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 60 L = = 3.75 8 0.8 300 120 say 4 Therefore this building should be equipped with 4 such lifts 120 As interval is = 30 4 so the quality of service is satisfactory. K.F. Chan (Mr.) Db of 2 Page Db 1 of 12
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 Lift travel with J=0, m (number of floors) Speed, m/s Building Usage Luxury flats Offices Bed lifts 0.25 to 0.4 - - 5 0.50 10 (3) 10 0.63 15 (4) 15 (4) - 1.0 20 (6) 20 (6) 20 1.6 25 (8) 32 (10) 45 2.5 40 (12) 50 (15) 100 5.0 100 (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 1,000m 2 per floor and arrival rate>15%, is :- Good service Average service Poor service One lift per 2 floors One lift per 3 floors One lift per 5 floors K.F. Chan (Mr.) Db of 2 Page Db 2 of 12
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.) Db of 2 Page Db 3 of 12
Example on number of lifts UNIVERSITY OF HONG KONG A lift system is to be designed for a 20-storey high regular multi-tenancy office building. Area of each floor is 450m 2, with interfloor height of 3.5m. It can be assumed that the occupancy is 9m 2 /person with daily occupancy of 90% only. What will be a suitable number of lifts? Answer 450 There are = 50 persons/floor 9 Total population is 50 x 20 = 1,000 persons Effective population is thus 1000 x 90% = 900 persons. 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 13.5 0.8 = 16.8, say 16 persons. 300 = 10 trips 30 135 = 13.5 persons. Thus rated capacity 10 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/s 2. So initial design is 6 lifts each of 16 persons rated capacity, 3.15m/s contract speed and acceleration of 1m/s 2. K.F. Chan (Mr.) Db of 2 Page Db 4 of 12
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.) Db of 2 Page Db 5 of 12
Example on RTT and number of lifts A multi-tenant prestige office has 50 floors above the ground floor. Each floor is 1000m 2 in area. The daily occupancy rate is 90%. The ground floor has a height of 7m; height of 1 st floor to 4 th floor is 5m each. The 24 th floor is a mechanical floor with a height of 3m only. Floor height of the other floors is all 3.5m. 36 th to 50 th floor are grouped into a high zone served by 4 lifts each of contract capacity of 16 persons, 8m/s contract speed, 1m/s 2 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 15m 2 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: S = N 1 1 1 N P S 1 15 1 1 15 ( 16 0.8) = = 8.8 K.F. Chan (Mr.) Db of 2 Page Db 6 of 12
H = N N 1 i = 1 i N P For N=15, CC=16, H=14.3 from table 2.13 of CIBSE Guide D The floor to floor height from G/F up to 35 th floor is 7 + (4-1+1)x5 + (24-5)x3.5 + 3 + (35-25)x3.5 = 131.5m This is the distance the lift express travels before reaching the zone served. The unequal floor distance for G/F and 24 th 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 v a H J + d f S <. Time to jump to first stop is a H J + d v S + a v f 14.3 131.5 + 3.5 8 8.8 = + 1 8 = 25.15s Again, it is obvious that lift is unable to reach contract speed in subsequent jumps because H d f v S >. Time for jumping subsequent stops is a a K.F. Chan (Mr.) Db of 2 Page Db 7 of 12
( S 1 ) 2 H d S a f 14.3 3.5 = ( 8.8 1) 2 8.8 = 37.2s 1 UNIVERSITY OF HONG KONG 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 < ). Time to express return to G/F is a ( J H ) v + + a v d f ( ) 8 131.5 + 14.3 3.5 = + 1 8 = 30.69s 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 16 0.8 1.2 + 1.2 = 30.72s ( P )( t l + t u ) = ( )( ) 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 16 0.8 300 4 = 103pax 148.3 The percentage population served in the 5-minute uppeak interval is thus 103 = 11.5% 900 K.F. Chan (Mr.) Db of 2 Page Db 8 of 12
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 ( 16 0.8 300) 148. 3 = 6 lifts Moreover, with 6 lifts, the uppeak interval is reduced to recommendation of 20 25s, thus acceptable. 148.3 =24.7. This is within the 6 K.F. Chan (Mr.) Db of 2 Page Db 9 of 12
Average number of passengers re-visited UNIVERSITY OF HONG KONG The average number of passengers carried per trip is generally assumed by the industry to be 80% of the rated car capacity. There are other justifications for this 80% assumption:- a) Circulation problems within the car (e.g. passengers at the back of a crowded car always wish to exit at the first stop!) causing delays to the car s journey. b) Obstruction of doors (e.g. collision of doors with passengers bodies and items carried) thus causing delays owing to door recycling. c) Statistical effects: a facility which must respond to a demand will respond more quickly if it is only loaded to 80% of its capacity. d) Claustrophobic effects: i.e. passengers dislike of crowded conditions. Computer analysis confirms that (c) is a plausible reason. To avoid passengers being left behind a queue to wait for the next lift, it is necessary to assume a lower than 100% utilization factor for car occupancy. This assumption arises as statistical theory tells that, as the utilization of a facility increases towards its maximum, the probability of immediate availability of that facility reduces. Therefore to achieve maximum utilization of a facility it is necessary to have a queue of applicants waiting (like an airport) However this is not considered satisfactory for a lift system. Therefore, the design utilization has to be lower than 100% to accommodate for statistical variations. The probability of the immediate use of a facility is shown diagrammatically in the following figures with respect to system utilization. As system utilization increases, the probability of a passenger being left behind increases, until at 100% utilization there is a high probability of being left behind to queue. The shape of the curve has been shown to apply to such diverse facilities such as access to a telephone line, availability of a lavatory, a free bank teller, etc. (Figure adopted from Fig. 3.5 of CIBSE Guide D) (Figure adopted from Barney, Elevator traffic handbook, theory and practice) The shape of the solid curve indicates the wisdom of selecting an 80% car loading as a design criterion. Usually, the 80% point is considered to be the knee of the curve for most system utilization judgments (c.f. Pareto analysis) Values less than 80% do not fully utilize the installation, and values above 80% quickly result in poor service times. K.F. Chan (Mr.) Db of 2 Page Db 10 of 12
,% (Table adopted from CIBSE Guide D) K.F. Chan (Mr.) Db of 2 Page Db 11 of 12
Effect of traffic supervisory system The conventional calculation procedure based on the RTT expression requires that lifts present themselves at the main terminal evenly spaced by a period of time equal to one interval. 2 factors can upset this situation:- 1) As shown in the following figure, the uppeak 5 minutes is part of a continuous process. Hence lifts will already be transporting passengers at the onset of the uppeak. As a result lifts will tend to be randomly dispersed around the building. A severe condition occurs when the distribution of lifts becomes so disturbed that lifts BUNCH together and move round the building together. The effect of bunching is to reduce the quality of service by making most passengers wait longer. (Figure adopted from Barney, Elevator traffic handbook, theory and practice) 2) The traffic supervisory control system during most of the day is arranged to deal with interfloor traffic. The peak periods, uppeak, down peak and lunch time 2 way traffic are generally supervised by special algorithms, which must be switched on when required. The changeover from interfloor to uppeak or down peak control is achieved by monitoring, say, car load, and when this exceeds a predetermined value the appropriate control algorithm is selected. Thus the uppeak algorithm must be active just before the peak 5 minutes starts, if it is to be effective. It is assumed in the round trip calculation that during uppeak all lifts are express to the main terminal after depositing the last passenger at the highest reversal floor. This is all the uppeak traffic supervisory algorithm can do for uppeak service. However the time when the uppeak control is switched in is important if it is too late, only those cars with completed trips will be traveling to the main terminal. Those lifts just starting, or part way through, a round trip can only become available at the main terminal some 2 or 3 minutes later, halfway through the uppeak period! It is thus essential to detect the uppeak condition well before it takes off to ensure the full lift system handling capacity is available at the lobby. K.F. Chan (Mr.) Db of 2 Page Db 12 of 12