Vertical Transportation Design and Traffic Calculations – Part Eight


In article “Vertical Transportation Design and Traffic Calculations – Part Six”, we started explaining how to perform the following lift traffic design calculations:
1- Calculation of the Number of Round Trips for a Single Car,
2- Estimation of Population,
3- Calculation of the Average Number of Passengers per Trip (P),
4- Calculation of the Uppeak Handling Capacity (UPPHC),
5- Calculation of the Waiting Interval (Passenger Waiting Time),
6- Calculation of The percentage population served (%POP),

Also, in article “Vertical Transportation Design and Traffic Calculations – Part Seven”, we explained the following two important calculations used for lift traffic design calculations:
7- Estimation of Arrival Rate,
8- Calculation of the Round Trip Time RTT,

Today we will continue explaining the last important calculation used for lift traffic design calculations, which is the Calculation of the quality of service for a lift system.

9- Estimation of Quality of Service (Grade of service)





There are (6) methods to estimate the quality of service for lift systems as follows:

  1. Based on The interval of car arrivals at the main terminal (INT),
  2. Based on The performance time (T),
  3. Based on Average System Response Time Performance  (ASRT),
  4. Based on Passenger Average Travel Time to Destination (ATT),
  5. Based on Passenger Average Journey Time (AJT),
  6. Based on Rule of thumb for single lift.



           

1- Estimation of Quality of Service based on the interval of car arrivals at the main terminal

  • Actual average passenger waiting time is the time between the instant of passenger arrival until the instant of the actual arrival of the lift.
  • Actual average passenger waiting time would be the best indicator of the quality of service that an installed lift system could provide:  the shorter the average passenger waiting time the better the service.
  • Unfortunately, average passenger waiting times cannot be measured easily owing to the difficulty of determining the exact arrival instant for each passenger.
  • What can be measured is the time for the lift system to respond to the landing call registered by the first arriving passenger. Some lift companies define the lift arrival time as the time when the arrival signal is given (lantern/gong) or call registration is cancelled. This will give optimistic results for performance as this signal can be as much as eight seconds earlier than the actual lift arrival.
  • Some designers, therefore, use the interval of car arrivals at the main terminal as an estimation of service quality. But it must be remembered that the interval is part of the evaluation of handling capacity which represents the quantity of service of a lift system.
  • In general terms, when considering office buildings, the interval can be used to estimate the probable quality of service, as shown in Table-1 while Table-2 gives guidance for values of suitable intervals for other types of buildings.



Interval (s)
Quality of service
≤20
Excellent
25
Very good
30
Good
40
Poor
≥ 50
Unsatisfactory

Table-1: Probable quality of service in office buildings

Building type
Arrival rate (%)
Interval (s)
Hotel
10-15
30-50
Flats
5-7
40-90
Hospital
8-10
30-50
School
15-25
30-50
Office( multiple tenancy)


Regular
11-15
25-30
prestige
15-17
20-25
Office( single tenancy)


Regular
15
25-30
prestige
15-17
20-25

Table-2: Percentage arrival rates and up-peak intervals for different types of buildings

Important note:
  • Caution must be exercised when using interval as a Quality of Service indicator as passenger waiting time depends on car load.


  • Table-3 gives numerical values for the average car load as a percentage of rated car capacity versus performance represented by passenger average waiting time (AWT) divided by average interval (INT) at that car load.


Car load (%)
AWT/INT (%)
Car load (%)
AWT/INT (%)
30
0.32
75
0.74
40
0.35
80
0.85
50
0.40
85
1.01
60
0.50
90
1.30
70
0.65
95
1.65

Table-3: Uppeak performance - numerical values

  • Based on Table-3, Caution must be taken if cars are allowed to load above 80% then the average passenger waiting time increases substantially. At the conventional assumed car loading of 80%, average passenger waiting time is 85% of the calculated interval. But at a 90% car loading, the average passenger waiting time has extended to 130% of the calculated interval. For loadings greater than 90%, the average passenger waiting time increases rapidly and in theory at 100% would be infinite.
  • However, for car loads between 50% and 80% it is possible to develop an approximate equation for the AWT as

AWT = {0.4 + [(1.8 x P/RC)-0.77]2} INT

Where RC is the rated car capacity

Example#1:

A speculative, regular 10 floor (above the main terminal) building is to be built. Each floor is 1200 m2 of usable space. What is the basic specification of the lift system?

Solution:

The basic parameters that specify any lift system are:
  • uppeak percentage population served,
  • uppeak handling capacity,
  • uppeak interval.


So, we are going to calculate these above values.

Step#1: Estimate the building’s population

From below table, you can estimate the building’s population

Building  type
Population estimate
Hotel
1.5–1.9 persons/room
Flats
1.5–1.9 persons/bedroom
Hospital
3.0 persons/bed space*
School
0.8–1.2 m2 net area/pupil
Office (multiple tenancy):
regular
10–12 m2 net area/person
prestige
15–25 m2 net area/person
Office (single tenancy):
regular
8–10 m2 net area/person
prestige
12–20 m2 net area/person
* Patient plus three others (doctors, nurses, porters, etc.).
Above Table indicates that 10–12 m2/person should be considered for regular office building with multiple tenancy.
Assume 12 m2, this gives 1200/12 = 100 persons per floor.
The total population will then be 10 floors ×100 persons per floor =1000 person.
Assume 80% daily occupancy, ie: this gives a design population of 800.


Step#2: Estimate Arrival Rate

From below table, you can estimate the Arrival Rate

Building type
Arrival rate (%)
Interval (s)
Hotel
10-15
30-50
Flats
5-7
40-90
Hospital
8-10
30-50
School
15-25
30-50
Office( multiple tenancy)


Regular
11-15
25-30
prestige
15-17
20-25
Office( single tenancy)


Regular
15
25-30
prestige
15-17
20-25


Above Table indicates 11–15% arrival rate for regular office building with multiple tenancy; assume 12.5%.

Step#3: calculate the required handling capacity in 5 minutes

Required handling capacity in 5 minutes = 12.5% x 800 = 100 persons.

Step#4: Estimate Interval

Above Table in step#2 indicates an interval of 25–30 s for regular office building with multiple tenancy. This is a speculative building so to save capital expenditure
assume 30 s.

the result:
The lift system should be sized to be able to handle 100 persons with a 30 s interval.


Example#2:

Example#1 requires a lift system to be sized to be able to handle 100 persons with a 30 s interval. Design such a system and estimate the average passenger waiting time AWT and quality of the service.

Solution:

Step#1: calculate the Number of Round Trips over 5 minutes

If the interval is 30 s, then a lift group must provide Number of Round Trips over 5 minutes = 5 minutes / RTT = 300 / RTT = 300 /30 = 10 trips over five minutes.

Step#2: Calculate the Average Number of Passengers per Trip (P)

UPPHC = P x Number of Round Trips over 5 minutes
P = UPPHC /Number of Round Trips over 5 minutes = 100 / 10 = 10 Passengers

Step#3: Estimate Rated Car Capacity

Step#2 means that each car must load with 10 passengers to handle 100 persons in five minutes.

P = CC x 80/100
10 = CC x 80/100
CC = 12.5 person

A 13-person lift is the nearest standard size as per below table:


Rated load
(kg)
(RL)

Max area
(m2)
(CA)

Rated capacity
(persons)
(CC)

Actual capacity
(persons)
(AC)

Design capacity
(persons)
(DC)

Capacity factor
(%)
(CF)

Actual load
(kg)
(AL)
320
0.95
4
4.5
3.6
90
338
450
1.30
6
6.2
5.0
82
465
630
1.66
8
7.9
6.3
79
593
800
2.00
10
9.5
7.6
76
713
1000
2.40
13
11.4
9.1
70
855
1275
2.90
16
13.8
11.0
69
1035
1600
3.56
21
16.9
13.5
64
1268
1800
3.92
24
18.6
14.9
62
1395
2000
4.20
26
20.0
16.0
62
1500
2500
5.00
33
23.8
19.0
58
1785
Car loading and car capacity



Step#4: Estimate the average passenger waiting time AWT

If a 13 person car is used, the percentage car load = 10/13 = 77%.

From Table-3 in above, a car load of 77% indicates that AWT/INT will be 78.4%.

The estimated AWT will be 0.784×30=23.5 s.

Step#5: Estimate the quality of the service

Based on Table-1 in above, since The estimated AWT =23.5 s. so, the quality of service will be very good.




2- Estimation of Quality of Service based on the performance time T

  • The performance time (T) has the most effect on the RTT. It is easily measured as it is the time taken between the instant a stationary lift starts to close its doors until the instant the doors are 800 mm open at the next adjacent floor.
  • For a 3.3 m interfloor height, Table-4 shows the probable performance of an installed lift system for various values of T.


Value of T (s)
Quality Of Service For The Lift System
8.0-9.0
Excellent system
9.0-10.0
Good system
10.0-11.0
Average system
11.0-12.0
Poor system
>12.0
Consider system replacement

Table-4: the performance time (T) as an indicator of quality of service




3- Estimation of Quality of Service based on Average System Response Time Performance (ASRT)

  • For office buildings, we can estimate the quality of service provided by an installed lift system based on either the percentage of calls answered in specified time intervals, or the time to answer a specified percentage of calls.
  • Table-5 indicates the percentage and time values for several grades of service over one hour of peak activity in an office building. An hour of peak activity is taken in order to obtain sensible and realizable results.


Grade of Service
Percentage of calls answered in time of
Time to answer Percentage of calls (s)
30 s
60 s
50%
90%
Excellent
>75
>98
20
45
Good
>70
>95
22.5
50
Fair
>65
>92
25
55
Poor/unacceptable
<65
<92
>25
>55

Table-5: Office Building Average System Response Time Performance




4- Estimation of Quality of Service based on Passenger Average Travel Time to Destination (ATT)

  • The passengers travelling to the upper floors of a building zone become annoyed if a lift takes too long to reach their floor. Strakosch (1998) states that for most people 100 s is a tolerable travel time which can be further tolerated to some 150 s of travel time if two people exit at each stop. He regards 180 s as the absolute limit.
  • The passenger average travel time (ATT) is the average period of time, in seconds, which an average passenger takes to travel from the main terminal floor to the requested destination floor, measured from the time the passenger enters the lift until alighting at the destination floor.
  • A quick rule of thumb, which has been used to evaluate this time, is to use the formula of: adding one half of the uppeak interval for a group of lifts to one quarter of the uppeak round trip time for the individual lift in the group:


ATT = 0.5 x UPPINT + 0.25 x UPPRTT

  • This rule of thumb ignores how quickly a car expresses back to the main terminal floor after the last passenger has alighted. So, A better rule of thumb, found by comparison to calculations, is to add one half of the uppeak interval to one half of the uppeak round trip time:


ATT = 0.5 x UPPINT + 0.5 x UPPRTT


  • But for A more accurate estimate of how long it takes the average passenger to reach their destination is to modify the round trip time Equation:


RTT = 2H tv + (S+1)ts + 2P tp

  • And calculate ATT to the midpoint of the local travel for any group of lifts. This means travel for a distance of H/2 with the number of stops being S/2 and a transfer of P/2 passengers boarding the lift and P/2 passengers alighting. The resulting formula is given below:


ATT = 0.5H tv + 0.5 S ts + P tp

  • If there is any express travel through a number of floors (Ej), the formula becomes:


ATT = 0.5H tv + 0.5 S ts + P tp + te

  • The above Equation calculates ATT to the midpoint of the local and express travel for any group of lifts. This will be to a point halfway between the lobby and the high call reversal floor (H). Also the equation takes account of the passenger transfer times and the express travel. To illustrate this consider a lift carrying eight passengers serving a building with 22 floors above the main terminal. What is the position of the average destination floor? Using Table the below table, the column for a 10 person rated load allows 8 passengers in the car. Following this column down to the line corresponding to 22 floors shows that the highest reversal floor is Floor 20. So, The average destination floor is thus Floor 10.

 
Values of H and S with respect to number of passengers carried in car

The Quality of Service based on Passenger Average Travel Time to Destination (ATT) is estimated as per the below table:

Time
Aim for
Poor
ATT
<60s
>70s




5- Estimation of Quality of Service based on Passenger Average Journey Time (AJT)

  • The passenger average journey time is the average period of time, in seconds, measured from the instant an average passenger first registers a landing call (or arrives at the landing), until alighting at the destination floor.


Thus the passenger average journey time = the average passenger travel time (ATT) + the average passenger waiting time (AWT).

  • The average passenger travel time (ATT) is simple to calculate, but the average passenger waiting time (AWT) depends on car loading, which can only be determined after the car size has been selected as explained in above. The passenger average travel time plus one half of the uppeak interval will give a close approximation for evaluation purposes.


AJT = 0.5H tv + 0.5 S ts + P tp + te + 0.5 INT

  • The passenger average journey time is more accurately obtained by adding the average passenger waiting time to above Equation and is given by:


AJT = 0.5H tv + 0.5 S ts + P tp + te + AWT

  • The average waiting time AWT should be estimated from Table-5 according to the car loading.
  • The Quality of Service based on Passenger Average Journey Time (AJT) is estimated as per the below table:


Time
Aim for
Poor
AJT
<80s
>90s




6- Estimation of Quality of Service based on Rule of thumb for a single lift

  • The following is a useful rule of thumb for the general level of service provided by a single lift serving several floors:


excellent service
one lift per 3 floors
average service
one lift per 4 floors
poor service
one lift per 5 floors

  • However, these rules of thumb may be overridden by the need to achieve a specified interval or handling capacity.




In the next article, we will explain step by step the two methods for Lift Traffic design calculations with solved examples. Please, keep following.
The previous and related articles are listed in below table:
Subject Of Previous Article
Article
Applicable Standards and Codes Used In This Course,
The Need for Lifts,
The Efficient Elevator Design Solution
Parts of Elevator System Design Process
Overview of Elevator Design and Supply Chain Process.

The Concept of Traffic Planning,
The (4) Methods of Traffic Design Calculations,
Principles of Interior Building Circulation:
A- Efficiency of Interior Circulation

B- Human Factors

C- Circulation and Handling Capacity Factors:
Corridor handling capacity,
Portal handling capacity,
Stairway handling capacity,
Escalator handling capacity,


Passenger Conveyors (Moving Walkways and Ramps) handling capacity,
Lifts Handling Capacity.
D- Location And Arrangement Of Transportation Facilities

Traffic design calculations:
1- Calculation of the Number of Round Trips for a Single Car,
2- Estimation of Population,
3- Calculation of the Average Number of Passengers per Trip (P),
4- Calculation of the Uppeak Handling Capacity (UPPHC),
5- Calculation of the Waiting Interval (Passenger Waiting Time),
6- Calculation of The percentage population served (%POP),



7- Estimation of Arrival Rate,
8- Calculation of the Round Trip Time RTT,








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