LoCa-HSS: A circular approach to the structural retrofit and vertical extension of obsolete social housing

In response to current decarbonisation targets and the widespread obsolescence affecting a significant portion of the Spanish residential building stock, this study proposes an intervention strategy for mid-twentieth-century social housing neighbourhoods based on the Low-Carbon Hybrid Structural System (LoCa-HSS). The approach in grounded in the hybridisation of existing reinforced concrete structures with lightweight, low-carbon systems, enabling vertical extensions, dwelling enlargement, and the incorporation of lifts to address multiple forms of obsolescence simultaneously. The research adopts a phased and practice-oriented methodology, conceived to ensure both replicability and applicability in real-world contexts. It begins with the selection of a representative case study, for which a comprehensive architectural proposal is developed. Building upon this design, a hybrid structural system is defined and subsequently evaluated from both structural and environmental perspectives. Finally, the proposed solution is validated through structural verification and a comparative assessment of its embodied carbon against that of an equivalent newly built reinforced concrete structure, as well as relevant environmental ratios for residential building structures. The results demonstrate that the proposed system does not require strengthening of the existing structure or foundations, as it does not lead to a significant increase in transmitted loads. This results in both economic and environmental advantages. Furthermore, the system is conceived as a standardisable and replicable solution, taking advantage of the typological and structural repetition that characterises mid-twentieth-century residential developments in Spain, thereby offering a scalable strategy applicable to a wide range of existing housing contexts.