Keywords: discrete truss design, ant colony optimization, genetic algorithm, local search, meta-heuristics.

Within the framework of the finite element method, we present in this paper an efficient new hybrid meta-heuristic - named in other context ANGEL - for solving discrete size optimization of truss structures. ANGEL combines ant colony optimization (ACO), genetic algorithm (GA) and local search (LS) strategy. The procedures of ANGEL attempt to solve an optimization problem by repeating the following steps. For the first time, the ACO searches the solution space and generates structure designs to provide the initial population for GA. After that, the GA is executed and the pheromone set in the ACO is updated when the GA obtains a better solution. When the GA terminates, the ACO searches again by using the new pheromone set. The ACO and the GA search alternately and cooperatively in the solution space. This study also proposes an efficient local search procedure, which is applied to yield a better solution when the ACO or the GA obtains a solution. The local search procedure plays a very important part in the proposed ANGEL method. During the optimization process, both ACO and the GAs use a population of agents or individuals to represent solutions, and the information collected by the population influences the next generation of the search. In the local search procedure, before the maintenance, we refine the first evaluated solution. In the main procedure of ANGEL we use the repetition of following steps:

• The ACO with local search, and
• The GA with local search.
• Finally, a fine-tuning search is conducted.

In this paper we applied ANGEL for discrete minimal weight design of space trusses with elastic-plastic collapse constraints. The geometrically and materially nonlinear space trusses are formulated as a large displacement structural model. The method of elastic-plastic collapse analysis is based on a path-following method [1]. The applied method is a combination of the perturbation technique of the stability theory and the non-linear modification of the classical linear homotopy method. With the help of the higher-order predictor-corrector terms, the method is able to follow the load- deflection path even in case of elastic-plastic material law.

1

A. Csébfalvi, "A non-linear path-following method for computing the equilibrium curve of structures", Annals of Operation Research, 81: 15-23, 1998.

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