With the building and construction sector accounting for a significant portion of final energy use and emissions, and society’s increasing focus on environmental awareness and improvements, the impor- tance of sustainable construction has never been greater. The utilisation of timber in construction offers clear sustainability benefits and has the potential to replace traditional functions of concrete. One notable replacement is the use of Cross-Laminated Timber (CLT) in composite structures with steel, enabling the potential creation of greatly performing floor systems which has seen a limited amount of research. The research explores the application of Steel-CLT Composite floor systems and assesses composite beam performance as well as predictive methods for analysing their behaviour in hopes of advancing the body of knowledge on this method of environmentally friendly construction. The literature study revealed the current knowledge on steel-CLT composite floor systems and connections. Two particular floor systems, with and without the use of grout, are highlighted as well the potential for future exploration. The steel-CLT composite connection utilised in the floor system significantly impacts performance and composite action as the use of grout is concluded to increase initial-, secondary stiffness and load carrying capacity of the connection. However, restricting the use of grout can promote circular con- struction practices. In this research the behaviour of steel-CLT composite beams is predicted by the use of a numerical finite element model with shell elements and an analytical approach based on the simplified analysis of Eurocode 5(EN1995-1-1): Mechanically jointed beams. The analytical method can predict the bending stiffness with a margin of error just slightly above 10%. However, it should be noted that the analytical method incorporates an incomplete representation of the composite connection used in the beam. On the other hand, the finite element model is able to accurately capture the pre-plasticity behaviour of the steel-CLT composite beam. This is achieved by incorporating the complete load-slip behaviour of the shear connection. A parametric study was conducted to assess the importance of closely spacing composite shear connections in a steel-CLT composite beam to achieve a higher level of composite action. The study also demonstrates that by utilising connections with higher initial stiffness, secondary stiffness, and ultimate capacity, this effect can be further strengthened. As a result, the force resisted at the service- ability limit state experiences a notable boost due to the implementation of both a stronger and stiffer connection, as well as their close spacing. Shear connection position optimisation by concentration of connectors near the supports showed to have a small positive effect on beam deflection. After evaluation of steel-CLT floor systems the research concludes that a steel-CLT composite floor system can be successfully constructed, offering environmental benefits and reduced structural weight while achieving the required structural performance for the set design conditions.

Recently, the construction industry has been confronted with changing regulations, especially with the advent of sustainability goals and circular economy ambitions. The Dutch government wants to accomplish its ambitious initial objective of using 50% fewer primary resources by 2030. Further, by 2050, the goal is to have a waste-free economy that relies mostly on sustainable and renewable resources and reuses both products and raw materials. Steel is a fundamental building component in the construction industry. However, due to changing market and industry conditions like lack of availability of raw materials for the production of virgin steel, exponential rise in prices of virgin steel and production of virgin steel being a high carbon and energy-intensive process - the use of virgin steel is becoming a not feasible option. Therefore, to meet the ambitions of the Dutch circular economy and to comply with the changing regulations like the demonstration of the Environmental Cost Indicator (ECI) as part of the Life Cycle Assessment (LCA) and the introduction of carbon taxes or EU ETS (European Union Emissions Trade System) is forcing the construction industry to limit the adoption of virgin steel. Thus, the reuse of structural steel elements is discussed highly in the building industry as an alternative. However, the reuse of steel encounters several barriers to its implementation. A combination of these barriers results in a paradoxical tension during the decision-making process in the design phase of the project is that “whether is it feasible to first locate existing reclaimed/demountable materials available for reuse from the market and then design around them [Material-Driven Design] or design first with the intention of sourcing/ identifying the required materials later during the procurement phase [Form-Focused Design] ?" The contradictory tension is called the design-acquire paradox. In this research, Biopartner 5, a project which has successfully implemented large-scale reuse of structural steel elements, is used as a case study to analyse the processes involved in adopting the reuse of steel. The successful implementation was facilitated by several favourable conditions that do not match the current market scenario and industrial conditions. Thus, the approaches adopted cannot be used unanimously for all projects that intend to consider the opportunity of reusing steel. The Ambidextrous Management Approach is identified as a feasible solution to deal with paradoxical tensions. In this research, with the principles of ambidexterity, an intervention is made into the existing industrial practices adopted to develop an Ambidextrous Process Tool. The tool can be used by various parties involved in the design phase of projects which intends to reuse steel. The tool aims to improve the potential and optimise the reuse of structural steel elements in buildings by mitigating the design-acquire paradoxical tension. The process tool emphasises balancing the exploration to improve the traceability of materials available in the market and the exploitation of identified materials. Furthermore, the tool focuses on embracing the potential of both Material Driven Design and Form-Focused Design simultaneously without making a trade-off between them.