Firefighting and evacuation stairs rely on the protection of closed stair fire doors.
When a 20-mph wind enters a standard one square metre window of a post-flashover apartment fire, the creation of a flow-path as openings occur behindadvancing firefighters (such as a smoke shaft, an automatic opening window vent, or a stairwell door) may create blow-torching like fire conditions in the corridor. If the inlet for the wind was a floor to ceiling balconysliding door, the increase in volumetric flow and velocity through the apartment and into the corridor is dramatically increased further.
In 2008, the New York University, in collaboration with the Fire Department of New York City (FDNY) and the National Institute of Standards and Technology (NIST), conducted a study of wind-driven fire dynamics in high-rise structures. The objective of this study was to improve the safety of firefighters and building occupants by developing a better understanding of wind-driven fires and wind-driven firefighting tactics, including structural ventilation and suppression.
Importantly, the simulated wind and fire conditions in this series of experiments are closely comparable to those occurring at a fire at Harrow Court, Hertfordshire in 2005, where two firefighters, and a building residentthey were attempting to rescue, were tragically killed whilst caught in the wind-driven flow-path that occurredon the 14th floor. The fire at Harrow Court featured in a BBC 2 documentary broadcast 30 October 2018, and was considered as one of five significant fires in the UK which occurred prior to the Grenfell Tower fire, which called for changes to regulations and policies. Since the late nineteen eighties there have been five UK firefighters killed in similar wind driven high-rise fires.
A series of 14 experiments were conducted in New Yorkto evaluate the ability of positive pressure ventilation fans (PPV), wind control devices (WCD), and exterior water application via floor below nozzles (FBN), also known as high-rise nozzles (HRN), to mitigate the hazards of a wind-driven fire in a structure. The results of the burn experiments were also validated by scientific simulation techniques. Previous research has shown that PPV fans positioned correctly are able to create a pressure sufficient enough to keep products of combustion out of the stairwell by creating a higher pressure than the fire creates. These experiments follow the guidelines produced by the previous research to determine if the fans can be effective at keeping the combustion products out of the stairwell under wind driven conditions.
Experiment 7A in the 14 live fire series utilized a simulated 20 mph wind, created by a large PPV unit at height. A wind driven fire was created from the fire ignited in a furnished bedroom. As the fire transitioned post-flashover, flames were forced out of the apartment by the 20-mph wind and into the common area corridor with the stair door closed, when conditions dramatically worsened further as the stairwell door was opened,allowing the hot gases to flow throughout the stairwell. These conditions would not be conducive to an interior direct frontal firefighter attack. Stairwell temperatures were in excess of 600ºC with the stair door open.
This live fire test burn highlighted the importance of door control. A wind driven condition was created on the fire floor and then the stairwell door was opened. With the bulkhead door also open the flow path between the fire floor and the top of the stairwell increased from less than 50 ºC (122 ºF) to over 400 ºC (752 ºF) in less than 15 s. The stairwell temperature peaked at 700 ºC (1292 ºF) in 35 s at which time the fire floor stairwell door was closed. Closing the door lowered temperatures to less than 200 ºC (392 ºF) quickly.
Every experiment began with all of the doors and windows closed with the exception of the door to thefurnished bedrooms and the main entrance door from the public corridor to the fire apartment. Once wind driven conditions were achieved many different openings were made to simulate the operations of a fire department. These operations included opening the front lobby doors for access, entering the stairwell, opening the door from the stairwell to the fire floor, and opening the bulkhead door at the top of the stairwell(this may have similar impact of the creation of a flow-path leading to untenable conditions for firefighters, where a corridor smoke shaft or AOV opens). The experimental duration varied between 23 min and 53 min depending on the growth of the fire and the impact of the tactics.
The flows out of the fire apartment were as high as 5 m/s with the stairwell door closed. The flow out of the fire apartment with the stairwell half opened (simulating firefighting access) was approximately 5 m/s and increased to 20 m/s (45 mph) with the bulkhead (roof) door opened at the head of the stairway. Note: The opening of a corridor smoke shaft or AOV located to the rear of firefighters could be expected to initiate a similar flow-path and increase in velocity/temperatures with an exterior wind entering the fire compartment.
When the wind-driven condition was created it forced ‘blowtorching’ like flames, all the way from the ignition bedroom to the stairwell door in the corridor. As the conditions developed, the area in the flow path became untenable for firefighters very quickly, to a point where conditions would not be conducive to an interior direct frontal firefighter attack.
Pressure in the stairwell.
Average pressure measurements in the stairwell are shown in Figure 3.3-21. The differential pressuremeasurements were made at 1.2 m (4 ft) on the stairwell landing of every floor and referenced to an apartment on the same floor. With all of the doors to the stairwell closed the pressures remained below 5 Pa and only fluctuated slightly. Once the bottom right windowpanefailed in the bedroom the pressures in the stairwell increased to approximately 12 Pa and steadily increased to 35 Pa to 45 Pa. Next, the front lobby doors and first floor stair door were opened. This allowed the pressure to vent and created a flow out of the first-floor lobby. At 600 s the pressures from top to bottom on each floor were 40 Pa, 30 Pa, 24 Pa, 16 Pa, 11 Pa, 8 Pa and 5 Pa respectively. A fan was added at the front lobby due to
the smoke coming out of the front doors, which increased the pressure in the stair further.
The bulkhead door was opened and the pressure in the entire stair dropped below 10 Pa as the gases moved to the vent. After the smoke moved up the stairwell, the fan was moved from the lobby to the base of the stairwell.This increased the stairwell pressures to a range of 5 Pa to 18 Pa. Adding a second PPV fan at floor 5 increased the stairwell pressure range to 5 Pa to 22 Pa. The bulkhead door was closed, and the stairwell pressure range increased to from 5 Pa to 30 Pa. Opening the fire floor door 0.08m (3 in) and allowing more wind driven flow into the stair increased the pressure range further to 20 Pa to 44 Pa. With the fire floor stair door completely opened the pressure range in the stair was 20 Pa to 58 Pa. After the living room window failed and the flow was split the pressures became steadier at 16 Pa to 24 Pa.
Positive Pressure PPV in the stairwell.
The use of the two 27-inch PPV combustion engine fans greatly improved conditions in the stairwell. They were not able to keep them completely free of smoke under wind driven conditions, but they were very effective and moving the flow up the stairwell and holding the smoke and hot gases when the fire floor stair door was closed. With the door open the flow was slowed but not stopped. After the bulkhead (roof) door was opened the temperatures in the public hall and the stair continued to increase until the fan was moved into place at the base of the stair on the first floor. Then the 5th floor door was opened, and another fan was added at that location. It is important to note the temperatures in the stair were reduced and kept at ambient conditions with two fans. A small increase in temperature occurred when the stair door on the 7th floor was opened 3 inches.
NIST Report 1629 – Wind Driven Fires (593 pages)
Flow-path created at the Empire State building fire in 1990 https://www.linkedin.com/pulse/how-1990-fire-new-yorks-empire-state-building-changed-paul-grimwood/?trackingId=eydcQivMQHyiE8jtaYTGqQ%3D%3D
Wind driven fire video (exterior signs)https://www.youtube.com/watch?v=S-NHrXBM56M
Wind driven fire video (interior view)https://www.youtube.com/watch?v=AlhIxDSpmcg
Fire curtains at the stair door (or not)https://www.linkedin.com/posts/paulgrimwood_high-rise-fires-and-wind-driven-flow-paths-activity-7118632654542921729-qneU?utm_source=share&utm_medium=member_desktop