PSI - Issue 78

Stefano Sorace et al. / Procedia Structural Integrity 78 (2026) 349–356

350

1. Introduction Building superelevations made with light-frame timber structural solutions are attracting growing interest from companies 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, resulting in less 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 possibility of future dismantling and recycling, or readaptation to new functional and usage needs, which is an essential requirement from the point of view of sustainability in the building sector; the aesthetic appeal; the versatility of finishes and integration into the plant and energy improvement project, etc. Based on these considerations, a study on the addition of light-frame timber structures on top of reinforced concrete (RC) buildings has recently been started, in the context of the Italian ReLUIS-DPC Project 2024/2026, WP12 – “Steel, timber and composite civil and industrial constructions”, Sub Task 12.2.2 – Timber top additions. The study aims at (a) evaluating the structural performance of traditional “platform-frame” top additions, providing contributions in terms of design and finite element modelling, and assessing the consequences of these additions on the seismic response of the underlying structures; (b) exploring the possibilities offered by the incorporation of supplemental damping systems, with sizes suitable to hide them behind the oriented strand board (OSB) sheathing panels of the light-frame structure, with the objective of substantially limiting the increase in seismic demand on the existing structures caused by the upper extension. This paper presents the results of the first section of this research, in which a real case study building was selected for the possible application of the two structural solutions. The building is a two-storey RC residential block designed in the 1990s, when the municipality where it is located was classified as a non-seismic zone. Subsequently, the new classification of the Italian territory provided for by the 2008 edition of the national Technical Standards placed the municipality in a moderate seismic zone, confirmed by the 2018 update of the Standards (IMIT 2018). The building is characterized by an eccentric position of the RC core surrounding the stairwell, which causes a notable irregularity in plan and thus significant torsional seismic response effects, making the case examined particularly challenging. The top addition planned in the architectural project for the height expansion of the building extends to approximately 70% of the flat roof level. The supplemental damping strategy adopted for incorporation into the timber structure consists of a dissipative bracing (DB) system equipped with small-sized pressurized fluid viscous (PFV) spring-dampers. The assessment analysis of the building in current state, the modelling criteria and the design of the timber superstructure in traditional and dissipative configurations, and a comparison between the seismic performance offered by the two solutions are summarized in the following sections. 2. Geometrical and structural characteristics of case study building The case study building is located in a medium-low seismicity site in Italy. Its plan dimensions are 36.3 m  22.2 m, with a total height above ground of 6.55 m. The ground and first storey heights are 3.4 m and 3.15 m, respectively. Fig. 1 shows the structural plan of the ground storey, including the X and Y axes of the Cartesian reference system assumed in the finite element analyses (being Z the vertical axis), and a longitudinal and a transversal section. The structure of the first and roof floor is 280 mm thick and made of 240 mm-high partly prefabricated R/C joists, parallel to Y , clay lug bricks, and a 40 mm thick upper RC slab. The beams in longitudinal direction, parallel to X , have an in-depth cross section of 1000 mm  280 mm, in the four internal alignments, and 350 mm  280 mm, in the two perimeter alignments. All beams in transversal direction have in-depth section of 400 mm  280 mm, except for the one facing the “C-shaped” RC core enclosing the flights of stairs (C-3/C-4 span), sized 1000 mm  280 mm, and those belonging to the A alignment, with section of 300 mm  280 mm. The 200 mm thick RC core has dimensions of 5555 mm along X and 3000 mm along Y . All columns have rectangular section with the larger side parallel to Y , equal to 600 mm on the ground storey, except for columns located on A-2, A-3, A-4 and A-5 fixed lines (Fig. 1), whose side is 400 mm long. On the first storey, the Y -parallel side is 600 mm long, for all columns belonging to the perimeter longitudinal alignments (1 and 6), and 400 mm long, in the four internal alignments (2 through 5). The X -parallel side is equal to 200 mm for all columns on both storeys. On the ground storey, the distance between the columns is equal to 6 m in the internal longitudinal alignments, and 3 m in the