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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 5.10.10.1, with additions
and the proposed requirements. underlined, and deletions struck through.
Table 3.16. Proposed changes to Article 5.10.10.1 of the AASHTO LRFD specifications.
5.10.10.1 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 (5.10.10.1-1) 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.
5.10.10.2 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).
