Embodied carbon premium for vanity height: A case for the exclusion of decorative spires in the design of tall buildings

In measuring the height of a tall building, the Council on Tall Buildings and Urban Habitat (CTBUH) recognises three categories: "height to architectural top"; "height to highest occupied floor"; and "height to tip". The "height to architectural top" category, which includes decorative spires, is used to define the influential CTBUH annual rankings of the "World's Tallest Buildings". The inclusion of decorative spires in the category ranking has created an incentive for developers to maximise the vanity height of tall buildings, defined as the height difference between a tall building's architectural top and its highest occupied floor. The aim of this paper is to demonstrate the detrimental influence of spires and, by extension, vanity height on the embodied carbon of structural systems for tall buildings. This influence is evaluated using three tall building scenarios of varying heights (50, 70, and 90 storeys). Two finite element models, with and without spires, are parametrically designed for each scenario. All the modelled structural systems comprise a reinforced concrete tube-in-tube lateral load resisting system. A hybrid life cycle inventory analysis approach is used to quantify the embodied carbon of spires as well as the resulting increase in the embodied carbon of structural systems. The findings of this study indicate that even basic spires can lead to an increase of up to 14.2% in the embodied carbon of structural systems for tall buildings, underscoring the imperative to eliminate such elements in the design and construction of tall buildings to minimise overall embodied carbon.