In this study we present an efficient new hybrid metaheuristic for solving size optimization of truss structures. The proposed ANGEL method combines ant colony optimization (ACO), genetic algorithm (GA) and local search (LS) strategy. In the presented algorithm ACO and GA search alternately and cooperatively in the solution space. The powerful LS algorithm, which is based on the local linearization of the constraint set, is applied to yield a better feasible or less unfeasible solution when ACO or GA obtains a solution. Test examples show that ANGEL can be more efficient and robust than the conventional gradient based deterministic or the traditional population based heuristic methods in solving explicit (implicit) optimization problems. ANGEL produces highly competitive results in significantly shorter runtimes than the previously described approaches.

In this paper, a genetic algorithm is proposed for discrete minimal weight design of steel planar frames with semi-rigid beamto-column connections. The frame elements are constructed from a predetermined range of section profiles. Conventionally, the analysis of frame structures is based on the assumption that all connections are either frictionless pinned or fully rigid. Recent limit state specifications permit the concept of semi-rigid connection of the individual frame members in the structural design. In a frame with semi-rigid joints the loading will create both a bending moment and a relative rotation between the connected members. The moment and relative rotation are related through a constitutive law which depends on the joint properties. The effect, at the global analysis stage, of having semi-rigid joints instead of rigid or pinned joints will be that not only the displacements but also the distribution of the internal forces in the structure must be modified. In this study, a simplified beamto-column connection is presented which was specified in EC3 Annex J. In order to capture the changes in the nodal force and moment distribution in terms of joint flexibility, the ANSYS finite element analysis is applied. The structural model is formulated as a combination of 3D quadratic beam elements and linear torsional springs. Present work deals with the effects of joint flexibility to the optimal design problem. The design variablesincluding joint properties -are discrete. Results are presented for sway frames under different load conditions.

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In the real-world truss optimization problems, the optimal performance obtained using conventional deterministic methods can be dramatically degraded in the presence of sources of uncertainty. The source of uncertainty may be the variability of applied loads, spatial positions of nodes, and section and material properties. In this paper, we present a new theoretical model and a problem-specific metaheuristic approach when the only source of uncertainty is the variability of the applied load directions. The essence of the novel conception is independent from the theoretical description of the uncertainty which may be either probabilistic (stochastic) or possibilistic (fuzzy). In the presented unified (nonprobabilistic and nonpossibilistic) approach, the varying load directions are handled as uncertain-but-bounded parameters. The result of the optimization is a robust minimal-weight truss design, which is invariant to the investigated load uncertainty type. The well-known ten-bar plane truss example with the most popular direction settings will be used to illustrate the validity and efficiency of the presented approach. In the presented example we replaced each nominal load direction by an angle set around the nominal value. The detailed description of the problemspecific robust metaheuristic based on the previously developed ANGEL metaheuristic will be presented in a forthcoming paper.

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