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Lift Design, Important Factors and Traffic Constraints

Er. Parbhakar Dwivedi
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While a lift designer designs the lift's dimension, speed, location and number thereof, the architect on the other hand is responsible to make all the necessary provisions in the building plans.

Efficient passenger transportation, both horizontal and vertical is the life line of any building, and hence, it is necessary that the architect take expert advice at conception stage. With a team approach, various aesthetic and conceptual ideas can be incorporated and optional solutions offered.

The net effect should be a building properly designed for good access and efficient transportation of the people using it or otherwise chaos prevails and the bad name reflects on the team and not on one individual.

When considering the traffic design of a new building the major building dimensions should be known. Unfortunately it is often the case that the architect responsible for the building conception will have fixed the building core limiting the space available for the lift system or even will have defined the number of shafts, their dimensions and travel. This removes one very important degree of freedom for the lift traffic designer.

Of course, at the low end of the market there may be only one lift or its dimensions may be fixed to conform with statutory regulations or to accommodate furniture, etc. But as the lift system moves 'up-market, initial design decisions become more important.

These fundamental constraints cannot be altered (at least not very much) when redesigning for modernisation as of course the building actually exists, However, there is one advantage that the building population to be served is known.

Human Constraints

A lift system has to be acceptable to the passengers using it. The passengers shall feel confident about the way they are handled, i.e. taking care of their physiological and psychological barriers.

i. Physiological Constraint

The physiological constraints limit the manner in which a passenger may be moved in the vertical plane. The human body is uncomfortable if its internal organs are caused to move in the body frame. This occurs when the body is subjected to acceleration or deceleration, i.e. the g effect. The magnitude of the effect depends on an individual's age, physical and mental health, and whether a sudden movement is expected. It is not clearly established the level of acceleration at which harm may be caused to the human body, but it is known by experience the levels of acceleration or deceleration, which have been found to be generally acceptable. These are shown in Fig. 18.1. Note there is no limit to the velocity at which a passenger may travel in an enclosed lift car, but that acceleration/deceleration should be limited to about (1-1.5 m/s²) and jerk (rate of change of acceleration) to 2m/s3. It is the latter effect-jerk-which causes the most discomfort. If jerk is not allowed to exceed 2 m/s3 and is maintained constant the discomfort may be minimised.

ii. Psychological Constraint

As would be expected, psychological constraints are more subtle. A passenger expects a grade of service from a lift system. The same passenger expects a different grade of service at different times of the day and at different locations. For example, an office worker will not be too fussy when travelling up a building to work, but will become annoyed if he cannot quickly leave at night. In contrast the same office worker would not expect the same grade of service from a lift in a residential block. This constraint can be categorised as the passenger waiting time constraint. In general the maximum waiting time in an office block should not exceed 30 sec. and in the residential block should not exceed 60 sec. Waiting time is the prime psychological constraint. Tolerance will lengthen to about 120 sec. only if a few passengers are being served at each floor. Finally if monotony is relieved by a changing scene our passengers may tolerate a ride as long as 150 sec.

A secondary psychological constraint is transit time in the car after the passenger boards. Here the passenger is dependent on his fellow passengers in the car and other passengers making landing calls. A passenger travelling high up a building becomes intolerant of stops after about 90s of travel. Again the tolerance level depends on whether he is travelling in company of and on other passenger’s behaviour. For instance, one passenger boarding or alighting is obviously more selfish' than two or three transferring at a time.

There are other psychological effects such as aesthetic appearance and ‘gentle' doors, which add to a passenger's confidence in a lift system and overcome the fears of some persons who are afraid of such machines.

Traffic Constraints

The users of lift system, i.e. the passengers impose on the lift system, the need for it to respond to different traffic patterns. Consider the passenger demand in an office building as represented by the number of individual calls, aggregated for up and down call directions. This office building is subject to a strict time regime of fixed starting, break and leaving times. It illustrates clearly the different traffic patterns of morning up-peak, evening down-peak, mid-day four-way traffic and random (balanced) inter-floor traffic.

i. Up-peak Traffic

An up-peak traffic condition exists when the dominant or only traffic flow is in an upward direction with all or the majority of passengers entering the lift system at the main terminal of the building.

Up-peak occurs in considerable strength in the morning when prospective lift passengers enter.

Passenger demand rate for an office building. The total demand on the lift system is the sum of up and down direction demands building, intent on traveling to destination on the upper floors of the building, to a lesser extent an up-peak occurs at the end of the mid-day. It is considered that if a lift system can cope efficiently with the morning up peak, then it will cope with other patterns of traffic, such as down peak and random inter-floor traffic the arrival rate profile for morning up-peak.

The up-peak condition results from employers requiring their employees to arrive at work by a specific starting time, Human nature then exacerbates the condition as the majority of employees feel that in conscience all they must do is to be in a building before the defined starting time and that the employer then has the responsibility to transport them to their work station. The arrival rate profile for the morning up-peak thus takes as the envelope of the curve describes the arrival profile, in terms of the instantaneous passenger arrival rate, in calls per hour for a period of one hour. Note the gradual build-up prior to the official starting time and the more rapid decay afterwards.

The modern trend to flexi-time working will give way to alleviate the up peak situation, but unfortunately it cannot be applied to all classes of employment.

The profile of designers terms of a 5 min peak rate taken as a percentage of the building population. A second definition commonly used for the up-peak profile is to state the percentage of the building population that is likely to arrive over 30 min of peak activity.

ii. Down-peak Traffic

A down-peak traffic condition exists when the dominant or only traffic flow is in a downward direction when with all or the majority of passengers leave the lift system at the main terminal of the building.

To some extent, down-peak is the reverse of the morning up -peak occurring at the end of the working day, and to a lesser extent at the start of the mid-day break. The evening down peak is usually more intense than the morning up-peak with up to 50% higher demands with duration of up to 10 min. illustrates these effects.

iii. Other Traffic Situations

It is possible to find office buildings where no dominant traffic flows occur, especially where flexi-time working is used. Sometimes the up-peak situation occurs twice, but at a lower intensity, and obviously traffic patterns are different in institutional and residential building; but often dominant pattern similar to those defined above do emerge and hence case design procedures.

iv. Requirements for Traffic Design and Control

It is extremely difficult to compare competitive tenders, where no standardised methods of specification or common design procedures are used. Each manufacture and lift consultant appear to use different methods, and are often not keen to explain their approach. Those methods that are published are often sketchy and some are inaccurate. Such a situation is confusing and thus a need exists to provide an easy to use, acceptable and standard design method. In addition, the use of modern control systems radically alters some of the design assumptions.

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