PSI - Issue 78

Gennaro Vesce et al. / Procedia Structural Integrity 78 (2026) 936–943

937

1. Introduction Vertical additions on existing buildings represent an optimal solution to reduce the horizontal expansions of the cities. In fact, the concept of a city above the city - widely used since from the past to counteract the urbanization phenomena in areas with restricted horizontal spaces available - facilitates the vertical people allocation without additional soil consumption and minimizing the effects on the environment (Kremer et al. 2023, Sanei et al. 2025, Li et al. 2025). Lightweight timber vertical extensions represent a renewable and flexible solution for improving the resilience and the environmental sustainability along with the life cycle of the constructions, enabling the reuse and requalification of existing buildings (Hafner and Stork 2019). Moreover, lightness of the material adds the advantage of introducing reduced masses with respect to other construction materials (e.g., masonry or reinforced concrete) representing a fundamental benefit for constructions in seismic-prone areas (Branco and Neves 2011). To date, real applications of multi-story timber vertical extensions varying from one up to more than ten additional storey are available worldwide, the great part of them made with light or heavy timber frames or Cross-Laminated Timber (CLT) panels (Kremer et al. 2024). Although timber vertical additions are gaining importance in urbanized contexts, their design does not follow common shared rules. This because no indications addressing towards rationale and standardized design processes supporting the practitioners are available in the national and international standards (Bartolotti et al. 2023). To this aim, the Chapter 8 of the Italian Building Code (IBC 2018) dedicated to existing buildings reports a single sentence underscoring that in case of vertical extensions the existing building must be verified with respect to seismic actions calculated according to the current design earthquake provided by the code, i.e. corresponding to those required for new constructions. Basically, this sentence, precautionary and general, needs to be supported by scientific results capable of capturing and differentiating the real effects of the vertical extensions on existing structures depending on masses and stiffness ratios between the upper and lower structure (Faiella et al. 2020, Bartolotti et al. 2023, Sandoli et al. 2025). On the other hand, by applying the approach suggested by the IBC 2018, strong, invasive and expansive retrofit interventions may be required for the lower structure. To date, very few studies addressing towards the understanding the seismic behavior of existing buildings with vertical extensions are available in the technical literature. The behavior of masonry buildings with steel vertical extensions, having an intermediate isolation system, is investigated in Faiella et al. 2020. Whereas the effectiveness of light timber frame vertical extensions on reinforced concrete buildings is analyzed in Bartolotti et al. 2023, highlighting as the effect of the timber upper structure may be also not negative in term of supplementary forces for the lower structure. Recent insights on the dynamic of masonry buildings with CLT vertical additions are also collected in Sandoli et al. 2025. They investigated the role of some design parameters affecting the dynamic behavior of such constructions, such as vibration periods, participating masses and modal shapes as a function of number of additional timber storey and of the mechanical connections. The present paper reports some remarks on the seismic demand arising in masonry buildings with CLT vertical extensions, delivering basic concepts for the computation of the seismic actions to perform linear and nonlinear static analyses. The research, setting a step forward with respect to the results achieved by the authors on this topic, focuses on the role of the mass ratio on the evaluation of the seismic demand accounting for the effects of the vibration modes. To this aim it has been selected masonry buildings made with a single-story CLT vertical addition, and elastic modal analyses were conducted on a finite element model accounting for the effect of different mass ratios between the upper and lower structure. 2. Research background and motivations The results herein collected are part of a research project, under development in the context of the Italian DPC ReLUIS 2024-2026 (Work Package 12), aimed at providing standards contributions for civil and industrial steel, timber and composite constructions. Among those, a challenging topic is that concerning the definition of the optimal design rules for existing buildings with timber vertical additions. The present research is motivated by the recent results concerning the structural behavior of masonry buildings with CLT vertical additions recently reported in Sandoli et al. 2025, recalled in this Section. The core of the research was