Fire Technology
Subtitle
Passive safety systems are part of the building structure and can include, but not restricted to, Fire walls,Fire Doors, Smoke doors, egress routes, compartmentation, fire service access and surface finish selection. (Ref MS Schedule 3.1).
Fire Walls. These have Fire resistance properties that have a Fire Resistance Level (FRL), able to resist the passage of flames from one compartment to another. Fire Doors. Openings in fire walls having an FRL to maintain the FRL of that wall. Marked internally and Externally with "FIRE SAFETY DOOR DO NOT OBSTRUCT". Also to open have a mechanism that must be a single downward action, capable of being activated with a single hand without the use of a key. (BCA D2.21, D2.23 and AS1851). Smoke Stop Doors. Resist the passage of smoke from one compartment to another. Marked internally and externally with "SMOKE STOP DOOR DO NOT OBSTRUCT" (BCA2.23). Example of a damaged Smoke Seal) Therefore it is vitally important that these doors are remained closed and latched shut at all times, not held open with items such as wedges and fire extinguishers. They can, however be fitted with an automatic door closing mechanism that releases the door in the event of a fire alarm activating. (References- BCA General provisions and International Fire Engineering Guidelines). (Typical (Automatic door closing mechanism) Green and White Diagonal Stripes. Fire Exit Sign Writing. Exit Sign and Green Strobe light. See Specifications (Typical shopping centre doors SA)
All FRLs are expressed in this order ie, 60/120/90 (60 mins structural Adequacy. 120 Mins Integrity and 90 mins Insulation). A dash would mean that there is not requirement for that critera ie fire doors would be expressed -/60/60 as there is not a requirement for Structural Adequacy. Metal tags should be affixed for the door set (Ref BCA D2.20 C3.15 AS1851 & 1905). 
door set metal tags) 
An exit must not be blocked at the point of discharge. If the exit leads to an open space or road the unobstructed width must not be less than the minimum width required or 1 m whichever is the greater. (BCA D1.10 & D1.6, MS Schedule 3.2).
The International Fire Engineering Guidelines (IFEG) define untenable conditions by using the following critera.
1. The height of the Smoke layer.
2. The toxicity of smoke.
3. The temprature of the smoke.
4 The radiant heat.
5. Direct Contact.
Untenable conditions occur when the smoke layer is greater than 2.1 m above floor level the smoke temperature is greater than 200C and rhe radiant heat is 2.5kW/m2.
Or when the smoke is lower than 2.1 m above the floor the temperature is more than 60 to 100C.
Options available to manage smoke.
1. Smoke dilution. Extraction systems, Pressurisation systems, Natural ventilation, Sprinklers?
.
2. Containment systems. Baffles, Smoke Curtains, Doors, Windows.
Powered Smoke extraction.
The advantages of these systems is that they remove the many toxic gasses and hot smoke avoiding flashover and preventing smoke spread into other zones thus attaining a smoke free zone which allows a greater egress time for occupants. The zone is then kept within tenable conditions that also give more time for the fire service to intervene. This also prevents widespread property damage. Smoke extraction systems can also be placed away from the affected area connected by ducting avoiding the risk of burning the supply fan, wiring etc. The main disadvantages of these systems include the location and type of detector with respect to the exhaust inlet. The noise that is produced can prove very frightening for the general public. Designers also have the added problem of avoiding plugholing. Once the smoke is outside where is it going to go? Back inside, or even into another building via the air conditioning systems. Once this system is in place it requires an ongoing maintenance programme which is very costly.
Plugholing explained. This Phenomenon can occur when the supply fan is too powerfull causing not only smoke to enter the exhaust but air as well, if there is an excess of air a plugholing effect is created.
Natural Ventilation Systems.
The advantages are permanently opened vents or opening on alarm discharging smoke to the outside. This system requires low maintenance. However the disadvantages out weigh the advantages. This system is affected by the wind the temperature of the smoke and the discharge point. Other considerations would be the external wind speed and direction entering through the openings and disturbing the smoke layer. 
Barriers / Smoke Curtains. Advantages of barriers / Curtains etc is that they can divide large spaces at high level these barriers are normaly made of glass. They can also work in conjunction with the smoke extraction systems either the powered or the natural openings. However the disadvantages a building with a low ceiling height barriers would not be an ideal solution due to the limited height available. (An example of glass smoke barriers working in conjunction with a smoke extraction system above a 2nd floor atrium in a shopping centreStair Pressurisation systems.
Fans are activated to increase the pressure in the stairwell to greater than the surrounding floors. If a fire door from a floor is opened the pressurised air from the stairwell will rush to the opening so stop smoke from entering the stairwell. Systems must be designed so that the air introduced to the stairwells will be able to flow to the most remote doors without creating an excessive pressure differential across any exit door. Pressure must be controlled so that the force at the door handle to open any door does not exceed 100N (Approx 10Kg) and doors are not prevented from closing and latching. Pressures must be restored within 10 seconds after the doors are opened and closed. Noise levels in the stairwell should also be considered.
(Please note it is standard practice to evacuate a multi storey building fitted with an Emergency Warning Intercommunication System (EWIS) and having an effective Emergency Control Organisation (ECO) in place, by the affected floor, 2 above and 1 below. This practice avoids conjestion in the stairwells and allows the affected floor to egress first). (Ref AS 3745 & AS 1670).
Compartment Pressurisation systems.
Maintaining pressure and high noise levels could provide difficulties when using this system.The advantages to system is very simular to the powered extraction system however they create a high pressure than that of adjacent spaces. They need three main components.
1. A supply fan. This pressurises the area.
2. A pressure relief to avoid over pressure.
3. An air release to release the smoke and hot gasses.
The main disadvantages of these systems are that to maintain the pressure difference would be fairly hard to control in an area resembling for example a shopping mall , due to the amount of openings and larger areas may need higher pressures to work effectively. Other considerations would be the noise produced, how and when the system is going to start (the type and location of detectors).
All these systems have various considerations as far as impact on the spread of smoke the height of the smoke layer, the development and the temperatures produced. The wrong design affects ASET (Available Safe Egress Time) which impacts on egress routes and alternative building solutions. Can we guarantee how may people will be in a zone at any one time? Is everyone going to travel at 2.5m/s? Can we also assume that up to 20% are mobility impaired? Are all the smoke management systems are working correctly? These questions will have a major impact on the movement of people. The smoke management system must be able to maintain the base of the smoke layer at the design height to ensure a safe evacuation of people. To design a smoke management system to mantain the smoke layer at a height which is tenable we need to know how much smoke is produced. The smoke produced is a product of: The size and the location of the fire, ie is it in the centre of the room, in a small room, or against a wall etc. All these factors influnces the the volume of the smoke produced. We can find this out by using mathmatical equations. The flow of smoke into a plume is the "Volumetric" flow rate described as (m3/s) or "mass flow rate" (Kg/s). other equations allow the mass flow rate to be converted into a Volumetric flow rate. The picture below demonstrates how quick smoke can build up after only a few minutes. Smoke travels at appros 2ms 5mph faster than an average human walking speed. A simple event line that illustrates the development of a building fire, that also demonstrates the relationship between Passive, Active systems and Wardens the reduction in tenability and the ability of the occupants to escape.
