UHPC and/or larger strand size and/or two-span continuous layouts. The effect of implementing continuity with the combination of UHPC and a larger strand diameter was shown to be much more significant than just increasing the concrete compressive strength or the strand diameter or using two-span continuous layouts. However, the use of longer full-span girders poses significant challenges for fabrication, transportation, erection, span-to-depth ratios, and live and dead load deflections of prestressed concrete bridges and, consequently, should be considered carefully for the final design of the bridge.