PSI - Issue 17

Hayder Al-Salih et al. / Procedia Structural Integrity 17 (2019) 682–689 Al-Salih/ Structural Integrity Procedia 00 (2019) 000 – 000

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motion. This restraint approximates how the axial stiffness of a concrete deck restrains out-of-plane motion of a bridge flange. At the girder mid-span a cross-frame was installed and attached to a connection plate. The connection plate was welded to the girder web only, creating the web gap region found on older structures. Load was vertically applied to the cross-frame through the use of a hydraulic actuator, the attachment of which can be seen in the top left corner of Fig. 2a. Vertical displacements at the end of the cross frame produce realistic distortion-induced fatigue loading in the web gap region, resulting in load applied vertically and out-of-plane with respect to the girder web.

Fig. 2. a) Distortion-induced fatigue subassembly and b) bifurcated crack in the web gap region

4.2. Complex Crack Geometry

The previously existing fatigue crack was loaded cyclically at a range from 2.2 to 25.5 kN (0.5 to 5.75 kips) for 73,000 additional cycles, causing crack propagation and bifurcation. This is highlighted in Fig. 2b, where black lines have been drawn over the fatigue crack. One branch of the bifurcated crack, segment B-C, propagated vertically up the stiffener-to-web weld, while segment B-D grew horizontally into the girder web. For the purposes of crack characterization, these will be presented as two separate cracks with a shared initiation site: the vertical branch (A-B-C) and the horizontal branch (A-B-D). Inspection of the web gap revealed the total length of the vertical branch was 90.3 mm (3.55 in.) and the total length of the horizontal branch was 75.1 mm (2.95 in.).

4.3. Loading Protocol

Finite element analysis was used to determine appropriate levels of load for the distortion-induced fatigue subassembly. An analytical model of a steel girder bridge was evaluated with the AASHTO fatigue truck. Differential girder displacements determined in the model were scaled for the test specimen resulting in a target deflection of 1.25 mm (0.05 in.). This deflection corresponded to an applied actuator load of 6.6 kN (1.5 kips), and seven load cases were defined both above and below this threshold. Each load case, presented in Table 1, had a minimum load of 0.89 kN (0.2 kips) simulating dead load.

Table 1. Loading Protocol for Distortion-Induced Fatigue Testing Load Case Load Range, kN (kips) LC1 0.89-2.2 (0.2-0.5) LC2 0.89-4.4 (0.2-1.0) LC3 0.89-6.7 (0.2-1.5) LC4 0.89-8.9 (0.2-2.0) LC5 0.89-11.1 (0.2-2.5) LC6 0.89-13.3 (0.2-3.0) LC7 0.89-15.6 (0.2-3.5)