Robert J. Dexter

FATIGUE DESIGN AND TESTING OF MODULAR BRIDGE EXPANSION JOINTS

Sealed expansion joint systems are becoming more widely used to minimize drainage and corrosion damage at bridge deck joints. Unfortunately, at this time there are no general design or fatigue design provisions for modular bridge expansion joints (MBEJs) in the U.S. As a result of this lack of design guidance, premature fatigue failures have been documented at the welded and bolted connections of these MBEJ due to truck wheel loads. Research sponsored by the National Cooperative Highway Research Program (NCHRP) is currently underway to develop performance-based specifications for the fatigue-resistant design of these MBEJ systems.

The effect of welding discontinuities on the variability of fatigue life

Large-scale I-section beams were welded from A710 (HSLA-80) steel plates. There were plain welded beams, similar beams with two attachment details fillet welded to the tension flange, and similar beams with transverse groove welds. The welding procedures used produced a wide range of welding discontinuities, particularly at groove welds which intersected the longitudinal fillet welds without a cope hole. The 162 beams were fatigue tested in four-point bending in a test matrix which included various stress ranges and minimum stress levels. The discontinuities at the origin of each fatigue crack were identified. These initiating discontinuities included microscopic weld toe discontinuities, porosity, inclusions, lack of penetration and hydrogen cracks. Fracture mechanics fatigue crack growth models were used to calculate the fatigue lives based on the initial discontinuity size. Various methods of idealizing the discontinuities as initial cracks were examined.

Propagation of Very Long Fatigue Cracks in a Cellular Box Beam

Large cellular box beams, 8000 mm long, were fatigue tested in four-point bending. A total of ten tests were performed in a test matrix which included several variations in load ratio. The nominal stress range varied from89 to 219 MPa. The box beams were fabricated from HSLA-80 steel. Crack growth rates in the base metal were obtained from CT specimens of various heats, orientations and thicknesses. In every test, a through-thickness crack developed at an intentionally poor weld detail after less than 30% of the total fatigue life. Nearly fixed crack growth rates were observed for a region of cracking, due to web restraints, redundancy aspects of the box beam design, and welding residual stresses. Cracks 1500 mm or more in total length still grew in a stable manner. The observed crack growth rates were related to macroscopic beach marking of the fatigue crack surface as well as microscopic striations. Correlation of the observed crack growth rates to the square of stress range was relatively good. The cracking history is decomposed into four separate stages. Using the concept of effective stress intensity factor range, crack growth rates predictions for each stage are presented.

Ductile Tearing of Welded Structural Details

Experiments were conducted at room temperature on 16 full-scale welded structural members containing fatigue cracks. These I-section and box-section members were fabricated from HSLA-80 steel that exhibits upper-shelf behavior at room temperature. These members consistently reached at least the net-section collapse load and deformed to several times the yield-point displacement. The experiments were analyzed with a variety of ductile-fracture models including the three levels in the British Standards Institute document PD 6493, a stable tearing analysis using finite-element analysis to calculate J, and a limit-load analysis. These models were studied in order to evaluate their usefulness and degree of conservatism relative to experimental results.

Comparison of Fatigue Resistance of Various Longitudinal Weld Joints

Various types of one-sided and two-sided longitudinal weld joints were made in the web/flange connection of large-scale HSLA- 80 I-section beams. Fatigue experiments were conducted on 78 beams. As expected, there is a decrease in the fatigue strength of the one-sided weld. Among the one-sided welds, the differences in the fatigue strength were slight. The findings show that existing design rules for longitudinal fillet welds in bridges can be used for ships. The data provides a rational basis for the selection of longitudinal weld joints, fit-up tolerances, and NDE requirements.