Approximating lateral stiffness of stories in elastic frames

Despite advances in the computer analysis of frame buildings for lateral loads, there remains a need for simple models that provide accurate estimates of response. The concept of lateral stiffness is reviewed, and it is concluded that a single value can be used to represent the stiffness of a story in an elastic, rectangular frame with fixed base that is subjected to regular distributions of lateral load. Three existing expressions for approximating the lateral stiffness of stories are compared, but it is concluded that these are applicable only for uniform frames with girders that are flexurally stiffer than columns. An expression is proposed that includes three numerically derived factors that greatly improve the accuracy of stiffness estimates for regular and moderately irregular frames. The proposed expression simulates: (1) The effect of unequal heights for adjacent stories; (2) the influence of top and bottom boundaries; and (3) the stiffening effect of the fixed base in low-rise frames.