Strand Debonding for Pretensioned Girders (2017) / Chapter Skim
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From page 52... ...
52 3.1 Research Approach The effects of strand debonding on the performance of prestressed girders were examined experimentally through design, fabrication, and testing of six girders. The data obtained at prestress release and at various stages of load testing were utilized for this purpose.
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experimental research approach, Findings, and associated analytical Simulations 53 Table 3.1. Key details of the test specimens.
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54 Strand Debonding for pretensioned Girders 3.4 Transfer Length During fabrication of the girders, five vibrating wire strain gages were placed in the concrete near the centroid of the strands at each end of each girder. The strains measured by these gages were used to assess the in situ transfer lengths, which were compared with computed values.
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experimental research approach, Findings, and associated analytical Simulations 55 Eq. 3.2, , , , f P A Pe y y I M y y I c transformed e transformed transformed c transformed transformed sw c transformed transformed ( )
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56 Strand Debonding for pretensioned Girders length was taken either as 60db (db = strand diameter) per AASHTO LRFD Bridge Design Specifications or the value obtained from Eq.
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experimental research approach, Findings, and associated analytical Simulations 57 Figure 3.3. Measured and computed longitudinal concrete strains at soffit.
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58 Strand Debonding for pretensioned Girders Figure 3.3. (Continued)
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experimental research approach, Findings, and associated analytical Simulations 59 for some of the girders, the calculations based on the current AASHTO transfer length correlate better with experimental data, whereas the transfer length recommended in NCHRP Report 603 (Ramirez and Russell 2008) yields more accurate results for some other cases.
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Figure 3.4. Loading arrangements.
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experimental research approach, Findings, and associated analytical Simulations 61 3.5.2 Instrumentation During fabrication of the girders, a number of electrical resistance strain gages were bonded to transverse and longitudinal reinforcing bars. Moreover, a number of electrical resistance strain gages were bonded to the second-layer strands before casting AASHTO BT-54, AASHTO Type III-a, and AASHTO Type III-b.
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62 Strand Debonding for pretensioned Girders of the strands at each end. The locations and numbers of strain gages are summarized in Appendix G
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experimental research approach, Findings, and associated analytical Simulations 63 on the overall stiffness of the girders. This observation should be expected, as the relatively small area of prestressing reinforcement does not affect the stiffness; and debonding, which is localized near the girder ends, has little or no effect on deflection.
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64 Strand Debonding for pretensioned Girders Table 3.7. Comparison of normalized peak load and deflection at peak load.
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experimental research approach, Findings, and associated analytical Simulations 65 Failure mode: Shear compression End A Failure mode: Shear compression End B (c) AASHTO Type III-a Failure mode: Shear tension End A Failure mode: "Sliding shear" at the web-flange interface End B (b)
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Figure 3.8. (Continued)
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experimental research approach, Findings, and associated analytical Simulations 67 Figure 3.10. Photo collages of crack patterns.
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68 Strand Debonding for pretensioned Girders North face South face Max.
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experimental research approach, Findings, and associated analytical Simulations 69 North face South face Max.
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70 Strand Debonding for pretensioned Girders As evident from Table 3.8, the average crack angles were essentially the same for the two ends of a single girder having different debonding ratios. The crack widths at End A, which had a larger debonding ratio than End B, were generally slightly wider than those at End B
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experimental research approach, Findings, and associated analytical Simulations 71 (a)
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72 Strand Debonding for pretensioned Girders For AASHTO BT-54 and Texas U-40, the R-O model in Figure 3.12(b) was used while the stress-strain relationships for the transverse steel in the other girders were characterized based on the elastic-plastic model shown in Figure 3.12(a)
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Figure 3.13. Concrete shear resistance vs.
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74 Strand Debonding for pretensioned Girders strain and l = the length over which the strand is debonded. The slip measured at the end of the girder on debonded strands will, therefore, be greater than the actual slip exhibited at the beginning of strand embedment (a distance l into the girder)
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Figure 3.15. Normalized shear-apparent slip relationships.
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76 Strand Debonding for pretensioned Girders 7. Comparing the measured slip of the End A and End B strands having unbonded lengths of 3 ft (the only strands available for such comparison)
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78 Strand Debonding for pretensioned Girders crack patterns and angle of cracks of the two girder ends, with different magnitudes of drs, were found to be small. The results are consistent with the hypothesis that bonded strand and nonprestressed tension reinforcement work together to resist longitudinal forces induced by shear (i.e., those calculated using AASHTO LRFD Eq.
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experimental research approach, Findings, and associated analytical Simulations 79 Table 3.12. FEM material properties.
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80 Strand Debonding for pretensioned Girders Figure 3.17. Measured vs.
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experimental research approach, Findings, and associated analytical Simulations 81 (d)
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82 Strand Debonding for pretensioned Girders (a)
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experimental research approach, Findings, and associated analytical Simulations 83 (d)
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84 Strand Debonding for pretensioned Girders Figure 3.19. Comparison of measured and computed longitudinal strains at release.
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experimental research approach, Findings, and associated analytical Simulations 85 Figure 3.19. (Continued)
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86 Strand Debonding for pretensioned Girders are shown along the girder centerline (strand location 1 shown in Figure 3.20) and near the web at strand location 14 (Figure 3.20)
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experimental research approach, Findings, and associated analytical Simulations 87 Figure 3.23. Stress in confinement reinforcement.
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88 Strand Debonding for pretensioned Girders H/4 + Lbearing. Figure 3.24 illustrates the relationship between the resulting stress (normalized with respect to the yield strength)
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experimental research approach, Findings, and associated analytical Simulations 89 bearing. An example is shown in Figure 3.25 showing the 36.8 in.
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90 Strand Debonding for pretensioned Girders Vcw and Vci was eliminated. Consequently, the need to determine whether debonding has any influence on the calculation of Vcw will no longer exist.
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experimental research approach, Findings, and associated analytical Simulations 91 4. VL and ML were calculated from the applied load at the time the first diagonal crack was visually observed in the experiment.
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92 Strand Debonding for pretensioned Girders for the BI-36 appear to be an anomaly and will be discussed later. Assuming the anomaly of the BI-36 can be explained, the data would indicate that the proposed changes to Article 5.8.5 requiring the principal tensile stress be checked in the webs of all prestressed girders with design strengths above 10 ksi would remain appropriate even for heavily debonded girders such as those described here.
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experimental research approach, Findings, and associated analytical Simulations 93 In the current version of the AASHTO LRFD Specifications, the crack angle used for determining Vs in conjunction with Vcw is calculated from Eq.
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94 Strand Debonding for pretensioned Girders The data shown in Table 3.17 indicate that, for all but the BI-36 girder, the total applied shear force at cracking exceeds Vcw, and the total shear force at failure exceeds Vcw + Vs in all cases except End B in NU-1100; however, this girder end was not loaded to failure. The results shown in Tables 3.16 and 3.17 indicate that no change would be needed to Article 5.8.3.4.3.
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experimental research approach, Findings, and associated analytical Simulations 95 A second explanation is that the critical section is in the hollow section of the box, which is 4 in. from the solid end diaphragm.
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