Two Main Methods for Yield Line Analysis of Slabs

There is a controversy about whether the classical yield lines analysis methods are in fact different methods or simply different ways to develop basically the same method. In this paper two methods are proposed that, without invalidating previous ones, really correspond to the two different ways of performing yield line analysis and therefore facilitate a better comprehension of the general problem of the failure of slabs. These methods are the nearly abandoned "normal moment method" and a new "skew moment method." In "normal moment method" only bending moments are supposed to act at yield lines. In "skew moment method," twisting moments in addition to bending moments act along yield lines. The "normal moment method" is general only if yield patterns are "correct," that is, they are composed by possible yield lines. If yield lines are "incorrect," or not possible, yield line analysis can only be performed, in general, by means of "skew moment method." As shown in this paper, many of the classical solutions of yield line analysis correspond to incorrect yield patterns. This work demonstrates that Johansenʹs "nodal force theory"—or "equilibrium method"—and "work method" are only partial applications of "skew moment method." This generalization of yield line analysis allows defining new equilibrium conditions not included in classical yield lines theory and permits obtaining more accurate solutions.