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Connection plate weld on a girder flange: Fatigue I and Fatigue II design check of a Category C′ detail (FHWA-NHI-16-016 Section 6.6)
| Tool | Welded Joint Fatigue |
| Source | Russo, F. M., Mertz, D. R., Frank, K. H., and Wilson, K. E. (2016). Design and Evaluation of Steel Bridges for Fatigue and Fracture: Reference Manual. FHWA-NHI-16-016, Federal Highway Administration, Washington, DC. Section 6.6, pp. 6.57 to 6.61; constants from Tables 6.3.3.2-1 and 6.3.4.2-1. |
| Level | Professional |
Problem
A cross-frame connection plate is welded to the bottom flange of a continuous composite plate girder at the 0.4 point of the end span, a Category C′ detail. The fatigue truck gives moments of +1,573 and −496 ft-kips; with 15 percent impact and a distribution factor of 0.75 lanes they are +1,357 and −428 ft-kips on a section modulus of 2,661 in³, a stress range of 8.05 ksi. The road carries 675 trucks a day in one direction on three lanes. Check the detail for infinite life and, if that fails, for a 75-year finite life.
Inputs to Enter
- Analysis: design check under truck traffic; detail category C′ (A = 44 × 10⁸ ksi³, threshold 12 ksi)
- Stress range of the fatigue truck 8.05 ksi; load factors 1.75 (Fatigue I) and 0.80 (Fatigue II)
- ADTT 675 trucks per day in one direction, lane fraction 0.80, 1.0 cycle per truck, design life 75 years
Answers
- Fatigue I: 1.75 × 8.05 = 14.09 ksi against the threshold of 12 ksi, ratio 1.17: infinite life is not provided (book 14.09 ksi > 12 ksi)
- (ADTT)_SL = 0.80 × 675 = 540 trucks per day; N = 365 × 75 × 1 × 540 = 14,782,500 cycles (book the same)
- Fatigue II: 0.80 × 8.05 = 6.44 ksi against (A/N)^(1/3) = (44 × 10⁸ / 14,782,500)^(1/3) = 6.677 ksi, ratio 0.965: finite life is provided (book 6.44 ksi < 6.68 ksi)
- Tool only: 83.6 years of this traffic reach the design line; the largest unfactored stress range that passes is 8.35 ksi
- Tool only: above 974 trucks a day in the lane the finite-life resistance falls below the factored threshold and only the infinite-life check can pass
- All categories at this stress range and traffic: A, B, B′, C and C′ pass (Fatigue II ratios 0.541, 0.690, 0.865, 0.965, 0.965); D, E and E′ fail (1.22, 1.53, 2.16)
Notes
The tool's default inputs are this case; press Submit. The stress range entered is the manual's rounded 8.05 ksi (8.047 from the moments). The other analyses of the dropdown load their own published cases: the cover plate evaluation of Section 8.3.3 of the same manual (lives of 42, 50, 57 and 63 years), its variable-amplitude Table 4.2.3-1 (effective stress range 4.96 ksi) and the riveted railway bridge of the JRC report EUR 23252 EN (damage 0.01668 per year, life reached in 1988).