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141Â Â Conclusions and Proposals 5.1 Conclusions An integrated multilevel analytical and experimental research program was conducted to investigate the potential benefits, fabrication challenges, and design issues of using 0.7-in. strands in pretensioned girders. From simulation results in conjunction with experimental data and observations, the primary conclusions of this study are as follows: 1. Current design procedures for prestressed concrete bridge girders, including the calculation of flexural and shear capacities and all required stress checks, are adequate for girders using 0.7-in. strands. 2. Permitted release stresses, the maximum allowed number of partially debonded strands, and prestressing bed capacity could limit the number of 0.7-in. strands that can be used in current prestressed girder shapes. Moreover, the span length of girder shapes optimized for 0.6-in. strands is not appreciably increased by using 0.7-in. strands. Reducing congestion by minimizing the number of strands required to achieve the same prestressing force appears to be the more beneficial application of 0.7-in. strands. 3. The use of 0.7-in. strands does not affect issues of girder stability per se. If, however, longer spans are fabricated using 0.7-in. strands, greater care must be taken in stability analysis although by revising hanging and dunnage support locations and providing adequately stiff support conditions in transport, stability checks can be satisfied. For exceptionally long girders, extending the width of the top flange has been shown to improve stability consider- ations. Girders with relatively thin bottom flanges, such as bulb tees, benefit more in terms of potentially increasing achievable span length by using 0.7-in. strands. However, such girders exhibit greater susceptibility to rollover while supported on dunnage or in transportation, thereby requiring greater consideration of these effects. 4. Similar to previous studies, experimentally determined transfer and development lengths were found to be less than those prescribed by the AASHTO LRFD Bridge Design Specifications (AASHTO, 2020). While overestimating the development length is conservative in terms of capacity calculation, overestimation of transfer length underestimates concrete tensile stresses at release, potentially leading to cracking in the girder end region affected by the transfer length. 5. Transfer and development lengths prescribed by NCHRP Report 603 (Ramirez and Russell, 2008) were observed to be more representative of the data measured in this study. 6. No deleterious effects from using 0.7-in. strands spaced at 2Â in. center-to-center were identi- fied analytically and experimentally. 7. The extension of bottom flange confinement reinforcement was found to be inadequate for cases with partially debonded 0.7-in. strands, but the extension of bottom flange confinement reinforcement to 1.5d beyond the end of the girder was adequate for cases with no debonded C H A P T E R Â 5
142 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders strands. The minimum amount of confinement reinforcement required (No. 3 at spacing not exceeding 6Â in.) was found to be sufficient to confine 0.7-in. strands. 8. Industry acceptance of 0.7-in. strands requires fabricatorsâ willingness to deal with an âunfamiliarâ strand and having the infrastructure, such as higher-capacity stressing jacks, prestressing beds, and harping hold-downs, to apply the greater prestressing force for 0.7-in. strands (35% greater than for a 0.6-in. strand). 5.2 Proposed Detailing Guidelines for Prestressed Girders Having 0.7-In. Strands The reported analytical and experimental studies indicate relatively few changes are necessary for the design and detailing guidelines of girders with 0.7-in. strands. Current AASHTO LRFD Bridge Design Specifications articles (AASHTO, 2020) are applicable to 0.7-in. strands unless noted in the following. Specific recommendations for inclusion in AASHTO LRFD Bridge Design Specifications articles are provided in Appendix L. 1. AASHTO LRFD Bridge Design Specifications Article 126.96.36.199.1 (AASHTO, 2020) splitting rein- forcement is sufficient for 0.7-in. strands to resist bursting stresses at release. However, in cases where the maximum number of possible 0.7-in. strands is used, the greater splitting forces could lead to potentially more congested vertical splitting reinforcing steel requirements at the beam ends. Therefore, it should be permitted to extend the required splitting reinforcement beyond the current h/4 limit. 2. The minimum bottom flange confinement reinforcement required by the existing AASHTO LRFD Bridge Design Specifications Article 188.8.131.52.2 (AASHTO, 2020) must be extended to at least 1.5d beyond the termination of the longest debonded length of 0.7-in. strands. 3. AASHTO LRFD Bridge Design Specifications Article 184.108.40.206.2 (AASHTO, 2020) confine- ment reinforcement does not account for the influence of support-bearing width and the impacts of a wheel load being close to the support. The minimum confinement required by Article 220.127.116.11.2 may be insufficient in some cases. The strut-and-tie approach discussed in Section 2.4.1 is a more comprehensive method for determining the required bottom flange confinement reinforcement near the support. 4. The existing transfer length from AASHTO LRFD Bridge Design Specifications Article 18.104.22.168.2 (AASHTO, 2020) is appropriate for strength calculations. A reduced transfer length, equal to that recommended by NCHRP Report 603 (Ramirez and Russell, 2008), should be adopted for girder end-region stress checks at prestress transfer. 5.3 Fabrication, Handling, Shipping, and Erection Guidelines 1. Fabricators need to check the adequacy of their prestressing bed and hold-down devices and provide this information to the engineer of record. Accordingly, the design may have to be changed based on the number of 0.7-in. strands that can be stressed or the jacking force may have to be lowered. More hold-down locations may need to be used if 0.7-in. strands are to be harped (deflected). 2. Current shipping strands by weight result in 35% less length of 0.7-in. strand on a given reel. Fabricators need to account for this information as part of their bidding process. 3. Fabricators must pay special attention to the greater binding tension of 0.7-in. strand reel, which is 1.85 times larger than that of 0.6-in. strand. This larger energy in the spool could pose a safety hazard when strands are pulled out of the reel. Workers are advised not to hold 0.7-in. strands tightly in their hands as they pull them out of reels; they need to let the
Conclusions and Proposals 143Â Â strands twist while being pulled out. Workers need to be informed that 0.7-in. strands are 35% heavier than 0.6-in. strands to prevent injury. 4. Use established procedures to check the stability of girders using 0.7-in. strands and (1) revise hanging and dunnage support locations as needed, (2) provide stiff support conditions in transport, and (3) extend the width of the top flange to enhance the stability of exceptionally long girders. 5.4 Suggestions for Future Research The following is a list of recommended future research activities. 1. The performance of additional girders utilizing 15-ksi concrete needs to be evaluated. 2. Companion girders with 0.6-in. and 0.7-in. strands should be tested to understand whether the recommended extension of 1.5d beyond the termination of the longest debonded length is also necessary for 0.6-in. strands. 3. Due to the marginally reduced cover when using the same center-to-center strand arrange- ments, the long-term performance of bridges using 0.7-in. strands under field conditions needs to be studied. 5.5 Proposed Revisions to AASHTO LRFD Bridge Design Specifications The following changes, indicated by underlines, are proposed. In TableÂ 22.214.171.124-1âProperties of Prestressing Strand and Bar revise the row for the strand to include 0.7-in. diameter as the largest strand size in the indicated specifications: Material Grade or Type Diameter (in.) Tensile Strength, fpu (ksi) Yield Strength, fpy (ksi) Strand 270 ksi 0.375 to 0.6 0.7 270 90% of fpu Bar Type 1, Plain Type 2, Deformed 0.75 to 1.375 0.625 to 2.5 150 150 85% of fpu 80% of fpu Revise TableÂ 126.96.36.199-1âCenter-to-Center Spacings by adding 0.7-in.-diameter strand to the row with a spacing of 2.00Â in.: Strand Size (in.) Spacing (in.) 0.7 0.6 0.5625 Special 0.5625 2.00 0.5000 0.4375 0.50 Special 1.75 0.3750 1.50 Revise Article 188.8.131.52.2âConfinement Reinforcement as follows: For the distance of 1.5d from the end of the beams other than box beams, reinforcement shall be placed to confine the prestressing steel in the bottom flange. With a 0.7-in.-diameter partially debonded strand, reinforcement shall be placed to confine the prestressing steel in the bottom
144 Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders flange from the end of the beam for a distance not less than 1.5d beyond the termination of the longest debonded length. The reinforcement shall not be less than No. 3 deformed bars, with spacing not exceeding 6.0Â in., and shaped to enclose the strands. For box beams, horizontal transverse reinforcement shall be provided and anchored by extending the leg of the stirrup or tie into the web of the girder beam. Add new Article 184.108.40.206.3 as follows: 220.127.116.11.3âHorizontal Transverse Tension Tie Reinforcement Horizontal transverse reinforcement provided to satisfy Articles 18.104.22.168.1 and 22.214.171.124.2 may also be used to satisfy this article. In lieu of the requirements of this article, steel sole plates shall be embedded at the girder ends. Articles 126.96.36.199.1 and 188.8.131.52.2 shall still be applicable when a steel sole plate is used. For all single-web beam sections with a bottom flange, horizontal transverse tension tie rein- forcement shall be provided to resist potential longitudinal splitting cracks in the bottom flange. For single-web sections, the strut-and-tie model and the associated equation shown in FigureÂ 184.108.40.206.3-1 shall be used to determine the required amount of horizontal transverse tie reinforcement. The horizontal transverse tie reinforcement shall be uniformly distributed over the length of the bearing plus a distance equal to one-quarter of the overall height of the beam or girder element in its final configuration (including a composite slab if provided) on the span side of the bearing. , where where Atie = area of tie reinforcement (in.2) bb = width of bearing (in.) cb = distance from the bearing reaction forces to the centerline of section (in.) FigureÂ 220.127.116.11.3-1âStrut-and-tie model for confinement
Conclusions and Proposals 145Â Â fy,tie = yield strength of tie reinforcement (ksi) hb = depth of bottom bulb (in.) Nw = total number of bonded strands aligned with web nf = number of bonded strands in one side of the outer portion of the web xp = the horizontal distance from girder centerline to the centroid of nf strands in the outer portion of the bulb (in.) yp = the vertical distance from girder soffit to the centroid of nf strands in the outer portion of the bulb (in.) Add new Commentary C18.104.22.168.3 as follows: The development of tension oriented transversely across the bulb of single-web flanged sections is a potential failure mode requiring tie reinforcement across the bottom flange to control the potential longitudinal cracking at the Strength I limit state. The nature of the result- ing failure, however, is related to excessive transverse deformation and cracking of the flange and is not likely to be catastrophic in nature. Minimum confinement reinforcement satisfying Article 22.214.171.124.2 contributes to the tie capacity, and in many instances will be sufficient to fully resist the horizontal transverse tie force calculated using Article 126.96.36.199.3. The horizontal portion of splitting reinforcement required by Article 188.8.131.52.1, if present, contributes to the tie capacity force calculated using Article 184.108.40.206.3. In some instances, these requirements may result in impractical horizontal transverse tie reinforcement details. An embedded sole plate would likely be more practical in such cases. Add the following reference to Article 5.15: Shahrooz, B. M., K. A. Harries, R. W. Castrodale, and R. A. Miller (2022) NCHRP Research Report 994: Use of 0.7-in. Diameter Strands in Precast Pretensioned Girders. Transportation Research Board, Washington, DC.