LITTLE P.ENG. FOR ENGINEERING CONSULTANCY SERVICES

The piping code stress equations are a direct outgrowth of the theoretical and investigative work discussed above, with specific limitations established by Markl in his 1955 paper. The stress equations were quite similar throughout the piping codes (i.e., between B31.1 and B31.3) until the winter of 1974, when the power codes, having observed that Markl was incorrect in neglecting intensification of the torsional moment in a manner analogous to the bending component, combined

Markl's investigation of the fatigue problem, following the earlier recognition of the maximum stress theory of failure, led to identification of the basic problem in the design of piping systems. Not one, but two different criteria must be satisfied, one for primary loads, which may lead to single application catastrophic failure, and one for cyclic, displacement-drivenloads that may lead to fatigue failure (especially in the vicinity of fittings and other discontinuities) a

The Welding Research Council's Bulletin 330, "Accuracy of Code Stress Intensification Factors for Branch Connections" documented a major attempt to re-assess the existing code requirements for the intensification of stresses at tees and other branch connections. The difficulty of this task was summed up in the bulletin by author E. C. Rodabaugh, who stated: "We would rate the relative complexity of i-factors for pipe, elbows and branch connections by the ratios 1:5:500. Thes

As noted previously, Markl's fatigue tests generated endurance curves for various fitting configurations, such as straight pipe, butt welded pipe, elbows, miters, welding tees, unreinforced and reinforced fabricated tees, mostly using 4" nominal diameter, size-on-size fittings. Markl noticed that the fatigue failures occurred not in the middle of his test spans, but primarily in the vicinity of the fittings, and in those cases, they also occurred at lower stress/cycle combina

In almost all cases the material fatigue curves are generated using a completely alternating stress; i.e., the average stress component is zero. Research has shown that the magnitude of the mean stress can have an effect on the endurance strength of a material, the trend of which is shown below: Note that as the mean stress increases the maximum permissible absolute stress (Sa + Sm) increases, while the permissible alternating stress decreases. The relationship between the al