# Topology Optimisation in Structural Steel Design for Additive Manufacturing
## 6 Recent Advances in Related Fields with Applicability in Structural Steel Design
### New Materials, Composites, and Polymers Design  
Materials design has been taking advantage of TO at a microstructural level and, conversely, promoting its development. As [PERSON] and [PERSON] [135] formulated it, the fundamental question in architected material design optimisation can be resumed to finding which microstructure will deploy sensible enhancements to macrostructural properties. The answer is complex and involves entrenched unit cell modelling, upscaling, and repetition [184,500,501]. Yet, diverse and frequently remarkable solutions can be attained, from useful elastic properties, including auxetic (NPR) materials (those with a negative Poisson's ratio), as popularized in [PERSON]'s work [502] and currently drawing much attention from the scientific community [503], to extreme thermal properties, optimised fluid permeability, and materials governed by non-linear mechanics for utmost energy absorption.  
Within the aforementioned exciting framework, recently published works include modelling methods for designing hierarchical structures employing non-uniform topologically optimised lattices [504], serving as enhanced energy absorbers in sandwich sheets [505] or facilitating manufacturing [506], even with new methods for mitigating non-smooth surfaces, as the Bezier Skeleton Explicit Density (BSED) Representation Algorithm [507]. The microstructural design for avoiding stress peaks has also been recently addressed [508], as uncertainty-resilient design for inter-diffusion interface issues has been brought forward [509], and advances in buckling of microstructures were published by [PERSON] et al. [510].  
The fabrication of these architected materials with AM techniques is yet another concern. Not seldomly published methods can be incomplete or vague, as [PERSON] et al.'s review pertinently points out, and offers mitigation by congregating some state-of-the-art answers [511].  
Even though new materials and, specifically, new material microstructures may still be far from promoting a change in steel alloys, structural steel design can find many important lessons in the formerly cited research. Both the tools and methods created for microstructural optimisation can be employed in the multi-purpose TO, but also solutions developed for a particular material microscale may have an employment in construction macrostructures composed of smaller members. That is also the case of recently developed methods for efficiently handling buckling in polymers optimisation [512], as well as for multimaterial TO (MMTO) [513,514], with its specific problems, as extensive local extrema, and techniques designed to overcome such obstacles. Furthermore, many important advances in TO methods and approaches are being developed within the materials design field, making it especially relevant, also for TO in civil and structural steel design.  
At a coarser scale, construction composite materials can also be enhanced under the assistance of TO. That is the case of the types of cement with an enclosure of rubber waste [515] or Carbon Fibre Reinforced Plastics (CFRP) [516].