Elevator and Escalator Consulting Engineers
For some years now there has been discussion as to how to deal with fires in high rise buildings.
The following excerpt from an article published in 1971 by G.T. Tamura, J.H. McGuire of the National Research Council Canada defines the problem:
“In the event of fire it is customary to evacuate buildings in order to minimize life loss, and occupants of low buildings can be evacuated in a relatively short time. The time for total evacuation by stair shafts, as currently provided, however, increases with building height and can exceed one half hour for a typical building over 20 storeys high. In very tall buildings, many occupants would be physically incapable of negotiating the stair shafts from the upper floors to the ground level, particularly under emergency conditions. Although smoke spread is a factor that must be considered for low buildings, it takes on much more serious significance for high-rise buildings because escape routes may become untenable before the occupants can be evacuated.”
Despite many conferences, papers and studies no consensus has been reached on the question of evacuation: whether to evacuate a building or stay in place and rely upon the firefighting forces to deal with any fire that occurs.
An article entitled Safety and fire prevention - Fire safety in high buildings published on The City of Toronto website (http://www.toronto.ca/fire/prevention/fireinsky.htm) says the following:
“In reacting to a fire in a high building, you must decide on two options: Do I leave the building to safety? or Is it safer to stay where I am? “
But no guidance is given to assist in making a decision.
Disasters such as the World Trade Center destruction have raised the question with greater force.
At the time of its destruction the World Trade Center had a nominal rental area of 10 million square feet in the two towers with a nominal population of 50,000 persons (http://www.nysm.nysed.gov/wtc/about/facts.html). It is estimated that at the time of the attack the number of occupants was ~18,000 (Estimating the World Trade Center Tower Population on September 11, 2001: A Capture–Recapture Approach http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2636613). Of the 18,000 people about 15 percent lost their lives. It appears, therefore, that some 85 percent of the people in the two towers were able to escape although many of these suffered injuries.
Whatever the weaknesses in our approach to high rise fires and how to deal with them, the survival rate for the World Trade Center disaster was remarkably high - particularly considering the exceptional nature of the attack.
A report issued 2013-04-10 entitled Landmark High-Rise Fire Study Evaluates Effectiveness of Crew Sizes, Elevator Use states that “41 percent of U.S. high-rise office buildings, 45 percent of high-rise hotels, and 54 percent of high-rise apartment buildings are not equipped with sprinklers, as compared with 25 percent of hospitals and related facilities. Moreover, sprinkler systems fail in about one in 14 fires.” (http://www.nist.gov/el/fire_protection/high-rise-fire-study-041013.cfm)
Although the seven percent failure rate of sprinkler systems is a matter of concern the consensus is that the first line of defence for any building is to have an automatic sprinkler system. A sprinkler deploys immediately before a small fire becomes a big fire. Which gives it a considerable edge over any firefighting crew.
Ultimately, if we are going to build buildings several hundred floors high (for the sake of brevity we will refer to such buildings as “tall buildings”), we have to consider using elevators should total building evacuation be necessary - as it was in the case of the World Trade Center. Even in those cases where total evacuation is not considered necessary or desirable it is likely that a given number of floors (those containing or adjacent to the fire) would require evacuation. The analogy has been made between ocean liners and tall buildings; the lifeboats are to be used only in case the ship is in danger of sinking and the elevators are to be used only if the building is in danger of collapse.
ThyssenKrupp has proposed a procedure to use for total building evacuation (TKE Total Building Egress system). The basic idea of this system is that all the elevators in a group will go to the top floor of the floors served, open their doors at the same time, load people waiting on that floor into the elevators, then descend to the ground floor (or the designated exit floor). The elevators would then empty the next to the top floor and so on down. The key advantage to loading the elevators at the same time at one floor is that the tendency to panic is reduced since there would normally be enough room for all of the waiting passengers. For example, a group of six elevators could easily empty a typical floor with an occupancy of 100 persons. Obviously implicit in the design would be refinements to deal with “missed” or “late” waiting people. With a round trip time of two minutes, we might expect the elevator group to empty fifteen typical floors in half an hour. This also corresponds to normal design parameters that would dictate a handling capacity of 10 to 12 percent of the population in five minutes on balanced peak (balanced peak is a much more onerous traffic pattern since there are multiple stops in each direction with a consequent lengthening of the round trip time).
If, on the other hand, we are only evacuating floors at or adjacent to a fire, then the sequence of evacuation would proceed starting from the fire floor and then the two floors above and then the two floors below. The same method of all cars of the group running in unison would be used. To empty the five floors would then take about 10 minutes.
All of the schemes for dealing with fires and other emergencies in tall buildings require, to be effective, a complete closed circuit monitoring system of elevators, elevator hoistways, elevator machine rooms, stairwells and other critical building locations. The monitor connections should be hardened, either by protected conduits or by wireless transmission. This applies as well to the connections for the standard elevator monitor system that displays the position, direction and functional status of each elevator.
In various conferences dealing with tall buildings consideration has been given to Areas of Refuge. The central idea is that every few floors (typically five to ten) an area with strong fire and smoke protection would be provided and that occupants on the other floors would take the stairs down to these Areas of Refuge where they would be better protected. It does seem that we should be able, rather than rely on the stairs and the instructions for accessing the Areas of Refuge, to make each elevator lobby an Area of Refuge.
Stairways, as a means to exit a building have their problems. Many people have been trapped in smoke filled stairwells. The standard stairway width can be problematical in that it does not easily allow counter-flow or transfer of people with physical disabilities and people that have been incapacitated; in tall buildings these difficulties are exacerbated. It is not unlikely that if building occupants had to walk down several hundred floors some of them would suffer cardiac or other problems; quite likely some would find the trip impossible.
Although it might not be practical to design a structure that could survive the impact of a large aircraft (as was the case with the World Trade Center), a thick structural concrete core might have a better chance than a structural steel core that could melt at high temperatures.
“One WTC will incorporate advanced life-safety systems that exceed New York City building code requirements. From structural redundancy to dense fireproofing to biochemical filters, it will create a new standard for high-rise buildings. Extra-wide pressurized stairs, multiple backups on emergency lighting, and concrete protection for all sprinklers will ensure optimal firefighter access. Exits are designed to ensure easy evacuation, and all safety systems will be encased in the core wall, with the enhanced elevators.” http://www.wtc.com/about/freedom-tower“. “All of the building's stairs, communications, risers, sprinklers and elevators are encased within the building's concrete core wall, which is up to six ft thick in some places. Concrete also protects all of the building's sprinklers and emergency risers.” (http://www.theconcreteproducer.com/highstrength-concrete/freedom-tower--new-york-city.aspx)
These characteristics of One World Trade Center indicate the direction in which the design of tall buildings is moving.
In all of the above, the elevators play an indispensable role. Elevators, as previously manufactured would not really be up to the task; they are too easily compromised by fire, water and smoke. To allow the elevators to function under adverse conditions many of the elevator components require modification. Hoistway switches (interlocks, limits et cetera) should be designed to operate under water by using proximity switches rather than open contacts. Door operators and door operator controls should be sealed so as to render them impervious to water. Hoistway and machine room pressurization would be required (see How Fast Can We Go? article for some comments on hoistway pressure). The power and control connections within and to and from the elevator system would have to be protected against water and fire. Although this list seems formidable, the actual cost would not be prohibitive once the changes become part of a “standard” elevator. There is no reason, for example, why an interlock designed to operate under water should be more expensive than the currently available interlocks or why a waterproof door operator with its associated control should be significantly more expensive than those being manufactured today.