Exploiting Previous Structural Over-design to Tackle Urban Expansion

Abstract

In the face of the climate emergency and massive housing deficits in the UK, vertical extensions to existing buildings offer a sustainable and economical solution for increasing housing supply. In essence, the existing building becomes the foundation for a new building. However, uncertainty around the reserve structural capacity of existing buildings remains a key barrier to wider uptake, primarily due to a lack of experience in assessing this capacity beyond conventional load-balancing approach. This often leads to the conclusion that major structural strengthening is required—an outcome that is costly, time-consuming, and highly disruptive.
This research identified a range of over-design factors that contribute to the inherent reserve capacity in existing reinforced concrete (RC) buildings. These include conservative design practices, material over-specification during construction, long-term concrete strength gain, and the conservatism embedded in the historic UK structural design codes and the current Eurocodes used for present-day structural assessments. Minimum (conservative) and likely (typical) magnitudes of these overdesign factors were quantified using expert interviews, published data, available guidance and reasonable assumptions, forming the basis of a flowchart-based framework—developed for the first time—to systematically unlock and quantify the reserve structural capacity inherent in existing RC buildings constructed between the 1950s and 1980s, a period during which reinforced concrete dominated the UK construction market.
This framework was then applied to assess the reserve axial capacity of existing RC columns designed in accordance with historical practices from that period. The vertical extension potential using two extension strategies - the addition of concrete and timber storeys – was assessed based on the unlocked capacity. Results showed that existing columns are likely to possess significant reserve capacity, eliminating the need for column strengthening when undertaking vertical extension.
The reserve lateral capacity of the stability system of a representative 1960s RC building was evaluated by incorporating shear wall–frame interaction and previous over-designs. Results demonstrated that, under both extension strategies, the original building height could likely be approximately doubled without requiring superstructure strengthening, with Serviceability Limit State (SLS) criteria being the governing factor—provided that the foundation has sufficient capacity. This makes timber a highly promising alternative material for vertical extension, offering advantages such as lower embodied carbon and a lighter structural self-weight, which helps reduce foundation demands.

Date of Award	14 Jan 2026
Original language	English
Awarding Institution	University of Bath
Supervisor	Tim Ibell (Supervisor), Antony Darby (Supervisor) & Will Hawkins (Supervisor)