UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
INTRODUCTION TO LIFT TRAFFIC ANALYSIS
(This section on lift traffic analysis is taken from lecture notes prepared by Ir WK Lee)
Consider how a lift completes a trip in a building:
1. Lift opens doors and loads passengers at the main terminal
floor.
2. The lift closes door then accelerates, moves, and
decelerates to the 1st stop of car call.
3. The lift opens door for passengers to alight, then closes
door and repeat the steps for subsequent stops of car calls.
4. The lift closes door at the highest stop of car call, then
accelerates, moves, decelerates back to the main terminal
floor.
The complete process is called a round trip. The time taken is
called the Round Trip Time, RTT.
Consider the way in which a single lift car circulates around a building during uppeak traffic
condition :-
-
-
-
lift car door opens at main terminal floor
passengers board the lift car
lift car door closes
car runs to first stopping floor going through acceleration, traveling at rated speed if
achievable, deceleration and leveling (*)
door opens, passengers alight, door closes
o
o
repeat for an expected number of stops
o
o
highest stopping floor is reached
o
o
lift car express runs to the main terminal floor going through acceleration, travelling at
rated speed, then deceleration and leveling.
(*) If interfloor distance is too small, travel at rated speed may not occur.
For lifts serving high zone only, the travel to the first stop traverses substantial lower floors, thus the
lift may achieve rated (contract) speed for this first stop travel.
K.F. Chan (Mr.)
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
(Adopted from CIBSE Guide D)
ROUND TRIP TIME
Round Trip Time (RTT) is the time which a lift car shall use to complete one cycle of travel, i.e.
from its opening of doors at the terminal floor, move and then back to the terminal floor to open
the doors again. Whilst depending on rated load, real time demand, length of travel and number
of stops, RTT between two and three minutes are typical values. RTT of express lifts for one
specific floor is usually shorter.
RTT is not the same during uppeak period, general period, and downpeak period.
CIBSE Guide D gives a model of round trip time based on the assumption that contact speed is
reached for a single jump, while this is applicable in many cases, a model was developed by Ir
WK Lee at HKU to cater for the fact that contract speed is not reached for high speed lifts
frequently used in HK. This model instead of the CIBSE model is presented in this set of lecture
material.
K.F. Chan (Mr.)
Da of F
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
Assumptions
The calculation of round trip time relies on a number of assumptions:1) The busiest traffic is the morning 5-minute uppeak
2) The traffic profile is ideal
3) All floors are equally populated or present equal attraction
4) Interfloor heights are assumed equal
5) The traffic supervisory system is assumed ideal
6) Passengers arrive uniformly in time (rectangular probability function)
7) Lift car is 80% loaded for each travel
8) Acceleration and deceleration are uniform (and possibly equal. In real lift, acceleration >
deceleration usually)
9) Lift car jerks up to rated acceleration (or deceleration) instantly. (In practice, jerk is limited to
1.2m/s3 or below for the sake of passenger comfort)
10) Heights between adjacent possible stops are equal
11) No interference or floor call
12) Various lost times, such as passenger disturbance, dispatch intervals, loading intervals, etc.
are negligible, i.e. passenger transfer times will be “brisk” and passengers do not misbehave.
(Adopted from CIBSE Guide D)
K.F. Chan (Mr.)
Da of F
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
Mathematically,
RTT   moving times   stop times
RTT  upward moving times  downward moving times   stop times
RTT =
upward (acceleration, moving at contract speed + deceleration) +
Downward (acceleration, moving at contract speed + deceleration) +

(door operating times + passenger transfer times)
Average number of passengers
Industry experts and consultants state in practice the lifts are not observed to fill with passengers
to the number permitted by the name plate, but a lower value, particularly in larger lifts.
It is generally accepted that the average number of passengers, P, during an uppeak traffic
condition can be taken as 80% of the rated capacity, i.e.
P = 0.8 CC
where CC is the contract capacity of the lift in number of persons.
K.F. Chan (Mr.)
Da of F
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
Expected number of stops, S
Basset JONES in 1923 published a method of calculating the expected number of stops for floors
with equal populations
1a) The probability that one passenger will leave the lift at any particular floor is
1
, where N
N
is the number of floors above the main terminal
1b) Therefore the probability of one passenger will not leave the lift at any particular floor is
1
1
N
2a) It is assumed that each passenger is independent of all others, so the probability that NO
passenger from a lift containing P passengers will leave the lift at any particular floor is
1

1  
N

P
2b) Hence the probability that a stop will be made at any particular floor is
P

1 

1

1


 

N  


3) The expected number of stops, S, for N floors will thus be
P

1 

S  N 1  1   
N  


K.F. Chan (Mr.)
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
Expected highest reversal floor, H
1a) The probability that one passenger will leave the lift at any floor is
1
N
1b) Hence the probability that one passenger will NOT leave the lift at any floor is 1 
1
N
1c) The probability that NO passenger will leave the lift at any given floor is
1

1  
N

P
where P is the number of passengers in the lift car
2)
The probability that the lift will travel no higher than the ith floor is equal to the probability
that no passenger leaves the lift at the
Nth, (N-1)th, (N-2)th, …..and (i+1)th floors.
This can be written as
1

1  
N


=

P
P
P
1  
1  
1 

1  ….. 
1 
 1 
1 
 1 


i  2 
N - 1 
i  1
N - 2


P

1 
1 
1  
1 
1 
1  N 1  N  1 1  N  2 .....1  i  2 1  i  1 


 



 (N - 1) (N  2) (N - 3) (i  1) (i) 
 N (N - 1) (N - 2) .... (i  2) (i  1) 


 i 
 
N
P
P
P
P
3) Now probability that ith floor is the highest floor attained is equal to
probability that the lift travels no higher than the ith floor
minus
probability that the lift travels no higher that the (i-1)th floor, viz
P
 i 
i - 1
  

N
 N 
K.F. Chan (Mr.)
P
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
4) Hence the average highest floor reached, H, is
H 
N

i 1
i - 1 
 i
i   

 N  
 N 
P
P
1
0
( )P  ( )P
N
N
Thus H =
+ 2(
2 P
1
)  2( ) P
N
N
3
2
 3( )P  3( )P
N
N
…………..
 ( N  3)(
 ( N  2)(
 ( N  1)(
 N(
N P
)
N
N3 P
N4 P
)  ( N  3)(
)
N
N
N2 P
N3 P
)  ( N  2)(
)
N
N
N 1 P
N2 P
)  ( N  1)(
)
N
N
 N(
N 1 P
)
N
Therefore
1
2
N 3 P
N 2 P
N 1 P
H  N  ( ) P  ( ) P  ....  (
) (
) (
)
N
N
N
N
N
or
N 1
 i 
H  N   
i 1  N 
P
(It is reported that the above uniform probability distribution function was first proposed by
Schroeder in 1955.)
CIBSE published table (2.13) showing H and S for different values of N and P in Guide D.
Values of H and S are shown overleaf.
K.F. Chan (Mr.)
Da of F
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
The following table shows values calculated for H and S. Those highlighted in red are shown in Table 2.13 of CIBSE Guide D
CC
5
6
8
10
12
16
18
20
24
28
30
P
4.0
4.8
6.4
8.0
9.6
12.8
14.4
16.0
19.2
22.4
24.0
N
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
H
S
H
S
H
S
H
S
H
S
H
S
H
S
H
S
H
S
H
S
H
S
3.6
2.7
3.7
3.0
3.8
3.4
3.9
3.6
3.9
3.7
4.0
3.9
4.0
3.9
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.4
3.0
4.6
3.3
4.7
3.8
4.8
4.2
4.9
4.4
4.9
4.7
5.0
4.8
5.0
4.9
5.0
4.9
5.0
5.0
5.0
5.0
5.2
3.1
5.4
3.5
5.6
4.1
5.7
4.6
5.8
5.0
5.9
5.4
5.9
5.6
5.9
5.7
6.0
5.8
6.0
5.9
6.0
5.9
6.1
3.2
6.2
3.7
6.5
4.4
6.6
5.0
6.7
5.4
6.8
6.0
6.9
6.2
6.9
6.4
6.9
6.6
7.0
6.8
7.0
6.8
6.9
3.3
7.1
3.8
7.4
4.6
7.5
5.3
7.6
5.8
7.8
6.6
7.8
6.8
7.9
7.1
7.9
7.4
7.9
7.6
8.0
7.7
7.7
3.4
7.9
3.9
8.2
4.8
8.4
5.5
8.6
6.1
8.7
7.0
8.8
7.3
8.8
7.6
8.9
8.1
8.9
8.4
8.9
8.5
8.5
3.4
8.7
4.0
9.1
4.9
9.3
5.7
9.5
6.4
9.7
7.4
9.7
7.8
9.8
8.1
9.9
8.7
9.9
9.1
9.9
9.2
9.3
3.5
9.6
4.0
10.0
5.0
10.2
5.9
10.4
6.6
10.6
7.8
10.7
8.2
10.7
8.6
10.8
9.2
10.9
9.7
10.9
9.9
10.1
3.5
10.4
4.1
10.8
5.1
11.1
6.0
11.3
6.8
11.5
8.1
11.6
8.6
11.7
9.0
11.8
9.7
11.8
10.3
11.9
10.5
10.9
3.6
11.2
4.1
11.7
5.2
12.0
6.1
12.2
7.0
12.5
8.3
12.6
8.9
12.6
9.4
12.7
10.2
12.8
10.8
12.8
11.1
11.7
3.6
12.1
4.2
12.6
5.3
12.9
6.3
13.1
7.1
13.4
8.6
13.5
9.2
13.6
9.7
13.7
10.6
13.8
11.3
13.8
11.6
12.5
3.6
12.9
4.2
13.4
5.4
13.8
6.4
14.0
7.3
14.3
8.8
14.4
9.4
14.5
10.0
14.7
11.0
14.7
11.8
14.8
12.1
13.3
3.6
13.7
4.3
14.3
5.4
14.7
6.5
14.9
7.4
15.3
9.0
15.4
9.7
15.5
10.3
15.6
11.4
15.7
12.2
15.7
12.6
14.1
3.7
14.5
4.3
15.2
5.5
15.6
6.5
15.8
7.5
16.2
9.2
16.3
9.9
16.4
10.6
16.6
11.7
16.7
12.6
16.7
13.0
14.9
3.7
15.4
4.3
16.0
5.5
16.5
6.6
16.8
7.6
17.1
9.3
17.3
10.1
17.4
10.8
17.5
12.0
17.6
13.0
17.7
13.4
15.7
3.7
16.2
4.3
16.9
5.6
17.4
6.7
17.7
7.7
18.1
9.5
18.2
10.3
18.3
11.0
18.5
12.3
18.6
13.3
18.6
13.8
16.5
3.7
17.0
4.4
17.8
5.6
18.2
6.7
18.6
7.8
19.0
9.6
19.1
10.4
19.3
11.2
19.4
12.5
19.6
13.7
19.6
14.2
17.3
3.7
17.9
4.4
18.6
5.6
19.1
6.8
19.5
7.9
19.9
9.8
20.1
10.6
20.2
11.4
20.4
12.8
20.5
14.0
20.6
14.5
K.F. Chan (Mr.)
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
CC
5
6
8
10
12
16
18
20
24
28
30
P
4.0
4.8
6.4
8.0
9.6
12.8
14.4
16.0
19.2
22.4
24.0
N
22
23
24
28
30
32
33
38
H
S
H
S
H
S
H
S
H
S
H
S
H
S
H
S
H
S
H
S
H
S
18.1
3.7
18.7
4.4
19.5
5.7
20.0
6.8
20.4
7.9
20.9
9.9
21.0
10.7
21.1
11.5
21.3
13.0
21.5
14.2
21.5
14.8
18.9
3.7
19.5
4.4
20.4
5.7
20.9
6.9
21.3
8.0
21.8
10.0
22.0
10.9
22.1
11.7
22.3
13.2
22.4
14.5
22.5
15.1
19.7
3.8
20.3
4.4
21.2
5.7
21.8
6.9
22.2
8.0
22.7
10.1
22.9
11.0
23.0
11.9
23.2
13.4
23.4
14.7
23.5
15.4
22.9
3.8
23.7
4.5
24.7
5.8
25.4
7.1
25.8
8.3
26.4
10.4
26.6
11.4
26.8
12.4
27.1
14.1
27.2
15.6
27.3
16.3
24.5
3.8
25.3
4.5
26.4
5.9
27.1
7.1
27.6
8.3
28.3
10.6
28.5
11.6
28.7
12.6
29.0
14.4
29.2
16.0
29.2
16.7
26.1
3.8
27.0
4.5
28.2
5.9
28.9
7.2
29.5
8.4
30.1
10.7
30.4
11.7
30.6
12.7
30.9
14.6
31.1
16.3
31.2
17.1
26.9
3.8
27.8
4.5
29.0
5.9
29.8
7.2
30.4
8.4
31.1
10.7
31.3
11.8
31.5
12.8
31.8
14.7
32.0
16.4
32.1
17.2
30.9
3.8
31.9
4.6
33.4
6.0
34.3
7.3
34.9
8.6
35.7
11.0
36.0
12.1
36.2
13.2
36.6
15.2
36.8
17.1
36.9
18.0
K.F. Chan (Mr.)
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
Poisson Probability Distribution vs rectangular probability distribution
The above derivation of H and S assumed that passengers arrived at a constant inter-arrival
interval (according to the particular level of arrivals existing) and that the lift arrival at a constant
interval to take the intending passengers to their destinations.
In practice passengers do not conveniently arrive in batches equal to 80% of the rated car
capacity nor do they register the same number of destinations during each trip. The effect of
this randomness is to cause the lifts to take different times to carry out a round trip and they
become unevenly spaced. This effect is called bunching.
The overall passenger average waiting times will increase and queues develop.
It is generally accepted that passengers arrive into a lift system according to the Poisson
probability theory. Work by Alexandris (1975) confirmed that Poisson distribution must be a
good approximation to the actual empirical distribution.
It can be shown that the Poisson probability distribution functions will always give smaller
H and S than that obtained by the rectangular probability distribution. That is to say, the
equations for H and S above always yield more conservative results and thus are used in
most design handbooks.
Average passenger transfer time
The average passenger transfer time (entry or exit) can be taken as 1.2 seconds each way. This
time could be reduced for single cars, but increased for groups. The time may be increased for
small door openings and reduced for large door openings.
For situations where passengers are elderly, or have no reason to rush, the transfer time should be
increased to 2 seconds.
ISO 4190-6 considers a passenger transfer time of 1.75 seconds suitable for residential buildings.
K.F. Chan (Mr.)
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
Door operating times
Centre opening doors open and close more quickly and the symmetrical reaction against the car
frame reduces sway. Side opening door takes more time because it has to open the full width.
The door operator and its control must meet the following requirements:
 the opening and closing speeds must be independently adjustable,
 for high performance lifts, the opening and closing speeds must be automatically adjustable
according to prevailing traffic conditions at the floor,
 safety edges must be fast acting and tolerant of mechanical impact.
Advanced opening is a time-saving feature widely used in office buildings to improve
performance. This allows the doors to commence opening once the car speed is below 0.3m/s
and the lift is within the door zone (typically  100mm, maximum  200mm) However, it can
be disturbing to elderly users and may not be suitable in some buildings.
Typical door closing and opening times:
Opening and closing times for
stated door width, seconds
Door Type
Advance
Normal
Closing
opening
opening
0.8m
1.1m
0.8m
1.1m
0.8m
1.1m
Side opening
1.0
1.5
2.5
3.0
3.0
4.0
Biparting
0.5
0.8
2.0
2.5
2.0
3.0
(Adopted from table 3.8 of CIBSE Guide D)
K.F. Chan (Mr.)
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
Zones
Let’s consider a high rise building, say 36 floors above the main terminal floor with 12 lifts.
We may divide the number of lifts into zones, 1 zone, 2 zones, 3 zones,….. 12 zones etc.
correspondingly there will be 12 lifts, 6 lifts, 4 lifts, ….. 1 lift per zone. Indeed there is no rule
saying that each zone has the same number of floors; nor each zone has the same number of lifts.
Our concern is whether the lift design of each zone can provide satisfactory service.
The zones limit the number of floors served, thus limits the number of possible stops, and so
shorten the door operating times and some of the lift travel times.
In each of the upper zones, the lifts are provided with an initial jump of J number of floors, and
the first jump as J+H/S number of floors. So its complete travel shall be J+H number of floors.
The provision of zones is extremely common in commercial buildings, but less common in hotel
and hospitals. The common zone arrangement is to optimize a size of continuous floors to the
same zone. Often, the upper zones have relatively smaller number of floors so that the saving in
stop times may counterbalance the effect of the longer travel.
In Hong Kong, because residential buildings are high rise, the provision of zones is also common.
Yet due to a relatively small number of lifts, a staggered-floor arrangement for the zones is
common. This provides an advantage that when one set of lifts is not working, still the residents
do not have to walk up many floors when using the other set of lifts.
Although the provision of zones lowers the RTT, but with a fixed number of lifts, zoning
decreases number of lifts in each zone, so the waiting time at the main terminal floor should in
general increases.
Nowadays, for very tall buildings, it is common to use Express or Shuttle Lifts to take top-floor
passengers to a transit floor for their lift zones. This provision shall modify RTT and grade of
services. Also the use of double-deck lifts is becoming common.
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UNIVERSITY OF HONG KONG
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ELEC 3105 2010 Building Services
K.F. Chan (Mr.)
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UNIVERSITY OF HONG KONG
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ELEC 3105 2010 Building Services
Contract speed and acceleration
For new high rise commercial buildings, contract speed between 4 and 8m/s is common, with
acceleration and deceleration up to 1.2m/s2 and 1m/s2 respectively. Deceleration above 1 m/s2
and jerk above 1.2 m/s3 will cause discomfort to passengers. For residential buildings, contract
speed is usually slower, while acceleration and deceleration are more or less the same.
Observe from the CIBSE table 2.13 that with N less than 15 and P more than 10, an average
jump, i.e. H/S, is less than 2 floors.
The first jump may reach contract speed in upper zones because the longer distance makes
acceleration to that speed possible. However, for subsequent jumps, this is seldom possible. To
simplify calculation, it is assumed that each subsequent jump is an average jump:
Average highest travel = H x floor height
Expected number of stops = S
Thus average jump = H/S x floor height
When heights of floors are irregular, then adjustment to the calculation should be made. For
example, the main terminal floor is usually taller than the rest of the building. Furthermore, in
high rise buildings, there may be mechanical floors to house building services equipment or
refuge floor to inhibit spread of fire between sections of floors.
K.F. Chan (Mr.)
Da of F
Page Da 15 of 27
UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
Recall mathematically,
RTT = upward (acceleration, moving at contract speed + deceleration) +
downward (acceleration, moving at contract speed + deceleration) +

RTT =
(door operating times + passenger transfer times)
time to jump to first stop +
time to jump from first stop to subsequent stops then to highest reversal floor +
time to express return from highest reversal floor to main terminal floor +
door operating times +
passenger transfer times
For simplicity, assume that acceleration = deceleration = a m/s2.
From the principle of linear motion, assuming that the lift is able to reach rated acceleration
v
instantly, the time needed for a lift to accelerate to contract speed is
. The distance travelled in
a
v2
this acceleration period is thus
. If a lift is able to reach contract speed in a jump, the distance,
2a
v2
d, must be greater than or equal to twice of this, i.e.
. This implies that a lift will not reach
a
contract speed in a jump if
v2
d
a
v
d
i.e.

a
a
1)
Time for lift to reach first stop
a.
Assume that the lift will reach contract speed in the jump to the first stop.
The time needed for a lift to accelerate to contract speed is
in this acceleration period is thus
v
. The distance travelled
a
v2
.
2a
Similarly, the time needed for the lift to decelerate from contract speed to stop is
The distance travelled in this deceleration period is thus
K.F. Chan (Mr.)
Da of F
v2
.
2a
Page Da 16 of 27
v
.
a
UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
 v2 
Now then the lift has to travel d  2  at contract speed, if d is the total distance
 2a 
of jump. The total time for this jump is thus
 v2 
d  2 
 2a   2 v  , on re-arranging this term becomes
 
v
a
v d
 
a v
Now the distance that the lift has to travel in the first jump is the J floors + the first
H
H

, meaning the total distance is  J  d f .
S
S

Therefore time for lift to reach first stop if contract speed is reached in the first jump
average interfloor jump, i.e.
becomes:

H

 v  J  S d f


 
v
a






The criteria for judging if the lift is able to reach contract speed is
H

 J  d f
v
S
 
a
a
b.
Again from the principle of linear motion, for a lift unable to reach its contract speed
t
in a jump, the maximum speed it can attain is a , where t is the time for that jump,
2
assuming that the lift is able to reach rated acceleration instantly.
t
The average speed in the whole of the jump is a , thus the distance, d, travelled is
4
2
t
d  a . Rearranging
4
d
t2
a
H

Now the distance that the lift has to travel in the first jump is  J  d f . Therefore
S

time for lift to reach first stop if contract speed is not reached in the first jump
K.F. Chan (Mr.)
Da of F
Page Da 17 of 27
UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
H

 J  d f
S
becomes 2 
a
The criteria for judging if the lift is not able to reach contract speed is
H

 J  d f
v
S
 
a
a
2)
a.
b.
Time for each subsequent jump
Similarly, we can deduce from principle of linear motion that time for each interfloor
jump is
H
 d f
S
2
a
assume all jumps equal and contract speed not reached.
H
d f , so time for lift to jump from first stop
As the height of each interfloor jump is
S
to highest reversal floor becomes
 H

df 

S  12 S 
a 



3)
Time for lift to express return to main terminal floor
J  H d f
 v J  H d f 
v

 
 provided
a
a
v
a

4)
Door operating time
S 1t o  t c 
5)
Passenger transfer time
Ptl  tu 
Therefore
K.F. Chan (Mr.)
Da of F
Page Da 18 of 27
UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services

H 
 H


 J  d f 
df  


v J  H d f 
v
S 
S
RTT    
 
  S  12
  S  1to  tc   P tl  tu 
v
a  a
v


a









[This model was developed by LEE, W.K., at HKU based on first principles of linear motion]
K.F. Chan (Mr.)
Da of F
Page Da 19 of 27
UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
Uppeak interval
Interval is the average time between successive lift car arrivals at the main terminal floor, during
any traffic condition, with cars loaded to any level.
Uppeak interval is the average time between successive lift car arrivals, at the main terminal
floor, during the uppeak traffic condition, with cars loaded to 80% of the car capacity.
In an installation of one lift car the uppeak interval, UPPINT, is equal to the RTT. In a system
comprising several cars, the uppeak interval is given by
UPPINT =
RTT
L
where L is the number of cars
Average waiting time =
Interval time
2
The concept of an INTERVAL is useful during up and down peaks, where cars call at the
terminal floor during every trip, but must be treated with caution during other traffic conditions.
Uppeak handling capacity
The uppeak handling capacity, UPPHC, of a lift system is defined as the number of persons that
can be transported from the main terminal to the upper floors of a building during a 5-minute
uppeak activity during which demand is heaviest.
For single car installation:UPPHC =
300P
UPPINT
For multiple car installations:
UPPHC =
300PL
RTT
Grade of service (approx) =
AWT + 0.5xupward moving time + 0.75xtransfer times + 0.5xdoor times
Grade of service may be taken as Average Journey Time.
The 5-minute handling capacity should match the 5-minute arrival rate.
Usually number of lifts in each zone, L, should be limited to 6 or less, otherwise, passengers will
find it confusing.
K.F. Chan (Mr.)
Da of F
Page Da 20 of 27
UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
Percentage population
The percentage of a building’s population, % POP, served during the peak 5-minute uppeak is
given by :
% POP
=
UPPHCx100
U
where U is the effective population of the building
The 5-minute handling capacity is usually expressed as the percentage of the total population
requiring lift service during a 5-minute period. This %POP may vary from 10% to 25%.
If no information is available on the flow rate to be expected, use table 3.2 of CIBSE Guide D.
(Tables adopted from CIBSE Guide D)
For office buildings with flex-hours, population will arrive at staggered times and the arrival rate
can be typically 5% lower than that indicated in the above table.
K.F. Chan (Mr.)
Da of F
Page Da 21 of 27
UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
Quality of service
While the quantity of service is represented by the handling capacity, usually taken as the
UPPHC, the quality of service is a more complicated issue.
The quality of service is represented by passenger waiting time.
Some designers use the interval of car arrivals at the main terminal as a representation of service
quality. Note that “interval” is part of the evaluation of handling capacity which represents the
quantity of service of a lift system as well.
For office buildings the probable quality of service is related to interval as follows :Excellent
Good
Satisfactory
Poor
Unsatisfactory
< 20s
25s
30s (*)
40s
> 50
(*) 50 and 90 seconds considered satisfactory for hotels and residential buildings.
K.F. Chan (Mr.)
Da of F
Page Da 22 of 27
UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
Example on RTT calculation
An office has 20 floors above the ground floor. Each floor has an area of 2000m 2. The floor
to floor height is 3.5m. Assume that the occupancy is 9m2/person and the daily occupancy is
90% only. There are 24 lifts each with contract capacity of 24 persons each. The contract
speed is 4 m/s, acceleration 1 m/s2 and deceleration 1 m/s2 too. The bi-parting door width is
1.1m, advance opening strategy is employed. .
Calculate the round trip time based on first principle of linear motion and the percentage
population served in the 5-minute uppeak interval.
Comment on the installation.
Answer
Without specific data, population can be estimated from data given in CIBSE Guide D, table 3.1.
In this case the effective population can be calculated as
2000
 20  0.9  4000 pax
9
The arrival rate can be estimated using table 3.2 of CIBSE Guide D.
the arrival rate as 15%.
In this case let’s assume
Therefore, the lift system has to handle more than 4000 x 0.15 = 600 pax in the 5 minute uppeak
period.
Now N = 20, CC = 24 thus P = 24 x 0.8 = 19.2
N 1
 i 
H  N   
i 1  N 
P
= 19.4 (can be found from CIBSE Guide D table 2.13)
P

1 

S  N 1  1   
N  


19.2
 
1 
 201  1    = 12.5
  20  
K.F. Chan (Mr.)
Da of F
Page Da 23 of 27
UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
Now RTT
= time to jump to first stop
+ time to jump from first stop to subsequent stops then to highest reversal floor
+ time to express return from highest reversal floor to main terminal floor
+ door operating times + passengers transfer time
As H = 19.4, S = 12.5
H
 19.4 
Average jump distance is  d f  
3.5  5.4m
S
 12.5 
It is obvious that the lift will not reach contract speed between jumps including the first jump
because
v 5

a 1

H
 d f
5.4
S

 2.3
a
1
Therefore time to jump to first stop
H
 d f
S
2  
a
 19.4 

(3.5)
12.5 

2
1
= 4.7 seconds
Time for subsequent jumps
 H
  d f
S
 S  12  

a


K.F. Chan (Mr.)






Da of F
Page Da 24 of 27
UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
  19.4  
3.5 
 
12.5  


 12.5  1 2


1




=53.6 seconds
Time to express back to main terminal


v J  H d f

a
v
4 0  19.4 3.5

1
4
= 21 seconds
Door operating times
S 1t o  t c 
From CIBSE Guide D table 3.8,
tO=0.8, tC = 3.0
Thus door operating time
= (12.5+1)(0.8+3)
= 51.3s
Passenger transfer time
Ptl  tu 
= 19.2 (1.2 + 1.2)
= 46.1s
RTT = 4.7 + 53.6 + 21 + 51.3 + 46.1 = 176.6s
Uppeak interval 
176.6s
 7.4 seconds
24
Average 5-minute uppeak handling capacity
 24 
24  0.8
 300  782 persons
176.6
Percentage population served in the 5-minute uppeak interval
K.F. Chan (Mr.)
Da of F
Page Da 25 of 27
UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services

782
2000
 20  0.9
9
 19.6%
Now uppeak interval is 7.4s < 20 seconds, thus quality of service seems to be excellent.
However, it can be seen from above calculation that the lift does not reach contract speed except
when it expresses from the highest reversal floor to the main terminal, which contributes less
than
21
 11.9% of the RTT.
176.6
Furthermore, 24 lifts in a group is extremely confusing to passengers.
Moreover, walking distance within each floor and structural requirement for such a large area
may require 2 lift cores.
Therefore, zoning must be considered.
K.F. Chan (Mr.)
Da of F
Page Da 26 of 27
UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 2010 Building Services
Notations
a
acceleration/deceleration, m/s2
CC
contract (rated) capacity, persons
df
average interfloor height
H
expected highest reversal floor
J
number of floor transit before the zone
Jxdf
the distance the lift express traveled before the reaching the zone served
L
number of lifts
N
number of floors in the zone above main terminal
P
expected number of passengers (typically taken as 0.8 of CC)
RTT
average round trip time
S
expected number of stops
tc
door closing time (between 2s and 4s depending on types of doors, but slower
than door opening time)
to
door opening time (between 0.5s and 3s usually, faster if advance opening is used)
tl
passenger loading time (usually taken as 1.2s)
tu
passenger unloading time (usually taken as 1.2s)
UPPHC
average uppeak handling capacity (persons served in the 5-minute uppeak
interval)
UPPINT
average uppeak interval
v
Contract (rated) speed, m/s
% POP
percentage of population served in the 5-minute uppeak interval
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