In the heart of Peterborough town centre, 130 new residential units have been added above an existing two-storey retail building which remained operational.
Our client wanted to extend the site with additional storeys and commissioned SD to investigate feasible options. Initially we undertook extensive analysis of archive records and drawings to understand the existing structure and its loading capacity.
Records showed the building had undergone various modifications between 1952-1976 by various owners. Although changes to the building were made fairly recently, design information for the existing structure was limited.
Based on our experience, we knew the variability of structural materials would vary greatly with each modification, and that historic codes for strength, loading and safety would differ to the standards we use today. We conducted site surveys to identify critical information such as steel types as well as depth and diameter of the substructure piles.
Various tests were undertaken to understand the loading capacity, and discrepancies between the available design information and physical building were identified. Many of the steel columns were noted to be significantly weaker than anticipated.
A typical approach to test the diameter and depth of piles is to undertake GPR surveys and sonic testing from above the pile itself. This method wasn’t available as access was restricted due to the ground floor retail space remaining operational. Through significant review of historic information we developed an alternative solution: tunnel beneath the existing building to locate the piled foundations and apply sonic testing methods from the side of the pile to verify the expected pile capacities noted.
This, combined with geotechnical ground model analysis, provided us with an in depth understanding of the foundation capacity. We developed three cost-effective structural solutions which achieved the client aspirations to maximise massing and usable space. To avoid foundation strengthening works, saving our client money and reducing embodied carbon, we developed a unique lightweight structural solution for the additional three storeys above the retail space.
Given the increase in building height, the building needed to be upgraded to resist disproportionate collapse. Vertical ties would typically be installed to satisfy this requirement; however, as these ties could not be extended to the operational ground floor, an alternative arrangement had to be devised.
Vertical ties were installed at the masonry walls down to the first floor steelwork, and these were in turn supported on a series of continuous cantilevered steel mega-frames at first floor. Horizontal ties at the top of the first floor steels provide additional strength, should the perimeter masonry walls become damaged and structurally unstable.
The key to the success of this project was our client’s understanding of the design and build process, and their appreciation of the varied and exhaustive testing required for a project like this.
The whole client and design team collaborated closely to ensure the buildability of the scheme over the phased three year programme.