Tenability in stairwell of highrise office buildings.

Abstract

The commonly adopted fire evacuation strategy for office buildings is total evacuation which involves simultaneous evacuation of all building occupants upon fire alarm activation. Total evacuation for building with high occupancy load will cause congestion within stairwell which often results in long queuing time at the stairwell door. Long queuing time on the fire floor causes the stairwell door to remain open for extensive period, and the smoke from the fire floor will enter the stairwell resulting in untenable conditions within the stairwell. This can have devastating effects on egressing occupants. This research utilises the state-of-the-art modelling tools such as FDS and FDS+Evac to study high-rise office building with two means of escape. The aim was to demonstrate that for certain building parameters, phased evacuation should be adopted instead of total evacuation to ensure an acceptable tenability level within the stairwells which permits safe evacuation of the occupants. A total of 48 unique simulations were identified based on varying building parameters such as floor area, building height, fire protection system, evacuation strategy and ceiling height. Relevant New Zealand Building Code (NZBC) Compliance Documents such as C/AS5, C/VM2 and D1/AS1 were used as guidelines to define the various modelling parameters such as fire growth and combustion characteristics, fire safety systems, modelling rules, evacuation parameters and geometries to ensure the modelled building achieves the minimum NZBC requirements. The use of FDS+Evac for simulating evacuation timings without the effect of smoke was validated to some extent against hydraulic models and relevant trial evacuation experiments found in the literature. The results demonstrate that for high-rise office buildings up to 20 storeys with floor area not exceeding 510 m² served by either conventional or scissor stairwell, the tenability within the stairwell can be maintained during total evacuation by having at least a Type 6 automatic fire sprinkler system. For high-rise office buildings between 10 to 20 storeys with floor area of 5000 m² or 2450 m² served by either conventional or scissor stairwell, those buildings are required be protected by Type 7 automatic fire sprinkler system with smoke detection and phased evacuation are also required to maintain tenability within the stairwell. The effect of stair arrangement on the tenability in the stairwell is more evident in highrise office buildings with high occupancy load where scissor stairwell is found to be worse than conventional stairwell. This is due to the nature of scissor stair arrangement which forces smoke to flow up the stairwell in a corkscrew manner, concentrating smoke along a specific path. This phenomenon is found to be detrimental to the tenability conditions in the stairwell.