Click for next page ( 62

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 61
61 ment (WWR) of the same area per unit length may be 3.8 Proposed Revisions to used to substitute for the #3 bars. The same amount of the AASHTO LRFD Bridge confinement steel must be provided at the bonded ends Design Specifications of all debonded strand groups. The research team proposes the following changes to Arti- Appendix G provides two examples for the design of end cle 5.10.10 of the AASHTO LRFD Bridge Design Specifica- zone reinforcement using the AASHTO LRFD specifications tions (18). Table 3.16 presents Article, with additions and the proposed requirements. underlined, and deletions struck through. Table 3.16. Proposed changes to Article of the AASHTO LRFD specifications. Factored Bursting Resistance C5.10.10.1 The bursting resistance of pretensioned anchorage This provision is roughly equivalent to the zones provided by vertical reinforcement in the ends of provisions of Section 9.22.1 in AASHTO Standard pretensioned beams at the service limit state shall be Specifications (1996). Results of tests conducted by taken as: the Florida Department of Transportation were taken Pr = fs As ( into where: account. fs = stress in steel not exceeding 20 ksi As = total area of vertical reinforcement located Additional research by Tuan et al. (PCI Journal, within the distance from the end of the beam (in.2) 2004) and Tadros et al. (NCHRP 18-14, 2009) shows that distribution of the 4% reinforcement h = overall depth of precast member (in.) such that at least one half of that reinforcement is concentrated in the end h/8 of the member while The resistance shall not be less than 4 percent of the the balance of the 4% is distributed over a distance prestressing force at transfer. from h/8 to h/2 provides for arrest of the cracking at The end vertical reinforcement shall be as close to the member end and for well distributed cracks in the end of the beam as practicable. the balance of the end zone. Vertical reinforcement located within the distance from the end of the Since the crack control reinforcement is required to beam shall be provided to resist at least 2 percent of the prestressing force at minimize the crack width, and not for strength, transfer. Also, the total amount of vertical reinforcement located within the there is no need to develop the full yield strength distance h/2 from the end of the beam shall be provided to resist at least 4 beyond the locations of the top and bottom cracks, percent of the prestressing force. The reinforcement in the end h/2 shall be not which are assumed for design to be at the junction less than that required for shear resistance. between the web and the flanges. The bar Crack control reinforcement shall be anchored beyond the anticipated extreme anchorage into the flanges should be designed for a top and bottom cracks, an embedment adequate to develop at least a stress = 30 maximum stress of 30 ksi which was found ksi. (NCHRP 18-14, 2009) to be conservative. Confinement Reinforcement C5.10.10.2 For the distance of 1.5d at least 60 strand diameters from the end of the beams, Welded wire reinforcement (WWR) of the same other than box beams, reinforcement shall be placed to area per unit length may be used to substitute for confine the prestressing steel in the bottom flange. The the #3 bars. reinforcement shall not be less than No. 3 deformed bars, with spacing not exceeding 6.0 3.0 in. and shaped to totally enclose the strands. The same amount of confinement steel must be provided at the bonded end of all debonded strand groups. For box beams, transverse reinforcement shall be provided and anchored by extending the leg of stirrup into the web of the girder. Add the following references to the list of references of Section 5: Tuan, C., Yehia, S., Jongpitakseel, N., and Tadros, M., "End Zone Reinforcement for Pretensioned Concrete Girders," PCI Journal, Vol. 49, No. 3, May-June 2004, pp. 68-82. Tadros, M.K., Badie, S.S., and Tuan, C., "Evaluation and Repair Procedures for Precast/Prestressed Concrete Girders with Longitudinal Cracking in the Web," NCHRP 18-14, Contractor's Final Report, November 2009 (published as NCHRP Report 654).