I currently work in Arup’s Advanced...
How well optimised do you think the structure of the building you are sitting in is? By that I mean, how close to its limit is the material holding up the building? How much of that material is redundant?
I think the answer to the last question must be: too much. But how can we reduce the amount of material in buildings, to make them leaner and greener, while also maintaining simplicity of construction and doing so at a price clients can afford?
Embedded in structural engineering design codes are probability analyses which have ensured that structural collapses of finished buildings have become very rare events indeed.
What surprises many people, especially those with backgrounds in other areas of design, is that in structural engineering the structure is designed at its most heavily-loaded point, and this solution is copied-and-pasted everywhere else in the building, forming a regular, but inefficient structure.
In addition to this simplification, at the scale of the individual element the simplifications are even greater. Each element size is determined by the part of the element that is most highly stressed. For example, a beam works hardest in the middle, which dictates how deep it needs to be. Despite the fact the force in the beam varies along its length, structural engineers specify constant depth beams in almost all applications, meaning much of the material in a beam is very lightly utilised. If it is remembered that the design loads are likely to have been determined for a more heavily loaded beam in another part of the building, it becomes clear just how much material in a building is not working particularly hard.
Dr. Julian Allwood and his team at Cambridge, working with Arup, have crunched some numbers on this. If elements were shaped to reflect the distribution of force within them, and each one sized only for the loads it was likely to carry in the lifetime of the building, the amount of material used in the structure of most buildings could be reduced by something close to a third. With such huge potential savings on offer, why do structural engineers design like this?
Unsurprisingly, economic drivers are what lie behind this exorbitant waste of material. Relative to design and construction time, material is cheap. The associated cost of manufacturing a component in a bespoke shape to match the likely loads on each individual member is much greater than that of the extra material used in a regular design. Simple designs with off-the-shelf components also carry less risk and can be constructed more quickly.
If the materials used in construction had a low environmental impact, then perhaps the structural engineering profession could afford to be less concerned by such over-simplification. The concrete and steel industries, however, both have significant carbon footprints, and the enormous volumes of these materials used in construction each year means that any systematic reduction in their use could single-handedly make a measureable impact on global carbon dioxide emissions.
So in a time when there is a legal and even moral imperative to tackle our industry’s carbon emissions, is designing such inefficient structures acceptable, or even ethical? As Nick O’Riordan points out in We must consider capital carbon, the focus for carbon-reduction policies has been on operational carbon. We must start counting capital, or embodied carbon, before we can start rewarding those who do work hardest to achieve the most with the material they use. This change will come from outside, from regulation, outside assessors, and the demand of more environmentally-aware clients.
Much of the effort, however, must also come from within the structural engineering profession. It needs to recognise the contribution it makes to global emissions and work with suppliers in industry and contractors to develop new ways of manufacturing and assembling structural elements. Most importantly, structural engineers must explain to clients on each and every job how a little more investment in design time can make a huge difference to the carbon footprint of their operations.