Realising the reserve capacity in existing reinforced concrete columns

Under the urgent global climate change situation and massive dwelling shortages in the UK, vertical extension and building conversions are viable approaches to supply dwellings sustainably and economically. However, in such reinforced concrete (RC) buildings, columns are often strengthened in related projects, which can be costly and time-consuming, while causing disruption to the residents or the daily running of business. In fact, this paper shows that strengthening could be avoided, as RC columns have more redundant capacity than one might have suspected. This paper quantifies this reserved structural capacity for the first time. There are many reasons for structural over-design inherent in today’s RC columns, emanating from the original structural design, construction, and present-day structural assessment stages. Each unlocks some spare capacity in existing RC columns. This paper groups the guaranteed ‘minimum’ and the ‘likely’ values of ‘overdesign factors’ to offer up values for minimum over-design and likely over-design in RC columns. It is well understood that seismic actions significantly influence buildings’ structural capacity. However, this paper exclusively focuses on the non-seismic zones of the UK. It transpires that for typical UK 1960 s purpose-built flats the guaranteed ‘minimum’ capacity of the columns that can be unlocked through following the ‘minimum’ path is 32%. This is already a significant unlocked capacity, in wait of exploitation. But, even better, it is found to be likely that such columns are only working at 15% of their potential capacity, which gives a ‘likely’ reserve capacity of 85%. In summary, significant capacity in an RC column can be unlocked from following both paths, which is readily available for retrofit vertical-extension exploitation. This would eradicate costly, disruptive strengthening. This paper provides the evidence base for such opportunity.