the flange goes to zero. However, the full stroke of the gate causes the beam to go past the vertical position, resulting in a positive tensile stress of 2.5 ksi on the same flange. When the gate is lowered again, the cycle is repeated. Thus, the stress range is [2.5 – (–7.5)], or 10 ksi. See Figure 11–2c. In the final example, consider the top flange of a simply supported beam that is part of a bridge-like structure within a building, across which forklifts travel. The dead load of the beam and the slab generate a compressive stress of –5 ksi. The forklift applies a live load, which increases the compression to a total of –15 ksi. The stress range is [–5 – (–15)], or 10 ksi. See Figure 11–2d. A comparison of the four illustrative examples shows that the stress range, as determined by subtracting the minimum stress from the maximum stress, yields the same 10 ksi value. In fatigue, all four loading conditions would put the same demand on the members and connections of such members, since the stress range is the same. This is true, despite the fact that loading conditions varied significantly: tension to tension (Example 1), no load to tension (Example 2), tension
to compression (Example 3), and compression to compression (Example 4). The static strength of the system must be established, and the design of the members and connections in terms of the static strength will be different. In Example 4, where the loading is compression to compression, fatigue cracks may initiate but will not propagate outside the region of the residual tensile stresses due to welding. To reduce the stress range, two options are available: reduce the loads or increase the material available to resist the loads. Note that increasing the steel strength will not help; if no change is made other than using a higher strength steel, the stress range will not change. Furthermore, the stress raisers associated with welds on lower or higher strength steel are the same. Thus, the steel strength does not affect the fatigue performance of welded steel applications. An increase in the steel strength will result in an increased ability to carry dead loads, but does not increase the live load capability. Ironically, using higher strength steel often increases the problems of fatigue, because when higher strength steels are specified, smaller members can be used to resist the
Figure 11–3. Stress range versus cycle life.
DESIGN GUIDE 21 / WELDED CONNECTIONS—A PRIMER FOR ENGINEERS/ 115