WSP’s strategy to safely evacuate the building using the existing passenger lift system took 18 months to develop and gain the approval of City of London building control, the London Fire Brigade and the London Fire Brigade’s fire engineering team.
The evacuation system makes use of the existing building and vertical transportation design which features 24 double-decker lifts – the equivalent of 48 conventional lifts. The building is split into four zones, separated by ‘unbreachable slabs’ - fire-hardened slabs with a two-hour fire rating (vertically and horizontally). Each of the four zones has its own dedicated bank of lifts, with the double-decker lifts serving two levels. The two top floors of each zone just beneath the slab are the designated evacuation floors, which have protected transfer corridors linking the staircase to the lift lobby.
If a fire were detected, people would be evacuated from the fire floor and the floor above, and directed down the fire escape stairs to re-enter the top two floors in the zone below, directly under the ‘unbreachable slab’. Here, the passenger lifts would automatically turn into fire evacuation mode, travelling continuously between the ground floor and the zone where evacuees are waiting. With each bank of lifts having eight double-decker lifts, and each cabin holding 20 people, 320 people at a time could be evacuated.
Travelling at 6-8m per second, the lifts would move between the evacuation and ground floors in a random pattern, thus avoiding the creation of a piston effect, which would create pressure movements in the building and damage lift mechanisms. To save time, the lift doors are designed to close automatically like a train or “London tube” door rather than opening every time an obstruction is detected.
Fire-protected power supplies to the lifts are located on opposite sides of the building and linked so if one failed the lifts would still have a supply. Six backup generators are arranged in fire-separated banks of two units, each with their own dedicated fuel supply.
The lift evacuation strategy integrates with the fire strategy for the whole building, which includes compartmentation of the building, sprinklers, smoke and fire detection alarm systems, all designed to contain the fire within one zone. CCTV in the stairs and at the lift lobbies allow the fire control centre to observe and direct evacuees using the PA system.
There are also two fire lifts to be used initially by the building’s fire wardens to travel up to the site of the fire and evacuation zones to direct people down the stairs to the lifts below and evacuate anyone unable to use the stairs via the fire lifts before the fire brigade arrives to take over.