Evaluation of the Italian seismic code for the design of concentrically X-braced steel structures

A critical analysis of the design methods provided by the current Italian technical code for constructions NTC2008 [1] for steel structures with X braces is carried out, with the aim of providing simplified and more efficient design criteria able to ensure adequate safety under seism. NTC 2008 are generally substantially based on Eurocode 8 - Part 1 [2]. Even though the previous Italian prescriptions provided by OPCM 3431 [3] introduced some noticeable changes on the design of steel structures with respect to the European standard, integrating the evidences of extensive studies on the seismic behaviour, those were not included in the current technical standards, in a view of guarding the symmetry with the EC8 [4]. What is more, the current NTC 2008 has several cuts with respect to EC8, which were partially recovered in the explicative Ministerial Circular [5]. Therefore the need to fill this gap, incorporating and merging all the most updated achievements of research related to the design of seismic resistant systems motivated the Italian project RELUISDPC (2010-2013), specifically focused on these issues. In particular the research unit UNINA-ING task was the optimization of design criteria for seismic resistant steel braced structures. First results of this research activity were provided in De Lucia et al. [6] and Macillo et al. [7], with respective reference to X-braced structures and chevron braced structures, the latter being described in Castaldo et al. [8]. The present study is divided in two parts. The first one consists in the design, according to the standard rules of typical steel X braced structures by linear analysis, both static and dynamic. The aims is identifying any possible weakness in the current design criteria, with particular reference to both the applicability of the proposed procedures and the actual possibility to size the bracing crosssections. The second part consists in the assessment of the seismic response of examined structures designed according to NTC 2008. To this purpose, non-linear static analyses are performed in order to evaluate the most relevant behavioural issues, like the behaviour factor, the failure modes and the effectiveness of the capacity design rules.