UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 Building Services
EXAMPLE
A regular multi-tenant office building is served by lifts with RTT of 120s. The building
effective population is 400 persons and each car has a contract capacity of 8 passengers.
Calculate the required number of lifts, comment on the quality of service.
Answer
From CIBSE table 3.2, take the arrival rate as 15%, so
%POP = 400 x 0.15 = 60 person / 5 minutes
L
60
 3.75 say 4
 8  0.8 

300
 120 
Therefore this building should be equipped with 4 such lifts
As interval is
120
 30
4
so the quality of service is satisfactory.
K.F. Chan (Mr.)
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 Building Services
Contract Speed
Speed is generally not a dominant factor in the RTT equation but it does become significant if the
served floors are in an upper zone where a higher speed will permit the un-served zone to be
more rapidly traversed.
Recommended Contract Speed
Speed, m/s
0.25 to 0.4
0.50
0.63
1.0
1.6
2.5
5.0
Lift travel with J=0, m (number of floors)
Building Usage
Luxury flats
Offices
Bed lifts
5
10 (3)
10
15 (4)
15 (4)
20 (6)
20 (6)
20
25 (8)
32 (10)
45
40 (12)
50 (15)
100
100 (30)
100 (30)
-
There is no theoretical upper limit to lift rated speed and it does not affect passenger comfort.
But it is limited by practical factors such as the maximum sheave diameter, rope-bending radius,
rope wear, safety etc. Moreover, it is important to limit the acceleration to approximately 1.2
m/s² in order to provide a good ride quality.
The HK Fire Services Department stipulates that all fireman lifts must be able to travel to the
highest floor from the fire control entrance level in 60 seconds.
Number of lifts
A general rule of thumb for office building of floor area 1,000m2 per floor and arrival rate>15%,
is :Good service
Average service
Poor service
K.F. Chan (Mr.)
One lift per 2 floors
One lift per 3 floors
One lift per 5 floors
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 Building Services
Design sequence
The design sequence for sizing a lift system for its uppeak performance is :a) Determine population per floor
b) Determine if Express Lifts are required
c) Select number of zones and their respective floors (suggest each zone serving 200 to 400
persons, and zones may have unequal number of floors)
Then for each zone
d) Find arrival rate, work out 5-minute uppeak demand.
e) Find J
f) Find N
g) Select contract speed, acceleration, and deceleration.
h) Select P
i) Find H and S
j) Use H/S as average jump (average flight)
k) Calculate RTT
l) Calculate 5-minute uppeak handling capacity per lift
m) In conjunction with the 5-minute arrival rate, determine number of lifts, L
n) Calculate the UPPINT and other parameters
o) Counter check if UPPINT and %POP are within the recommendation of table 3.2 of
CIBSE Guide D
p) If meeting constraints and specification, proceed to other zones. If not OK, iteration.
Finally
q) Check if the capital and running costs are acceptable.
K.F. Chan (Mr.)
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 Building Services
Example on number of lifts
A lift system is to be designed for a 20-storey high regular multi-tenancy office building. Area of
each floor is 450m2, with interfloor height of 3.5m. It can be assumed that the occupancy is
9m2/person with daily occupancy of 90% only. What will be a suitable number of lifts?
Answer
450
= 50 persons/floor
9
Total population is 50 x 20 = 1,000 persons
Effective population is thus 1000 x 90% = 900 persons.
There are
Assume 15% arrival rate in the 5-minute peak (from CIBSE Guide D table 3.2) the arrival rate
will thus be 900 x 15% = 135 persons / 5 minutes
For average quality of service, UPPINT is 30 seconds. So there shall be
minimum in the peak 5-minutes. Each trip is to take
should be
300
= 10 trips
30
135
= 13.5 persons. Thus rated capacity
10
13.5
= 16.8, say 16 persons.
0.8
For average quality of service, there should be one lift per 3 floors, say 6 lifts in this building.
Now from table 3.7 of CIBSE Guide D, for lift travel of 3.5 x 20 = 70m, let’s take contract speed
as 3.15m/s with acceleration of 1m/s2.
So initial design is
6 lifts each of 16 persons rated capacity, 3.15m/s contract speed and acceleration of 1m/s2.
K.F. Chan (Mr.)
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 Building Services
Remarks
The standard lift traffic design uses the uppeak calculation, i.e. with only an up flow of
passengers, to determine the likely performance of a lift system. It is generally accepted that if
the uppeak traffic pattern is sized correctly all other traffic patterns with also be adequately
served. (#)
Fortunately it can be shown that a lift system possesses 50% more handling capacity during
down peak than uppeak. This is because during down-peak a lift car fills at 3, 4 or 5 floors and
then makes an express run to the main terminal.
(#) Exceptions:-
hotel at meal times, hospitals at visiting times, buildings with trading floors
(insurance company, stock markets).
Unequal interfloor distance
In case of unequal floor distances, and if condition permits, the rule of thumb is to add together
the incremental floor distances, extra to the standard interfloor height, multiply by 2 to account
for both directions of travel, and divide by the rated speed to obtain the additional RTT.
K.F. Chan (Mr.)
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 Building Services
Example on RTT and number of lifts
A multi-tenant prestige office has 50 floors above the ground floor. Each floor is 1000m2 in area.
The daily occupancy rate is 90%. The ground floor has a height of 7m; height of 1st floor to 4th
floor is 5m each. The 24th floor is a mechanical floor with a height of 3m only. Floor height of
the other floors is all 3.5m. 36th to 50th floor are grouped into a high zone served by 4 lifts each
of contract capacity of 16 persons, 8m/s contract speed, 1m/s2 acceleration and deceleration. The
lift door is biparting type, 0.8m wide, advance opening control is employed.
a. Calculate the round trip time, and the % population served in the 5-minute uppeak period
with this lift installation.
b. Are 4 lifts sufficient for this zone? If not, recommend number of lifts.
Answer
When no data is available, population can be estimated from table 3.1 of the CIBSE Guide D. In
this case let us assume that the occupancy is 15m2 per person. With 90% daily occupancy rate,
the occupancy per floor is
1000
 0.9  60
15
The total population in the zone is thus 60x15=900pax
Table 3.2 of CIBSE Guide D recommends that the arrival rate in the 5-minute uppeak period to
be taken as 17%, so the handling capacity in that 5-minute uppeak period should not be less than
900x0.17=153pax
a)
The probable number of stops and highest reversal floor can be calculated by the following
formula:
P

1 

S  N 1  1   
N  


160.8 
 

1
S  151  1  
  8.8
  15 

N 1
 i 
H  N   
i 1  N 
P
For N=15, CC=16, H=14.3 from table 2.13 of CIBSE Guide D
K.F. Chan (Mr.)
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 Building Services
The floor to floor height from G/F up to 35th floor is
7
(G/F)
+ (4-1+1)x5
(1/F to 4/F inclusive)
+ (23-5+1)x3.5
(5/F to 23/F inclusive)
+ 3
(24/F)
+ (34-25+1)x3.5
(25/F to 34/F inclusive, meaning up to the floor level of 35/F)
= 131.5m
this is the distance between floor of G/F and floor of 35/F
131.5m is the distance the lift express travels before reaching the zone served.
The unequal floor distance for G/F and 24th floor will be accounted for in the “time to jump to
first stop” and in the “time to express return back to main terminal floor” in the calculation
below.
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 +
passenger transfer times
Now it is obvious that the lift is able to reach contract speed in the first jump because
H

 J  d f
v
S
 
. Time to jump to first stop is
a
a

H

 v  J  S d f


 
v
a







 14.3  
 8 131.5   8.8 3.5 


 
 = 25.15s
8
1



Again, it is obvious that lift is unable to reach contract speed in subsequent jumps because
K.F. Chan (Mr.)
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 Building Services
H
 d f
v
 S  . Time for jumping subsequent stops is

a
a
 H

df 

S  12 S 
a 



 14.3

3.5 

 = 37.2s
 8.8  12 8.8
1 



Furthermore, as the lift is able to reach contract speed in its jump to the first stop so it is more
than obvious that the lift is able to reach contract speed in the return to main terminal floor (or
we can check if
v

a
J  H d f
a
). Time to express return to G/F is
 v J  H d f 
 

v
a

 8 131.5  14.33.5 
 
 = 30.69s
8
1

From table 3.8 of CIBSE Guide D, the door opening and closing time can be taken as 0.5s and 2s
respectively. Door operating times is thus
S 1t o  t c  = 8.8  10.5  2 =24.5s
Passenger loading and unloading time can be taken as 1.2s for each passenger according to
CIBSE Guide D. Therefore, total passenger transfer time becomes
Ptl  tu  = 16  0.81.2 1.2 = 30.72s
Thus RTT = 25.15+37.2+30.69+24.5+30.72 =148.3s
For 4 lifts, number of passengers served in the 5-minute uppeak period is
4
16  0.8  300
= 103pax
148.3
The percentage population served in the 5-minute uppeak interval is thus
K.F. Chan (Mr.)
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UNIVERSITY OF HONG KONG
Department of Electrical & Electronic Engineering
ELEC 3105 Building Services
103
= 11.5%
900
b)
With 4 lifts, the number of passengers served during the 5-minute uppeak period is only 103pax.
This is lower than the 153pax calculated above thus not acceptable. Furthermore, table 3.2 of
CIBSE Guide D recommends an interval of 20 – 25s. With 4 lifts, the uppeak interval is
148.3
=37.1s, this is also considered to be longer than the recommended interval for a prestige
4
multi-tenant office.
The number of lifts should be increased to
153
= 6 lifts
16  0.8  300 148.3
Moreover, with 6 lifts, the uppeak interval is reduced to
148.3
=24.7. This is within the
6
recommendation of 20 – 25s, thus acceptable.
K.F. Chan (Mr.)
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