Principles of Lift Traffic Design 4.1 THE NEED FOR LIFTS
Lifts are installed into buildings to satisfy the vertical transportation needs of their occupants and
visitors. They are necessary to provide a comfortable means of transportation to the different levels in a building. Some of these requirements are written into statutory regulations.
The transportation capacity of the lift group in a building is a major factor in the success or failure of a building as a place to work, live or receive a service. Like toilets, lifts should be available and easy to use without a second thought. Unfortunately this is not always the case and speculative building often results in the installation of an imperfect lift system.
In offices and other commercial buildings, lifts are installed to aid the efficient movement of the
occupants around the building, when performing their work tasks. This has the benefit of saving time, and hence money.
These financial considerations do not apply for residential property; quite the opposite, money is saved by not providing a lift, and statutory regulations have been framed to ensure suitable lifts are installed. In Britain, for example, it is recommended that a lift be installed in all residences where there are four or more storeys, and that two lifts be installed where a building contains more than six storeys.
The increase in the numbers of high and medium rise buildings since the Second World War has been a challenge to the lift industry. The four decades between 1945–85 have seen the acceptance of
automatic cars, the introduction of better traffic and control systems, and the inclusion of the digital computer in equipment. Improvements have also occurred in the engineering design and engineering installation of lift systems. The acceptance of traffic design methods has been slower and has only really become accepted since the early 1970s.
4.2 FUNDAMENTAL DESIGN CONSTRAINTS
The planning and selection of transportation equipment is a very involved subject. Although the basic calculations are relatively simple, the theory on which they are based is complex. The results obtained need to be tempered with a great deal of working experience of existing buildings, in order to ensure satisfactory design results.
When sizing a lift system for 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 not have taken professional advice from a lift specialist and may well have fixed the building’s core dimensions, thus limiting the space available for the lift system or, even worse, may have defined the number of shafts, their dimensions and travel. This removes one very important degree of freedom from the lift traffic designer. Building circulation, both horizontal and vertical, is the lifeblood of any building, and hence if a successful building is to be designed it is essential that the architect take expert advice at conception. This does not imply that the lift designer will take over the core
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design, but simply that by means of a team approach various aesthetic and conceptual ideas can be considered early on and design and optimal solutions offered.
Often the result of a team and professional approach will be a better sized lift system design, possibly with less shafts or fewer shafts travelling the whole height of the building, or a rearrangement of service floors and main terminals. The net effect should be a building properly configured for good access with sufficient handling capacity to serve the proposed population and its circulation needs.
Of course, at the low end of the market, there may be only one lift in a building, or its dimensions may be fixed to conform with statutory regulations, or to accommodate the carriage of furniture, etc. But as the lift system moves “up-market”, initial design decisions become more important.
When redesigning for the modernisation of an existing lift installation, the fundamental constraints mentioned above cannot be altered (or not very much) as the building actually exists. However, there is often the advantage that the building population to be served is already known.
4.3 HUMAN CONSTRAINTS
A lift system has to be acceptable to the travelling public. The most important requirement the public demands is safety. These aspects are covered by the safety standards promulgated at national, continental and world wide levels. This requirement is most important so that passengers may feel confident about the way they are handled. However, passengers are human and are subject to constraints, which fall into two categories: physiological and psychological, the body and mind. 4.3.1 Physiological Constraints
The physiological constraints (the effects of movement on the body) 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 within the body frame. This occurs when the body is subjected to acceleration or deceleration, the well known g effect. The magnitude of the effect on an individual depends on an
individual’s age, physical and mental health, and whether the individual is prepared for the experience of a sudden movement. It is not clearly established what the level of acceleration is at which permanent 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, when riding in a lift. These are shown in Figure 4.1.
Note that there is no limit to the velocity at which a passenger may travel in an enclosed lift car, as speed is not noticeable to the passenger. But the values of acceleration/deceleration (rate of change of velocity) should be limited to about one eighth of gnl or 1.5 m/s2 and the values of jerk (rate of change of acceleration) to 2.0 m/s3. The affect of an acceleration of one eighth of gn on a body weighing 80 kg travelling in an upward direction is that it then weighs 90 kg. Likewise the same body subjected to a deceleration, while travelling in an upward direction, would weigh 70 kg.
1gn is the acceleration of a body due to gravity, numerically equal to 9.81 m/s2.
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Figure 4.1 Ideal acceleration, velocity and distance travelled curves for a single floor jump (a) Acceleration profile: note maximum jerk 2.0 m/s3 and maximum acceleration 1.5 m/s2. (b) Velocity profile: note maximum speed 1.5 m/s.
(c) Distance travelled: note total distance 3.0 m.
It is the jerk values (not a very scientific sounding name, sometimes called shock), which cause the most discomfort. If the value of jerk is allowed to exceed 2 m/s3 for any length of time (tenths of seconds), discomfort will be experienced. Whereas velocity and acceleration/decelerationprofiles can be specified and controlled in drive systems, jerk cannot. Constant values of jerk require that the
acceleration/deceleration profile increase/decrease at a constant rate, and this is not always possible. It is perhaps fortunate that these human constraints do exist as they ease the design of lift drive systems considerably!
4.3.2 Psychological Constraints
As would be expected, psychological constraints are more subtle. A passenger expects a good service from a lift system. But an individual 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 annoyed if delayed when travelling up a building to work, but will become very annoyed if delays occur when leaving at night. In contrast, the same office
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worker would not expect the same grade of service from a lift in a residential block. This constraint can be categorised as the passenger’s waiting time constraint. In general, the average waiting time in an office block should not exceed 30s and in the residential block it should not exceed 60 s. Waiting time is the prime psychological constraint.
A secondary psychological constraint is the transit time, or travel time, in the car after the passenger boards. Here the passenger is dependent on the fellow passengers in the car and other passengers on the landings making calls. A passenger travelling high up a building becomes intolerant of stops after about 90 s of travel. Again the tolerance level depends on whether the passenger is travelling in
company of friends or colleagues and on the other passengers’ behaviour. For instance, one passenger boarding or alighting is obviously more “selfish” than two or three transferring at a time. This
psychological constraint has been summed up by Strakosch (1967) as “a person will not be required to ride a car longer than a reasonable 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.
4.4 TRAFFIC PATTERNS
As the users of lift systems, the passengers impose on the lift system the need for it to respond to different traffic patterns. Consider Figure 4.2, this shows 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, midday traffic and random (balanced) interfloor traffic.
Figure 4.2 Passenger demand for an office building
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At the start of the day there is a larger than average number of up-hall calls. This is due to the building’s occupants arriving to start work. This traffic pattern is called the morning uppeak.
Late in the day there is a larger than average number of down-hall calls. These are due to the building’s population leaving the building at the end of the working day. This traffic pattern is called the evening down peak.
In the middle of the day there are two separate sets of uppeaks and two down peaks. This represents a situation where the occupants of the building take two distinct lunch periods (ie: 12.00 to 13.00 and 13.00 to 14.00). This pattern is sometimes called two-way traffic.
During the rest of the day the numbers of up-hall and down-hall calls are similar in size and over a period are equal. This traffic pattern is called interfloor traffic, sometimes qualified as balanced interfloor traffic.
In practice this pattern may not be observed exactly as shown, as many companies have adopted a “flexitime” attendance regime. It does, however, serve as a model for discussion.
4.4.1 Uppeak traffic
This traffic condition is shown diagrammatically in Figure 4.3.
Definition 4.1: An uppeak traffic condition exists when the dominant, or only, traffic flow is in an