The advent of supertowers coupled with an ageing population have prompted engineers and fire officials to take a fresh look at fire safety and evacuation options for high rise fires.
The World Trade Center attack has also made clear the need for some incidents to facilitate full building evacuation of high rise buildings.
In current international high rise design, this may include the use of an increased number of exit stairways, the use of safe areas or refuge floors within the building and the use of elevators to facilitate emergency evacuation of building occupants.
“Most people can evacuate moderate distances with relative ease. As the difficulty of egress increases, the effective mobility of occupants decreases,” said Denny Verghese, senior fire performance engineer at Meinhardt. “Evacuation in tall buildings should therefore consider the limitations on the physical ability of its human occupants. It is well known these limitations are only increasing due to age and health related factors that affect the population of most large cities.”
Defend In Place
A “defend in place” strategy provides a means to safely remain within the building during a fire. Defend in place strategies are common in most healthcare buildings, as they contain a large proportion of occupants with some level of mobility impairment. They have now become more commonplace in tall buildings with the trend of baby boomers wanting to remain in urban centres.
“Options to evacuate or defend in place are essential for tall buildings. Depending on the type of building and the nature of the occupants, these options can aid in providing a sound basis for the performance of a building in the event of a fire,” Verghese said.
Retired Fire Department of New York deputy chief Vincent Dunn, having worked through 9/11, is critical of the firefighting strategy of “defend in place,” however, particularly for high-rise office buildings. He argues that this has proven to be unworkable and unrealistic, as lightweight high-rise office buildings have become less fire resistive and the floor areas have become larger.
The defend-in-place strategy is based on three factors:
- That a high-rise building is fire resistive and occupants can stay in the building without being exposed to smoke and fire.
- That firefighters can extinguish a fire in a high-rise office building.
- That the occupants of a high-rise office building will comply with the fire chief’s instructions.
Dunn said that today, high-rise office buildings are not fire resistive because smoke spreads readily throughout the air systems, while the advent of large, 10,000-square-foot office floor areas are beyond the firefighter’s hose stream extinguishing capability.
“High-rise firefighting has become similar to a 'controlled burn;' firefighters protect the stairs while the floor contents burn,” he said. “Since 9/11, it is very unlikely occupants of a high-rise office will comply with the fire chief’s instructions to remain in place while a fire is being extinguished.”
If the tendency for occupants is to go, and if stairs are not viable, what is the option?
In the United States, National Elevator Industry Inc (NEII) codes and standards analyst Brian Black says there are changing views on the role of the elevator in emergency occupant egress.
“The reasons behind prohibiting the use of elevators in a fire were based on historical concerns that have become modern anachronisms in new buildings,” he said.
“Due to today’s advances in building design and elevator technology, working elevators don’t become inoperable in fire situations, trapping passengers as the environment becomes untenable. There isn’t necessarily a serious loss of power to the building or a shutdown of the elevator system due to intrusion of water into the elevator shafts, capturing passengers engulfed by smoke or fire.”
For persons using wheelchairs or those with a limited capacity to use exit stairs, elevators are really the only viable option in emergency circumstances, but Black admits that using elevators to evacuate non-disabled building occupants in building fires remained an idea outside of the norm.
He added, howeverm that with extremely high-rise construction in particular, using elevators versus the exit stairs can shave hours off of the time it takes for building occupants to move from close proximity to a fire to the safety of the outdoors.
“Many have theorized that had the building elevators in the 9/11 World Trade Center attacks remained operational as a means to evacuate the upper stories of the buildings, more lives could have been saved,” he said. “In fact, in the 18 minutes after the North Tower was struck, hundreds of workers chose to evacuate the South Tower and reach safety on the ground floor by using the building’s elevators.”
Following the attacks, the American Society of Mechanical Engineers (ASME) A17 Elevators Standards Committee established a task group on the use of elevators for occupant evacuation. Within nine years the National Fire Protection Association (NFPA) and International Code Council and ASME Elevator Code all had new provisions.
These permitted extremely high-rise buildings to use elevator systems to safely remove building occupants from the upper stories of a building during fire. These occupant evacuation elevators are housed within specially designed elevator lobbies with hoistways and machine rooms protected from the intrusion of fire, smoke and water.
In fact, the International Building Code has incentives for installing these types of elevators where a building exceeds 420 feet in height.
In Australia, Society of Fire Safety national president Dr Weng Poh agrees that designing appropriate vertical transportation systems is a solution to evacuating tall buildings but that national codes are currently a barrier.
“The main issue is egress since the Building Code of Australia (BCA) does not allow a lift to be used in the event of a fire,” he said. “In fact, presently, there is no BCA solution to evacuate very tall buildings.”
“The Australian Building Codes Board has issued a guideline document regarding the design of lifts as an alternative solution for evacuation but it only contains various factors to be considered and no concrete solutions.”
Wider stairwells, advanced sprinkler systems and alarms that give precise instructions during an emergency all make buildings safer.
“But to really make these systems work you really need to educate the occupants,” said Chris Jelenewicz, an engineering program manager at the Society of Fire Protection Engineers, speaking to the New York Times after a high rise fire claimed a number of lives.
“Many people don’t know whether they live in a fireproof building and don’t know whether they should stay or go during an emergency,” Johnson said.
Outside The Box
A number of alternative escape solutions have been proposed since 9/11 that do not require structural changes.
Helicopter: A rooftop heliport located a safe distance from antennas, mechanical installations and other hazards could provide a means of evacuation. However, not every building can have a heliport and evacuees, especially the disabled, cannot always get to the roof.
Parachutes: At least six companies are selling escape parachutes but this extreme method of egress is only suitable for the athletic and requires the jumper to be a suitable distance up.
Slides: Similar to the chutes used in airplane evacuation, slides have been listed as an alternate means of egress in the building codes of several US cities. Such slides, however, may not be feasible above the height that is reachable by fire department ladders.
Wires: These are stretched between buildings so that people can slide from one affected building to another, but again gives no consideration to those who are less mobile.
Tubes: A vertical tube of flexible material has been installed in some buildings in Asia and Europe. It can serve multiple floors, but unless it is located outside the building, it may be subject to the same hazards as stairways and elevators.
Collapsible elevators: This system has been developed in Israel and requires the installation of collapsible cubicles on the roof, which deploy along a rail on the outside of the building in an emergency. New York City has denied a permit for the system because of the potential bottlenecks as people try to enter the cubicles.
These are all interesting concepts, but they are not really practical or viable on a mass scale.
“The fire engineering community is continuing to grapple with this challenge,” Poh said.