Strand Debonding for Pretensioned Girders (2017) / Chapter Skim
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11 2.1 Research Approach A multiple component analytical study was carried out with the objective of demonstrating the effects of strand debonding on prestressed girder design and behavior. An initial analysis of present AASHTO limits to strand debonding based on two previously reported experimental programs was conducted (Section 2.2)
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12 Strand Debonding for pretensioned Girders 2.2.1 Shahawy et al.
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analytical research approach and Findings 13 until the two sides of the equation are made equal. This new value of Vtotal is the predicted girder capacity if the failure is due to insufficient longitudinal steel at the critical section, VT@dc.
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14 Strand Debonding for pretensioned Girders (c) shear capacity at face of support (VT@support)
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analytical research approach and Findings 15 that the debonding itself had little effect on girder capacity provided that the remaining bonded steel is adequate to resist the tension demand.
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16 Strand Debonding for pretensioned Girders and SERVICE III requirements; that is, L = min[LSTRENGTH I, LSERVICE I, LSERVICE III] ≥ Lmin, in which each length is defined in the following sections.
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analytical research approach and Findings 17 + + ≤ ′0.60 Eq. 2.2b , , M M M S f WS barrier wall LL top of girder composite c slab The subscript "girder" refers to the properties of the precast girder alone whereas the subscript "composite" refers to the full composite section.
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18 Strand Debonding for pretensioned Girders The concrete compression limit of 0.6f ′ci and tension limit of 0.24√f ′ci were also checked at midspan at the time of prestress transfer. In this calculation, the total number of strands in the section, N, was considered and the self-weight of the girder was included.
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analytical research approach and Findings 19 (a) I girders and bulb-tees (b)
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20 Strand Debonding for pretensioned Girders (a) I girders and bulb-tees (b)
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analytical research approach and Findings 21 approximately correspond to L/h = 23.2 and 10, respectively (Figure 2.4)
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22 Strand Debonding for pretensioned Girders Commercial software was used to conduct all 3D FEM analyses. The software is focused on reinforced concrete structures and is widely used for both design and research.
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analytical research approach and Findings 23 validated and a mesh-sensitivity study conducted using experimental data from Burgueño and Sun (2011) , as described in Appendix C
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24 Strand Debonding for pretensioned Girders Type IV, Nebraska NU-900, AASHTO BIV-48 (in adjacent box arrangement) , Texas U-54, and BT-72 girders were considered.
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analytical research approach and Findings 25 for directly in the model) was taken as 0.56fpu = 151.2 ksi in all cases.
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26 Strand Debonding for pretensioned Girders walls distributed over five girders)
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analytical research approach and Findings 27 (a) Type IV girder (b)
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Model Parameters (See Table 2.3) Stress Checks (See Table 2.4)
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analytical research approach and Findings 29 girder near the girder end, as is clearly shown in the crack patterns at prestress transfer shown in Appendix D For all cases considered (except 20 and 21)
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30 Strand Debonding for pretensioned Girders Some BIV and NU-900 sections also fail to meet the Aps fps/T ≥ 1.0 criterion at dv/2 and dv. Despite not meeting the criterion, redistribution is adequate such that significant reserve capacity is still available.
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analytical research approach and Findings 31 2.4.2.2.5 Performance of Different Girder Cross Sections. Little difference was observed in the overall behavior of AASHTO Type IV and NU-900 girders.
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32 Strand Debonding for pretensioned Girders 2.5.1 Motivating Example Partial debonding of strands is required to reduce concrete stresses and mitigate cracking in girder end regions. Nonetheless, the details associated with partial debonding can, in fact, cause local cracking and potentially lead to early and abrupt ultimate-limit-state failure, particularly at girder ends (Ross et al.
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analytical research approach and Findings 33 3. Struts and ties are anchored at nodes corresponding to centroids of groups of bonded prestressing strand.
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34 Strand Debonding for pretensioned Girders and girder geometry, whereas Ross et al.
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analytical research approach and Findings 35 Girder geometry Cases Range of debonding ratio Tie forces Cases satisfied by 60 ksi ties… Girder H S N at midspan L V/ tmax tdr = 0 No.
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36 Strand Debonding for pretensioned Girders BT-72 N = 24 BT-72 N = 38 BT-72 N = 48 (a)
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analytical research approach and Findings 37 AASHTO Type VI N = 24 AASHTO Type VI N = 48 AASHTO Type VI N = 76 (b)
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38 Strand Debonding for pretensioned Girders NU-1800 N = 24 NU-1800 N = 48 NU-1800 N = 60 (c)
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analytical research approach and Findings 39 NU-1100 N = 24 NU-1100 N = 48 NU-1100 N = 60 (d)
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40 Strand Debonding for pretensioned Girders -50 0 50 100 150 200 250 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 Debonding ratio, dr compressive strut predicted fy = 60 ksi #3 ties @ 6 in. #4 ties @ 6 in.
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analytical research approach and Findings 41 each girder/spacing combination, all possible debonding patterns of strands in the outer portion of the bulb (i.e., varied nf) that satisfied the following criteria were considered: 1.
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42 Strand Debonding for pretensioned Girders tie forces are greater for wide flat bulbs (NU-1800) than for thinner deep bulbs (AASHTO Type VI)
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analytical research approach and Findings 43 restraint would be expected from this bearing; thus, the observed splitting should be expected. Increasing the bearing to full width (24 in.)
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44 Strand Debonding for pretensioned Girders 2. The strands located in the planes of the webs are not debonded; this detail allows the necessary tension force also to be developed in the same plane as the compressive strut.
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analytical research approach and Findings 45 The developed length of bonded strand i at OA is (Eq.
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46 Strand Debonding for pretensioned Girders This principal stress is oriented at an angle from Eq.
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analytical research approach and Findings 47 (a) Girder G2 (note debonding of two strands at acute corner [left side]
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48 Strand Debonding for pretensioned Girders force at line OA shown in Figure 2.16 being greater in the "realistic" case than in the "design" case?
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analytical research approach and Findings 49 (a) Cross section (b)
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50 Strand Debonding for pretensioned Girders 2. G5-A exhibited cracking very similar to that seen in the field (Figure 2.15)
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analytical research approach and Findings 51 Elevation - Longitudinal stress (Vertical dimension exaggerated 200%) Reverse plan (soffit)
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