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Steel fiber replacement of mild steel in prestressed concrete beams
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Steel fiber replacement of mild steel in prestressed concrete beams
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    Technical Report; September 2006 ¿ August 2010
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  • Abstract:
    In traditional prestressed concrete beams, longitudinal prestressed tendons serve to resist bending moment and

    transverse mild steel bars (or stirrups) are used to carry shear forces. However, traditional prestressed concrete I-beams

    exhibit early-age cracking and brittle shear failure at the end zones despite the use of a high percentage of stirrups (4.2%).

    Moreover, producing and placing stirrups require costly labor and time. To overcome these difficulties, it is proposed to

    replace the stirrups in prestressed concrete beams with steel fibers. This replacement concept was shown to be feasible in a

    TxDOT project (TxDOT project 0-4819) recently completed at the University of Houston.

    The replacement of stirrups by steel fibers in highway beams requires a set of shear design provisions and guidelines

    for prestressed Steel Fiber Concrete (PSFC) beams. The development of rational shear provisions with wide applications

    must be guided by a mechanics-based shear theory and must be validated by experimental tests on I- and box-beams. A

    rational shear theory, called the Softened Membrane Model (SMM), has been developed at the University of Houston for

    reinforced concrete beams. This theory satisfies Navier’s three principles of mechanics of materials, namely, stress

    equilibrium, strain compatibility and the constitutive relationship between stress and strain for the materials.

    The first phase of the research consisted of testing 10 full-size prestressed PSFC panels. This was done to establish the

    effect of fiber factor and the level of prestress on the constitutive models of steel fiber concrete and prestressing tendons.

    From this data a set of constitutive models was developed to predict the behavior of prestressed PSFC. Notable findings

    include the fact that increasing steel fiber content has a beneficial effect on the softening properties of prestressed PSFC.

    Additionally, the findings show that increasing steel fiber content increases tension stiffening in prestressed PSFC under

    tensile loading.

    The second phase of this research project generalizes the SMM shear theory for application to prestressed PSFC beams.

    This was achieved by feeding the new constitutive models of fiber concrete and prestressing tendons into a finite element

    program (OpenSees). The accuracy of the new shear theory was evaluated by testing full-size prestressed PSFC I- and

    box-beams that fail in shear modes. The developed finite element program was used to simulate the shear behavior of the

    beams with acceptable accuracy. Finally, a design equation and recommendations were provided for use when designing

    PSFC beams. Using the design equations, a series of four design examples, was also provided.

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