The idealization of beam-column connection behaviour as being either flexible or rigid, as used traditionally in engineering practice, is not adequate, because it leads to very different mechanical responses. In this work, it is observed that, the choice of appropriate model for these connections influences the structural behaviour, i.e. the values and distributions of internal stresses, the displacements and structural stability.

Therefore, an appropriate choice of stiffness values for connections becomes very important in the design of steel frames. Furthermore, the knowledge of the real behaviour of connections enables a more rational use of the material properties and consequently to design more efficient structures. The consideration of intermediary stiffness in beam-column connections is a way to become more realistic in the structural analysis.

This paper investigates the influence of semi-rigid connections in the mechanical behaviour of steel plane frames and proposes a methodology for optimum design of these structures to take advantage of this knowledge. The resulting optimization problem seeks the structural cost minimization through the determination of optimum values of the stiffness of the beam-column connections, associated with classical sizing optimization.

A numerical model is developed using finite element techniques to study the behaviour of beam-column connections. The geometrical nonlinear behaviour of beam-column connections and the material nonlinear behaviour of the structural system are considered. The structural analysis model uses two finite elements to model plane steel frames: a frame element and a spring rotational element. The frame element, BEAM23, is used to represent the beams and columns of the structure and, additionally, a spring element, COMBIN39, is used to model beam-column connections to simulate their non linear semi-rigid behaviour.

A six-storey plane steel frame is shown as an example of the methodology proposed in this paper. This model was investigated by Freitas [1]. Six rotational springs for beam-column connections, one for three connections of each storey are considered. The results show that the consideration of the actual behaviour of semi-rigid beam-column connections and structural non linear behaviour in the optimum design of steel plane frames leads to various feasible configurations, local minimums, modifying significantly the distribution of stresses and displacements in the structure.

1

J.P. Freitas, "Optimum Design of Steel Frames considering semi-rigid connections and the Structural Instability", MSc Dissertation, State University of Norte Fluminense, Campos dos Goytacazes, Brazil, 2010.

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