BCSA’s UK structural steelwork: 2050 decarbonisation roadmap, produced with input from SCI and launched at the end of 2021, identified ‘design efficiency’ as one of the key ways for steel construction to achieve its targets. Design’s estimated potential to contribute 18% of the ‘change’ needed is clearly very significant, and much of that potential is available to be exploited right now. SCI’s guide to Semi-continuous Braced Frames (P183) was published in 1997, and remains entirely valid. It was written at a time when SCI was focused primarily on ‘structural engineering’, and had only one or two experts working on ‘sustainability’. Today these previously separate disciplines have effectively merged as far as much of the guidance and advice SCI offers is concerned, with a major focus of the steelwork industry in the UK now being ‘sustainable structural engineering’.
Reducing the weight of steel by 18% may sound a lot, but some years ago Prof Julian Allwood and his team concluded that UK steel construction designers were using (in multi-storey buildings) twice as much steel as was necessary in order to satisfy design codes, standards and regulations. Much of this overdesign is down to non-technical issues, including low design fees, and the order of the design and procurement process (which means information such as some load values is lacking when needed).
A combination of low design fees, and the
traditional UK approach to split responsibility for the design of the beams and
columns, and the design of the joints, combine to make so-called simple design
very attractive. However, an obvious outcome of simple design is that taking
zero moments in the joints means that sagging moments in the simply-supported beams
are maximised for a given level of loading. Semi-continuity offers many of the
benefits of ‘full’ continuity, in terms of reducing beam sagging moments,
deflections and therefore sizes, whilst retaining most/all of the benefits of
simple construction in terms of fabrication cost and erection ease. This is
because many joints that are assumed to act as nominal pins, perhaps with some
‘thickening’ of components, do in fact offer reasonable stiffness and strength.
Stiffeners and haunches are not needed to achieve reasonable levels of
semi-rigidity and partial strength, to use terminology from
EN1993-1-8.
P183 describes how to apply semi-continuous design to braced frames. It is based on the premise that standard joints will be used, although 25 years after it was published we should be able to offer greater design/detailing freedom. In 2022 SCI and BCSA will be working together to produce guidance on sustainable design. This will include revisiting the semi-continuous frame design principles and rules. Designers today can use tools to determine joint strength, stiffness and rotation capacity, although levels of accuracy (of the last two) may be open to some question. SCI will be developing an easy-to-use tool, using Trimble’s TEDDs platform, in 2022. This will allow key characteristics of ‘typical’ types of joint to be determined using the well accepted EN1993-1-8 component method. As TEDDs is part of the Trimble family this may also allow integration with their frame analysis and design software, which will really make semi-continuous design simple.