of Off-Centre braced system. H.A. Mosalman Yazdi & N.H. Ramli Sulong. 1. Introduction. In designing structures against lateral loads, braced frame systems are ...
Genetic algorithm in locating the optimum midconnection of Off-Centre braced system
H.A. Mosalman Yazdi & N.H. Ramli Sulong
1. Introduction In designing structures against lateral loads, braced frame systems are commonly used. In general, they are divided into two groups; concentric and eccentric. Concentric braced systems are more desirable because of relative good stiffness, along with their easy construction and economy aspects, hence it is more popular than eccentric braced frames (Moghaddam et al. 2005). Eccentric braced frames need more construction accuracy, thereby it decreases construction speed and requires more cost. Hence, designers do not prefer this method in spite of possibility for allowing more openings, better stiffness performance and higher energy dissipation. Other advantages of these system compared to concentric braces is because of that they mainly yield in bending and, therefore their hysteresis behaviour is close to ideal elasticplastic systems without significant deterioration and degradation of strength and stiffness (Engelhardt and Popov 1989, Tsai et al. 1993). Researchers try to develop this system’s collapse mechanism because partial and local failure modes are responsible of the worsening of the energy dissipation capacity leading to an increased risk of collapse under destructive ground motions (Kim and Choi 2005, Bosco and Rossi 2009, Mastrandrea and Piluso 2009a,b). However, the fact that they should undergo severe
excursion in the nonlinear range during heavy earthquakes has led to the development of various expedients and innovations to either increase the ductility and the capacity for energy dissipation, or reduce stiffness by using base isolation systems or other relevant techniques. For example, the knee bracing system proposed by Aristizabal-Ochoa (1986) has advantage because it changes plastic deformation from simple yielding to plastic bending. Therefore, a much better performance in terms of hysteresis behaviour can be achieved because the diagonal braces provides most of the elastic lateral stiffness to eliminate pinching in the hysteresis of structures, where the brace is designed to resist compression without buckling (Balendra et al. 1990). Significant numbers of knee bracing systems with different configurations have been proposed to improve its capacity in energy dissipation (Lotfollahi and Mofid 2006, Mofid and Lotfollahi 2006). In this article, one of the invented Off-Centre braced (OCB) systems which is capable of providing certain amount of seismic isolation to the structure will be described. This system, as illustrated in Figure 1, consists of three brace elements, where the diagonal members BO and OC are not straight and have an eccentricity, i.e. Point O is connected by the third member to the connection of beam to column. This system with its special design allows architect to have more opening in the panels. In spite of many studies on this system focusing on tensile brace members under seismic loads (Moghaddam and Estekanchi 1995, Moghaddam and Estekanchi 1999), many evidences from recent earthquakes
have shown that the connection point O in Figure 1 (when the braced elements are subjected to compression) has instability in out-of-plane. This is because of 2-dimensional assumption in the design and consequently it caused a lot of damages under severe earthquake conditions (Saffari and Yazdi 2010). In this regards, some complexities arise in determining the location of connection point. Although at first look, it seems like a simple structural analysis problem, but considering many parameters affecting the system’s stiffness such as the brace element connection point from the frame elements, cross
section area of the brace elements and dimensions of the panel and opening, therefore finding the best connection point is not trivial tasks. In this article, a method based on genetic algorithms (GAs) is proposed to replace the trial and error method for finding the connection point and facilitate the design of this system. In this regards, investigation on operators such as selection, crossover, mutation and elitism has
been carried out to have a proper design. In addition, a feasible area based on the frame and opening dimensions is introduced to ensure valid connection point in the computations of generation for faster convergence.
Full available at : http://www.tandfonline.com/doi/abs/10.1080/15732479.2011.654125
