# Topology Optimisation in Structural Steel Design for Additive Manufacturing
## 9 Discussion and Research Perspectives  
In opposition to the previous section, where future trends have been collected from distinguished experts' opinions and research articles, the current section aims to analyse the whole revision work and, from a critical assessment, depict a current status and infer future tendencies. Adding to relatively scarce previous Review Articles, the current article was able to provide an embracing picture of TO, and DfAM developments occurred in the last 5 years. Considering the concentration of cutting-edge research in a few research centres, as well as a significant asymmetry between the leading funding agencies and the remaining ones, the study of patents in TO was regarded as an indispensable step for understanding the role of the industry in pushing TO forward. Nevertheless, it also has been found that the current surge in TO is partly owed to many newcomers entering the field. Other reasons can be attributed to the close link between TO and AM and the later significant emergence. Approaches and methods in TO have been found to compose a broad, yet heterogeneous, fabric of resources. Those encompass and blend techniques originated in Discrete Optimisation and Optimisation of Continua and offer an increasing palette of the forthcoming TO classification options. Nevertheless, the latest research trends suggest a post-SIMP era, where such a method's current predominance is now challenged both by very interesting research on Level-Set methods, as well as evolutionary approaches maturity and pervasiveness in many engineering applications, especially fuelled by Genetic Algorithms. Arguably, this trend may be related to the significant controversy over the ESO and BESO methods, which, despite continuous enhancements, seem to be deprecated to GA methods as the first choice in many new practical applications. Observing the most prolific research centres recent output, it is possible to speculate that research in TO methods and algorithms is likely to evolve towards computationally demanding solutions, such as multiscale projection and giga-resolution solutions, as well as further developments in surrogate-based optimisation, more complex techniques for addressing stress constraints and optimisation under uncertainty. Bringing TO into the engineering practice is another issue, as non-experts are particularly dependent on existing programmes, codes, and commercial software. Regarding this matter, the SIMP method is expected to remain preponderant, considering its current dominance in commercial software. However, the herein depicted investigation showed a clear future tendency for the software market leaders to offer hybrid approaches and to patent their own methods. Another observation from the current revision on programmes and codes is the unveiling of a more profuse set of alternatives, compared to what is frequently referred. Such information may assist both practitioners and early-stage researchers to find convenient alternatives and means for benchmarking solutions. In structural steel design, TO is now mostly constricted to prototyping and nodes in tensegrity structures. Its large-scale employment depends on several factors, including reliability in additively manufactures alloys properties, the ability to account for multiple alternate loading and multiple local instabilities in TO, better addressing the non-linearityand the existence of comprehensive and code-compliant practical methodologies for practitioners. Yet, it is interesting to observe that while many researchers find the mass reduction in steel members and connections due to the employment of TO extremely rewarding and unattainable otherwise, others still put the economic viability threshold further than what has been attained so far. Arguably, the type of member and connection seems to play a decisive role in the economic viability of TO.