Publication Type: Conference Paper
Source: Pedestrian and Evacuation Dynamics PED 2012 (2012)
The counterflow condition on the escape stairs is a situation that may occur during the evacuation from buildings when the downward flow of evacuating occupants passes the upward flow of firefighters heading to the fire floor. It does not always take place because in majority of lower buildings evacuation starts and ends before fire brigade arrives on the scene. In case of taller buildings the conflict between the two movements is more likely to happen due to certain factors: the firefighting and evacuation strategy adopted for the building, the use and purpose of the building, the number of staircases and sizing of the stair, fire warning systems available, the quality of management and staff training, the availability of firefighters lifts and the distance to the fire station.
The evacuation strategy in taller buildings is usually either simultaneous or phased. In case of simultaneous evacuation the counterflow condition is more likely in case of long pre-movement times that are usually the result of inadequate warning systems or poor training. In this case the movements are more likely to coincide. An important issue here is the sizing of the stairs and the philosophy here varies slightly from country to country. For example in UK the sizing of the stairs for simultaneous evacuation is directly related to the whole number of occupants in the building which implies that the total number of people must have enough standing space within the staircase. In most countries however stairs are sized to the highest floor population based on assumption that floors are well compartmented and only the fire floor and the one above must be evacuated immediately while other floors will have more time. This concept is quite close to the phased evacuation which allows narrower stairs, however phased evacuation is usually gradual and controlled by warning system, internal communication, well trained staff and the fire control centre. In case of phased evacuation, due to a delayed evacuation of some floors, the likelihood and the impact of counterflow increases.
In British regulations (ADB) this is acknowledged by a requirement for checking whether the effect of fire fighters on phased evacuation is possible. This should be taken into account while deciding for the number of staircases in the building assuming that one may be discounted due to fire brigade activities. In practice the most likely occasions where counterflow is expected are tall offices, hotels and the buildings that are not tall enough to have firefighting lifts while having significant population. The problem of counterflow can be even bigger in cases where disabled people are evacuated.
The aim of this work is to study the effects of counterflow in connection with the requitements of regulations and most common evacuation strategies. The second aim is to provide data for numerical modeling and calculations. To achieve these goals a series of experiments was conducted on a staircase with and without a simultaneous counter flow of ascending firemen. Experiments took place in a typical representative staircase of an office building. The effect of the fire brigade intervention on evacuation movement on stairs was examined for several different initial densities. 78 people were involved the experiments – 73 as evacuees and 5 as firefighters. In every run the same number of students was involved but to simulate different floor densities, they were placed on different segment of the stair case. The lower floor density was applied the more levels of the staircase were occupied. The start of every run was signalized by loud whistle audible for all people on the stair case. The evacuees were asked to move calmly and not to run downstairs. They were also asked to move all at once at the start of every run to keep the floor density from the beginning of the run to the moment when they meet firefighters.
For data collection a full measuring installation was prepared. It was composed of two IR directional sensors (one for every exit door) connected with the counting unit to the computer with software. The sensors were mounted on the top of both door openings and they were giving impulse to the counting unit any time when the person was passing the door in the outside direction. The sensors were calibrated to measure anybody who is at least 140 cm tall and they should recognize every person even in high floor density. The calculating unit was set up to give information to the computer about number of evacuated people in one second time steps. Additionally video cameras were used. One was placed on first floor which was the best place to observe the influence of firefighters movement on the evacuation process. The second video camera was used by one of the firefighters to record the fireman‘s perspective during the counter flow movement.
The results and discussion will be presented in a paper. A factor accounting for the counter flow is proposed for egress stair sizing calculations in cases where counter flow is identified as a potential problem. The experimental results are compared to simulations performed with current popular evacuation packages: EVAC, STEPS, Building EXODUS, Pathfinder. Various aspects of software application, necessary adjustments and the implications for evacuation studies in the context of fire safety will be discussed.