# Topology Optimisation in Structural Steel Design for Additive Manufacturing
## 5 Topology Optimisation in Structural Design of Steel Elements and Joints
### Steel Elements Design
Specific issues of tall buildings have been addressed by recent TO studies, including the development of a genetic algorithm-based method for optimising outrigger systems [404], using swarm optimisers [279], considering linearised buckling in the TO process [405], developing a method for optimising bracing systems under adaptive multimodal load patterns [406; 407], and conceiving structural systems for tall buildings based on the Optimisation of Continua using Reissner-Mindlin (or Mindlin-Reissner) shell elements [407]. Loading is, undeniably, a major challenge for TO in buildings structural systems, especially when multiple actions, combinations, and modes are considered and more so when the structure-load interaction is strong and influences the latter intensity. That is the case of seismic loading, in which specific, and usually extensive, code prescriptions apply. Subjected to such constraints, research in Topology Optimisation of seismically loaded structures is still exiguous. Nevertheless, the works of [PERSON]'s group on shear walls [408] and on different ductility steel Moment Resisting Frames (MRF) [409] can be highlighted as well as [PERSON] et al. 's [410] braces optimisation to the non-linear dynamic analyses of earthquakes time-histories. A similar complexity can be found in fixed offshore structures for oil and gas or wind energy production. Even though such structural systems usually have a simple and intelligible conceptual design, wave and extreme loading require rather complex engineering [411]. For those reasons, recent developments in the conceptual design optimisation of jackets [412], including geotechnical aspects [413; 414] and fatigue design [415] are very significant. Likewise, the method by [PERSON] et al. [416] for multicriteria optimisation of modular steel towers is expected to have an impact on the industry. Long-span structures have also been an object of study in TO, even if the research output is scarce in quantity. Within this subject, one can mention the Topology Optimisation of domes through a Colliding Bodies Optimisation (CBO) method by [PERSON] [417] and the general space-frame steel roof optimisation method with Genetic Algorithms by [PERSON] and [PERSON] [265]. Concerning bridge engineering, the spotlight is in the DTU TO group steel girder optimisation for super-long suspended spans, employing computational morphogenesis [418] (Figure 10). Other significant advances, which may have an impact on structural steel design optimisation, include [PERSON]'s work on TO under extensive and non-linear deformations [165] and [PERSON] et al. 's [419] studies on contact pressure and friction. As steel design and detailing is deeply affected by fabrication and erection procedures, which usually constrain engineering options in a more extensive manner compared to construction with other common materials, a further investigation into recent research papers devoted to considering such aspects into TO is due. For such an end, and regardless of a deeper review on Additive Manufacturing for TO in Section 7, one shall refer to recent reviews [420; 421], and keynote [422] on employing welding robotics in the so-called Wire and Arc Additive Manufacturing (WAAM) of topology optimised steel members and connections. However, the most common methods of creating steel in AM use powder as a feedstock and laser or electron beams as binding mechanisms [423]. Even if high-quality surfaces and accurate geometries are achievable with such means [424], steel properties are TO variables with broader uncertainty, due to its complex and repeated heating and cooling cycles [425]. Control, inspection, and testing will have an increasingly paramount role in enabling steel fabrication processes reliant on Topology Optimisation [129] and will add to the well-known hindrances of cost, aversion to change, and lack of technical knowledge [426].