Fully Stressed Design of Frame Structures and Multiple Load Paths

Fully stressed design is an optimization heuristic that is widely practiced for member sizing of frame structures. When performed iteratively, such as with the stress-ratio method, it usually converges rapidly and yields a reasonable structural design. A previous paper has demonstrated that some frame structures are capable of being proportioned in many different ways, each of which is a distinct fully stressed design with a unique load path. Furthermore, many of these fully stressed designs are unstable fixed points under the stress-ratio iteration, and are consequently unobtainable by conventional methodology. This paper relates this behavior to Hardy Cross's notion of normal, hybrid, and participatory action. Parameter studies reveal that certain combinations of lateral-to-gravity load ratios and height-to-width ratios tend to produce a greater multiplicity of fully stressed designs. The multiple designs are classified according to eight commonly observed material distribution patterns, and their load paths are examined. The eight structural types are shown to have relative advantages in terms of material economy and resistance to sidesway. Practical construction-related issues are addressed by linking the sizes of multiple members through a common design variable.