Keywords: cross-laminated timber, timber-concrete composite, CLT-concrete composite, shrinkage, composite factor, vibration, damping ratio, in-situ testing.

Cross-laminated timber–concrete composite (CCC) slabs are an increasingly adopted flooring solution that combines the structural efficiency of CLT with the compressive strength of a concrete topping. Despite their growing use, experimental data on the serviceability performance of realised CCC systems remain limited. This paper investigates both the vibration behaviour and long-term deflection response of a CCC floor supported by a steel framing system, through in-situ measurements and a large-scale numerical parametric study. Vibration measurements were conducted on a completed building in Budapest, where the slab was excited by single-person and group walking at controlled frequencies. The measured fundamental natural frequencies ranged from 13.5 Hz to 18 Hz, satisfying the Eurocode 5 minimum requirement of 8 Hz. Damping ratios ranged from 2% to 3%, consistent with values reported in the literature for similar systems. The parametric study examined the influence of concrete creep, timber creep, concrete shrinkage, and connection efficiency on long-term deflections over a 50-year service life, using a two-layer finite element model with 36,864 simulation runs. Concrete shrinkage was identified as the dominant factor, increasing deflections by up to 29%. The results offer practical guidance for the serviceability design of CLT–concrete composite floors.

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