Behind the Project: Pig Patch House
Pig Patch House provides some unique insight into working with exposed timber in the UK. Recently, we sat down with Associate Sanja Buncic to hear how she and the Entuitive team approached this special project.
Thanks for sitting down with us today, Sanja. Can you please tell us a little bit about Pig Patch House?
Pig Patch House is a single-family bespoke house built in England, UK. It is built from Nur-Holz timber panels used in the walls, Brettstapel panels used in the floors and the roofs, and Nu-Span insulated precast concrete floor planks over RC ground beams on mini piles for the ground floor and foundation structure.
Entuitive provided structural engineering and fire engineering services on this project.
Sounds like an interesting project! Were there any challenges you faced and, if so, how did you solve them?
There were a few challenges with this project. These included assessing the Nur-Holz panels to act as deep beams and balustrades over large door openings, designing the floor panels to span over 6m and carry the load from the first-floor structure and the roof, and fire design of the building.
We came to the project a bit late, meaning the architect had already finalized the design. While we weren’t able to put our own spin on the design, the challenge for us came with how to make the design work structurally.
Assessing & Designing the Nur-Holz Panels
For instance, the ground floor plan showed an open living, dining, and kitchen area directly under a master bedroom and first floor roof terrace. The span of over 6m, meant that we were not able to use a Nur-Holz floor structure. Instead, we used Brettstappel panels for the first floor and roof, helped locally with a single glulam beam. The ground floor structure was built in insulated precast panels called Nu-Span and was supported on ground ring beams installed over mini piles. There were two reasons for using this type of the foundations system. The first one was the presence of trees in cohesive soils such as clay which have influence on volumetric changes of the soil, and the second one was the reduction in overall concrete and reinforcement.
Most of the walls and soffits were left exposed which had some major impact on satisfying the Building Regulations Part B in Britain.
We worked closely with Rombach, the fabricator, and Bright Forest, the supplier of these panels and were responsible for the design of all connections and wall and floor panels.
One of the important issues to resolve during prefabrication of the panels was position of all mechanical and electrical services.
Fire Engineering an Exposed Timber Structure
Following the Grenfell Tower tragedy in June 2017, the UK government introduced a ban on the use of combustible materials in cladding. Even though only specific types of buildings were included in this ban, there was a chain reaction within the industry. Suddenly, a lot more people became aware of the issues surrounding fire regulations and started to pay attention.
For a building of this size in the UK, we would generally follow the guidance from the documents called Approved Documents to satisfy Building Regulations. Approved Documents are meant to speed up the timing of, minimize the costs of, and prioritise the design process.
However, the Approved Documents are not the only option for the fire safety design. The design team can deviate from the prescriptive guidance if they demonstrate the building is still safe and meets the requirements of the Building Regulations. In the case of Pig Patch House, there was a desire to leave the Nur-Holz panels untreated and fully exposed, although this would not satisfy the internal spread of flame requirements in Approved Document B.
Options for satisfying the prescriptive requirements include treating the exposed surfaces or encapsulating them. A third option proposed is to use a performance-based approach, accepting that the exposed timber does in fact exhibit a faster spread of flame but mitigating this through a holistic fire strategy. Knowing that spread of flame is important to the early stages of fire growth and tenability, the mitigation strategy must similarly address these early stages of the fire.
The proposed solution was to leverage fire modelling to demonstrate performance and compare it back to the performance of a guidance-compliant fire strategy. Option 1 represents the baseline solution. This is a building that meets all prescriptive requirements of ADB, including fire-retardant treated wood with improved spread of flame. Option 2 is the proposed solution, where the exposed timber is not treated. By improving the detection throughout the dwelling as opposed to a single detector in corridors, evacuation is similarly improved and the overall level of risk is improved. The specific scenario shown below is a fire in the bedroom with smoke compromising the exit corridor for the bedrooms on the first floor.
With fire models developed for both schemes, the amount of time for unsafe conditions to develop is quantified and compared against the evacuation times. Since occupant evacuation is a stochastic process, the risk to occupants is quantified for all scenarios using probability distributions and the resulting risk from each scenario is quantified with an event tree analysis. An example of this is shown below.
This type of analysis is able to demonstrate that the holistic fire strategy achieved the required level of performance despite aspects not meeting prescriptive guidance.
What was your favourite part of the project?
Learning about this new glueless CLT because it’s a bit different in terms of the design than regular CLT.
Any final thoughts?
It was big learning curve – we learned a lot about fire and building regulations approvals. On a building like this where the timber is almost fully exposed and not encapsulated, it’s very important to have an open and direct conversation with the building control officers, so that they can buy into your strategy and provide their input.
Thank you for sitting down with us today, Sanja! We learned a lot.