Design of Concrete Bridge Beams Prestressed with CFRP Systems (2019) / Chapter Skim
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30 3.1 Introduction The parameters that influence the behavior and design of CFRP prestressed beams were investigated in experimental and analytical programs. The experimental program included testing of material samples, small-scale beams and prism, and full-scale beams to validate the existing models and to develop design guide specifications.
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Research Results 31 Type of Prestressing CFRP Type of Prestressing Prestressing CFRP Profile Type of Loading Number of Beams Beam ID CFRP Cable (C) Pretension (Pr)
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32 Design of Concrete Bridge Beams Prestressed with CFRP Systems to investigate the fully bonded case and the other three beams were unbonded. Unbonded posttensioning was only used for beams with CFRP cables.
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Research Results 33 combined tensile capacity of the reinforcement (the number of cables multiplied by the rupture load of one cable) was similar regardless of the type of prestressing CFRP.
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34 Design of Concrete Bridge Beams Prestressed with CFRP Systems with unbonded CFRP cables: one with a straight cable (CPouSF) and one with both straight and draped cables (CPouDF)
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Research Results 35 of the midspan. However, the cracks in the unbonded post-tensioned beams were concentrated in a few wide cracks near the constant moment region; these forked as the load was increased.
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36 Design of Concrete Bridge Beams Prestressed with CFRP Systems After preparation of the anchorage systems, the specimen was inserted inside a steel hollow structural section. During the test, the prestressing CFRP cable or bar was subjected to a sustained load under a constant strain condition in a self-reacting configuration as shown schematically in Figure 3.8.
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Research Results 37 The test results were used to develop expressions to calculate the stress relaxation of prestressing CFRP cable and bar systems (DfpR)
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38 Design of Concrete Bridge Beams Prestressed with CFRP Systems The anchorage losses (presented in Figure 3.11) were subtracted from the total stress relaxation of prestressing CFRP systems to obtain the stress relaxation of the prestressing CFRP cables and bars.
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Research Results 39 levels (0.5, 0.6, and 0.7 times the design tensile strength of the prestressing CFRP, fpu)
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40 Design of Concrete Bridge Beams Prestressed with CFRP Systems at three prestressing levels. As shown, high concrete creep and shrinkage rates occurred during the first 100 days after prestress transfer; creep and shrinkage strains became constant as time passed.
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Research Results 41 prestressing levels of 0.6 fpu and 0.7 fpu decreased by 0.0002 and 0.0003 in./in., respectively. There was also a reduction in the longitudinal strain of the prestressing CFRP bars inside the concrete prisms with initial prestressing of 0.6 fpu and 0.7 fpu indicating an average loss of the prestressing force of 30% to 40% of the jacking stress in prisms prestressed with CFRP bars.
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42 Design of Concrete Bridge Beams Prestressed with CFRP Systems prisms and plain concrete specimens were 6.64 × 10-6 and 6.80 × 10-6 (/°F) , respectively.
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Research Results 43 3.2.3 Harping Characteristics of the Prestressing CFRP The hold-down device geometry, harping angle, and type of prestressing CFRP were considered in this study. The radius of curvature of the prestressing CFRP was varied by changing the diameter of the hold-down point (deviators)
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44 Design of Concrete Bridge Beams Prestressed with CFRP Systems Premature failure initiated by splitting 20 in. diameter Harping device Figure 3.19.
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Research Results 45 a need to limit their application in prestressed concrete beams. However, the harping devices used in the tests provided a tensile capacity retention of more than 92% of the design tensile strength of prestressing CFRP cables for harping angles between 10° and 20°.
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46 Design of Concrete Bridge Beams Prestressed with CFRP Systems then calculated using the experimental transfer length for all test specimens according to Equation 3.10 (the average values were 1.0 and 1.3 for prestressing CFRP bars and cables, respectively)
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Research Results 47 and Table 3.5 shows a comparison of the ultimate loads and deflections; these indicate that the FEA estimates are in good agreement with the experimental results. More details are provided in Appendix E
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Experimental FEA ∆ ∆Beam ID Load, (kips) Deflection, ∆ (in.)
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Research Results 49 • Reducing the span-to-depth (a/d) ratio from 6.05 to 4.0 (1/3 reduction)
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50 Design of Concrete Bridge Beams Prestressed with CFRP Systems ID Parameters Peak Load (kips) Deflection (in.)
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Research Results 51 AASHTO LRFD (2017)
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52 Design of Concrete Bridge Beams Prestressed with CFRP Systems 3.5 Reliability Analysis A Monte Carlo Simulation approach was used to calibrate the strength reduction factor for CFRP prestressed beams failing due to the rupture of the CFRP tendons. A total of five bridges with different span lengths, roadway widths, girder positions, and number of girders were considered; these are listed in Table 3.8.
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Research Results 53 Section Type Span Length (ft.) Girder Spacing (ft.)
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54 Design of Concrete Bridge Beams Prestressed with CFRP Systems Variable Distribution Bias COV Reference Model error Normal 1.15 0.14 Current Study Height of deck Normal 1.00 0.03 Okeil et al.
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Research Results 55 = 4.0 = 0.8 Figure 3.28. Reliability index versus resistance factors (Type BT72)
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