Browsing by Subject "Prestressed"

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    Anchorage of shear reinforcement in prestressed concrete bridge girders
    (2014-06) Mathys, Brian Thomas
    The Minnesota Department of Transportation has typically used epoxy coated straight legged stirrups anchored in the tension zone as transverse reinforcement in prestressed concrete bridge girders. With the straight legs of the U-shaped stirrups anchored into the bottom flange of the girders, this configuration is readily placed after stressing the prestressing strands. American Concrete Institute (ACI) and American Association of State Highway and Transportation Officials (AASHTO) specifications require stirrups with bent legs that encompass the longitudinal reinforcement to properly anchor the stirrups. Such a configuration is specified to provide mechanical anchorage to the stirrup, ensuring that it will be able to develop its yield strength with a short anchorage length to resist shear within the web of the girder. AASHTO specifications for anchoring transverse reinforcement are the same for reinforced and prestressed concrete; however, in the case of prestressed concrete bridge girders, there are a number of differences that serve to enhance the anchorage of the transverse reinforcement, thereby enabling the straight bar detail. These include the precompression in the bottom flange of the girder in regions of web-shear cracking. In addition, the stirrup legs are usually embedded within a bottom flange that contains longitudinal strands outside of the stirrups. The increased concrete cover over the stirrups provided by the bottom flange and the resistance to vertical splitting cracks along the legs of the stirrups provided by the longitudinal prestressing reinforcement outside of the stirrups help to enhance the straight-legged anchorage in both regions of web-shear cracking and flexure-shear cracking. A two-phase experimental program was conducted to investigate the anchorage of straight legged epoxy coated stirrups that included bar pullout tests performed on 13 subassemblage specimens which represented the bottom flanges of prestressed concrete girders in a number of configurations to determine the effectiveness of straight legged stirrup anchorage in developing yield strains. Additionally, four girder ends were cast with straight legged stirrup anchorage details and tested in flexure-shear and web-shear. The straight leg stirrup anchorage detail was determined to be acceptable for Minnesota Department of Transportation M and MN shaped girders as nominal shear capacities were exceeded and yield strains were measured in the stirrups prior to failure during each of the tests.
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    Experimental Evaluation of Post-Cracking Deflection Behavior in Prestressed Concrete Beams
    (2022-08) Kulzer, Alex
    Precast prestressed concrete beams are more efficient and effective than the traditional reinforced concrete beam, but their performance is more difficult to predict. This experimental thesis was run to quantitatively compare deflections between laboratory precast prestressed concrete beams to ACI 318-19 and PCI Design Handbook (8th edition) design methods. A full design and testing procedure is detailed for 12 rectangular beams with varying levels of partial prestressing to compare the effects of mild steel in prestressed beams. The rectangular beams are designed with uncracked, transition, and cracked sections at a constant service load. Testing procedures are detailed for full sized, typical double tee sections designed by Metromont. This research analyzed and commented on the performance of the different rectangular beams and double tees in relation to calculated deflection prediction curves. Additional comments are made on the cracking of the rectangular beams in relation to previous studies. Results from the rectangular beams deflections show a consistent low prediction in initial stiffness, varying results at service, and higher inaccuracy in post cracking predictions with increasing mild steel when using the effective moment of inertia and bilinear methods for predicting deflections. Cracking patterns show similar results from previous studies and vary when mild steel is introduced. Findings from this research conclude that the current methods for predicting deflections are not reliable in all situations.