## 6 Results and Discussion
### Benchmarking SEMDOT Solutions against SIMP-Based TOSCA Optimisations  
The PhNLFEA to the SIMP-based TOSCA optimisation of the same original plate to a volume fraction of 55% showed an ultimate strength of 827 kN, achieved with an ultimate longitudinal displacement of 26.5 mm [21]. Comparatively, the optimal np-SEMDOT-optimised solution was able to resist 827 to 859 kN at a longitudinal displacement of 27.1 to 33.7 mm. Not only did the previously depicted results perfectly match in terms of strength and displacement capacity--the latter with a slight superiority of 2% to 4% for the np-SEMDOT solution--but the complete force-displacement relation with its stages through elastic and plastic domains (see Figure 18) was also almost coincident for both plates, except for the best (\(ER=0.50\) and \(r_{min}=4\)) np-SEMDOT solution, which adds a further displacement plateau, increasing the ultimate displacement by approximately 27%. Assessing the failure mode of each solution, one can observe that np-SEMDOT was more successful in involving four out of the five bolt holes in the ultimate plastic phenomena, whilst the TOSCA solution essentially mobilised significant plastic damage in three [21]. This may explain why the former achieved a superior ultimate displacement. Recent findings on bolted connections have shown that reaching the ductility requirements for topologically optimised parts largely depends on ensuring that all critical sections--the hole sections--have the same relation between loading and resistance. The current results corroborate these findings. In the context of the performed PhNLFEA, np-SEMDOT and SIMP-based optimisations can be concluded to be equivalent for engineering purposes. Significantly, both approaches meet the behavioural requirements of strength and ductility for steel connections in the context of structural engineering, and are code-compliant, making them suitable for real-world applications. Figure 18: Ultimate capacity and force-displacement relation for the np-SEMDOT best solution and for the baseline SIMP (in TOSCA software) optimised solution. Another significant aspect that should be considered in comparing the two solutions is the accuracy in ensuring the volume fraction. In TOSCA and np-SEMDOT, the target volume fraction was defined for 0.55 (55%), but in neither of those was the outcome exactly 0.55. While the TOSCA solution's volume fraction was slightly under 0.550, both np-SEMDOT solutions resulted in volume fractions of 0.528. Therefore, while for engineering purposes, the solutions would be referred to as 55% target volume fractions, for analysis purposes it should be highlighted that the np-SEMDOT solutions slightly outperformed the reference solution, with marginally less material. Concerning the boundary smoothness of each solution, a critical issue for manufacturing, a comparison between the np-SEMDOT and SIMP-based solutions illustrated in Figure 19 shows that, for external boundaries, the np-SEMDOT solution's large irregularities were less exuberant, meaning that it made large portions of inefficient material less likely to accumulate. On the other hand, both solutions were equivalent in terms of small irregularities or roughness. Concerning the manufacturability of the SIMP-based solution, it is important to note that several specimens of excellent quality have already been produced by subtractive manufacturing using standard cutting technology (as shown in Figure 20a). Figure 19: Superposition of the edges of the np-SEMDOT (ER = 0.25 and \(r_{min}\) = 2) and SIMP-based solutions. Figure 20: Manufacturing the (**a**) SIMP-based and (**b**) np-SEMDOT-based solutions. A further step has been taken with the np-SEMDOT solution in the form of a prototype produced by additive manufacturing (as shown in Figure 20b), proving its expected excellent manufacturability. Another critical issue in the assessment of the potential for mass employment of TO resources is its ease of use by practitioners.