The Sustainable Structures Trilemma
by Richard O’Hegarty
Efficiency, Longevity and Flexibility
The definition of what a sustainable structure is, is becoming number-ized.
Embodied carbon calculations are being used to quantify how sustainable a building’s structure is or isn’t.
As an engineer, I’m all for measurements and calculation, and I’m particularly pro embodied carbon measurement. As they say:
“If you can’t measure it, you can’t improve it” (Lord Kelvin…apparently)
But understanding what you are trying to measure and improve should always be Step 1. Steps 2,3 or 4 might then include the use of embodied carbon calculation.
If you don’t know what you are measuring, you don’t know what you are improving.
Assessment by precise embodied carbon calculation is not the definition of what is, or isn’t, a sustainable building. It is just a (very) helpful tool. Appropriate use of this tool can approximate the global warming potential of a building within a specified scope and can help guide design decisions early on to ensure minimum embodied carbon.
Efficiency - Using the right amount of material
As well as using lower carbon materials, designing efficient structures helps to ensure the minimum amount of material is used to meet specific criteria. Thereby keeping the embodied carbon figure low. There are lots of structural efficiency examples which focus on optimising geometries and material combinations.
But optimising material efficiency to achieve lower carbon buildings is not the full picture.
If it was so simple all buildings would be the same. They aren’t. And nor should they be.
A building achieving the lowest possible embodied carbon figure is possibly sustainable, but it’s only part of the sustainability triangle.
Longevity - Making the materials and components last
On another corner of the spectrum, we have examples of timeless structures (e.g. the Pantheon) whose embodied carbon over a 50-year lifetime performs poorly under current embodied carbon criteria, due largely to less efficient usage of material. Over their real lifetime, however, their impact is dwarfed by the number of build-demolish cycles of a building designed for 50 years.
Constructing a timeless building requires both durable materials and an appreciation that, whatever is being built, is going to be needed/wanted by future generations. 2000 years is an extreme case and many of these examples we point to are an effect of survivorship bias.
Understanding what future generations might need is always going to be unpredictable while using durable materials and bigger components to survive future extreme weather events is going to increase the embodied carbon.
So, should we make buildings last longer, or make them lower carbon?
Reducing upfront embodied carbon emissions, at the cost of a reduction in durability, is a philosophy that many practitioners are edging towards…some knowingly, some unknowingly. Those who are knowingly taking this carbon-over-longevity pathway argue that it’s more important we reduce emissions today, and that we will figure the rest of the problem out later.
Philosophically, I tend to edge towards the other side of the spectrum, the build-to-last side, and would accept an upfront embodied carbon premium if you could ensure the building will last. The hope in this case is that we figure out ways to adapt buildings in the future if their use becomes obsolete in future generations.
Both approaches have issues.
Which brings us to the third corner of this trilemma.
Flexibility - Designing for a different future, in a different location
Can we design buildings so their components can be used to make other things in the future?
While the concept is endearing and has attracted lots of attention, there are some challenges worth discussion. Making connections for disassembly means more complex design conditions, which usually means more material…and hence more embodied carbon.
It also typically means we won't be able to benefit from the structural superiority of highly efficient inseparable composite materials and components. In a precast concrete sandwich panel, for example, reducing the sections overall thickness can be achieved by enhancing the composite behaviour between the concrete sections, which in turn requires thicker and more robust connections between the different materials. Making them more difficult to separate[1].
The law of entropy and the tendency for increased disorder over time is also a challenge for this design-for-disassembly philosophy. Reusing components can reduce material demand and embodied carbon of new buildings, but it’s likely that once disassembled, the component will not be used as efficiently as it was in its first life. Either because bigger safety factors are applied or because a perfect use case does not exist close enough or at the time its needed. Maximising the use of disassembled components requires an appreciation for space and time. Which in turn might require low carbon transport and some sort of storage facilities.
So, in cases where buildings have been constructed of reused components you often have more materials used overall. This is almost certainly a worthwhile penalty to pay if it ensures the use of virgin material and energy is reduced overall.
It’s just something worth considering
So, what then is a sustainable building?
I think the beauty of buildings is that they all have different needs and functions. Understanding these needs in conjunction with emerging tools and methods, underpinned by an appreciation for future needs, is probably a good start to building something sustainable.
Structural efficiency, longevity and flexibility all play a role.
Try not to get stuck on just one corner of the triangle.
‘The delicate balance between efficiency and durability is a theme Richard explores thoughtfully in his article—a conundrum I, too, find myself grappling with in my own work. Should we prioritise minimising carbon emissions today, knowing this could impact the longevity and adaptability of a building? Or should we favour durable, potentially replaceable elements that may not align with immediate efficiencies but promise resilience for future generations? Our industry is rife with such “rules of thumb”—some urging us to minimise carbon now, others advocating for a future-focused approach. Yet, as he rightly observes, the unpredictable nature of what lies ahead is the curious, sometimes unsettling beauty of our field.
Richard draws our attention to the Pantheon—a structure that stands in stark opposition to much of modern design thinking. Its enduring presence challenges us to reconsider: should we not aspire to build with longevity and adaptability at the forefront, while embracing low-carbon methods? Or is such a philosophy an ideal rendered impractical by the relentless pressures of time and budget in contemporary practice?
Reflecting on his article, I find myself drawn to the idea of revisiting the philosophical underpinnings of ancient design. The Pantheon, as Richard suggests, embodies a mindset rooted in permanence and evolution—a commitment to building not just for the now, but for the ages. Perhaps the key lies in blending this timeless perspective with modern imperatives, ensuring that our designs honour both the demands of the present and the unknowable needs of the future. ‘ - James Morton
Richard O'Hegarty is a distinguished researcher at the forefront of building and infrastructure decarbonisation. With a PhD in façade engineering, Richard’s expertise lies in developing innovative solutions to tackle the environmental challenges posed by the built environment.
Richard serves as the Research Lead at RKD Architects and as the Lead Researcher for the Building in a Climate Emergency research group at University College Dublin. In these roles, he leads groundbreaking projects focused on reducing the carbon footprint of buildings across their entire lifecycle – from construction to operation.
His research has been instrumental in addressing critical questions about the sustainability of the built environment. Recent projects include:
Investigating whole-life carbon in the Irish built environment (funded by IGBC).
Monitoring the U-value performance of building envelopes (SEAI).
Assessing and optimising heat pump technologies (SEAI).
Exploring low-carbon, high-performance concrete cladding (EU H2020).
Developing building-integrated solar thermal façades (Irish Research Council).
With over 50 peer-reviewed publications, Richard has contributed extensively to advancing knowledge in energy efficiency, renewable energy technologies, life cycle analysis, concrete sustainability, and structural design optimisation.
Through his work at RKD and UCD, Richard is helping to identify the challenges and craft the solutions necessary to decarbonise the built environment, ensuring a sustainable future for generations to come.
Find the same article on the RKD website here: The Sustainable Structures Trilemma - Research
[1] O′Hegarty, R., West, R., Reilly, A., Kinnane, O., 2019. Composite behaviour of fibre-reinforced concrete sandwich panels with FRP shear connectors. Engineering Structures 198, 109475. https://doi.org/10.1016/j.engstruct.2019.109475