Strand Debonding for Pretensioned Girders (2017)

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96 4.1 Conclusions Through a study combining extensive parametric, analytic, numeric, and experimental com- ponents, a better understanding of the use of debonded prestressing strands resulted. This study was premised on the concept that debonding prestressing strands are necessary to control stresses resulting from prestressing force released in prestressed concrete girders. This is particularly true with heavily prestressed girders and the adoption of larger strand diameters, where harping strands is not always a practical or sufficient option. The primary conclusion of this study is that debonding strands—in itself—is not detrimental to prestressed girder performance provided the requirements for longitudinal reinforcement (AASHTO LRFD Article 5.8.3.5—described in Section 1.3.1 of this report) to resist the additional tension due to shear are met. This finding confirms and consolidates those of previous research; indeed, the analyses conducted in this study have shed light on and explained some inconsistent findings of previous work. This study also addressed design details associated with debonded strands and girder end regions for which a number of recommendations resulted. These recommendations are reported in the following sections. 4.2 Suggested Detailing Guidelines for Prestressed Girders Having Debonded Strands Based on the reported experimental and analytical studies, the following design guidelines are proposed. Some of the guidelines are the same as currently used when the number of strands is not greater than 25 percent. In this section, all references are to 2016 AASHTO LRFD section numbering. In all girders, regardless of section shape: • The total number of debonded strands shall not exceed 60% of the total number of strands unless test results or successful past practices indicate that a larger percentage of strands may be debonded. • The number of debonded strands in any horizontal row within the bottom flange height other than the bottom row shall not exceed 80% of the number of strands in that row. • Tensile force in prestressing reinforcement (Aps fps) shall exceed the tensile force of the nonprestressed reinforcement (As fs) at all sections. Development of straight and bent-up strands as well as overhangs, if present, should be taken into account for determining the value of fps. C h a p t e r 4 Conclusions and Suggestions

Conclusions and Suggestions 97 • No more than 40% of debonded strands, or four strands, whichever is greater, shall have the debonding terminated at any section. (No change from current AASHTO LRFD Arti- cle 5.11.4.3). • Satisfy AASHTO LRFD Articles 5.10.10.1, 5.10.10.2, and 5.8.3.5. In single-web flanged sections (I-beams and bulb-tees) as summarized in Figure 4.1: • No more than 50% of the bottom row of strands shall be debonded. • The outermost strands in all rows located within the full-width section of the flange shall remain bonded. Full width is understood to mean the full width of the bottom flange less a distance accounting for the chamfer—typically 2 in. on both sides. • With the exception of the outermost strands, strands further from the section vertical center- line shall be debonded prior to those nearer the centerline. • Strands in the flange within the web width should remain bonded. • Debonded strands shall be symmetrically distributed about the vertical centerline of the cross section of the member. • Full flange width bearing shall be provided at supports. Full flange width bearing is not neces- sary if a steel sole plate is provided. The width of the sole plate shall be at least one-half of the width of the bulb. In multi-web sections having a “bottom flange” (voided slabs, box beams, and U-beams) as summarized in Figure 4.2: • No more than 50% of the bottom row of strands shall be debonded. • Bearings placed below webs not connected by an end diaphragm shall engage a width equal to twice the extension of all webs at supports. • For girders supported at their corners, the strands located within a width equal to twice the extension of all the webs shall remain bonded. • For girders supported at their corners, strands shall be debonded from the centerline outward. Figure 4.1. Proposed details for single-web sections.

98 Strand Debonding for pretensioned Girders • For girders supported across their width, debonded strands shall be uniformly distributed across the flange width between webs. In solid slabs: • No more than 50% of the bottom row shall be debonded. • For slabs uniformly supported across their width, debonded strands shall be uniformly dis- tributed across the width. • For slab beams supported at their corners, strands above the bearings shall not be debonded. Proposed specification changes in the format of a Working Agenda Item based on the eighth edi- tion of the AASHTO LRFD Bridge Design Specifications (forthcoming) are provided in Appendix I. 4.3 Suggestions Regarding Transverse Tension Ties at STRENGTH I Ultimate Limit State The development of tension oriented transversely across the bulb of single-web sections was identified as a potential failure mode requiring tie reinforcement through the bulb to control associated longitudinal cracking at the STRENGTH I limit state. The nature of the resulting failure, however, is related to excessive transverse deformation and cracking of the bulb and is not likely to be of catastrophic nature since reinforcement satisfying AASHTO LRFD Arti- cles 5.10.10.1 and 5.10.10.2 contribute to the tie capacity, and in many instances will be suf- ficient to fully resist the tie force. Furthermore, although the tie force is not a function of strand debonding per se, the guidance for debonding patterns presented above were developed partially (a) Supported at the corners (b) Supported across full width Figure 4.2. Proposed details for multi-web sections without end diaphragms.

Conclusions and Suggestions 99 with the intent of minimizing the tie force. The following requirements were developed to design reinforcing to fully resist the tie force: • For single-web flanged sections, the STM shown in Figure 4.3 shall be used to determine the required amount of tie reinforcement required, where t is the tie force to be resisted (Eqs. 4.1 and 4.2): [ ]( ) ( ) ( )( )= φ − + − Eq. 4.1t V n x h y x c yNu f p b p p b pw where 2 1 Eq. 4.2c b n Nb b f w( )( )= − and dimensions are defined in Figure 4.3. • The reinforcement determined in these requirements shall be uniformly distributed over the length of the bearing plus a distance equal to one-quarter of the overall height of the girder (including the composite slab if provided) toward the midspan of the girder. In some instances, these requirements may result in impractical confinement reinforcing details. An embedded sole plate would likely be more practical in such cases. • The steel sole plates shall be embedded at the girder ends. • AASHTO LRFD Articles 5.10.10.1 and 5.10.10.2 shall still be applicable when steel sole plates are used. Proposed specification changes in the format of a Working Agenda Item based on the eighth edition of the AASHTO LRFD Bridge Design Specifications are provided in Appendix I. 4.4 Web Shear Cracking Analysis of experimental results shows that using a principal tensile stress of 0.11 fc′ ksi is a reasonable lower bound for predicting shear cracking in webs for girders with debonded strands. However, in sections with concrete end diaphragms, the Mohr’s Circle approach of AASHTO LRFD Article 5.9.2.3.3 does not correctly predict stresses in areas near the diaphragm, as these Figure 4.3. STM for confinement.

100 Strand Debonding for pretensioned Girders are “disturbed” regions where the assumptions implicit in a Mohr’s Circle analysis do not apply. Mohr’s Circle under predicts the actual web stresses in these areas. A finite element approach was needed to correctly assess the stresses in these areas. Since a finite element is impractical in a design situation, a reduced stress limit of 0.08 fc′ is a practical solution until more research can provide another approach. 4.5 Suggestions for Future Research The following is a list of recommended future research activities. 1. The effects of skewed girder ends should be investigated experimentally. The effects of both large flange prestressing forces and the ability for debonding to mitigate these should be considered. 2. The STM should be evaluated further through a testing program focused specifically on transverse cracking. The stiffness of bearing and embedded sole plates should be the pri- mary test parameters. 3. The detailing guidelines and proposed code revisions need to be examined for concrete strengths less than 10 ksi. 4. Other girder shapes, such as voided and solid slabs, should be tested.