Keywords: steel structures, composite and steel semi-rigid joints, composite construction, genetic algorithms, structural optimization.

One of the most important aims of the engineering science is the development of cost effective structural elements usually achieved with the implementation of new design philosophies and procedures. A typical example of these new trends can be identified in the composite and steel portal frame design that was traditionally based on rigid or pinned beam to column joint assumptions. Numerous investigations were conducted over the last few years to assess the actual response of semi-rigid joints and led to various formulations that enable the evaluation of the joint main characteristics in terms of stiffness, strength and rotation capacity like the recommendations present in the current Eurocodes 3 and 4 [1,2].

Despite the significant knowledge growth the composite semi-rigid design is still facing some resistance from structural engineers. This can be partly explained by the fact that few design standards allow or even encourage the use of this design philosophy. On the other hand, the semi-rigid design procedure involves repetitive and exhaustive tasks arising from the fact that numerous geometric and mechanical properties have to be considered. All these aspects, added to the various interconnected relations of the involved variables, inhibit the optimum joint configuration to be easily obtained.

When the search for optimal structural joints are considered within this context the genetic algorithm (GA) proves to be a valuable tool for optimizing steel and composite semi-rigid joints. Focusing on the objective of minimizing the effort spent in the structural design, the present paper presents an automatic optimization procedure implemented to determine the optimum joint design. This optimum joint has the required flexural strength and initial stiffness aiming at the minimum joint fabrication cost.

This paper used the procedures for the endplate beam to column steel and composite joint design according to the component method proposed in the Eurocodes 3 and 4 [1,2].

The example results indicated that the composite flush endplate joints performed better, in terms of flexural capacity and cost viability, than their equivalent extended steel endplate joints but the initial stiffness results of the later joints were in general higher. The results indicated that with the incorporation of the longitudinal bars and the reinforced concrete slab in the composite joint models led to a substantially higher flexural capacity than the non-composite joint models. This was particularly relevant when beams with heavier section were adopted.

1

Eurocode 3, EN1993-1-1, previous term Eurocode 3. "Design of steel structures, general rules and rules for buildings", European Commission for Standardization, 2005.

2

Eurocode 4, EN - 1994, "Design composite steel and concrete structures. Part 1.1 General rules and rules for buildings", European Commission for Standardization, 2002.

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