Top additions of existing buildings with timber-based structural solutions are attracting growing interest from companies and operators in the sector, as well as from the professional world, for the considerable potential they offer. Indeed, they present several advantages, including: the greater intrinsic lightness of the wooden structure as compared, for a same structural performance, to solutions made with other structural materials, with consequent lower additional masses, particularly important for interventions in seismic zones; the flexibility and speed of installation (with elements that can be assembled on site or entirely prefabricated); the equally simple possible future dismantling and recycling, or re-adaptation to new functional and usage needs; the aesthetic pleasantness; the versatility of finishing and integration into the plant and energy project, etc.

Based on these considerations, a research study on light-frame timber superstructures to be installed on top of reinforced concrete buildings located in medium-to-high seismicity zones has been recently started. The study has a double objective: to evaluate the possibilities offered by traditional “platform-frame” interventions, and to explore new configurations based on the incorporation of supplemental damping systems to be easily hidden behind the OSB structural sheathing panels. In the first part of the research activity, a representative real case study was selected, consisting of a three-storey reinforced concrete (RC) building, characterized by an eccentric position of the RC core surrounding the stairwells and the elevator shaft, which determines a remarkable irregularity in plan and thus significant seismic torsional response effects. The top addition extends for about 65% of the existing flat roof floor.

The paper reports: a seismic assessment analysis of the building in current conditions; the modelling criteria and the design of the light-frame timber superstructure in traditional configuration and in the presence of the supplemental damping system, constituted by dissipative braces incorporating small-sized pressurized fluid viscous spring-dampers; comparisons between the seismic performance of the superstructure, as well as of the entire building structure, in the two different design hypotheses.