Sustainable architectural top additions with light-frame timber structures also incorporating dissipative braces

A study on sustainable architectural top additions for reinforced concrete (RC) buildings based on the use of light-frame timber (LFT) structures is presented in this article. The objectives of the study are: a) to evaluate the possibilities offered by traditional LFT solutions; b) to propose a new LFT configuration that incorporates a small-sized dissipative bracing (DB) system, easily hidden behind the sheathing panels of the timber structure; c) to extend the installation of the DB system to a small number of perimeter spans of the underlying RC structure in order to achieve its seismic retrofit with no architectural intrusion and interruption in the use of the building. For the simulated application of these structural solutions, a representative real case study is selected, consisting of a two-storey RC residential building characterized by an eccentric position of the RC core surrounding the stairwell. This causes significant seismic torsional response effects in plan, making this case particularly challenging. A single-storey architectural top addition is designed for the building, covering 63% of the flat roof level. The results of the study show that: thanks to its intrinsic lightness, the traditional LFT structure produces a moderate increase in inter-storey drifts and stress states in the structural members of the underlying RC structure, as compared to asbuilt conditions; the incorporation of the DB system in only eight spans of the LFT structure in the longitudinal direction of the building, and four spans in the transversal direction, allows to meet the basic design objective of almost completely annulling the increase in seismic demand caused by the construction of the upper addition; the seismic retrofit of the RC structure is achieved by extending the installation of the DB system only to 8% of its transversal spans, situated on the side façade, and no longitudinal spans, without any architectural impact on the building.