VERTICAL TRANSPORTATION SYSTEMS: HOW TO DO AN ELEVATOR TRAFFIC ANALYSIS

As  architects, developers and building owners race towards adorning the landscape with high-rise commercial and institutional buildings, one of the roles of the mechanical services engineer is to quantify the parameters and criteria which represent quality in the vertical transportation service. At the inception stage of any project the design team establishes design requirements. They establish unique requirements of the building or anticipated occupancy and their influence on the vertical transportation systems (which for this discussion are the elevators).  These requirements are translated into design criteria which combined with the projected population is utilized to conduct an elevator traffic analysis.

Before I proceed further, it is necessary to throw in some definitions of important terms used in elevator traffic analysis.

DEFINITIONS

·        Average lobby time or average lobby waiting time. The average time spent by a passenger between arriving in the lobby and leaving the lobby in a car. This is a key selection criterion.

·        Handling capacity (HC). The maximum number of passengers that can be handled in a time given period—usually 5 minutes, thus the term 5-minute handling capacity. When expressed as a percentage of the building’s population, it is called percent handling capacity (PHC). This is a key selection criterion.

·        Interval (I) or lobby dispatch time. The average time between departures of cars from the lobby.

·        Registration time. Waiting time at an upper floor after a call is registered.

·        Round-trip time (RT). The average time required for a car to make a round trip—starting from the lower terminal and returning to it. The time includes a statistically determined number of upper-floor stops in one direction and, when calculating elevator requirements based on up-peak traffic, an express return trip.

·        Travel time or average trip time (AVTRP).  The average time spent by passengers from the moment they arrive in the lobby to the moment they leave the car at an upper floor. This is a key selection criterion.

Carrying out an elevator traffic analysis will require the following design considerations:

·        Intended usages of the building, height, number of floors, etc.

·        Expected characteristics of “internal traffic” by occupants as follows

o   Domestic floors (less demand on elevator service)

o   Office floors (higher demand on elevator service)

o   Commefrcial floors e.g. shops (high population density and traffic)

o   Restaurants (peak demand at launch and dinning)

·        Level and quality of elevator service required

·        Types of elevator systems to be used : (traction or hydraulic)

o   Passenger elevators

o   Service elevators

o   Fireman’s elevator

o   Dumbwaiter (e.g. for restaurant)

From the above, an assessment of building population and number of persons using the service is estimated. Also the critical traffic period and occupancy loading of the elevator car are assumed.

Depending on the class of service, the basic design criteria for lift performance are:

·        Five minutes handling capacity (HC)

·        Average interval time or waiting time (AWT)

·        Maximum passenger transit time

This analysis can be performed by manual calculations or by computer programs. Also some elevator design sites now have online calculators to help in determining these parameters (e.g. http://www.kone.com/COUNTRIES/EN_MP/TOOLS/Pages/default.aspx ). However they will not show their calculations, just the results.

A typical elevator traffic analysis I did is given below. The building used in this analysis is a four-floor mixed-use building. The ground and first floor are office spaces. The second floor holds 36 nos. single rooms en-suite while the third floor holds a four bedroom family unit.

ANALYSIS TYPE:  UP-PEAK TRAVEL ANALYSIS

·        BUILDING AND POPULATION

No. of floors: 3

Building Population: 296

Floor Name	Height (m)	Floor Level (m)	Description	*Floor Area (sq.m)	Density Sq.m/pers	Gross Population Estimate	Vacancy Factor	Net Population
Pent House	3.15	12.95	Single Family			8	10%	7
2nd Floor	3.15	9.8	Multi Residence			108	10%	97
1st Floor	3.15	6.65	Office	1036	10	104	10%	94
Grd. Floor	3.5	3.5	Office	759	10	76	10%	68
D E S I G N          P O P U L A T I O N								266

*Floor area indicates net lettable area.

·        ELEVATORS

Recommended Control: Schindler ID

ELEVATOR	A	B
Rated Load (kg)	400	400
Weight per person (kg)	80	80
Passenger/Deck gross	5	5
Max. Car Filling	80 %	80 %
Passenger/Deck net	4	4
Maximum speed (m/s)	1.0	1.0
Maximum acceleration (m/s2)	0.5	0.5
Drive Jerk (m/s3)	0.5	0.5
Door width (mm)	900	900
Opening time (s)	1.9	1.9
Closing time (s)	2.6	2.6
Transfer time per person (s)	1.0	1.0
Minimum transfer time (s)	1.0	1.0
Travel height (m)	12.95	12.95

UP-PEAK TRAVEL ANALYSIS

Building characteristics

Type:               Mixed-use (Office on two floors, multi-dwelling on 3^rd floor and Single family dwelling on pent-house)

Tenancy:         Multiple

Floors above terminal floor:            3

Floor Height: 3.15m

Population above terminal floor PATF:      188

Recommended Handling Capacity Factor HCF:     12 %

Recommended Interval:                                           30

Calculating Handling Capacity, HC:

                            HC = PATF x HCF / 100

                             = 188 x 12 / 100

                             = 22.56  (ie 23 persons/5 minutes

Lift Travel Limit, LTL:

                        LTL = Floor Height x Floors above terminal floor

                            = 3.15 x 3

                            = 9.45 m

From HC and LTL, recommended lift speed = 1.6 m/s

The Round Trip Time (RTT) in seconds of a single elevator during up-peak traffic is given by:

RTT = 2Ht_v + (S + 1)t_s + 2N_pt_p

where,

H         is the average highest reversal floor

S_av               is the average number of stops

N_p                is the average number of passengers in the car (assuming an 80% occupancy)

t_v         is the time to transit between two adjacent floors at rated speed (s)

i.e. t_v= df_H / v

where,

df_H       =  average interfloor height (m)

v          =  rated speed (m/s)

t_s          =  time lost at each stop (s)

[ i.e. t_s = t_f (1)+t_o+t_c−t_v = T −t_v ]

where:

tf_H (1) =  single floor flight time (s)

t_o         =  door opening time (s)

t_c         =  door closing time (s)

T          =  performance time (s)

t_p         =  average one away passenger transfer time (s)

RTT = (2 x 3.3 x (3.15/1.6)) + (2+1) x (5.0 + 1.9 + 2.5 – (3.15/1.6) + (2 x 4 x 1.0)

        = 13 + 22.3 + 8

        = 43.3 s

The up-peak interval (UPPINT), in seconds, for the 2 lift cars is given by

UPPINT = RTT/2

              = 43.3 / 2

             = 21.65 s

The up-peak handling capacity (UPPHC) in persons/5 mins is given by

UPPHC = 300 N_pL/RTT where L = No. of lift cars

             = 300 x 4 x 2 / 43.3

              = 55.43

The percentage of the total building population (POP) above the terminal floor that can be served during up-peak is given by:

% POP = (UPPHC / POP) x 100

             = (55.43 / 188) x 100

             = 29.5 %

Average waiting time (AWT) is usually taken as 60 – 70 % of UPPINT

Therefore

AWT = 0.65 x 21.65 s

          = 14 s

RESULTS

Round Trip Time, RTT	43.3 s
Up-peak Interval, UPPINT	21.65 s
Up-peak Handling Capacity, UPPHC	55.43
Percentage Population Handled % POP	29.5 %
Average waiting Time, AWT	14 s

From the Schindler standards and recommendations, 2 nos. 400 kg lifts for this facility has a minimum rating of 5.0

Regulations and Standards:

·        British Standard BS 65655: Part 1 – 11  Lifts and Services Lifts

·        Code of Practice on the design and construction of Lifts and Escalators (EMSD)

Contact me if you need more information.

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