Octavo grado

The evacuation of people from commercial buildings is a concern as long as there are emergencies of all kinds. In many countries, including Australia, performance-based fire safety engineering models are built to allow a flexible approach to building design. However, the problem is finding the quickest way of evacuating people from a commercial building be it a hotel, airport or shopping centre. There are computer-based models. In research undertaken by Owen M, Galea E, and Lawrence P (1996), the EXODUS model simulates escape patterns of occupants and provides valuable information for the design of circulation paths and exits. Lo, Huang, and Yuen (2007) look at the mathematical methodology of the evacuation process. The model that is discussed is confined to simply one level exit or entry. Difficulties arise when there are multi-entries and exits on a number of levels. This can relate to enclosed shopping centres that are located in central business districts and high-density areas such as Hong Kong. However, many enclosed shopping centres are being built in major Australian cities that are multi-storey or close to other commercial buildings.

Building exits and pedestrian flow

The issue of building exits and the flow of pedestrians from a commercial building is raised in the paper by Zhao, Yang, Li, Zhu, and Zou (2006). The issue is that when the evacuation of a building is required, what is the relationship between pedestrian flow and the exit structure? The writers put forward the

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theory that the movement of pedestrians is not unlike the behaviour of granular material that can exhibit both solid-like and fluid-like behaviour. These peculiar properties give rise to at least three important „states‟ in granular flows; namely, dilute flow, dense flow, and the jammed state. The phenomenon of crowding can be considered as a transition from dilute to dense flows and that of jamming is a transition from dense flow to a jammed state. Movement during an occupant evacuation can also be considered as a kind of discrete flow. Especially when the density of occupants is high, people in the pedestrian flow can do nothing but go with the crowd. Their walking speed is constrained by the other people in the flow; individual ability and psychology play a reduced role in their movements and the occupants act just like granular material. The state of the pedestrian flows can be dilute, dense, and jammed. In dilute flow, occupants can walk at their expected speed, and their individual ability and psychology make individual behaviour different from each other. In dense flow, occupants act more like granular material and their walking speeds are restricted by the other occupants. Of greatest interest is the dilute-to- dense transition in pedestrian flow and the relation between the exit design and this state of transition. The pedestrian flow in evacuation has essential differences from the flows of inanimate granular material according to Zhao, Yang, Li, Zhu, and Zou (2006). The walking speed of occupants is confined to a relatively small range while particle velocity can be increased endlessly in gravity or some other fields. The forces in granular flow are gravity, friction, etc., while pedestrian flow may be more complex because of various „social forces‟ which can be simulated by physical attributes such as position attraction, attraction and repulsive force caused by nearby occupants, attraction of movement direction, and repulsive force caused by fire. The interactions among the occupants in pedestrian flow are more complex than that among inanimate particles. In the case of granular flow, there are only collision, extrusion and so on, complying with the deterministic physical rules. On the contrary, different psychologies of occupants will arouse different behaviour: on the one hand, all occupants try to avoid colliding with each other; on the other hand, occasionally they want to gather together with their relatives.

Behaviour is also different at a bottleneck. Grains can pile up around the bottleneck. When they pile up to a certain degree, phenomena similar to „avalanches‟ occur; namely, a large quantity of grains flows through the exits suddenly. If the occupants pile around the bottleneck there can be potentially dangerous phenomena such as crushing and trampling. But sometimes occupants queue for evacuation at the bottleneck or move up and down uneasily, especially in an evacuation with the intervention of staff, the avalanche phenomenon will seldom happen. Gwynne, Kuligowski, Kratchman, Milke (2008) discuss exit width in their article. The size of the exits can have a profound effect on the number of people passing both into and out of a commercial building. The ideal situation is to get the maximum size exit without having undue influence on the buildings‟ services such as the air

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conditioning systems. If they are too narrow, they hinder the movement of people. Figure 3.1 is used in computer simulation models to ascertain the movement of people out of a building. It is assumed that in an emergency, inward movement would be confined to emergency personnel only and that other people movement will be restricted.

Figure 3.1 Computer simulation flow chart

The computer model should show the most efficient way to move the people from the building in an emergency. This is conditional of the variables as stated above in Figure 3.1 being correct and as close as possible to reality. However, with all computer simulations, various scenarios are undertaken simultaneously according to Owen M, Galea E, and Lawrence P (1996).