Reserve capacity and vertical extension potential in steel-frame buildings

Reusing existing buildings is key in transitioning to a circular economy and meeting associated decarbonisation targets. Many adaptive reuse strategies, including vertical extensions, result in increased loads that need to be resisted by reserve structural capacity and/or strengthening. Previous work has identified structural under-utilisation within existing buildings and numerous sources of this have been suggested. However, understanding of associated contributions to reserve capacity and how this influences a building’s ability to be extended is limited. To address this, 11 scenarios, each modelling a different source of under-utilisation, were developed and applied in the design and reappraisal of 11 252 hypothetical steel-frame office buildings. An average reserve capacity of 10% was found in optimal Eurocode (EC) designs, rising to around 20% for British Standards and over 30% in the conservative or defensive application of ECs. Office to residential conversions were found to unlock an average reserve capacity of more than 20%. Across the 11 scenarios, 9–89% of considered cases were shown to be extendable without the need for strengthening. This demonstrates potentially significant extension potential within steel framed buildings, but that this varies with frame configuration, design/appraisal approach, and the extension’s use and structural material.