Although it can be frustrating, many seemingly clear-cut questions in the sustainability world end up being answered "it depends."
One example of this is the classic concrete vs steel question covered in the SEI Sustainability Committee's excellent FAQ: Which is better, concrete or steel?
Another example is the reuse of existing buildings. Studies have shown that reusing existing buildings can be an excellent way to reduce energy usage since there is no need to replicate the energy expenditure that is already embodied in the existing structure. However, a recent study in New York found that this isn't always the case -- at least for the specific midcentury skyscrapers considered, and for the specific replacement considered.
The study found that the reduced energy usage of a new building would make up for the embodied energy expenditure in 15-30 years, well within the design lifespan. The reduction in energy use is from designing for very high operational energy efficiency, but also from providing a larger structure with more floor area. The larger floor area means that the site can have more occupants than the existing structure.
A key factor in this part of the energy calculation is the consideration of the city as a whole as the unit for analysis. If the analysis only considered the difference in energy use between the new building and the existing building, the calculated energy payback period would be over 100 years. By assuming that all of the new tenants are relocating from buildings that have a similar energy performance to the original building, the savings are amplified and the payback period is reduced significantly.
What does this have to do with structural engineering? One issue raised by the study authors (in this reporting about the study) is that the existing buildings do not have enough capacity to support heavier facade loads from double-glazing or other systems. Keeping future flexibility in mind when designing the structural system allows for these types of retrofits to occur in the future. The potential need for extra capacity must be weighed against material efficiency in the present, so there is plenty of room for engineering judgement.
