Effective Column Length—Tier Buildings

The revision of the AISC Specification adopted in November, 1961 contained a large number of new provisions reflecting extensive research sponsored by the Institute during the previous decade. Yet none of these have provoked as much discussion (and misunderstanding) as those in Sect. 1.8 which relate allowable axial stress in compression to an effective column length instead of to the actual unbraced length as in earlier editions. Oddly enough, the underlying theory here is by no means new. Why, then, is it only now receiving recognition in American specifications? The answer lies in two recent developments both of which place greater emphasis than previously on the possibility of overall frame instability; the introduction of high strength steels and the trend towards light wall constructions.