Last edited 23 Sep 2015

Lifts for office buildings


[edit] Introduction

There are three key sources of design standards for lifts in office buildings:

  • British Standard 5655 part 6.
  • CIBSE Guide D.
  • Transportation Systems in Buildings and British Council for Offices (BCO) best practice guide.

The criteria for design includes:

  • Morning up-peak handling capacity.
  • Occupation density.
  • Average interval.
  • Car occupancy.
  • A staircase allowance.

[edit] Morning up-peak handling capacity

Morning up-peak handling capacity is expressed as a percentage of the building’s designed population that needs to be moved in a five minute period. Historically, the general consensus estimated this at 15%. However, the increasing use of flexible hours has reduced mass arrival numbers and the introduction of sophisticated computer controls systems suggest that the 15% figure might be radically reduced.

A 2004 morning peak survey of nine West End and City of London office buildings by Stanhope (Lift performance and specification) concluded 6% of an assumed population using 14m2/person recorded actual reality. Furthermore the survey concluded that lunchtime patterns were more relevant as lifts are being utilised in both directions as people enter and leave the building simultaneously. The Stanhope survey suggested 7% is an adequate design capacity figure.

[edit] Occupation density

Occupation density should include a 15% allowance for absenteeism. Multiple tenancies occupy space more densely than single tenants and trading floors have particularly dense spatial requirements. 10m2 per person is a very conservative figure that matches British Standards. BCO however believe 14m2 to be more realistic for standard offices. NAO audits carried out in 1999 on public sector buildings suggested 19m2 per person was a realistic density figure.

[edit] Average interval time

Average interval time is the theoretical time for a single lift to complete the cycle of receiving passengers on the ground floor, discharging them on the various upper floors and returning to the ground floor. Where there are banks of lifts, the average interval time is divided by the number of lifts in the group. These calculations are based on averages and not the upper limit. 20 to 30 seconds provides a range from standard to prestige.

Waiting times are not the same as average interval times:

  • 75% car loading of a 30 second average interval time equals 22.5 seconds waiting time.
  • 85% car loading of a 30 second average interval time equals 25.5 second waiting time.

It is waiting times that are the driver of public perception and are of interest to letting agents.

[edit] Car occupancy

Car occupancy is 20% less than the stated capacity. A 16 person car in practice will carry 13 people.

[edit] Staircase allowance

A staircase allowance is suggested by CIBSE, with 50% first floor occupants preferring to use the staircase if in close proximity to a lift.

[edit] Conclusion

Standards and criteria established in the mid-seventies are open to challenge. The Stanhope survey suggested that updated criteria might result in the following:

  • Occupation density: 14m2 per person (nett lettable area).
  • Lunchtime loading:
  1. 5% down, 3% up 12:00 to 12:40 per 5 mins.
  2. 3% down, 3% up 12:40 to 13:20 per 5 mins.
  3. 3% down, 5% up 13:20 to 14:00 per 5 mins.
  • Capacity factor: 60% of car rating.
  • Average waiting time: 25 secs lunchtime.
  • Inter-floor traffic: 1% of design population 12:00 to 14:00 hrs.

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki

[edit] External references

  • Stanhope position paper – Lift performance and specification published February 2004