Bottom Flange Reinforcement in NU I-Girders

Authors

George Morcous Ph.D., University of Nebraska-LincolnFollow
Kromel Hanna Ph.D., University of Nebraska-Lincoln
Maher K. Tadros Ph.D., P.E., University of Nebraska - LincolnFollow

Date of this Version

2010

Document Type

Article

Citation

Report SPR-1(10) P331 Final Report 26-1120-0050-001

Comments

Copyright 2010 Mid-America Transportation Center

Abstract

The 1996 edition of AASHTO Standard Specifications for Highway Bridges stated that nominal confinement reinforcement be placed to enclose prestressing steel in the bottom flange of bridge girders from girder ends to at least a distance equal to the girder’s height. The 2004 edition of AASHTO LRFD Bridge Design Specifications changed the distance over which the confinement reinforcement was to be distributed from 1.0h to 1.5h, and gave minimum requirements for the amount of steel to be used, No.3 bars, and their maximum spacing, not to exceed 6.” Research was undertaken to study what impact, if any, confinement reinforcement had on the performance of prestressed concrete bridge girders. Of particular interest was the effect confinement had on the transfer length, development length, and vertical shear capacity of the aforementioned members. First, an analytical investigation was carried out, and then an experimental investigation followed which consisted of designing, fabricating, and testing eight 24” tee-girders and three NU1100 girders. These girders had different amounts and distributions of confinement reinforcement at girder ends and were tested for transfer length, development length, and shear capacity. The results of the study indicated that: 1) neither the amount nor distribution of confinement reinforcement had a significant effect on the initial or final transfer length of the prestressing strands; 2) at the AASHTO predicted development length, no significant change was found on the nominal flexural capacity of the tested girders regardless of the amount and distribution of confinement reinforcement; and 3) despite the improved anchorage of prestressing strands at the girder ends when higher levels of confinement reinforcement are used, the ultimate shear capacity of tested girders was found to be considerably higher than nominal capacity even when low levels of confinement reinforcement were used.