Elevator Evacuation Algorithms

Elevator Evacuation Algorithms M-L. Siikonen and J. S. Sorsa KONE Corporation P.O. Box 7, 02151 Espoo, Finland Corresponding author: [email protected] Abstract In emergency situations, the practice has been to return elevators to the exit discharge level, and then shut down. After that, the elevators are not available for the building occupants until the emergency is over. In this paper, we study the use of the elevators in an emergency evacuation. We first introduce the theoretical egress time and the handling capacity calculation for an elevator group, which is based on floor-by-floor evacuation. We also describe a specialized elevator evacuation algorithm, which automatically, with or without landing call information, dispatches elevators to the occupied floors and shuttles passengers to the rescue level. The algorithm serves floors in a priority order and detects floor occupancy automatically. We compare this algorithm to a normal algorithm and to two staircases in evacuation. For that purpose, we run simulations of test buildings with realistic transport arrangements and obtain performance measures such as crowding levels of the lobbies, passenger service times and total evacuation time. On this basis, we propose the best algorithm for different types of emergencies.

Introduction It is estimated that over 800 buildings exceeding 200 meters will be built in 2012 [1]. In the beginning of 2010, the 828-meter tall Burj Khalifa with 160 floors was opened. It exceeds the height of the previous record holder, Taipei 101, by over 300 meters, and Taipei 101 lost its five-year leadership as the tallest building in the world. In tall buildings such as these two examples, evacuation plans are tailored specifically for the building. People travel to the safety area using either protected elevators all the way [2, 3] or, first stairways to refuge floors and then protected elevators to the safety area [4, 5]. In an emergency, elevators are driven either by attendants or an automatic evacuation algorithm. In fire evacuation, in addition to enhanced or protected elevators, fire-, smoke- and water-proof elevator lobbies have to be designed [6, 7]. In tall buildings, typically two stairways are reserved for evacuation. The capacity of a stairway depends only on its width. Consequently, stairway capacity is constant but egress time increases since the total population increases by the number of floors [8]. Elevators are planned according to the total population, which makes egress time when using elevators independent of the number of floors. Ac-

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cordingly, the total evacuation is faster by stairways than by elevators only up to a certain population density [9]. For high population densities, phased evacuation by stairways is used to avoid congestion. In another study, it was found that theoretically elevator evacuation becomes faster than two stairways in buildings having more than 2 500–3 000 occupants [10]. In residential buildings, the population rarely exceeds 2 000 persons; hence, in most such cases, stairways are the fastest way out. Our challenge here is to determine the best methods for self-evacuation in buildings of more than 200 meters in height. The best evacuation methods for different emergency scenarios can be found by using simulation [11], and then selected as an integral part of the building security plan. A default evacuation mode is automatically switched on by the alarm system. Evacuation continues up to one to two hours depending on the fire-resistance time and emergency power resources. In this paper, we study elevator evacuation algorithms in emergency scenarios. Throughout the paper we assume that the elevators can be used in total evacuation: either the emergency is a non-fire situation, or the elevators are designed as safe to use during a fire. Since fundamental elevator performance is set by the elevator cycle time from start to start, we review the parameters that define it. By using these parameters, we formulate equations for evaluating elevator group performance both in up-peak and in floor-by-floor evacuation. Our results show how evacuation time relates to up-peak performance, according to which elevators are currently planned. After the theoretical part, we describe elevator evacuation algorithms, simulate complete building evacuation with them, and then compare the obtained results to a benchmark evacuation with two staircases.

Elevator Cycle Time The cycle time, TC, is the time between two elevator starts, which consists of the flight time, tf, and the stop time, ts, and is further divided into the components shown schematically in Figure 1 below. The stop time consists of door operation delays, start delay and passenger transfer time. Start delay, tst, is spent closing the safety circuit before the elevator starts to move. When the elevator approaches the floor level, the doors can be opened in advance some 15–30 mm before reaching the floor. This advance opening time, tado, reduces the stop time. The door opening time, tdo, is defined as the time until the doors are 800 mm open, which is assumed to be enough for passengers to move in or out of the elevator. The doors stay open for a dwell time but in the calculations it is assumed that photocell beam is always broken by the passengers. After the beam is restored, it still takes the time equal to the photocell delay, tph, until the doors start to close. During the stop, M passengers transfer to/from the elevator, each taking the transfer time tm. Finally, the door closing time, tdc, is de-

Elevator Evacuation Algorithms

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fined as the time until the doors are closed and locked. Typical values of the delays included in the stop time are shown in Table 1 below.

Start of door closing

Photocell restored

Door fully open 800 mm open Elevator stops

Start of door opening

Elevator starts

Door closed

Start of door closing

Flight time

Stop time

Time

Stop time Cycle time

Fig. 1. Cycle time components

Table 1. Typical values of the cycle time components Start delay Door opening Door closing Advance door opening Photocell delay Transfer time (s) time (s) time (s) time (s) (s) (s) 0.7

1.2

3.0

0.7

0.9

1.0

The flight time depends on the travel distance, s, rated speed, acceleration, and jerk of the elevator, v, a, and k, respectively [12]. The minimum distance needed by the elevator to reach rated speed is smin  v 2 a  va k .

(1)

The flight time, depending on whether the rated speed is reached or not, is if s  s min , s v  v a  a k , tv   2  4s a  a k   a k , if s  s min .

(2)

Finally, the cycle time is

TC  tv  t ado  t s  tv  t ado  t dc  t st  t do  Mtm  t ph .

(3)

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Elevator Planning and Egress The core of tall buildings, including elevator shafts and stairwells, is optimized to occupy as little of the floor area as possible. Usually elevator groups are zoned so that one elevator group serves particular floors of the building. If a building is higher than 20–25 floors, it is divided, for instance, between two elevator groups serving low- and high-rise zones, both starting from the main lobby. In this manner, the building can be divided even into ten zones. In mega-high-rise buildings, however, this kind of banking results in a large core area and the rentable area becomes too small. To overcome this problem, mega-high-rise buildings are most often designed with sky lobbies. In the sky lobby arrangement, shuttle elevators transport passengers from the main lobby to the sky lobby. From the sky lobby, the passengers travel to their destinations with local elevators. This reduces the building core area so that the building is still economically feasible. Elevator groups are usually planned for the morning up-peak traffic since it is the most demanding situation for the elevators. Up-peak handling capacity is calculated from the roundtrip time, TRT, during which the elevator loads M persons in the main lobby, serves an expected number of car call stops, S, and expresses back to the main lobby from the expected reversal floor H. It is customary to set M equal to 80% of the rated capacity of the elevator. The up-peak roundtrip time with an average floor-to-floor distance s becomes [13] TRT  2 Hs v  ( S  1)t s  2 Mt m .

(4)

The handling capacity of an elevator group with L elevators is HC  300 ML TRT .

(5)

The handling capacity, given in persons per five minutes, is usually scaled relative to the building population and given as a percentage. This relative handling capacity is used as a design criterion. Typically, a handling capacity of 12–16% of the population in five minutes is required in offices, but only 5–7.5% in residential buildings. This reflects the fact that traffic demand in offices is much higher than in residential buildings. In addition, the building filling time can be calculated from the relative handling capacity. The criteria given above correspond to filling times of 31–42 minutes in offices and 68–100 minutes in residential buildings. Evacuation is not generally considered in elevator planning since elevators are currently shut down during an emergency situation. Buildings with heavy downpeaks, such as hotels in Islamic countries with specific praying times, make an exception to the rule. With a full collective control system, elevator-group handling capacity in down-peak is 1.5–1.8 times the up-peak handling capacity without congestion. In down-peak, the elevator dispatching algorithm has more degrees of freedom in serving the calls, which results in shorter building emptying times.


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The emptying, or egress time of a building with L elevators can be calculated from the roundtrips, where the elevators transport population Pi from floor i to the level of discharge [14]. In each roundtrip, Mi,j persons are transported. As in the up-peak case, we assume that Mi,j is at most 80% of the rated elevator capacity. The number of roundtrips needed to empty floor i is J i  Pi M  ,

(6)

and the total egress time, TE, to evacuate N floors to the level of discharge is TE 

N

Ji

 TRT ,i, j

L.

(7)

i 1 j 1

By counting the total number of roundtrips, an average egress roundtrip time can be calculated. If this roundtrip time is substituted into equation (5) then we get the elevator egress handling capacity. The egress times vary in office buildings from 20 to 28 minutes for the above-mentioned up-peak handling capacity criterion, and from 44 to 67 minutes in residential buildings. Table 2 shows the calculated egress times and handling capacities in conjunction with the corresponding up-peak figures. According to the results, the theoretical egress handling capacity is 2.5 times the up-peak handling capacity. The reason is the number of car call stops in up-peak, in evacuation situation elevators shuttle between two floors. Table 2. Calculated elevator group egress and up-peak performance Egress Time (min) Evacuation HC (%/5 min) Filling Time (min) Up-Peak HC (%/ 5 min) 10

64

25

20

20

32

50

10

26.8

24

66.7

7.5

40

16

100

5

60

12

133.3

3.75

80

8

200

2.5

Elevator Evacuation Algorithms Three evacuation types have been recognized: fractional, staged, and total evacuation [6]. An elevator evacuation algorithm based on one of these evacuation types can automatically switch on when an alarm occurs. During evacuation it is critical to communicate correct and up to date information to the evacuees. In the case where the elevators are used in evacuation, this becomes even more important. People then have to choose whether to wait for the elevator, or to start de-

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scending the stairs. Elevator displays and voice announcements can be used to inform the status of the elevator group, for example: “Elevator evacuation in progress”, “Next elevator arrives in 5 minutes”, or “2 elevators arriving, room for 40 persons”. If such information is available, the occupants are able to decide their means of egress.. Later, authorized personnel can change the mode [15]. Automatic elevator evacuation is interrupted if smoke, heat or water is detected in the lobbies, the elevator shafts or the machine rooms. After the emergency, an authorized people such as firemen, return the elevators back to their normal operation. In fractional evacuation, a focused group of building occupants such as disabled people is rescued by the firemen using attendant drive. In staged evacuation, first the emergency floor and a couple of floors above and below it are rescued in a priority order, while the rest of the building occupants may remain at their floors waiting for further instructions. After the first stage of the evacuation has completed, the rest of the building can be emptied in a total evacuation. Complete evacuation with self evacuation mode is more efficient compared to attendant-driven manual modes since the dispatching algorithm has information of all the served floors simultaneously. In a fire situation, automatic dispatching algorithm can operate with or without call buttons depending on whether the elevators are protected or not. In a non-fire emergency situation, it is possible to serve landing calls as in normal operation. During evacuation, elevators accept destination calls only to the level of exit discharge. The normal operation mode of the existing elevator dispatching algorithms can also be used for the evacuation. In the normal mode, however, the dispatching algorithm gives its own priorities in serving the floors. Interconnected Full Collective (IFC) control has already been used by the relay control systems. In IFC, an empty elevator is always sent to the topmost landing call. If a car is already serving that call, the next vacant elevator is sent to the second highest call. Then landing calls are served one by one on the way down so that only one car stops to each landing call at a time. In modern full collective control systems, mathematical methods, such as forecasting or Genetic Algorithm (GA) [16], are used in the optimization. Different optimization objectives can be set to minimize, for example, average passenger waiting or journey time. The optimization results in about equal service at all floors. If all the people waiting do not fit in a car, they give a new landing call after the fully loaded car has left. If the car is not full, the elevator serves the next landing call on the way towards the rescue floor. Destination Control (DC) is an elevator system where passengers give the destination call using a numeric keypad already at the lobby [17]. In evacuation, a floor warden can use the keypad to enter the number of evacuees at the lobby. Then, the dispatching algorithm can to send automatically as many cars as are needed to empty each floor. In the floor-by-floor algorithm mode the floors are evacuated one by one starting from the floor with the highest priority, e.g. from the emergency floor or the topmost floor. If landing calls exist, elevators are dispatched to the corresponding floors in priority order. The algorithm takes into account slow reaction times since


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elevators are dispatched to occupied floors when requested. Then, elevator may stop to several landing calls during a trip depending on a set load limit (Fig. 2) Update occupant count from - defaults - forecasts - measurements - DC panels

Automatic Floor-by-Floor evacuation mode

Do landing calls exist?

Express to the area of safety No Yes

No

Evacuation time elapsed?

Yes

Yes No

Define priorities to occupied floors to be evacuated

Is the load below a set limit?

Dispatch an elevator to occupied floor according to the floor priority order

Decrease floor occupant count by the car load

Load the car at the occupied floor

End automatic evacuation and return cars to area of safety

Fig. 2. Flow chart for total evacuation floor-by-floor

In case there are no landing calls, floors are served according to the occupant count, which can be based either on default values, measured people, forecasts or external inputs. External input can be, for example, processed image data, or exact figure dialed by the floor warden using the destination control keypad in the lobby. According to the number of occupants, the dispatching algorithm sends as many cars as needed to rescue the waiting occupants from the floor. The algorithm keeps track of the number of occupants on each floor by decreasing the occupant count by the car load each time the elevator leaves the floor. For safety reasons, the algorithm dispatches elevators to the floors in priority order until the evacuation mode is turned off, or automatically after a defined time, e.g. two hours.

Simulation Study using Evacuation Algorithms In the simulation benchmark study, egress times and passenger service times are compared in three building zones for a total evacuation scenario. The Building Traffic Simulator, BTS, [18, 19] is used to simulate the scenarios. In the simulation, occupant arrival time at the exits is adjustable, here passengers arrive at elevator or stairway lobbies within 30 seconds. The egress time is defined as a time

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from the arrival time until the moment when all people have been rescued to the safety area. Passenger journey time consists of passengers’ waiting times and egress times in staircases and with elevators. The building data as well as the elevator group sizes, loads, speeds and other parameters are shown in Table 3. Elevator groups are planned according to 15% up-peak handling capacity criterion. Table 3. Building and elevator group definitions Travel Population Group size Car Size Speed Acceleration Jerk (m/s3) (m) (persons) (elevators) (persons) (m/s) (m/s2)

Zone

Served Floors

Low-rise

0, 1-20

66

1440

7

24

3.5

1.0

1.6

High-rise 0, 41-60

198

1440

8

24

8.0

1.2

2.0

The elevators are used in five evacuation modes with diverse dispatching algorithms. For comparison, evacuation with two 1 200mm-wide staircases is simulated. A person queuing at a landing enters a staircase if his journey time is longer than the one’s descending in the staircase and passing the floor. In zoned evacuation mode people first descend 20 floors by the stairway from the high-rise area to the refuge floor where the elevators bring them down; in the low-rise area people use only stairways. The simulation results are shown in Table 4. Table 4. Egress and passenger service times with different evacuation algorithms Zone

Parameter (min)

GA normal

Egress time

13.3

14.1

Low-rise

Waiting time

5.5

6.3

Journey time

6.8

High-rise

GA Evac Fl-by-Fl mode algorithm

Fl-by-Fl Eq.7

Refuge Floor

Stairs

14.5

13.1

11.4

11.4

6.7

6.7

0.3

0.3

7.4

8.1

7.4

6.5

6.5 23.5

Egress time

15.2

16.8

16.9

14.7

14.7

Waiting time

6.3

6.9

7.4

7.6

1.7

0.4

Journey time

7.7

8.3

8.7

8.6

7.7

17.5


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Av. Queue Length per Floor 80

GA Evac Mode

70

GA normal Fl-by_Fl algorithm

Persons

60

Fl-by-Fl Eq.7

50 40 30 20 10 0 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Floor Index

Fig. 3. Average passenger queues per floor in the high-rise area

The combined use of stairs and elevators with refuge floor is the fastest in all rises. Stairways are the fastest in the low-rise area, but in the high-rise elevators are clearly faster. Modern normal GA algorithm gives the shortest passenger waiting and journey times. Also average queue lengths over time are the most balanced with normal GA algorithm, as shown in Figure 3.

Conclusion In tall buildings, the number of emergency elevators, and the dispatching algorithm have a direct effect on how many people can be rescued in an emergency situation. The one-hour Required Safe Egress Time (RSET) has been proposed [2]. This target could be reached in most existing buildings if all the passenger elevators were used during evacuation. In 200+ meters high buildings people may have to use several transportation devices on the way down and a one-hour criterion can be too tight. According to this paper elevator group egress time varies from 10 to 60 minutes depending on the designed up-peak handling capacity, but more precise results are obtained by simulation. As a conclusion of this study, in complete evacuation the egress times are the shortest with staircases up to 20-40 floors depending on the population density per floor. A combined usage of stairways and elevators gives the shortest egress times, which is in line with earlier studies [10, 20]. The normal modern dispatching algorithms provide passengers the fastest egress and the best service level.

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For self-evacuation in a fire situation, floor-by-floor evacuation algorithm is needed to set priority order to the floors to be evacuated. The floor-by-floor mode is more efficient, if it also serves landing calls instead of knowing just the number of people waiting at the floor. The algorithm utilizes all the available information to evacuate the building as fast as possible but is not dependent on any specific piece of information. With proper signalization devices the control system can communicate of the status of the elevator evacuation to the occupants since the dispatching algorithm is capable of estimating the future position of the elevators.

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