Seismic Analysis and Design Optimization of Light Timber Frame Vertical Additions on RC Low-Code Buildings

Vertical additions to existing buildings offer economic and environmental advantages but pose concerns regarding seismic safety, particularly in aging structures lacking adequate seismic design. This study proposes a strategy to minimize the seismic impact of light timber frame (LTF) vertical additions (VA) while ensuring benefits such as modularity, prefabrication, and sustainability. A reinforced concrete (RC) archetype building, representative of 1970s Italian residential structures, serves as the case study. The design of a two-storey LTF VA is performed using a parametric optimization approach with a simplified equivalent 3degree-of-freedom (3-DOF) model. Key design parameters include the number of timber walls and sheathing-to-framing nail spacing. The optimization process was conducted ensuring that the second story of the addition remained elastic, while the first story was allowed to yield and dissipate energy through hysteretic behaviour. Seismic performance was assessed using seven spectrum-compatible ground motions, comparing the as-built configuration of the case study building with its performance after the addition. The assessment was conducted via nonlinear time history analysis on both refined finite element (FE) models and the simplified 3-DOF equivalent model to evaluate the accuracy of the simplified approach. Findings reveal that optimized LTF VA can enhance seismic performance and reduce structural vulnerability compared to non-optimized configurations. While the simplified modelling approach has inevitable limitations, the study underscores the importance of well-designed timber vertical additions in improving seismic resilience of existing buildings.