PSI - Issue 5

M.H. Hebdon et al. / Procedia Structural Integrity 5 (2017) 1027–1034 Liu et al. / Structural Integrity Procedia 00 (2017) 000 – 000

1032

6





3

       6

2 10



C

N



(4)

3

e

4. Fatigue assessment

4.1. Procedure

A fatigue life evaluation was performed for the critical connections according to the following steps: Step 1: Establish the bridge model with the critical details and fatigue truck load. Step 2: Conduct the FE analysis under the passing fatigue truck load to obtain the time history response (i.e. stress vs. time step) of the details. Note that the vehicle loads move forward after each load sub-step to simulate the movement of the vehicle. Step 3: Calculate the stress histogram and number of cycles by the rain-flow counting method. The stress ranges under the cut-off limit were ignored because they were considered ineffective on the fatigue of the details. Step 4: Calculate the equivalent stress and further obtain the fatigue life according to Eqs. (3) and (4).

4.2. Stress responses

Fig. 7 depicts the von Mises stress contour plot of Connection L18-2 under self-weight, where a significant stress concentration exits in the weld detail of the floor-beam with stringers. The complex configuration of the connection is primary reason for the stress concentration in the weld detail.

Unit: MPa

-20.0 -6.7 6.2 19.3 32.4 45.5 58.6 71.7 97.9 84.8

Fig. 7. (a) The von Mises stress contour plot under self-weight.

Fig. 8(a) presents the stress-time histories at the weld detail of the stringer-to-cross-beam connections (i.e. Connection L16-2 to L20-2) along the bridge under the passing of the FLM-3, where the connections are located near the wheel tracking. It is observed that each connection has a similar response. However, the connection L19-2 has the largest response among stringer-to-floor-beam connections, although connection L18-2 is closest to the eyebar chain transition point. The maximum stress range of connection L19-2 is at least 5.9% than that of other stringer-to-floor beam connections. It seems that the resulting stress redistribution occurring at the may have an influence on the stress response of the critical details. In addition, the influence of each axle can be clearly identified from the peaks or valleys of the stress-time histories, as shown in Fig. 7(a). This phenomenon has been observed and identified in bridge field monitoring (Guo et al., 2011). It reveals that such connections are subjected to significant stress ranges under passing trucks, which is the primary reason for significant fatigue damage in the weld detail of such connections. Note that the stress range, rather than the absolute stress, is the concern; therefore, stress due to self-weight is excluded. Fig. 8 (b) further illustrates the stress-time histories of connections (i.e. Connection L18-0, L18-1 and L18-2) under the FLM-3. With the increased distance between the connection and tire, remarkable change exits in stress peaks or stress valleys among the stress-time histories. The maximum stress ranges of connection L18-0, L18-1 and L18-2 are 22.2 MPa, 5.076 MPa and 50.76 MPa, respectively. It is indicated in the commentary that the influence of tire pressure can affect significant stress ranges on the region of the structure under/near the wheel tracking while the effect.