344 Summary

1. Structural elements where the live load is a large percentage of the total load are potentially susceptible to fatigue. Many factors in fabrication can increase the potential for fatigue, including notches, misalignment and other geometrical discontinuities, thermal cutting, weld joint design (particularly backing bars), residual stress, nondestructive evaluation and weld defects, intersecting welds, and inadequate weld access holes.

2. The fatigue design procedures in the AASHTO and AISC specifications are based on control of the stress range and knowledge of the fatigue strength of the various details. Using these specifications, it is possible to identify and avoid details expected to have low fatigue strength.

3. Low-cycle fatigue is a limit state for members and connections repeatedly cycled in the inelastic range, such as for seismic loading. Low-cycle fatigue can be predicted using strain-range versus cycles curves derived from the stress-based S-N curves for high-cycle fatigue.

4. Welded connections and thermal-cut holes copes, blocks, or cuts are potentially susceptible to brittle fracture. Many interrelated design variables can increase the potential for brittle fracture, including lack of redundancy, large forces and moments with dynamic loading rates, thick members, geometrical discontinuities, and high constraint of the connections. Low temperature can be a factor for exposed structures. The factors mentioned above, which influence the potential for fatigue, have a similar effect on the potential for fracture. In addition, cold work (e.g., from rotary straightening or punching holes), flame straightening, weld heat input, and weld sequence can also affect the potential for fracture.

5. The AASHTO specifications require a minimum CVN notch toughness at a specified temperature for the base metal and the weld metal of members loaded in tension or tension due to bending. Almost two decades of experience with these bridge specifications have proved that they are successful in significantly reducing the number of brittle fractures.

6. Surveys of CVN for wide-flange shapes sold in North America show that 99.9% of this steel meets the AASHTO bridge steel requirements for service down to — 34° C. Therefore, under most circumstances, there is no need to specify CVN for shapes used in buildings.

7. Achieving the required minimum CVN toughness in the girder flange-to-column flange groove welds is critical for good performance in the prequalified steel moment connections. Lot testing should be considered, or it may be necessary to specify electrodes that typically far exceed the minimum toughness levels. Either step would assure that minimum CVN requirements represent a realistic lower-bound of weld toughness deposited in the field.

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