The impact of Wind Driven Fires should be analysed for CLT Tall Building Designs

There is no greater hazard to the firefighter than a building fire that is impacted by a near perpendicular wind driven air-flow. A fire where the involved ‘moveable’ fire load is ‘fanned’ by the wind heading directly into an open or failed window can create havoc inside a building, trapping and killing occupants and firefighters within seconds. Several well documented tragedies have seen multiples of firefighters lose their lives over the past few decades and a further number of ‘near misses’ have occurred that came close to repeating such tragedies. At one such fire in the south of UK, as firefighters were about to enter the apartment the entire door and frame fell away into the corridor as searing heat and flame filled the common spaces within seconds. This fire would burn through adjacent 30 minute fire doors to involve other apartments within half the allotted FR time frame. The fast-moving high heat and smoke conditions would then pass through the stair door being held open by firefighting hose and fill the stairwell top to bottom from the 12th storey fire.

Wind driven fires do not form part of the design codes for buildings but perhaps they should when the structural frame itself is combustible. The increase in heat release has seen concrete stripped from the walls to near 20mm depths after the plaster had long fallen away. Such fires are commonly left to burn for some-time before firefighters are able to regain some control. What would such fire development do to an engineered CLT high-rise structure? How would the structural fire load add to the heat release generated by the moveable fire load?

A study by the Research Institutes of Sweden (RISE) in 2018 concluded:

1. Previous experiments involving under-ventilated fires in compartments with one ventilation opening and wind exposure in the direction perpendicular to the fa?ade opening has shown a reduction of maximum temperatures inside a compartment as well as a reduction of corresponding heat release rates with increasing wind velocity.
2. Previous CFD simulations and experiments indicated a reduction of air flow into and out of the fire compartment as a consequence of wind. It is, however, considered possible that the wind increases the airflow in some compartments if they have openings on more than one side.
3. A single zone model that includes the contribution of exposed wood in the calculation of compartment temperatures indicated that the char depth after a complete fire increases for a reduction of airflow into the compartment. The model included the assumption that delamination of CLT and fall-off of the base layer of gypsum board are prevented. The predicted char depths corresponding to a compartment with large openings and a compartment with minimum ventilation openings were, however, significantly lower than char depths corresponding to standard fire resistance test exposure of 2 hours which is relevant for most tall building designs. The damage of compartment fires is expected to be significantly higher if CLT delamination is not prevented.

Developing a better understanding of wind driven fires in the USA

However, The National Institute of Standards and Technology (NIST), the Fire Department of New York City (FDNY), and the Polytechnic Institute of New York University with the support of the Department of Homeland Security (DHS)/Federal Emergency Management Agency (FEMA) Assistance to Firefighters Research and Development Grant Program and the United States Fire Administration (USFA), conducted a series of wind driven fire experiments in a seven story building on Governors Island, New York. The objective of this study was to improve the safety of fire fighters and building occupants by developing a better understanding of wind driven fires and wind driven fire-fighting tactics, including structural ventilation and suppression. A series of 14 experiments were conducted in typically furnished rooms. Unlike earlier research into CLT fire development, a flow-path of fire development was created between openings on either side of the fire. The experiments were run until the fire burned down and smoke production was minimal. The experimental duration of the one and two-roomed apartment fires varied between 23 min and 53 min depending on the growth of the fire and the impact of the tactics in this traditional brick and concrete construction.

A wind-driven flow-path between openings on either side of the fire

During these experiments a public corridor and stairwell area was exposed to a wind driven, post-flashover apartment fire. The door from the apartment to the corridor was open for each of the experiments. The conditions in the corridor and the stairwell were of critical importance because that is the portion of the building that firefighters would use to approach the fire apartment or that occupants from adjoining apartments or adjacent floors would use to exit the building. All of the fires were ignited in furnished rooms of an apartment. Due to excess fuel pyrolysis/generation (lack of ventilation) the room of fire origin could not transition to flashover until windows self-vented and introduced additional fresh air with oxygen to burn. Without a wind imposed on the vented window, the fire did not spread from the room of origin and never left the apartment of origin.

Even with no externally applied wind, creating a flow path from the outside, through the fire apartment into the corridor and up the stairs to the open bulkhead on the roof increased the temperatures and velocities in the corridors and in the stairwell resulting in hazardous conditions for fire fighters and untenable conditions for occupants on the fire floor and above in the stairwell. With an imposed wind of 9 m/s to 11 m/s (20 mph to 25 mph) and a flow path through the fire floor and exiting out of the bulkhead door on the roof, compartment fire temperatures exceeded 1000 oC with floor to ceiling temperatures elsewhere along the route in excess of 400 oC (752 oF) and velocities on the order of 10 m/s (22 mph), were measured in the corridor and stairwell above the fire floor. These extreme thermal conditions are not tenable, even for a firefighter in full protective gear. These experiments demonstrated the “extreme” thermal conditions that can be generated by a “simple residential room and contents” fire and how these conditions can be extended along a flow path within a real structure when wind and an open vent are present.

How would similar fire conditions impact the fire development and structural stability within a CLT high-rise building? What if the fire involved large open-plan spaces? Has the research to date truly reflected how a 'real fire' may behave in these structures or are we to find out the hard way?

[email protected]