Keywords: seismic design, composite joint, partial strength connection, cyclic loading, hysteretic behaviour, rotational ductility.

The technological development of recent years has led to the increasing application of complex isolators, dissipating systems, hybrid and/or sophisticated active control systems in seismic design. Similar goals and performance levels can be attained through the ductile design of traditional structures, which can develop high dissipation without a significant resistance reduction. In general, high-ductility structures must withstand low seismic actions, characterized by a small return period, without undergoing appreciable damages. Conversely for high-intensity earthquakes, such structures must exploit their plastic resources in order to dissipate seismic energy, sustaining substantial damages but guaranteeing at the same time, overall stability.

In design practice, the possibility of relying appreciably on dissipation effects translates into lower seismic design actions than those called for in brittle structures, which can count on elastic resources alone. In turn, lower design actions, which standards provide through higher values of the behaviour factor, allow for lower design values of plastic resistance, and therefore smaller structural sections and lower weights as well. The savings in terms of structural weight, when coupled with sufficient ease of execution, may render ductile structures very competitive in seismic areas.

Solutions assuring the necessary ductility can be obtained not only through careful study of building morphology, structural schemes and construction details, but also through the rational use of materials. However, the adoption in design practice of steel-concrete composite solutions, perhaps the most convenient from both the constructional and economic perspectives, has to date been precluded by the lack of suitable constructional solutions. In fact, Eurocode 8 sets forth general principles for designing composite structures for seismic areas and imposes precise constructional and performance guidelines; however it does not furnish adequate information on the use of the various structural schemes, or on the associated constructional solutions and design methodologies, for which it often refers designers to codes and regulations regarding non-earthquake-resistant structures. Thus, it is necessary to conduct systematic studies of each and every potentially suitable structural scheme by analysing their constructional and economic requirements as well as their performance in terms of ductility and dissipative capacity. Such analysis must provide designers with precise rules regarding the constructional solutions suitable to each scheme and to the associated design methodologies necessary for evaluating their performances.

When analysing the possible structural solutions, it becomes immediately evident that the use of composite columns and beams, by stiffening the structural elements and therefore significantly limiting second-order effects, can allow the erection of buildings of considerable height without the need to use bracings. This advantage, combined with the introduction of partial strength joints, guarantees the formation of global dissipative frame mechanisms for seismic loadings, while at the same time avoiding unwanted storey or local mechanisms. If associated to suitable constructional solutions, such structural types can undoubtedly provide significant advantages in terms of both economy and performances.

Clearly, the choice of such structural systems in current design practice cannot leave the preliminary evaluation and mechanical definition of the different possible connection types and the associated structural details out.

In this paper we illustrate and discuss both the principles and design methodologies for building beam-to-column joints for high ductile, composite steel- concrete frame structures, made up of composite slabs and partially encased composite columns. The global mechanism is obtained by localizing dissipation phenomena in the beam-to-column joints and at the column bases.

The principles set forth have then been applied to the design of beam-to-column joints and columns of a full-scale frame structure that will be built and subjected to pseudo-dynamic tests at the ELSA Laboratory of the Joint Research Centre in Ispra.

Some full-scale substructures representing an internal joint have been subjected to monotonic and cyclic tests at the Laboratory for Materials and Structures Testing of the University of Pisa. Test results of interior joints entail a favourable hysteretic behaviour in terms of strength and rotation capacity.

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