Behavior of Bridge slab Ends at Expansion Joints

The Texas Department of Transportation (TxDOT) currently uses, for most of its bridges, the “IBTS” standard detail for bridge slab ends at expansion joints. That detail has evolved as a way of achieving increased transverse stiffness at slab ends, without using diaphragms. In the ITBS detail, slab ends are stiffened by a 2-in. (51-mm) increase in slab thickness and a slightly reduced reinforcement spacing for skewed slabs. Although the origin of this detail is unknown, it has been used successfully by TxDOT for decades.

Bridges in Texas are designed in accordance with AASHTO provisions. While AASHTO design loads are typically at the HS-20 level, several TxDOT districts have considered increasing those by a factor of 1.25, to what has been unofficially designated an “HS-25 level.” In part, this study seeks to understand the behavior of slab ends under HS-20 and HS-25 applied load levels. Two specimens have been constructed thus far, one constructed with 0º skew slab ends (Ryan 2003) and one constructed with 45º skew slab ends.

An alternate and possibly more economical detail was designed with a flexural capacity similar to that of the IBTS detail but without a thickened edge. Designated the Uniform Thickness Slab End (UTSE) detail, it was also instrumented and tested, and its performance was compared with that of the IBTS detail.

For the 45º skew specimen, a three-bay concrete slab, 21 ft 6 in by 33 ft 7 in. (6.5 by 10.2 m), was built composite with four steel girders. The IBTS and UTSE end details were constructed on opposite sides. Loads were applied to four test areas in the AASHTO design tandem load configuration to HS-20, HS-25 and overload levels, such that negative bending was maximized over the girder in the two 8-ft (2.4-m) bay slab ends, and positive bending was maximized in the two 10-ft (3.0-m) bay slab ends. All test areas were ultimately loaded to failure.

For the 45º skew specimen, the UTSE and IBTS end details performed well at HS-20 and HS-25 load levels. At these loads, reinforcing bar strains were insignificant (less than 0.1?y) and deflections were extremely small relative to girder spacing (between l/2000 and l/36000). Cracking was not observed at the HS-20 and HS-25 load level in the 8-ft (2.4-m) girder spacing bays. Only the UTSE detail, positive moment test area cracked at the HS-20 load level. The IBTS detail, positive moment test area cracked at the HS-25 load level.

In the 45º skew specimen, test areas had high reserve strength: where negative moment was maximized, the specimen failed at around 6.0 x HS-25, and where positive moment was maximized, the specimen failed at around 4.0 x HS-25. All test areas failed in punching shear, with the exception of the IBTS detail, positive-moment test area, which failed in one-way shear.

Comparisons have been made for the IBTS and UTSE end details constructed at both a 0º skew and 45º skew. An increase in applied loads from HS-20 to HS-25 load levels frequently resulted in a nearly proportional increase in midspan edge deflection and tensile strain in reinforcement. At both HS-20 and HS-25 load levels, tensile strains in transverse reinforcement and the deflection-to-girder-spacing ratio were both extremely small (always less 10% of yield strain and l/800 respectively). Slab ends usually remained uncracked until multiples of the HS-20 design load level, with the exception of the 45º skew, 10-ft (3.0-m) girder spacing, UTSE detail test area, which cracked at 1.0 x HS-20 (13 kips per load point, or 56 kN). When loaded to maximize negative moment, slab ends in 8-ft (2.4-m) bays usually failed around 6.0 x HS-20 (75 kips per load point, or 335 kN). When loaded to maximize positive moment, slab ends in 10-ft (3.0-m) bays failed at load levels higher than 3.8 x HS-25 (48 kips per load point, or 210 kN).