TRADA joined the ranks of organisations turning to digital platforms with its annual Better Timber Buildings conference going online in November.
The conference, which focused on CLT and mass timber buildings, was split into two, two-hour sessions a week apart. The first, looking at the issue of fire, was particularly neatly timed, falling as it did on November 5 – bonfire night.
Danny Hopkin, technical director at OFR fire and fire risk consultants, gave delegates an insight into an industry-led research project, which aims to establish a compliance road map and fire safe design envelope for mass timber buildings.
He explained how the Structural Timber Association (STA) Special Interest Group (SIG) has been formed to address challenges through a series of work packages to provide guidance and evidence to support the fire safe design of mass timber high-rise residential buildings (HRRB) and commercial buildings. The three funding partners are Stora Enso, Binderholz and KLH and supporting partners are OHR, which is delivering the research, and the steering group influencers, Henkel, B&K Structures and Eurban.
The three main goals are education and promotion of good practice, knowledge generation through small and large-scale experiments and dissemination of that knowledge and the six work packages (WP) are broadly split into two bodies of work with WP1-4 predominantly desk based and WP5 and WP6 experimental and lab based.
Mr Hopkin’s presentation focused on WP1, which concerns the development and demonstration of a compliance framework. The compliance framework will focus on the information that should be produced by the design team to demonstrate that an adequate level of safety is achieved.
The first piece of guidance from this project forms part of the STA’s suite of ‘Structural timber buildings in use guidance’ publications – ‘Volume 6: Mass timber structures; Building Regulation compliance B3(1)’. It can be downloaded free by STA members and nonmembers at www.structuraltimber.co.uk/ sectors/clt-special-interest-group and as more project outputs become available they will be posted on the same website.
“It is roughly an 18-month project and it is continuing at pace,” said Mr Hopkin. “We are up to WP4 [an HRRB compliance pathway] and will be moving towards large scale experiments in 2021, with a focus on commercial construction.”
Designing mid-rise timber structures to meet regulations and client needs was covered by Jonathan Roynan, technical director at Buro Happold, who looked at commercial developments, and Ben Price, technical manager at B&K Structures, who focused on residential properties.
Following the advice given in the STA document is key, said Mr Roynan, who gave some examples of how Buro Happold is keeping the timber element out of the building façade on taller buildings.
He also advised the early involvement of a structural fire engineer. “The sooner they are involved, the sooner you can have informed and knowledgeable discussions with your client and your client’s insurers,” he said.
“It’s very important that you have a structural fire engineer – not just a fire engineer – who understands mass timber and timber design and is able to do some analysis.
“The client needs to understand that in the current climate the design may need to evolve and change to reflect the structural fire engineering advice,” he said. For example, exposed timber may have to be encapsulated.
He added that there were ongoing discussions between industry and the insurance market.
“I think the insurance market understands that timber has many benefits and it is an opportunity, but it is also a risk and hopefully the work that SIG is doing in terms of understanding the material better and having the testing set up means that we can alleviate those risks and concerns.”
Mr Roynan said that structural engineers need to be more aware of fire stopping and compartmentation, which were subjects also touched on by Ben Price.
He looked at STA’s guidance and the three routes to compliance – charring, encapsulation and fire engineering – saying the choice of route depended on the size and type of building.
“The scope of where we can use the charring route to compliance is limited and for residential buildings we are going to be looking at it up to 11m,” said Mr Price. “The route map says that structure can be exposed, partially protected or encapsulated and you can demonstrate fire resistance through calculation methods like those in EC5 or by carrying out some testing.”
Encapsulation is typically used on taller buildings and is a lining solution – plasterboard fixed to CLT, for example – which has been tested and that can avert pyrolysis for the duration of the fire resistance proved.
The “last leg” of the route map, fire engineering, is appropriate for larger buildings where encapsulation isn’t desirable.
“The requirement there is to demonstrate auto-extinction and that is going to be very building specific,” said Mr Price. “You need a competent fire engineer to look at that and it’s not an easy calculation to do. It’s going to become easier as the STA guidance and the experiments that are planned for 2021 are completed but for now this is going to be a hard route for residential buildings.”
Another of the classical elements – water – was the subject of the second part of the Better Timber Buildings conference on November 11.
Jennifer Eriksson, building physics engineer with Stora Enso’s Building Solutions team updated delegates on a project she introduced at last year’s conference. The project, which is being conducted in collaboration with BM TRADA, is looking into the moisture dynamic of CLT.
There is a wealth of information on water uptake and drying of solid timber components and wood-based sheet materials. However, as CLT is a relatively new engineered timber building system, the information currently available is more limited. Whilst CLT is formed using solid timber laminations, the size and thickness of panels create some unique differences and opportunities compared to other timber systems.
The project’s aim is to investigate the moisture dynamic of CLT; investigate the most effective ways of protecting CLT panels from wetting during construction and find the most time and cost effective ways of drying CLT panels which have become wet. Its conclusion will be best practice guidance, which, in turn, will increase confidence in the industry.
The research is in two phases, the first of which was concluded in March 2020, just before the first lockdown. This involved 22 test CLT build ups being set up at BM TRADA’s High Wycombe site and wetted at regular intervals. Particular attention was paid to the influence different joint types had on moisture ingress, how effective end grain sealants and adhesive tape were and also on whether the CLT was in a horizontal or vertical plane.
Lewis Taylor, timber frame consultant at BM TRADA, described the painstaking work that had gone into setting up and monitoring the test rigs.
“CLT panels were 100mm thick and consisted of five lamella with moisture sensors placed in the centre of each lamination within the field of each panel as well as at joints,” he said. “Moisture content readings were recorded every hour for nine weeks – that’s more than two million readings used to form the data set used for analysis.”
Phase two of the project, which is looking at what factors influence the drying rate of CLT panels, is ongoing but has been delayed by Covid-19 restrictions. At the time of writing, however, the research team was hoping to have guidance available in early 2021.
