3422 Fatigue Loading

Since fatigue is typically only a serviceability problem, members and connections are designed for fatigue using service loads. Most structures experience what is known as long-life variable-amplitude loading, that is, very large numbers of random-amplitude cycles greater than the number of cycles associated with the CAFL [29]. For example, a structure loaded continuously at an average rate of once per minute (0.016 Hz), would accumulate 10 million cycles in 19 years. With long-life variable-amplitude loading, structures are usually designed so that only a small fraction of cycles, on the order of 0.01%, exceed the CAFL [29,30].

If the percentage of stress ranges exceeding the CAFL is greater than 0.01%, the history of N variable stress ranges can be converted to N cycles of an effective stress range that can then be used just like the constant-amplitude stress range in S-N curve analysis. Typically, Miner's Rule [31] is used to calculate an effective stress range from a histogram of variable stress ranges. Theoretically, this effective constant-amplitude stress range results in approximately the same fatigue damage for a given number of cycles as that for the same number of cycles of the variable-amplitude service history. If the stress ranges are counted in discrete "bins," as in a histogram, the effective stress range, SRe [29] can be calculated as where a¡ is the number of stress cycles with stress range in the bin with average value Sri divided by the total number of stress cycles (N).

Variable-amplitude fatigue tests conducted with various sequences in the variable-amplitude loading history have shown that Miner's Rule is reasonably accurate in most cases but can be unconservative with some load histories with unusual sequences. For this reason, some fatigue design specifications for offshore structures put a safety factor of 2.0 on life if Miner's Rule is used [23].

In the AASHTO specifications [15], the stress range from the fatigue design truck represents the effective stress range. No additional safety factor is used for Miner's Rule since it is relatively accurate for truck loading on bridges. For large numbers of cycles, the AASHTO specification has another check that involves comparing the stress range from the fatigue design truck to half of the CAFL. The rationale for this check is that if the effective stress range is less than half the CAFL, most of the stress ranges should be below the CAFL, but occasionally (about once a day) the stress range can exceed the CAFL with no significant effect.

Misalignment at a welded joint is a primary factor in susceptibility to cracking. The misalignment causes eccentric loading, local bending, and stress concentration. The stress concentration factor (SCF) associated with misalignment is where e is the eccentricity and t is the smaller of the thicknesses of two opposing loaded members. The nominal stress times the SCF should then be compared to the appropriate category. Generally, such misalignment should be avoided at fatigue critical locations. Equation 34.3 can also be used where e is the distance that the weld is displaced out of plane due to angular distortion. A thorough guide to the SCF for various types of misalignment and distortion, including plates of unequal thickness, can be found in British Standard BS 7910 [32].

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