Example of large compartment fire experiment; Example of large compartment fire experiment;

Large compartment fire experiments: expanding knowledge of building safely with timber

This article was amended on the 7 June 2021 to reflect the inclusion of a second experiment.

The environmental impact of construction is unsustainable in our warming world. Faced with the high carbon footprint of concrete and steel production, timber is becoming a compelling third option. Properly managed over its lifecycle, timber is a natural way to lock up carbon and unlock sustainable design. But as architects and engineers explore the speed, quality and human appeal of this sustainable material for larger and taller structures, fire safety standards and codes are yet to evolve to support low embodied carbon structures.

We are contributing to the growing body of knowledge by testing the fire performance of timber and sharing our insights as we all work to develop a safe, low-carbon future for buildings.

Testing resilience, at scale.

Cross-Laminated Timber (CLT) and glulam are the most affordable, efficient and flexible form of timber used for construction. Previous fire performance tests for these materials have focussed on structures under 90m2, the equivalent of an eight-person office. If timber is to make a full contribution to sustainable design for offices, educational and residential buildings, we need to consider its performance in much larger compartments.

Our fire safety team has been working at CERIB – a professional fire test facility in France – on a series of full-scale fire experiments using a 380m2 combustible compartment, with input from Hazelab at Imperial College London. That size of compartment represents an office of approximately 40 people.

Two experiments have now taken place, with the first occurring on 10 March and the second occurring on the 1 June. For both we were joined at CERIB by observers from the French Fire Prevention department and the Paris Fire Brigade.

Purpose built fire test structure Purpose built fire test structure
Based on previous large scale compartment experiments led by Imperial College, we designed our purpose-built test structure using masonry walls, an exposed 5-ply CLT ceiling and exposed glulam columns to mirror the aesthetic of modern timber office buildings.

Early observations from the experiments

To study how fires grow and develop in these larger spaces formed with exposed mass timber, we incorporated extensive instrumentation to measure temperatures and heat flux inside the building and within the CLT and glulam columns themselves. Together with cameras, this allowed us to monitor and measure fire spread, speed, duration and – with no firefighting intervention – the longterm fire decay and smouldering behaviour of CLT.

The spread of fire across the exposed CLT ceiling was faster than current estimates would predict, an important insight for the design of evacuation options and fire-fighting response. Once the fuel in cribs on the floor had been consumed and only glowing embers remained, the flaming combustion on the surface of the CLT panels also reduced, though small pockets continued to smoulder for a significant number of hours. We are using these results to drive our numerical modelling of the fire behaviour of exposed timber.

Use of a combustible CLT ceiling resulted in external flame extensions Use of a combustible CLT ceiling resulted in external flame extensions
The use of a combustible CLT ceiling resulted in substantial external flame extension from the openings, something we will continue to study in our future experiments.

Shared insight

With the two experiments now completed and results yet to be peer reviewed, we are looking to this work to inform future evacuation, structural design and fire-fighting planning. The design of the building at CERIB will allow us to vary both fuel load and ventilation to more fully understand how timber performs in a fire as we deconstruct, rebuild and re-instrument the structure. Properly understood and incorporated into new codes, we hope this work will contribute to the robust use of sustainable mass timber in buildings around the world.

Download a summary of our first fire experiment at the CERIB Fire Test facility 

Download a summary of our second fire experiment at the CERIB Fire Test facility