Preliminary Design And Traffic Analysis in Elevator

Contents Mention in this blog are furnished in detail in the Indian Standards document. Readers are advised to read the IS document.

1. Number of lifts and Capacity:

The number of passenger lifts, their capacities and speed, required for a given building depend on the characteristics of the building. User satisfaction can be obtained only by providing sufficient number of lifts with adequate capacity and speed in order to reduce passenger waiting times.

Few important aspects to be considered are:

• The number of floors to be served by the lift.
• The floor to floor distances to be travelled.
• The population of each floor to be served, and
• The maximum peak demand. This demand may be unidirectional, as in up and down periods, or a two way traffic movement.

In view of any variables, no simple formula is possible for determining the most suitable combination of lifts. It should be appreciated that all calculations on the traffic handling capabilities of lifts are dependent on a number of factors which vary according to the design of the building and the assumptions made on passenger movement.

In addition to the type of building, different authorities and manufacturers differ widely in their methods of calculation, due to variations in lift rates of acceleration and deceleration and door performance time. Therefore, the calculations made by different organizations will not necessarily agree. It follows therefore, that the result of such calculations can only be put to limited use of a comparative nature.

2. Preliminary Lift Planning:

2.1. General:

Methods of calculating the traffic handling capabilities of lifts were firs devised for office buildings. In due course detailed modifications were devised to suit other applications without altering the basic principles. The application to office buildings is still the most frequently used. Therefore, the following general method may be used as general guidance on preliminary lift planning for offices. A lift installation for office building is normally designed to service the building at a given rate and three main factors to be considered are;

• Population or the number of people who require lift service,
• Handling capacity or the maximum flow rate required by these people,
• Interval or the quality of service required.

2.2 Population:

The first point to be ascertained from the eventual occupier is the total building population and whether occupier number is likely to increase in the future. If a definite population figure is unobtainable an assessment should be made from net area, and probable population density. Average population density can vary from about one person per 4 sqm to one person per 20 sqm. It is essential, therefore, that some indication of the probable population density should be obtained from building owner, if no indication is possible, 5 sqm per person for general office building can be generally assumed.

2.3 Quantity of Service:

The quantity service is a measure of the passenger handling capacity of a vertical transportation system. It is measured in terms of the total number of passengers handled during each five-minute peak period of the day. A five-minute base period is used as this is the most practical time over which the traffic can be averaged. The passenger handling capacity should be approximately 10 percent to 15 percent of the estimated population that has to be handled in the building in five minutes for diversified tenancy office building and 15 percent to 25 percent for single purpose occupancy office building. For residential buildings, 7.5 percent is sufficient.

2.3.1 The handling capacity:

The handling capacity of a lift system is the total number of passengers that it can transport in a period of 5 min during the up peak condition with a specified average car loading.

Obviously, meeting the high instantaneous demand would require large and expensive system. Thus a compromise is necessary, where the intending passengers are required to wait a reasonable time for service during peak demand periods. A period of 5 min for the handling capacity has achieved general acceptance. Thus if it is possible to equate the passenger demand as expressed by 5 min percentage peak arrival rate with the handling capacity of a system, then a suitable configuration could be designed.

2.4. Quality of Service:

The quality of service on the other hand is generally measured by the passenger waiting time at the various floors. The following shall be the guiding factor for determining this aspect:

Quality of Service or Acceptable Interval

20 to 25 seconds Excellent

30 to 35 seconds Goods

36 to 40 seconds Fair

45 seconds Poor

Over 45 seconds Unsatisfactory

Note: For residential buildings longer intervals may be acceptable.

2.5 Waiting Time:

The theoretical average wait of all persons is one half the interval, the interval being the theoretical longest wait for any person. For example, if three elevators are provided and the average round trip time s 90 sec, the average interval for that period is 30 sec. Some people will get the service immediately with 0 sec wait and few may wait for the full 30 sec. The average waiting time in that case would be 15 sec. However, some times, some people may delay the elevator or the elevator may be filled and may bypass landing calls etc. Considering these conditions, the average wait time could be 55 to 60 % of the interval.

2.6 Traffic Peaks:

The maximum traffic flow during the morning peak period is usually considered as a measure of the vertical transportation requirement in an office building. The employees of all offices are subject to discipline and are required to be at their place in time. Consequently, the incoming traffic flow is extremely high and the arrival time is over a short period.

Sometimes it becomes necessary to reduce the maximum traffic flow by staggering the arrival of the employees so that different groups arrive at different times. This reduces the peak and also the requirement of lifts. However, many organizations may object to staggering and prefer to have all the employees arrive at the same time since it is claimed that staggering will affect the proper co-ordination of business.

2.6.1. Up-Peak Traffic:

An up-peak traffic condition exists when the dominant or only traffic flow is in upward direction with all or majority of passengers entering the lift system at the main terminal of the building. (The main floor is the arrival floor, that is the building entrance floor). Up peak occurs in considerable strength in the morning when lift passengers enter a building with intent on traveling to destinations on the upper floors of the building. The up peak generally occurs from the employers requiring their employees to arrive at work at a specified time. It is found that if a lift system can cope with the morning up peak, then it will cope up with other patterns of traffic, such as down peak and inter floor traffic. The traffic pattern is idealized by designers in terms of a 5 min peak rate taken as percentage of the building population.

2.6.2. The up-peak arrival percentage:

Is the number of passengers who arrive at the main terminal of a building for transportation to the upper floors over the worst 5 min period expressed as a percentage of the total building population.

A lift system is expected to respond to the peak demand in such a way as to quickly and efficiently transport passengers to their respective destinations without excessive passenger waiting time occurrence or formation of queues. This implies that the handling capacity of the lift system should be sufficient to carry all those passengers demanding service.

2.6.3. Down-peak Traffic:

A down peak traffic condition exists when the dominant traffic flow is in a downward direction with majority of the passengers leaving the cars at the main floor of the lobby. To some extent, down peak is the reverse of the morning up peak occurrence at the down peak is more intense than the morning up peak up to 50 % higher demands and with durations of up to 10 min.

2.6.4 Two way and four way traffic:

A two way traffic condition occurs hen the dominant traffic flow is to and from one specific floor, which is not the main terminal.

A four way traffic condition exists when the dominant traffic flows are to and from two specific floors, one of which may be the terminal floor.

They can arise from the presence of a refreshment floor which at certain times of the day attracts a significant number of stops and calls.

2.6.5 Inter Floor Traffic:

Random inter floor traffic can be said to exist when no discernable pattern of calls can be detected. Inter floor traffic is caused by the normal circulation of people around a building during the course of their business. Sometimes this traffic is called balanced two way traffic as it involves both up and down trips and it is balanced because passengers return to their original floors after moving about the building.

2.7 Speed:

It is dependent upon the quantity of service required and the quality of service desired. Therefore, no set formulae for indicating the speed can be given. However, the following general recommendations are made:

No of Floors Speed ( mps )*

4 to 5 0.5 to 0.75

6 to 12 0.75 to 1.5

13 to 20 Above 1.5

*Meters per second

2.8 Layout:

The shape and size of the passenger lift car bears a distinct relation to its efficiency as a medium of traffic handling. A study of most suitable proportions for these lifts reveal that the width of the well entrance is, in reality, the basic element in the determination of the best proportions. In other words, the width of the car is determined by the width of the entrance, and the depth of the car is regulated by the loading per square meter permissible under this standard. Center opening doors are the most practicable and the most efficient entrance units for passenger lifts.

2.9 Determination of Transportation or Handling Capacity during the Morning Peak:

2.9.1 The handling capacity is calculated by the formula:

H = 300 * Q * 100/ T *P

Where 300 = 5 minutes interval

H = handling capacity as the percentage of the peak population handled during 5 min period

Q = average number of passengers carried in a car

T = Waiting interval and

P = Total population to be handled during morning peak period ( it is related to the area served by a particular bank of lifts)

The value of ‘Q’ depends on the dimensions of the car. It may be noted that the car is not loaded always to its maximum capacity during each trip and, therefore, for calculating ‘H’ the value of ‘Q’ is taken as 80 percent of the maximum carrying capacity of the car.

The waiting interval is calculated by the formula:

T = RTT / N

Where

N = No. of lifts, and

RTT = round trip time. That is, the average time required by each lift in taking one full load of passengers from ground floor, discharging them in various upper floors and coming back to ground floor for taking fresh passengers for next trip.

2.10 The round trip Time(RTT):

The round trip time is the time in seconds for a single car trip around a building from the time the car doors open at the main terminal until the doors reopen when the car has returned to the main terminal floor after its trip around the building.

A round trip time should not usually exceed two to three minutes as the majority of this time can represent the journey time for some passengers with destination at top of the building, which is not desirable.

RTT is the sum of the time required in the following process:

• Entry of the passengers on the ground floor.
• Entry or Exit of the passengers on each landing floor.
• Door closing time before each starting operation.
• Door opening time on each passenger exit operation.
• Number of Probable stops.
• Acceleration periods.
• Stopping and leveling periods.
• Periods of full rated speeds between stops going up, and
• Periods of full rated speeds between stops going down.

It is observed that the handling capacity is inversely proportional to waiting interval which in turn is proportional to RTT. By reducing the RTT of a lift from 120 to 100 seconds its handling capacity increases by 20 percent.

The round trip time can be decreased not only by increasing the speed of the lift but also by improving the design of the equipment related to opening and closing of the landing and car doors, acceleration, deceleration, leveling and passenger movement.

These factors are discussed below:

• The most important factor in shortening the time consumed between the entry and the exit of the passengers to the lift car is the correct design of the doors and the proper car width. For comfortable entry and exit for passengers it has been found that most suitable door width is 1000 mm and that of car width is 2000mm.
• The utilization of center opening doors has been a definite factor in improving passenger transfer time, since when using this type of door the passengers, as a general rule, begin to move before the doors have been completely opened. On the other hand, with a side opening door the passengers tend to wait until the door has completely opened before moving.

The utilization of center opening doors also favors the door opening and closing time periods. Given the same door speed, the center opening door is much faster than the side opening type. It is beyond doubt that the center opening door represents an increase in transportation capacity in the operation of a lift.

2.10.1 An example illustrating the use of the above consideration is given below:

Net usable area per floor = 950 sqm

No of landings including ground = 15

Assuming a population density = 9.5 sqm per person.

Probable population in 14 upper floors = 14 * 950 / 9.5

Total population to be handled during peak

P = 1400 persons.

If the calculated RTT Taking 20 passengers lift with 2.5 mps. = 165 sec

Taking No of lifts N = 4

Waiting Interval T = RTT / N = 165 / 4 = 41 s

Waiting time = T/2 = 20.5 sec

Avg. No persons carried in car in each trip

Q = 20 * 0.8 = 16

H = 300 * Q * 100 / T *P

= 300 * 16 * 100/ 41 * 1400 = 8.3%

Taking No of lifts N = 6

T = RTT / N = 165 / 6 = 27.6 s

H = 300 * 16 * 100 / 27.6 * 1400 = 12%

Interval T = 27.6 s

Waiting time = T / 2 = 13.8 s

3. Quiet Operation of Lifts:

Every precaution should be taken with passenger lifts to ensure quiet operation of the lift doors and machinery. The insulating of the lift machine and any motor generator from the floor by rubber cushions, or by a precast concrete slab with rubber cushions prevents transmission of most of the noise. Some recommendations, useful in this connection are given in IS 1950.

4. Position of Machine Rooms:

It will be noted that all lifts conforming to IS 14665 ( Part 3 / Sec 1), have machine rooms immediately over the lift well, and this should be arranged whenever possible without restricting the overhead distance required for normal safety precautions.

Alternate machine positions should only be considered when there are special reasons justifying the addition cost, such as head room restrictions imposed by the planning authority for lifts serving the top floor.

It is desirable that emergency exit may be provided in case of large machine rooms having four or more elevators.

Two basic considerations, namely, the quantity of service and the quality of service desired, determine the type of lifts to be provided in a particular building. Quantity of service gives the passenger handling capacity of the lifts during the peak periods and the quality of service is measured in terms of waiting time of passengers at various floors. Both these basic factors require proper study into the character of the building, extent and duration of peak periods, frequency of service required, type and method of control, type of landing doors, etc.

For instance, they can with advantage be used to compare the capabilities of lifts in a bank with different loads and speeds provided the same set of factors is used for all cases.

5. RTT calculation Assumptions Leading to Traffic Analysis:

5.1 Entry & Exit of the passengers on the ground floor:

Usually the elevators are designed to wait for a fixed time of typically 8 seconds at the lobby for loading and unloading the passengers. It is found that typically an average of 0.8 seconds is required for transfer of passengers at the lobby. This means that up to 10 passengers can be loaded and unloaded within the fixed time of 8 seconds. If there are more Passengers, 0.8 seconds should be added for each passenger.

5.2 Entry or Exit of the passengers on each landing floor:

The lifts are typically designed for a dwell time of 2 sec when the lift halts at a particular landing, initiated either by a car call. Similarly the dwell time due to landing call is 4 seconds.

5.3 Door closing & Door Opening time at each landing:

Typically the elevators are designed to have a shorter door opening time for the passengers to exit the lift quickly. The door closing time is kept more to avoid the doors banging on the passengers who are entering the lift.

The table below provides some assumptions on typical door open a close times. This may vary from manufacturer to manufacturer. It will be a good idea to get these values from the manufacturers before doing further calculations.

Table 1: Typical Door Operating Times

• Door open time can be reduced by 1 sec in case of advanced door opening.
• Once the door is closed, it takes some time to ensure that the door is locked and for the elevator motor to build up torque to commence run. This delay is estimated as 0.5 sec and added in the above door operating table.

5.4 Probable number of stops (S):

• The number of people entering the lobby greatly influences the number of stops the elevator makes. The number of floors the elevator serves influences the number of stops an elevator is expected to make with a given passenger load. For example, an elevator that serves 8 floors and carries 8 passengers is most unlikely to stop at all the 8 floors.
• The nominal population in each floor also has direct influence on the number of stops made per trip. If one floor has a population of 100 and the other 10, the tendency for stopping at the floor with a population of 100 is much more.
• When there is an incoming traffic peak, the elevator makes predominantly up car stops and returns to the lobby with very small probability of in between stops. When everyone wants to leave the building after office hours, a different pattern of stops is likely.
• When persons are traveling in between floors, each person makes two stops, one for boarding and the other for leaving. Under extremely busy condition with inadequate elevators, it is possible for an elevator to make every stop up and every stop down, resulting in intolerably high waiting times.
• In estimating the probable number of stops per trip, we can reasonably be certain that in every elevator trip the elevator will stop in upper floors in proportion to the number of people in the car and the number of floors that elevator serves. George Strakosh in his book titled Vertical Transportation Elevators and Escalators has provided a statistical calculation of the probable number of passengers leaving the elevator at a given floor at the same time with the following formula:
• Probable number of stops per trip = S-S[S-1/S]p

S= Total number of stops above lobby

p= No of passengers carried on each trip

A simple calculation with the above formula for floors above lobby upto 20 floors and total capacity of 20 Passengers leave us with the following table:

Table 2: Probable Stops

Above Equation is the one used in planning elevators for buildings when little is known about future population and each floor is assumed to have equal population. Applying the formula, Table shows probable stops value for approximate numbers of passengers per per trip total number of upper floors served by an elevator.

5.5. Running Time:

Running time includes: 4.5.4 Number of Probable stops

• Acceleration & Deceleration periods.
• Starting, Stopping and leveling periods.
• Periods of full rated speeds between stops.

Assumptions:

a) Acc / Dec Rates

0.6 m / sec / sec for 0.7mps speed

1.0 m/sec/sec for 1mps speed

1.1 m/sec/sec for 1.5mps speed

1.2 m/sec/sec for 1.75mps speed

1.25 m/sec/sec for 2mps speed

1.35 m/sec/sec for 2.5mps speed

b) Starting + Stopping delay = 0.5 sec ( for DC injection at Zero speed)

c) S curve delays during acceleration + Deceleration = 1 Sec

Using the Newton’s law v = u + at and v*v + u*u = 2as

Where v = final velocity, u = initial velocity, a = Acc / Dec rate & s = distance travelled, the following table gives time for various speeds and different acceleration / deceleration rates

Table 3 : Time and distance during acc/ dec

During time other than the acceleration and deceleration, the lift will run at rated speed.

Time during rated speed = Distance travelled / Speed.

With the running time assumptions, above table and time during rated speed, time for floor to floor can be calculated.

The table below provides the total run time

Table 4 : Run Time

In addition to loading and unloading and door opening and closing, the other element in the round trip time of the elevator is the running time. Part of the running time is spent in accelerating to rated speed and decelerating it to stop. Typical floor to floor traveling time for elevators of various speeds and with typical acceleration and deceleration rates is given in table.

The above table indicates that increase in speed helps to reduce the time only when the distance travelled is more.

Sample calculation for 1.5mps speed and travel of 6 m.

From table 3, time for acceleration & deceleration is 2.7s and distance travelled during acceleration/ deceleration is 2.0m.

Time during rated full speed = (6 – 2) / 1.5 = 2.7s

Total time = Starting / Stop delay + S curve delay + Acc/Dec time + Rated speed run time

Total time = 0.5 + 1 + 2.7 + 2.7 = 6.9s

5.6. Traffic Calculation – Example

SOLUTION:

Summed up results of Examples 2 & 3:

It may be noticed that with advanced door opening, the performance of elevator with 1.5 m/s is comparable to 2 m/s speed, with all the other specifications remaining the same, as the round trip time has improved with reduction in door open times.

*For a single car, RTT in S No. 13 = Interval.

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