Assessment of the design provisions for steel concentric X bracing frames with reference to Italian and European codes

In the field of construction in seismic areas, the current Italian technical code for constructions NTC2008 is substantially based on the design criteria of Eurocode 8 (EC8), although with some differences. Focusing on steel structures with X Concentric Braces (CB), which is one of the most common seismic resistant structural typology in steel buildings, the paper illustrates a critical review of design methodologies specified in NTC2008, with the intent of providing simplified and more efficient design criteria and procedures able to ensure adequate safety levels under seismic actions, according to the modern design approach. The study is divided in two parts. The first part consists in the design according to the standard rules of typical steel X braced structures by linear analysis, both static and dynamic. The aim 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 cross-sections and the connected structural members, like beams and columns. The second part consists in the assessment of the seismic response of structures examined. 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 criteria. Based on the results obtained, a proposal for the enhancement of design criteria is presented.