An improved K-factor formula

The effective length factor, K, of a column is dependent on the lateral restraint imposed on it. The sway and no-sway alignment chart K-factors are special cases when the lateral restraint is zero and infinite, respectively. Therefore, they are not directly applicable to practical columns. A general formula is derived that is applicable to columns in fully braced, partially braced, and unbraced frames, i.e., to cases in which the lateral restraint can vary from minus infinity to plus infinity. The advantage of using the formula is demonstrated by comparing it with other methods. For example, the AISC LRFD Specification Commentary does not allow K-factors less than the limit imposed by Eq. C-C2-4b to be used when calculated by means of Eq. C-C2-4a. With the proposed formula this restriction is removed, thereby increasing the design range. It also shows that K-factor formulas based on the stiffness concept become increasingly unconservative as the lateral restraint becomes very large.