Optimum design of rigid and semi-rigid steel sway frames including soil-structure interaction

Doğan, Erkan
In this study, weight optimization of two dimensional steel frames is carried out in which the flexibility of beam-to-column connections and the soil-structure interaction are considered. In the analysis and design of steel frames, beam-tocolumn connections are assumed to be either fully rigid or perfectly pinned. However, the real behavior of beam-to-column connections is actually between these extremes. Namely, even the simple connections used in practice possess some stiffness falling between these two cases mentioned above. Moreover, it is found that there exists a nonlinear relationship between the moment and beam-to-column rotation when a moment is applied to a flexible connection. These partially restrained connections influence the drift (P- effect) of whole structure as well as the moment distribution in beams and columns. Use of a direct nonlinear inelastic analysis is one way to account for all these effects in frame design. To be able to implement such analysis, beam-to-column connections should be assumed and modeled as semi-rigid connections. In the present study, beam-to-column connections are modeled as “end plate without column stiffeners” and “top and seat angle with web angles”. Soil-structure interaction is also included in the analysis. Frames are assumed to be resting on nonlinear soil, which is represented by a set of axial elements. Particle swarm optimization method is used to develop the optimum design algorithm. The Particle Swarm method is a numerical optimization technique that simulates the social behavior of birds, fishes and bugs. In nature fish school, birds flock and bugs swarm not only for reproduction but for other reasons such as finding food and escaping predators. Similar to birds seek to find food, the optimum design process seeks to find the optimum solution. In the particle swarm optimization each particle in the swarm represents a candidate solution of the optimum design problem. The design algorithm presented selects sections for the members of steel frame from the complete list of sections given in LRFD- AISC (Load and Resistance Factor Design, American Institute of Steel Construction). Besides, the design constraints are implemented from the specifications of the same code which covers serviceability and strength limitations. The optimum design algorithm developed is used to design number of rigid and semi-rigid steel frames.