Principle of ASET and RSET - Coursework Example

Principle of ASET and RSET [Name] [Institutional Affiliation] [Date] Table of Contents Introduction 3 Meaning of ASET and RSET Concepts 3 Means of Escape 4 BS7974 Explanation on Means of Escape 5 Approved Document B1(ADB) with regard to Means of Escape 5 ASET and RSET Calculations 6 ASET Calculations 7 RSET Calculations 7 Reasonable and Tenable Condition 9 Issue of Visibility 10 Occupant Speed Along horizontal Travelling Line 11 Time Response to General Warnings 11 Example on Escape & Evacuation Times for an Hotel 12 Discussion and Analysis 12 Conclusion 13 References 14 Introduction Different legislations and researchers have come up with different explanations and application of ASET and RSET and the concept on means of escape in fire is covered in BS9999, BS7974 and Approved document B (ADB). BS7974 explores the principle of ASET and RSET, BS9999 derives some of the concepts from BS7974 whereas ADB offers some of the building regulations applicable in both UK and Wales on matters to do with providing means of escape when fire arises. Therefore, this report critically reviews the principle of RSET and ASET, the B1 Building Regulations’ requirement, how the ASET and RSET principle is incorporated in the UK’ s standard guidance, related calculations from the BS7974 modeling and discusses and analyses the results obtained from different topics with regards to this research. Further, the report outlines what a tenable and reasonable occupant condition means, elaborates the effect of visibility on the overall escape time and offers typical models or formula to be used in calculation of ASET/RSET. Meaning of ASET and RSET Concepts Available Safe Escape Time(ASET) refers to the available calculated time covering between the moment ignition starts and when the criteria for tenability has been exceeded within a given space in the building (British Standards Institution 2004). Further, Required Safe Escape Time(RSET) represents the available calculated time between the moment ignition starts and when all the occupants in a given place or enclosure can reach the place of safety. In most cases, a place of safety can be temporary and represents an escape route that is protected or simplly a protected compartment. Such a place should ultimately be outside the building and at some safe distance. ASET determination depends on the application of fire modeling or the empirical correlations. This is achieved by first establishing a fire design taking into consideration the types of combustibles available and the product yields that are associated to the situation (both carbon monoxide and primary soot). Further, the fire design already established is given as input to calculation tools such as the fire model so as to establish the duration after fire ignition when the space through which the occupants have to pass becomes untenable as a consequent of heat or smoke presence. RSET covers the alarm time, evacuation delay time or pre-movement time and movement duration (British Standards Institution 2004). Alarm time refers to the first time the occupants become aware of fire either through the manual or automatic fire systems. The pre-movement time covers the time for investigating if the fire is real, collecting the belongings and reaching for friends and relatives and this time varies from one occupancy to another. Finally, movement time entails the duration required by the occupants in order to access the protected exits, whether exterior to the building or an enclosure. Means of Escape The Approved Document B, BS7974 and BS9999 explore the concept of means of escape in different ways. Means of escape covers all the safe routes provided for the occupants to travel from any point within a building to a point of safety. BS7974 Explanation on Means of Escape The general principle for offering safe means of escape is to create sufficient capacity for all the occupants so that they can safely evacuate from a building (BSI 2004, 6). Approved Document B1(ADB) with regard to Means of Escape Any building design must have means of escape in the event of fire. This applies both to new and old buildings. According to the Approved Document B1, all buildings shall be required to be constructed and designed to have sufficient provisions for means of escape and early fire warnings (Great Britain 2007). In order to meet this requirement for means of escape, the escape routes should be of sufficient capacity and number and should be located at a suitable point for easy escape to a point of safety whenever fire arises. Besides, the escape routes should be lit properly and adequately, suitably signed, sufficiently protected from any fire effects and should have appropriate facilities to limit smoke ingress along the escape routes or remove and restrict smoke. All these depend on the purpose for which the building is intended, the height of that building and its size as well. What is more, the means for giving early warnings to the occupants of a building should be sufficient. The means of escape are designed such that, when fire arises, the occupants are capable of escaping to safe areas even without any external assistance. Approved Document B (ADB) stipulates the criteria applied for means of escape. For example, the preliminary principle for such a design is to provide alternative escape means from the building or institution. In case there is no direct escape route or where it is impossible to directly access a safe area, there should be a possibility for access to a relative place of safety like a protected stairway which should be on the path to the exit. The travel distance for the escape means should be reasonable (Great Britain 2007). The document outlines some of the unacceptable means of escape. These include the portable ladder, throw-out ladders, appliances and manipulative apparatus, and lifts (except for the installed evacuation lifts meant for evacuating the disabled people during fire incidents). Although the escalators are used by people who are evacuating, they should not be considered to be of sufficient exit capacity. However, the mechanized walkways can be accepted. The assessment of their capacity relies on their use while in static mode. In some situations, the escape route can be rendered impassable due to smoke, fire or fumes (Billington 2007). Therefore, the occupants should be able to run away from fire to the exit or other protected routes that lead to a safe place. The distance covered by both the protected and unprotected escape routes should be limited in the sense that occupants do not have to travel excessive distances. ASET and RSET Calculations The difference between ASET and RSET gives you the safety margin. Calculations for both ASET and RSET are significant and are broadly outlined in BS7974. The margin of safety is considered in ASET and RSET calculations (British Standards Institution 2004). The expression bellow shows the relation between margin of safety and both ASET and RSET. = ASET = RSET Safety factor should be applied in each calculation in order to take care of some uncertainties. ASET Calculations According to Babrauskas, Fleming & Don Russell (2010), ASET time can be predicted by way of estimating the time-concentration curves for smoke, major toxic products and heat and on how the ASET endpoints for such hazards are derived and estimated. RSET Calculations The time of escape depends on several factors such as fire detection, warnings, and other parameters related to movement and behavour of occupant during evacuation (British Standards Institution 2004). The determination and characterization of the evacuation behaviour of the occupant is broadly categorized into pre-movement behaviors and travel behaviour. Pre-movement behaviours entail occupants’ responses before starting to move to escape routes and cover duration when the occupants are inactive but do not include the duration for movement towards escape routes (Chu, Chen, Sun & Sun 2007). What is more, the travel behavour entails occupant physical movement into and through the escape route. The occupants’ pre-movement and travel behavour can be affected by factors such as the occupants predicting or seeing smoke while evacuating or even being exposed to fire effluent. For the purpose of quantifying RSET, a number of elements are used as shown in the expression bellow : Where; Evacuation time, = the duration between ignition and detection by either the first occupant or the automatic systems. This time depends on the available systems for detecting fire and the existing fire scenario. = the time between detection and the raise of general alarm. This time can range from zero to several minutes. = pre-movement duration The Figure bellow represents a Simplified Schematic Diagram for Processes entailed in escape time as compared to the time available for safe escape. Figure 1: Simplified Schematic for the Processes Involved in the Escape Time in Comparison to Time Available for Escape BS9999 and BS7974 on Fire Safety BS9999 derives some of its principles on BS7974. BS9999 aims to advance the approach between the actual prescription of the design for fire protection and the “full” approach to fire safety engineering where the BS7974 comes in the British Standard. However, both the BS9999 and BS7974 offer technical guidance on matters of fire safety. Reasonable and Tenable Condition The occupant condition depends on a number of factors such as heat and fire, thus affecting RSET and ASET. In the context of design, such factors contribute to both physiological and psychological considerations evaluated while determining the reasonable tenability limits (Spellman & Whiting 2010). When we can predict that the occupant within an enclosure is capable of saving oneself and is not incapacitated by fire, smoke or poisonous fumes, such a condition is said to be both reasonable and tenable. However, the conditions in any building enclosure are considered untenable after the endpoint for ASET calculation. Further, an untenable condition results when there exists a prediction that an occupant who is entering an enclosure or who is inside an enclosure is not likely to save oneself or has effectively been incapacitated to save oneself due to the nature of the effect emanating from exposure to heat, smoke or any toxic effluent from the fire. The sample criteria given for determination of tenability and ASET outlines that temperature has to remain less than 65oC, visibility remain above 10m and concentration of carbon monoxide should be bellow 1,400 ppm. In other words, if the escape condition is not conducive or too onerous, then the occupant condition in untenable and unreasonable whereas for a tenable occupant condition, the situation is not onerous. Issue of Visibility Visibility is something to worry about and can affect ASET. The occupants’ decision will largely be influenced by visibility due to fear of the suffocating and poisonous smoke. BS9794 assumes that the occupants cannot use an escape route whose visibility is not more than three meters with an extinction coefficient of about 0.76 (BSI 2004, 15). However, if occupants get into escape routes with this optical density, then their progress ability relies on both smoke irritancy and optical density. BS7974 approximates that when visibility is not more than 10 m, most occupants would not prefer to use that exit (British Standards Institution 2004, p. 15). Occupant Speed Along horizontal Travelling Line The expression bellow is used for calculation of the occupant’s speed along a horizontal travelling line (Anderson 2009). S = occupant speed along the travelling line k= 1.4 for the case of horizontal travel a=0.266 D = the density of the occupants (persons)/ square meters Time Response to General Warnings Mostly, the systems in place determine the response time to a given warning. In cases where there are automatic systems in place that can trigger immediate alarm, then the warning time design is zero. Where there are automatic systems for detecting fire and sending a pre-alarm to management or to the security, the minimum of 10 minutes is considered as warning time. However, where we do not have the automatic warning systems, the warning time is likely to take long or be unpredictable as per the actions of the occupants available (Gwynne, Purser & Boswell 2011). Thus, the example bellow illustrates the nature of response exhibited in large and complex buildings such as an hotel. Example on Escape & Evacuation Times for an Hotel An hotel presents an example for sleeping where the occupants are likely to be unfamiliar with the systems and the building. The hotels are large and their layouts are complex. Further, the occupants are likely to posses the feeling of irresponsibility to the alarms and announcements. The occupants might ignore large fires, especially when they feel not to be threatened directly. Therefore, due to these reasons, it is not possible to achieve an efficient and rapid evacuation system even where the systems for efficient fire management are in place and the staff properly trained. The alarms may not be relied upon to raise triggers for evacuation. In such a situation, personal addresses or voice alarms are considered most appropriate for initiating an evacuation. The calculation of escape times and evacuation time may not be effective and ranges from 40-60 minutes depending on the situation at hand. Discussion and Analysis The rules of RSET and ASET demonstrate that the process of evacuation is complex and depends on specific factors with regard to different buildings and their occupants (Poon 2014). Such a performance-based approach is useful because it allows factors to do with escape time and fire hazard to be incorporated and that the effect of alteration of such factors can be determined. In the analysis of ASET/RSET, designers must take into consideration a number of systems. For example, the value of ASET increases if the combustibles are limited. It can also be increased by provision of sufficient separation distance between the packages for fuel, providing fire suppression systems that are sufficient and providing passive or active systems meant to control smoke. Considering the RSET, the following the possible outcomes: The detention time can be lowered by strategically placing the smoke detectors. Pre-movement time can be lowered by introducing an occupant system for notifications, especially one that is capable of communicating live messages. The movement time can also be lowered if the exit signage is positioned and arranged strategically. Proper positioning of signs prevents the pinch points and unnecessary queuing (Chitty& Fraser-Mitchell 2003). Although the report outlines the role played by the principle of ASET and RSET, a lot needs to be done to sensitize the occupants about how to use the escape routes upon the raising of an firm alarm. The complexity of the escape route depends on the nature of the building or enclosure and the level of knowledge the occupants have about fire alarms. Conclusion This research has covered how different legislations and researchers explain the concept of ASET and RSET. The concept on means of escape is covered in BS9999, BS7974 and Approved document B (ADB). Whereas BS7974 uses the principle of ASET and RSET, BS9999 derives some of the concepts form BS7974.Further, ADB offers some of the building regulations applicable in both UK and Wales on matters to do with providing means of escape when fire arises. The report has also outlined what a tenable and reasonable occupant conditions means, elaborated the effect of visibility on the overall escape time and offerred typical models for calculation of ASET/RSET. References Top of Form Bottom of Form Top of Form Bottom of Form Top of Form Bottom of Form Top of Form Bottom of Form Top of Form Bottom of Form Top of Form Bottom of Form Top of Form Bottom of Form Top of Form Bottom of Form Top of Form Top of Form Top of Form Bottom of Form Bottom of Form Top of Form Top of Form Top of Form Top of Form ANDERSON, H. (2009). Fire safety. Chandni Chowk, Delhi, Global Media. http://search.ebscohost.com/login.aspx?direct=true&scope=site&db=nlebk&db=nlabk&AN=269869. BABRAUSKAS, V., FLEMING, J. M., & DON RUSSELL, B. (2010). RSET/ASET, a flawed concept for fire safety assessment. Fire and Materials. 34, 341-355. BILLINGTON, M. J. (2007). Means of Escape from Fire. Oxford, John Wiley & Sons. http://www.123library.org/book_details/?id=28235. BRITISH STANDARDS INSTITUTION. (2004). Application of fire safety engineering principles to the design of buildings. Part 6. London, BSI. CHITTY, R., & FRASER-MITCHELL, J. (2003). Fire safety engineering: a reference guide. London, BRE Bookshop. CHU, G., CHEN, T., SUN, Z., & SUN, J. (2007). Probabilistic risk assessment for evacuees in building fires. Building and Environment. 42, 1283-1290. GREAT BRITAIN. (2007). Approved document B. London, NBS/RIBA Publishing. GWYNNE, S., PURSER, D., & BOSWELL, D. (2011). Pre-Warning Staff Delay: A Forgotten Component in ASET/RSET Calculations. POON, S. (2014). A Dynamic Approach to ASET/RSET Assessment in Performance based Design. Procedia Engineering. 71, 173-181. SPELLMAN, F. R., & WHITING, N. E. (2010). The handbook of safety engineering: principles & applications. Lanham, Md, Government Institutes. Bottom of Form Bottom of Form Bottom of Form Bottom of Form Bottom of Form Read More