Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders (2022) / Chapter Skim
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27 Analytical Research Approaches and Findings 2.1 Introduction Design case studies were conducted to compare maximum achievable span lengths when using 0.6-in.
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28 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders 2.2.2 Assumed Design Loads All reported designs are for interior girders; the following additional assumptions were made.
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Analytical Research Approaches and Findings 29 straight strands, number of harped strands, number and length of debonded strands, and web reinforcement) are provided in Appendix B
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30 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders increase within a band of 1.0 to 1.22 times their corresponding cases having 0.6-in.
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Analytical Research Approaches and Findings 31 2.2.3.2 Influence of Type of Girder The influence of the type of girder on the increase in span length that may be achieved using 0.7-in. strands was examined by comparing the span length increases for girders of equal or nearly equal depths.
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32 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders Figure 2.4.
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Analytical Research Approaches and Findings 33 As described by Collins and Mitchell (1997) , one measure of the efficiency of a prestressed concrete section is the lever arm of the internal couple.
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34 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders In addition to the girder shape, which affects (e + kt)
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Analytical Research Approaches and Findings 35 to be reduced to satisfy the tensile and compressive stress limits at prestress transfer and the SERVICE I and SERVICE II limits near the end of the girder. It is noted that regardless of the outcome of this "side check," all designs progressed using the values of f ′ci = 0.85f ′c or 0.6f ′c.
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36 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders Table 2.4 summarizes the material properties used in the models.
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Analytical Research Approaches and Findings 37 For each of the four cases (each girder shape using 0.6-in.
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38 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders BT-72-6 ß support half span (66.25 ft)
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Analytical Research Approaches and Findings 39 BT-72-7 ß support half span (78.25 ft) shown midspan à Release of tendons Longitudinal stress: , = 0.43 ksi = 0.14 , = 7.90 ksi = 0.88 Crack pattern , ≤ 0.023 in.
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40 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders NU-2000-6 support half span (90.75 ft)
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Analytical Research Approaches and Findings 41 NU-2000-7 support half span (101.25 ft) shown midspan Release of tendons Longitudinal stress: , = 0.05 ksi = 0.02 , = 6.80 ksi = 0.76 Crack pattern , ≤ 0.016 in.
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42 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders Nw = total number of bonded straight strands at section (harped strands do not affect endregion STM)
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Analytical Research Approaches and Findings 43 composite slab if provided) toward the midspan of the girder; i.e., a length of H/4 + Lbearing = (h + tslab)
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44 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders 2.
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Analytical Research Approaches and Findings 45 (c) AASHTO-PCI BT girders Girder BT-54 BT-63 BT-72 Strand dia.
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46 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders 2.4.2.1 Transverse Tie Forces and Flange Reinforcement Near Girder Ends All 448 single-web design cases exhibit transverse tie forces that may be resisted without unrealistic reinforcement requirements.
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Analytical Research Approaches and Findings 47 in lower tie force demand, provided good practice debonding patterns (aimed at minimizing nf /Nw and xp) are adopted as described above.
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48 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders In terms of splitting reinforcement [AASHTO LRFD Bridge Design Specifications Article 5.9.4.4.1 (AASHTO, 2020)
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Analytical Research Approaches and Findings 49 A For the given release sequence [reported as outside-in]
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50 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders critical sections shown in Figure 2.13(b)
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Analytical Research Approaches and Findings 51 where no = the total number of released bonded strands outboard of the vertical section defined by hf Aps fps = the force in a single released strand (assumed to be 0.9 × 0.75Aps fpu) xpo = distance from the centroid of outboard strands to the vertical section defined by hf Ly = internal moment arm in Y-direction Lt = length of assumed tensile stress distribution Ly and Lt vary according to the number of cut strands, the shape of the cross section, and the location within the cross section at which the calculation is made.
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52 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders corresponding to the six outermost columns of strands of an FIB girder, using the recommendations of Ross (2012)
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Analytical Research Approaches and Findings 53 of x3po and hf. The values of Ly and Lt will be proportional to this stiffness, that is, a stiffer outstand will reduce peeling stresses along the critical section.
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54 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders Section (in.)
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Analytical Research Approaches and Findings 55 and (3) an "outside-in" release sequence.
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56 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders to 0.75APS fpu, and each case represents only the strands indicated being released.
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Analytical Research Approaches and Findings 57 To model peeling stresses, which are a local e ect, a half-span model of a WF-72 was used. e half span was made statically determinate with the following restraints intended to represent the restraint of a prestressed girder at prestress transfer: 1.
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58 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders (2012)
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Analytical Research Approaches and Findings 59 in essentially the same stress as 0.5-in. strands at 1.75 in.
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60 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders For simplicity, these analyses neglect the transfer of longitudinal prestress to the concrete; this assumption is believed to be acceptable since the focus of these analyses is on the high stresses at the free end of the prism.
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Analytical Research Approaches and Findings 61 The model assumes linear behavior having stiffness Ec through 0.5σcu. Beyond 0.5σcu, the stress-strain relationship through 0.3σcu (at εd)
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62 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders 2.7.1.2.3 Failure Surface.
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Analytical Research Approaches and Findings 63 2.7.1.2.4 Shear Retention. The ABAQUS smeared crack model also permits shear retention, that is, the degree of shear capacity retained in the cracked concrete model.
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64 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders Case (in.)
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Analytical Research Approaches and Findings 65 except that the gray region in Figure 2.22 indicates the region in which predicted tensile strains exceed those expected to cause cracking (σt0 > 0.69 ksi)
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66 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders (a)
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Analytical Research Approaches and Findings 67 ß square prism dimension = 3 × strand spacing à r = s/2 The section at which concrete stresses reported in Figure 2.25 is determined Case (in.)
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68 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders 2.7.1.5 Summary of Strand Transfer Analytical Study The Hoyer effect is expected to result in circumferential stresses at the strand-concrete interface that exceed the concrete stress tensile strength.
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Analytical Research Approaches and Findings 69 The analyses conducted considered the following conditions for each girder (PCI, 2015)
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(d) Longitudinal strand layout and dimensions required for stability analysis (a)
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Analytical Research Approaches and Findings 71 2.8.1.2 Calculation of Camber Camber (positive value is upward deflection) is calculated at each stage as: ∆ = ∆ + ∆ + ∆self ps ohang Eq.
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72 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders ( )
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Analytical Research Approaches and Findings 73 Condition Lift from bed Dunnage Transport Lift in field In place In place (top strands cut) Assumed age Release Release >28 days >28 days >28 days >28 days Strand (in.)
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74 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders stages up to placing the girder on its bearings.
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Analytical Research Approaches and Findings 75 weight is assumed to be 0.150 kips/ft3 in all cases. Other analysis parameters not indicated are the same as those given in Table 2.27.
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76 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders Girder Condition Lift from bed Dunnage Transport Lift in field In place Age Release Release >28 days >28 days >28 days Prestress losses 10% 10% 50% 50% 50% (ksi)
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Analytical Research Approaches and Findings 77 Girder Factor of safety Acceptance criteria Lift from beda Dunnage Transport Lift in field a Place on bearings WF100G (207 ft)
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78 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders Added flange width WF100G BT-72 OHWF-72 / / / As built 0.045 1.60 0.95 0.075 2.37 1.14 0.124 3.73 1.37 +12 in.
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Analytical Research Approaches and Findings 79 8. Peeling stresses are not unique to 0.7-in.
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