Tuesdays are CPD days at SD HQ.
Last week we heard from one of our Directors, Andy Simpson, about a project of his. Two Victorian buildings of masonry construction in Islington housed multiple apartments, and the buildings were vertically extended with an extra storey to create additional residential space – designed by another engineer.
The additional storey took the masonry building to five-storeys high, meaning the disproportionate collapse class changed from Class 2a to Class 2b. A Class 2b structure requires additional consideration, and typically vertical ties are installed to protect against collapse should an explosion occur.
Work was almost complete when it became apparent to the checking engineer on the project that the disproportionate collapse class change hadn’t been accounted for in the now completed design. SD had successfully worked with the developer client on a similar scheme before and so they contacted us and requested our immediate help.
We began by reviewing the scheme and considering our various options of retrospectively stabilising the design to account for disproportionate collapse and designing around constraints such as prohibited access to the apartments in the original building below. This rendered tying approaches impossible and so we concluded that an exhaustive Notional Removal approach was the most appropriate solution.
As the new floor steels were already installed, we decided to utilise these as a strong floor at the fourth level. The strong floor would separate the new extension from the original building in case of structural failure, ensuring any debris from an explosion in the new floor would collect on and be supported by the strong floor without disrupting the floors below. If structural failure occurred beneath the strong floor, the strong floor frame would span over the collapsed structure without causing additional damage to the original building. In both scenarios, the existing structural elements that remained post explosion were exhaustively checked to be able to support the loading in the accidental scenario.
Several clients have previously been advised against changing from Class 2a to Class 2b on masonry buildings but, in our experience, this is not always good advice. While not straightforward, it is code compliant and safe. As professionals, our clients look to us to maximise the potential of their assets, not just take the path of least resistance. In a world where we are in dire need of more housing and we need to minimise the embodied carbon that comes with demolishing and rebuilding, we must advise our clients well.
We pride ourselves on working with our clients to de-risk a site and offer opportunities to develop where they might have been advised not to before. Please get in touch with us if this sounds like something of interest.
