How structural engineers contribute to sustainability

As the performance in use of buildings improves and we begin to focus once again on the embodied carbon or embodied energy within the building’s fabric, we surely need to be able to approach the subject in a more rational and representative way. What is required is a process which recognises when efforts have been made to design in a way that produces real reductions and truly creditable performance - and rewards it appropriately.

In our day-to-day delivery of projects in the UK, BREEAM is the most common mechanism employed for assessing the sustainable credentials of a project, but I am afraid to say that for a structural engineer the invitation to attend a project BREEAM session is not something which invokes much enthusiasm. This is not because structural engineers are not interested in sustainability - quite the opposite. Structural engineers take sustainability very seriously but the framework for them to contribute, proactively and passionately is simply not there.

It seems crazy that you can achieve just about as many points for appointing an ecologist as you can for having a green solution for the frame of the entire building

BREEAM is well established and it does a number of things exceptionally well, but it falls short in spectacular fashion when it comes to earning points, or otherwise, for the production of a green structure. To be blunt, it does not even scratch the surface of this subject and it seems crazy that you can achieve just about as many points for appointing an ecologist as you can for having a green solution for the frame of the entire building. Within the framework of BREEAM there is no easy way in which to reward strategically important decisions such as an appropriate structural grid, meaning that if you have a flat slab on a 7.5m grid or one on a 12m grid there is no perceived difference in spite of one having perhaps a 250mm thick structure and the other a 450mm structure. There is no simple way of rewarding the reduced material content for post-tensioned or internally voided slabs (such as Bubbledeck). Reusing existing structural elements equally does not appear to be appropriately rewarded. Yet it is questions of this kind which present the opportunity to really reduce embodied energy or carbon. Halving the volume of concrete in a building as opposed to putting some cement replacements in the specification for the concrete mix undoubtedly achieves more, but the assessment process is disjointed and not representative of this sort of situation.

The energy performance of a residential building in use, including the performance of the fabric of a building, is modelled by performing a SAP Calculation. SAP is the government’s standard assessment procedure for energy rating of dwellings. This calculation is executed against a theoretical “model” building. Surely there must be room for a similar approach in structural terms?

It should be possible to develop a design which is used as a benchmark for the structural design. This could perhaps be related to building typology as grids for commercial offices will not be appropriate for residential, for example, but within those typologies it must be possible to establish a best practice position against which to consider the embodied energy performance. Alternatively it may be that after consideration of the model building that this is distilled into a simple target embodied energy/m2 within the structure which is rated on a sliding scale, similar to EPC or washing machine performance indicators, which result in the awarding of an appropriate number of points; these points must reflect the true contribution rather than be a token gesture in the way that they are at the moment given that around 5-20% of lifecycle carbon and up to 50% of the embodied impacts in a building may be in the structure.

BREEAM was a good start and I do not want to knock what it has achieved in raising the profile of sustainability and to embedding it firmly within the design process, but now we must lengthen our stride.

Gerry O’Brien is director of AKT II, consulting structural and civil engineers