Vertical Structural Irregularities

Abstract

This chapter explores five vertical structural irregularities: soft story (stiffness) irregularities, weight (mass) irregularity, vertical geometric irregularity, in-plane discontinuity in vertical lateral force-resisting element irregularity, and discontinuity in lateral strength-weak story irregularity. However, the chapter concentrates primarily on soft story irregularity and weak story irregularity. A soft story irregularity exists if the stiffness of any story is less than 70% of the stiffness of the story above it, or if the stiffness of the story is less than 80% of the average stiffnesses of the three stories above it. Weak story irregularities are difficult to detect because the concept of story strength is not well defined, even for somewhat simple systems, such as moment frames and X-braced frames. The chapter also discusses the consequences of vertical irregularities.