# Topology Optimisation in Structural Steel Design for Additive Manufacturing
## 5 Topology Optimisation in Structural Design of Steel Elements and Joints
### Joints Detailing  
In the current state-of-the-art steel connections (or joints, since both names are interchangeably used in structural engineering), TO is built of many mechanical engineering originated research endeavours and some developments made in the context of structural engineering research projects. Within the latter domain, innovative connections have been prototyped, breaking barriers of conventional manufacture constraints and delivering outstanding weight reduction [139]. Moreover, joints compactness, as regularly achieved by such designs, is critical for structural steel detailing, which is usually heavily constrained by space limitations. Some recent examples of topologically optimised and additively manufactured joints can be found in Figure 11. Herein, new concepts and interesting solutions for space structures can be found [422], yet the significant predominance of applications for lattice, reticulated, and generally tensegrity structures is evident. Such shortcoming is mostly due to the limited quantity of load combinations which can reasonably be considered for the optimisation process. Should multi-axial bending and shear add to the tension and compression stresses, the TO procedure would escalate several levels of complexity and yield less intuitive results, in which validation would become a critical issue [427]. In fact, experimental testing of these joint details under a multitude of load cases is an inevitable step towards the broader employment of TO in steel joints detailing [139; 427]. A further step towards the application of TO in joints design can be envisaged in [PERSON] et al. 's comprehensive method for optimizing and fabricating joints in three-like structures [428]. As such, the nature-inspired structural concept is particularly prone to TO, in which a solution for optimising and manufacturing its joints is paramount. Other interesting developments in the TO of joints among tubular elements can be found in the work by [PERSON] and [PERSON] [429; 430]. Concerning the optimisation of spherical nodes in space frames, the work by [PERSON] et al. [364] is particularly relevant for providing a holistic guideline, including fabrication concerns, practical issues with Grasshopper modelling, as well as results post-processing and interpretation. Likewise, the work by [PERSON] et al. [431] on the connections topology optimisation in Moment Resisting Frames (MRF) is deemed to assist structural designers in the task of managing the recursive task of designing frames and its joints. Figure 10: Super-long suspended spans optimisation with computational morphogenesis. (**a**) Case-study initial (blue) and interpreted (red) design [418] and (**b**) computational morphogenesis result [418]; (reproduced under the Creative Commons Attribution 4.0 International License, [[http://creativecommons.org/licenses/by/4.0/](http://creativecommons.org/licenses/by/4.0/)]([http://creativecommons.org/licenses/by/4.0/](http://creativecommons.org/licenses/by/4.0/)) accessed on 25 February 2021). Nevertheless, several and significant factors still hinder joints TO. Some of the most apparent problems lie in the materials properties uncertainty and fabrication cost. With a strong relation to AM techniques [139], those can only be solved by sensible technology developments, financial investments in the industrial capacity, and workforce know-how, as well as a stronger standards framework. Problems with the optimisation process have an outstanding issue in the incapacity of some algorithms for accounting non-linearity [422]. This is critical in several connection types, including the bolted ones, where plasticity plays a major role. However, significant advances have been made in the modelling of bolted connections, holes, bolt-hole contact, and friction in the context of TO [432; 433; 434; 435]. Furthermore, a very significant research published in the context of mechanical engineering systems is general enough to have a profound impact also on steel joints detailing.