Saturday, December 11, 2010

EFFICIENT EVACUATION METHODS IN TALL BUILDINGS

1.INTRODUCTION

According to European evacuation plans, elevators are returned to the main entrance when a fire alarm is raised. Only stairs are used for evacuation during an emergency situation, which is not always the most efficient way. Furthermore, the regulations tend to forget the disabled and elderly people who may not be able to use the stairs at all. The time from the beginning of an emergency until all people have reached an area of safety outside the building can be divided into three phases, recognition time, reaction time and egress time. Pre-movement time is the time before people start to egress the building. Its length is influenced by the signs of danger (e.g., people smell smoke), alarms, time of day and type of building. For example, one can expect that the pre-movement time is long in an apartment building at night. According to some studies, the pre-movement time is about three times longer than the actual movement time. Occupants become aware of the emergency after the recognition time. This is often the most critical part, when fire alarms have not yet been given. During the reaction time, people try to get information about the situation, the source of the emergency, gather personal belongings, perhaps their children and so forth. Training for emergency situations also affects the reaction time. During the actual movement phase, people move out of the building until everyone has reached an area of safety. Evacuation studies often concentrate on finding out the egress time since it can be estimated theoretically.

Critical time (Tcrit) is defined as a time limit when the evacuation of people is safe and probably no injuries or deaths will occur (Equation 1).

Tp + Tr + Te Tcrit (1)

2. EVACUATION BY ELEVATORS

2.1 Evacuation Times vs. Planning Criteria

Elevators are usually planned for the up-peak situation since it is the most demanding situation for the elevator transportation capacity. In residential buildings, a typical handling capacity is from 5% to 7.5% of the population in five minutes. In office buildings, a typical range of up-peak handling capacity is from 13% to 18% of the population in five minutes.

Down-peak or occupant evacuation is not normally considered at the planning stage of a building and in elevator arrangements. It has been stated that elevators should be capable of evacuating the population of a building within 15-30 minutes. A rough conclusion from the simulations is that elevators can transport about 1.5 times more passengers in down-peak than in up-peak. According to BTS simulations with modem-control systems, down-peak handling capacity with modem-control systems can even be 1.8 times greater than in up-peak. As an example, if elevators can transport 15% of the population in up-peak, the same elevators can transport about 22.5% to 27% of the population in down-peak. The reason is that the elevators have fewer car calls in down-peak and can reverse their direction more easily. This makes the elevator roundtrip time shorter, which increases handling capacity in down-peak.

The building filling and evacuation times for different up-peak handling capacities are shown in Figure 2. It can be seen, for instance, that with 15% up-peak handling capacity, the building filling time is about 33 minutes, and the average egress time is 19-22 minutes. The requirement of 15-30 minutes evacuation time is fulfilled with the up-peak handling capacity of 10% to 22% of the population in five minutes.

2.2 Mega-High-Rise Building

In a study of the behavior of people during an explosion below the World Trade Center (WTC) plaza in New York City in 1993 (Fahy and Proulx 1998), the mean reaction time of occupants was 11.3 minutes in Tower 1 and 39.9 minutes in Tower 2. According to this study, the total evacuation time was from one to three hours. This time included the times passengers had spent waiting or resting in areas of refuge. Results of the same order were obtained from the terrorist attack on the WTC on September 11, 2001, where, for example, the window washer took 53 minutes to exit the building by the stairs from the 50th floor. In the first incident in 1993, the occupants would probably have been able to use the elevators during the emergency. In buildings where sky lobby arrangements are used, the number of floors is at least 60, and the egress times by the stairs are very long. If elevators were used during the emergency, passengers above the sky lobby would have to use local and shuttle elevators during their way down to the main entrance. In mega-high-rise buildings, shuttle elevators may become a bottle-neck during evacuation if they are planned with normal up-peak criteria. According to Figure 3, local elevator groups can empty all passengers from upper floors within 20 minutes, if the up-peak handling capacities of local groups are in the range of 14% to 17% of population in five minutes.

In this case, during evacuation all passengers arriving from local groups to the sky lobby cannot be transported down by the shuttle elevators with the same rate as they arrive, and congestion will occur. In the previous example, passengers would be transported down within 20 minutes by the shuttle group, if its up-peak handling capacity was 25% of the population in five minutes. With a high handling-capacity requirement, a single-deck shuttle group takes a lot of building core space. Double-deck or triple-deck elevators require less core space, since they theoretically double or triple the handling capacity per shaft if they have two stops only.

2.3 Movement Time by the Stairs

The movement of people in stairs can be modeled as unified crowd flows. The maximum flow per stair width has been measured for different types of staircases, riser heights and treads depths. The flow models give optimistic results, since they assume that stairs do not become overcrowded. In case there is heavy congestion, the walking speed and also the occupant flow drop significantly. Simulations give more information about situations where the occupant density is very high and there is smoke and panic.

In the Melinek and Booth flow model (Melinek and Booth 1975), the evacuation situations can be roughly divided into two categories. In one case, there is congestion on the stairs and the occupant flow is at maximum all the time. In the other case, occupants can walk freely. The egress time is the maximum of these two.

where

t1 egress time (congestion)

tn egress time (free walk)

n number of floors

N number of people per floor and exit

Fs nominal occupant flow on stairs (persons/m/s)

W width of the staircase

ts walking time between adjacent floors (free walk)

3. EVACUATION BY ELEVATORS AND STAIRS

If there are several ways to escape from the building, occupants try to use the quickest one. If the environment is not familiar, people may have difficulties in finding the emergency exits, and they would rather use the same route by which they entered the building. Even if the environment is well known, it is difficult to choose the best route, since some of them become congested in an emergency situation. Therefore, at least one of the stairs should be placed in the proximity of the elevators so that the occupants can easily choose the best exit route.

An optimal situation for using the stairs and elevators would be that the last person from the stairs and the last person from the elevator exit the building simultaneously. Otherwise, the longest egress time from either of the exit ways defines the total evacuation time. It is difficult to

predict the proportion of people that would use elevators or stairs in an emergency situation. A case where half of the population use elevators and half of them use stairs was studied for the same buildings as in the previous section. According to Figure 6, in an office building with 30 floors and with 100 persons per floor, the egress time by stairs and elevators drops to about 13 minutes. From Figure 5 it can be seen that by stairs it is 26 minutes, and by elevators 22 minutes. Using both exit ways considerably decreases the egress time up to about 50-floor buildings with about 100 persons per floor. For 200 persons per floor, using equally stairs and elevators.

4. CONCLUSION

In this article, evacuation by elevators and stairs was studied. According to the simulation results, arranging people into zones during the evacuation does not decrease the evacuation time. Modern control systems can handle the down-peak situation effectively without zoning. Evacuation times in buildings with different number of floors and with different proportions of population per floor were studied for stairs and elevators. In planning stairs in a building, only the floor population and walking times and distances to the nearest exits and stairways are considered. According to the results, if either stairs or elevators are used, evacuation times by elevators become shorter for office buildings with 15 floors or more depending on the number of people per floor. If both stairs and elevators are used, evacuation time usually decreases compared to using elevators or stairs only. In a typical residential building the stairs are the fastest way for evacuation since the elevator-handling capacity is small. If only a small percentage of occupants use the elevators, the egress time decreases compared to using only the stairs in a residential building. If elevators are used in mega-high-rise buildings during an emergency situation, evacuation times can drop to 15- 30 minutes instead of two to three hours. In these buildings, shuttle elevators may become a bottle-neck during the evacuation and down-peak. Handling capacity of a shuttle elevator group with only two stops can be considerably increased with double-deck or triple-deck elevators.

5.REFERENCES

1. Fahy, R. F. and G. Proulx. (1998). “Human Behavior in the World Trade Center Evacuation.” Fire Safety Science – Proceedings of the Fifth International Symposium, pp. 713-724.

2. Lo, S. M., Z. Fang and D. Chen. (2001). “Use of a Modified Network Model for Analyzing Evacuation Patterns in High-Rise Buildings.” Journal of Architectural Engineering, June, pp. 21- 29.

3. Melinek, S.J. and S. Booth. (1975). An Analysis of Evacuation Times and the Movement of Crowds in Buildings. Borehamwood; GB: Building Research Establishment, Fire Research Station. (BRE Current Paper CP 96/75 FRS)

4. Proulx, G. (1995). “Evacuation Time and Movement in Apartment Buildings.” Fire Safety Journal, Vol. 24 (3), pp. 229-246.

5. Schneider, V. (2001). Application of the Individual-Based Evacuation Model ASERI in Designing Safety Concepts. International Symposium on Human Behaviour in Fire, Boston, 26/28 March, 17 pp.

6. Siikonen, M-L., T. Susi and H. Hakonen. (2001). “Passenger Traffic Flow Simulation in Tall Buildings.” ELEVATOR WORLD, August, pp. 117-123.

7. Strakosch, G. R. (1967). Vertical Transportation: Elevators and Escalators, John Wiley & Sons, Inc., p. 495.

8. Weckman, H., S. Lehtimäki and S. Männikkö. (1999). “Evacuation of a Theatre: Exercise vs. Calculations.” Fire and Materials, 23, pp. 357-361.

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