Printing Reinforcement Steel

Abstract

In light of the global attempts to reduce material use by the construction industry, this research focuses on combining topology optimisation with additive manufacturing of steel. It is investigated whether an automated procedure can be developed to generate reliable strut and tie models for reinforced concrete elements, while satisfying the constraints that apply to 3D-printing using the Wire and Arc Additive Manufacturing(waam) technique.
Additive manufacturing offers a fully automated production process where a large freedom in form can be achieved. Topology optimisation concerns with finding a good material distribution within a prescribed domain. A literature review was performed on current developments regarding both subjects. It was found that the waam-technique is very suitable for printing reinforcement designs. Sufficiently large models can be printed, and material properties can be achieved that match the properties of traditional reinforcement steel. This manufacturing process is expected to produce functional structures that can readily be used as reinforcement steel in buildings. Two main manufacturing constraints should be accounted for during design of the model. A minimum member inclination and a minimum member diameter are both expected to be necessary to ensure a smooth printing process.
Several different topology optimisation algorithms are discussed in the second part of the literature review, and it is determined which algorithm is most suitable to continue with in the rest of this research. Examples are presented that explain the functionality of three important optimisation schemes: Bi-directional Evolutionary Structural Optimisation(beso, Solid Isotropic Material with Penalisation(simp) and Ground Structure Optimisation(gso). It was found that all three can be used to analyse reinforced concrete. Each algorithm has advantages and disadvantages, so there is no obvious best choice. However, motivated by the easy access to member forces and availability of a very good Python implementation, it is chosen to use gso for the remainder of this research.
This Python script was modified to include the constraints that come with an additive manufacturing process. It was found that the minimum member inclination can straightforwardly be included. The new function that was proposed allows the user to specify a minimum inclination, and ensures that no members are generated within the design domain that violate this minimum angle. Experimenting with this new function revealed cases where material use increased significantly when this function was used. This lead to development of an alternative procedure to ensure a printable design. In this alternative procedure, an optimisation without any angle constraint is performed first. Then, in the form of a post-processing script, The complete model is rotated around two separate axes in an attempt to find a suitable printing orientation.
The third and final proposition that was done in this part, consists of a post-processing script for the minimum member diameter. Including this minimum diameter in the optimisation would require rigorous changes to the optimisation script. Therefore it was chosen to investigate the performance of this post-processing script first.
The case study that was performed in the third part of this research, proved that this post- processing script for member diameter is sufficiently efficient for practical implementation. Together with the ability to slightly suppress the amount of members that are generated in the design domain, the printing constraint for minimum diameter could relatively easily be enforced. A bigger challenge lies within ensuring the minimum member inclination. The 60◦ minimum that was set, proved to be very harsh on the solution space. In the example from the case study, no printable model could be generated without significantly reducing the material efficiency. However, it is argued that this minimum inclination constraint can possibly be relieved by recent developments in additive manufacturing techniques. An example of this could be a rotating printing surface, that has the potential to remove this angle constraint completely.
Overall, the experience of combining topology optimisation, additive manufacturing and strut and tie modelling has been predominantly positive throughout this research. The combination of a state of the art manufacturing technique and a more performance driven design process with a labour intensive traditional calculation procedure has shown promising first results. In the example in this research, 30% less material was required to accommodate the tensile forces in the concrete.