The implementation of circular economy principles in building activities holds the potential for substantial environmental, economic, and social benefits. Although extensive research has examined the impact of circularity strategies on various aspects of buildings, there is a significant gap in the literature focusing specifically on building components and systems (BC&S). Most existing studies develop indicators applicable to buildings as a whole or solely at the materials level. This study aims to address this gap by identifying and emphasising specific circularity criteria for BC&S, including structure, infill, and services. The primary objective is to elucidate the contribution of each system to the overall circularity of buildings, thereby prioritising the most impactful circularity aspects. At the component level, it is essential to consider the specific attributes of component assemblies that constitute a system. To enhance the practical application of these findings, the study is supplemented with relevant case studies demonstrating best practices for circularity in BC&S. These case studies provide empirical evidence and practical examples of how targeted circularity strategies can improve the sustainability and efficiency of building practices, thereby advancing the goals of the circular economy.

Circular Manufacturing (CM), understood as CE strategies adopted in manufacturing, takes a key position in decoupling industry growth from environmental impacts. To achieve a transition into circular economy in construction, a clear view on the state-of-the art is crucial. Construction materials such as concrete, cross-laminated timber or steel have an environmental impact during their production and circularity is not always given. Knowing that the design phase of a product defines a big part of its overall environmental footprint, this chapter discusses CM principles and most commonly pursued CM strategies for steel, concrete and timber. Effects and impacts on buildings and eventual challenges are discussed. Furthermore, Additive Manufacturing (AM), as a possible key driver of circularity is analysed. The reduction of material use is identified as key driver in order to reduce material flows, however structural safety and durability needs to be assured. The design and mixture of construction products and materials itself together with an efficient design process in the projects are essential pillars of CM. Prefabrication, modular construction as well as DfD and DfA are key principles that can be achieved with all the analysed construction materials but are more widespread in steel construction today.

With the emergence of advanced manufacturing techniques (e.g. additive manufacturing) and the availability of novel structural elements with non-conventional geometries, there is an increasing need to reassess the stability and buckling behaviour of compressed struts. Indeed, current buckling formulations adopted in design standards, such as the Eurocodes, account for material nonlinearity and imperfections proper of conventional manufacturing processes. Those imperfections relate mainly to residual stresses and global (member-level) and local (cross-section-level) initial geometrical imperfections, which were calibrated against extensive experimental data obtained from conventionally manufactured steel sections. This study investigates the compressive behaviour of steel struts with pronounced initial global geometrical imperfections by proposing analytical formulations to predict the axial compression resistance. The classical Perry-Robertson buckling approach is extended to explicitly account for large initial out-of-straightness, material plasticity, and strain hardening. The proposed analytical formulations provide practical design-oriented tools, as they can be represented through compression resistance curves derived for a wide range of imperfection amplitudes, thereby extending the applicability limits of current design buckling curves. Numerical simulations are conducted to validate the proposed formulations, considering different constitutive models, including elastic-perfectly plastic, elasto-plastic with hardening, and Ramberg-Osgood material behaviour. The results highlight the necessity for dedicated design methodologies tailored to innovative structural elements, such as additively manufactured slender steel struts.

Wire Arc Additive Manufacturing (WAAM) enables high deposition rates and large build volumes compared to other Additive Manufacturing (AM) technologies. Further advancements have been achieved through the Dot-by-Dot (DbD) WAAM strategy, which allows the fabrication of complex geometries, particularly intricate lattice structures, with enhanced control over material properties via thermal cycle regulation. However, DbD WAAM presents significant challenges in geometric accuracy, primarily due to its thermal cycle, where heat buildup and limited interpass temperature control lead to unsatisfactory geometries and defects, requiring manual interventions that compromise repeatability and reliability.This work introduces the initial findings of the CIRCULARWELD research project, focused on developing a comprehensive experimentally-validated modeling framework for DbD WAAM. Designed to optimize both process and product without the need for expensive and unreliable trial-and-error, the framework integrates Geometrical (PG), Thermal (TM), Microstructural-Mechanical (MM), and Mechanical-Structural (MS) digital models. These models investigate mutual interactions to link process parameters with geometrical accuracy, microstructural features, mechanical parameters, and structural performance. This paper presents preliminary results of the framework’s application to single DbD WAAM rods of different nominal diameters, focusing on initial model calibration and validation. Experimental results confirmed the efficiency of calibrated geometrical and thermal models, enabling good process repeatability and rods with controlled geometrical features and mechanical properties. The rods demonstrate mechanical and structural performances compliant with steel grade S275, suitable for structural applications.