The excessive use of steel and concrete as energy- and carbon-intensive construction materials have led to a great deal of research on environmentally friendly alternatives to replace conventional construction materials and methods that can reduce the negative environmental impact of the building industry. Application of timber as an environmentally sustainable and light-weight construction material has been highlighted in many studies, but, the widespread use of structural timber has been hindered by significantly different mechanical properties in longitudinal and transverse directions and its variability due to environmental conditions. The recent advancements in manufacturing engineered wood products such as cross-laminated timbers (CLT) with enhanced dimensional stability and similar mechanical properties in both directions have largely addressed the former drawbacks. Accordingly, it has been seen growing interest in mass CLT constructions and/or combining the light-weight CLT panels with steel and reinforced concrete to develop environmentally sustainable structural systems. One such system is steel-timber composite (STC) which comprises prefabricated CLT slabs connected to steel girders using mechanical connectors (e.g. screws and bolts). The adoption of STC floors in practice is however affected by lack of knowledge on the amount of achievable environmental benefit by the trade-off between embodied and operation energy consumption due to the lesser thermal mass of the timber compared to concrete. Furthermore, the long-term behaviour and vibration performance of the steel-CLT composite beams under service loads remains largely unexplored. This study demonstrates the environmental benefits (lower carbon footprint and energy consumption saving) of the STC system in the first step. Then, the hygro-mechanical properties of CLT are measured experimentally as input for numerical simulations. The acceptable long-term performance of the STC connections and beams under sustained service loads are demonstrated by long-term push-out and six-point bending tests in the following part. A simplified numerical model that takes advantage of fibre element is developed and validated against experimental data to predict the long-term creep induced deflections for a service life of 50 years. In the last part of this study, the vibration performance of the STC floors as a governing factor in the design of light-weight low-damping STC systems is studied experimentally and numerically.