To stick to climate change targets and reduce global carbon emissions, the building sector needs to adopt a circular economy. In the steel sector, reuse or elements can reduce carbon emissions which are created in today's steel scrap recycling practices. This thesis contributes to closing the knowledge gap on design, costs, and environmental impact score for working with reclaimed steel elements. The tool Steel-IT is created to research the business-case and environmental impact score of low-rise office buildings. Steel-IT uses a donor element database and new office design, which are specified by the user, and generates new and reclaimed steel design alternatives. Using a weight and activity-based costing method, the costs of these design are estimated. Life-cycle analysis is used to quantify the impact. By generating results and analysing the effect of the input in the tool Steel-IT, the following conclusions can be made regarding the business-case and impact score of reclaimed steel in low rise office buildings. A case study using Steel-IT shows that a 25% impact reduction against minimal cost increase is possible. For this, the match between design and donor elements needs to be good. This match can be searched for using Steel-IT. In terms of design, connection detailing is the most critical factor in the success of a donor steel skeleton. Donor steel elements should be sourced from an existing building to limit storage costs and should not have toxic conservation systems applied to them. Both induce extra costs and make the business case of donor steel less competitive with new steel. In the future, currently suggested emission taxes do not affect the business-case significantly as these taxes are low compared to the total building costs. The most helpful action to increase the business-case is to apply donor steel more often. If the experience with donor steel and its availability grows, reuse will become easier. Steel-IT can contribute to this as it contributes to closing the knowledge gap on costs and impact and can help to match demand and supply.

The Netherlands is currently in the process of transitioning from a linear economy to a circular economy, in accordance with the ”Nederland Circulair 2050” policy. To increase the circularity of buildings, several approaches can be integrated. In this research, the so-called Design for Deconstruction and Donor Structural Framework concepts are elaborated as possible approaches. The first concept focuses on taking the future de- and remountability of a building into consideration during the design process. This concept allows buildings that approach their end-of-life phase to be (partially) reused as structural components, on a new location. The second concept can be applied during the construction phase of a building, where structural components of an old building are dismantled and reused in the to be constructed building. The difference between the two concepts thus being the life cycle in which they are applied. Therefore, the resulting benefit of using a Donor Framework can be seen immediately, whereas the benefit of applying the Design for Deconstruction concept can only be stated in the future. Unfortunately, the current procedure to measure the sustainability score of a building, the Life Cycle Assessment methodology, does not take these concepts into account. This makes determining their impact on the environment hardly possible. Also, due to the fact that detailed information about a design is required, a Life Cycle Assessment is made only once the design is final. In this order, all design variables are set such that designing towards sustainability is not an option. This research focuses on solving the introductory problems and aims to enable sustainable material choices for a structural design possible in the early design phase. Both the Donor Structural Framework and the Design for Deconstruction concepts were taken into consideration. This main goal has been split into two sub–questions:- How to assess the environmental impact of a steel, concrete and timber load bearing structure in the early design phase? -How to implement the Donor Structural Framework and the Design for Deconstruction concept into the existing Life Cycle Assessment methodology? The research questions have been answered by executing the following approach: 1.A parametric model is used in which not only the geometry and structural calculations are included, but the Environmental Impact Calculation as well. In the event of a design change, the Environmental Impact Calculation is automatically reiterated, which means different designs can be compared quickly based on their environmental impact. The model constructed for this study is suitable for designs in steel, concrete and timber. For each material a reference design is created. The Bill of Materials of these designs serves as input for the Environmental Impact Calculation on which the materials were compared in a later research phase.2.First, an existing end-of-life allocation method has been adjusted to include reuse during both the construction phase (Donor Structural Framework) as the end-of-life phase (Design for Deconstruction). Secondly, the Building Circularity Index, which recognizes a ”circularity score”, has been implemented in this method. In this study the Building Circularity Index is assumed as the ”probability of future reuse of the building”. The modified method was implemented in the parametric model to enable a real-time Environmental Impact Calculation. This approach has been fully implemented into a parametric visual script, executed in the Grasshopper, a parametric environment plugin of Rhino which enables visual scripting. Input parameters are imported from Excel, the Grasshopper script calculates the environmental impact and exports the results to Excel where they are visualized in a dashboard. Ultimately, the developed parametric model has been divided into a part containing the geometry and structural calculations of the reference designs and a part where the newly developed Environmental Impact Calculation method is implemented. Combining the results of both parts in the total model, it becomes possible to assess whether a design is best built in a certain material in the early design phase. The final model can provide results with or without the use of a Donor Structural Framework and with or without application of the Design for Deconstruction concept. For the purpose of demonstrating the functioning of the model, a reference design in steel, concrete and timber was implemented as a basic geometry. This geometry was assumed equal across all designs and for comparability purposes, dimensions were fixed. Consequently, it can be concluded from the results of these reference designs that using a Donor Structural Framework results in a lower environmental impact than applying the Design for Deconstruction concept by maximizing the remountability of a structure. Until a lifespan of 75 years, using a timber donor framework is the most sustainable solution for the reference design. From 75 until 100 years this is the case for steel and from 100 years onward, a concrete design, whether or not using a donor framework, results in the lowest environmental impact. In the current design practice of a building, the default lifespan has been determined by the function of the building (Functional Service Life). By using the model developed here, this lifespan can be determined on the basis of sustainability requirements instead of functional requirements. The differences in environmental impact for different lifespans can easily be compared. Therefore, it is made possible to steer towards a certain lifespan, in order to determine the most sustainable construction based on the clients requirements. This is currently not possible in the Dutch construction industry. However, these results do have their limitations, as they should not be interpreted as general but rather specific conclusions. The following points of attention apply: -Results should not be interpreted as general results, but these results only apply on the three reference designs as elaborated further in the research. These reference designs are not optimized for every material used. -Changing input parameters can have a significant impact on the results. In addition, a number of important parameters (reuse percentage, material lifespan etc.) have been assumed due to insufficient existing research. -The developed allocation equations include the incineration of timber too favorably. This results in a significant deviation in timber environmental impact for lifespans much shorter than 75 years. This flaw can be either due to the model, or the impact parameters as stated in the NIBE EPD app. Lastly, it is recommended to further research the assumed parameters in this research, especially the material lifespan and the incineration impact parameters. As these parameters can have a major impact on the environmental impact of a specific design.