ASET and RSET Principles and Their Incorporation - Case Study Example

Just let me know if I need to change or add anything. Send me a message. Sorry for the delay. Thanks! Design Project Management Critical Review of ASET and RSET Table of Contents Contents Contents 2 1. Introduction Fire safety is critical requirement of UK’s building regulation thus there a number of standards and guiding documents for this purpose. In particular, Approved Document B (Fire Safety) has been published as a prescriptive guide in design and construction of buildings in relation to different aspect of fire safety such as means of escape, fire alarm and detection systems, fire-resisting constructions, and others. Similarly, although more technical in nature, BS 7974 and BS 9999 introduced fire safety through engineering methods such performance-based design using ASET or Available Safe Escape Time and RSET or Required Safe Escape Time that are dependent on existing knowledge of human behaviour in fires. The following sections critical review the principles behind ASET and RSET and its applicability to UK standard guiding documents for fire safety. It also shows some sample calculations how such performance-based fire safety design is done and issues surrounding the applicability of pre-movement time, tenability conditions, and visibility distance. 2. Critical Review of ASET and RSET Principles and its incorporation in standard UK guidance In BS 7974, travel time in terms of escape is considered as the time required for a person to reach a place of safety from a certain part of the building. Similarly, travel distance is considered the actual distance (measured from a certain part of the building to the nearest exit) that a person needs to travel in order to reach a place of safety. The difference however is the fact that consideration given to travel distance includes concerns over the integrity and layout of walls, partitions, and fittings (BS 7974, 2004, p.5). These principles applied on travel distance and time led to the development of performance-based design or FSE where ASET or Available Safe Escape Time and RSET or Required Safe Escape Time determines the possibility of escape during a fire. The principle behind ASET and RSET is good us it is generally intended to facilitate smooth evacuation as well as getting the most number of people out of the building in case of fire. For instance, margin of safety is calculated using difference between the available time (ASET) and required time (RSET) thus tmargin = t ASET – tRSET. In this equation, RSET is the product of different safety factors combined such as time from ignition to detection, time from detection to first alarm, occupants pre-movement time, and travel time as shown in Equation 1 below (BS 7974, 2004, p.7). Evidently, the margin of safety incorporates critical scenarios as well as the impact of unique human behaviours in fire Equation 1: tRSET = ∆tdet + ∆ta + (∆tpre + ∆ttrav) The above equation suggest that the time of detection (detected by automatic detection system or individual who saw the fire) or ∆tdet is added to the time when the general alarm is heard or ∆ta and therefore the total time before an individual decide and move towards place of safety. Similarly, the total between ∆tpre or pre-movement time (see Table 1) and ∆ttrav or travel time is considered because of the reality that not all people move towards the exit immediately after hearing the alarm particularly those that are in groups (human behaviour in fires). Clearly, BS 7974 recognised the fact that escape time depends on how individual will react and move towards the place of safety. For instance, in ∆ttrav or travel time, the average time for an individual to “walk” towards the exit is considered along with “flow time” or the rate of occupants flow through escape routes and exits determined by escape width and exits’ sizes (BS 7974, 2004, p.7). Behavioural scenarios play an important roles in BS 7974’s calculation of pre-movement and travel times. Table 1 shows how building occupants’ reaction to fire is influenced by different individual behaviours and environmental factors. Table 1 - Behavioural Scenarios (BS 7974, 2004) Category Occupant Alertness Occupant Familiarity with building Occupant Density Complexity of Enclosure Occupancy Types A Awake Familiar Low One or many Office or industrial B1 B2 Awake Awake Unfamiliar Unfamiliar High High One or few One with focal point Shop, restaurant, circulation spaces, Cinema, theatre Ci Cii Asleep Long term; individual occupancy Familiar Low Few Dwelling without 24 hours on site management Ciii Asleep Unfamiliar Low Many Hotel, Hostel D Medical care Unfamiliar Low Many Residential (institutional) E Transportation Unfamiliar High Many Railway station/Airport The above table suggest that it is entirely based on occupancy characteristics from some pre-defined building types which in reality may not be entirely applicable. For instance, buildings are not exactly the same while building occupants’ characteristics varies greatly in terms of culture, condition, and situation at the time of fire occurrence. For instance, it is important to note that when calculating RSET and ASET, it is important to be clear which part of the building is being considered and other factors affecting pre-movement time, speed of movement, movement on stairs and others. For instance, older people’s walking time may be lower than defined while movement of people in stairs is entirely different from horizontal corridors. Moreover, the effect of density-speed relationship in a certain point in time must be consider in calculating RSET and ASET otherwise it will yield incorrect results (NHS, 2009, p.30). Other possible scenario that may not be considered in developing RSET and ASET is the strong possibility that some people may return and run in the opposite direction when they realise that something valuable is left behind. Similarly, family members who already made the escape will try to rescue other family members and take the same escape route where other occupants are escaping. Approved Document B or the standard UK guidance on fire safety does not include ASET or RSET but it is recommending reference to BS 7974 in the design and assessment of fire safety measures in buildings (Approved Document B, 2007, p.14). In essence, the UK standard guiding document for fire safety recognized the usability and potential of both ASET and RSET in performance-based design and ensuring life safety. However, due to issues involving pre-movement time and occupancy characteristics, incorporation of ASET and RSET in the guiding document seems disadvantageous as the will affect its prescriptive purpose wherein means of escape is mostly dependent on fire-resisting design and construction of escape routes and alternative means of escape (Approved Document B, 2007, p. 15). 3. Means of Escape in ADB, BS 9999, and BS 7974 In Approved Document B, the term means of escape actually include both warning and ways to escape from fire. By definition, it is way occupants of a certain building can be notified and get out of the building using a safe route to a place of safety during a fire (Approved Document B, 2007, p.15). Similarly, BS 7974 (2004) consider means of escape as a safe routes where occupants can travel to a place of safety (p.4). Although not directly defined, means of escape is a combination of safe escape routes, emergency lighting, final exits, passageways, and other elements that can help occupants reach a place of safety (BS 9999, 2008, p.12). The “place of safety” on the other hand for all three documents is an open space outside the building (BS 9999, 2008, p.12), a place where there is no immediate danger (BS 7974, 2004, p.4), and free of the harmful effects of fire (Approved Document B, 2007, p.16). 4. Reasonable and Tenable Condition The idea behind for both ASET and RSET are based on the premise that occupants must evacuate before the building becomes “untenable” due to fire and smoke (BS 9999, 2008, p.21). This is also why the endpoint of ASET calculations corresponds to the time when the enclosure becomes untenable or the time that it’s too late from occupants to save their lives as shown in Figure 1 below (BS 7974, 2004, p.14). A reasonable condition therefore is the opposite or when time is still available for occupants to escape safely out of the building. Figure 1 – ASET and RSET Diagram BS 7974 (2004) is also clear that the tenability criteria is dependent on several factors such as the visual appearance of fire to occupants, location of escape routes in relation to occupants’ position in the building, exposure to radiant heat, and the willingness of occupants to go through an escape route filled with flames, smoke or toxic gases (p.17). This is because BS 7974 (2004) assumes that exposure to these elements have some behaviour modifying effects such fear of seeing smoke, impaired vision and irritation of the eyes, breathing difficulties, and others (p.14). However, it is important to note that unlike prescriptive guidance such as Approved Document B, this reasonable and tenable condition of BS 7974 does not include consideration on alternative means of escape as evidenced by the assumption that an escape route with flames and smoke signal tenability limit, beyond margin of safety, and the end point of ASET. In contrast, Approved Document B recognized the aside from the reality that fires do not start in two different places at the same time, occupants who cannot use the primary escape route can either go to a place of relative safety such as protected stairways or use an alternative route leading to the outside of the building (Approved Document B, 2007, p.16). In other words, such lost of means of escape or margin of safety does not necessarily mean “untenable” or inability to save one’s life as by nature, people have tendencies to escape by other means. 5. Pre-Movement Time and Visibility Issues Pre-movement time is generally defined as the time before occupants move towards an exit (BS 7974, 2004, p.4). As mentioned earlier, RSET is computed using pre-movement time which is dependent on occupants’ pre-movement behaviour such as periods of inactivity after being notified or being aware of the fire and the longest part of the total escape time (BS 7974, 2004, p.6). Similarly, BS 9999 (2008) recognise pre-movement time as the first move of occupants towards a place of safety (p.17). There is pre-movement time in Approved Document B (2007) because it is more focus on the alternative means of escape and fire-resisting design of the internal structure rather than computing the available and the required escape time (p.16). The problem in using pre-movement time as indicator of safe escape is the fact that it’s dependent on both occupancy characteristics and individual behaviour. For instance, not all occupants move at the same time, not all will respond quickly to the general alarm while their recognition of danger and necessity to escape varies considerably (BS 7974, 2004, p.7). Visibility is more identified with tenability and ability of occupants to move through smoke in both BS 9999 and BS7974. The issue however is that visibility also determines ASET and RSET as hesitations or unwillingness of occupants to travel towards an exit determines “untenability” and chances of occupants to survive the fire (BS 9999, 2008, p.20; BS 7974, 2008, p.15). Note that visibility necessarily determines the margin of safety as shown in Figure 1 and therefore critical in determining the total escape time. For instance, setting visibility limit to 3 metres will put occupants into a dangerous situation because of toxic gases while increasing visibility to 10 will keep them away from toxic effects. According to the European Guidelines which is somewhat similar to BS 7974 but more prescriptive in nature, smoke and other gases reduced visibility and cause irritancy in the eye and respiratory system. The distance of an occupant escaping from smoke and toxic gases is therefore important particularly in determining the possibility of finding a safe place, flow speed, and likelihood of using the exit on the other side of the smoke (CFPA-E,2009, p.15) 6. Example calculations Determining RSET or Required Safe Escape Time for building occupants of say a 10-storey modern hotel with sophisticated alarm and detection, fire suppression, and escape routes using the equation below will yield a different result when applied to a building with lower variations in occupancy characteristics. It assumed that the individual involved is fully awake, no disability, and physically fit to walk down from the 10th floor to the place of safety in front of the hotel at the ground floor. tRSET = ∆tdet + ∆ta + (∆tpre + ∆ttrav) where; ∆tdet = 8 minutes (ignition to detection) ∆ta = 3 minutes (detection to alarm) ∆tpre = 5 minutes (recognition + response time) ∆ttrav = 33 minutes (Walking time using 1.2 m/s from CFPA-E (2009) Table of Smoke Free escape routes) Thus; tRSET = ∆tdet + ∆ta + (∆tpre + ∆ttrav) tRSET = 8 + 33 + (12) tRSET = 41 + (12) tRSET = 53 minutes This result suggests that the individual mentioned earlier must make it to the place of safety 53 minutes max. The ASET value therefore will be larger as shown in Figure 1 as it includes the margin of safety as tmargin = tASET – tRSET. However, calculating ASET according to CFPA-E Guideline include consideration on the nature of the fire as combustion products will make it difficult to escape (CFPA-E, 2009, p.9). Therefore, ASET time should include detailed knowledge of fire from ignition to development and the hazardous conditions. ASET = tHAZ - tDET Where; tHAZ = time onset of hazard tDET = time of detection therefore; if tHAZ occurs after RSET or 53 minutes then the remaining ASET for other occupants in this scenario will be: ASET = 53 – 8 ASET= 45 minutes The margin of safety on the other hand will be lower as time pass by : tmargin = tASET – tRSET tmargin = 45 – 53 tmargin = -8 (minutes late) This calculation suggests that all occupants of the building must reach a place of safety within 45 minutes because a 53 minutes RSET will only save a few individual. 7. Discussion and analysis Analysis of ASET and RSET particularly in providing safe means of escape through time calculations and within margin of safety suggests that such approach is entirely dependent on the behaviour of building occupants. The pre-movement time as shown In Figure 1 suggest that the longer it takes for occupants to start and move towards a place of safety result to decrease travel time. Similarly, recognition time affects pre-movement and subsequent travel. In general, any changes in time from detection, alarm, recognition, response, and travel time affects the margin of safety or the chances of occupants to survive the fire. The mere fact that ASET and RSET are dependent on occupancy characteristic makes the margin of safety variable and inconsistent and cannot be applied in all situations. For instance, if the occupants are all male, the likelihood of longer travel time is less as males generally move quicker. According to BS 7974, there is a greater degree of interaction between pre-movement times, walking times, and exit flow times because alertness, walking or movement speed, and size of exit in relation to density are mutually dependent (BS 7974, 2004, p.12). However, although ASET and RSET takes into account behavioural modifiers and scenarios, there is no indication that it consider the possibility (actually happening in real life) of people’s varying circumstances. For instance, not all occupants leave immediately upon hearing the fire alarm and most of them will assess the situation first or wait for further warning. This is from the fact that most people do not generally want to abandon their homes or offices if no significant danger is visible (recognition time). Another is the reality that some people, particularly family members will generally return if they forget something important such as valuables, family members who were left behind, and other things they consider critical to the continuity of their lives. Other people are courageous enough to penetrate and pass the smoke in order to save their lives while others will stop and hesitate when they see even a small flame. Moreover, BS 7974 and BS 9999 seems not interested in the potential of alternative escape routes contrary to the prescription of Approved Document B. Both standards appears so much focus on calculating the margin of safety based on a single escape route as if occupants has no instinct and capacity to think and take a better alternative. In reality, buildings has more than one exit, they have protected stairs that can be use as relative place of safety, people normally look for other means of escape, and “untenability” does not necessarily translate to hopelessness. In general, the effectiveness of ASET and RSET in determining the margin of safety is scientifically correct but it did not consider all behavioural scenarios. The problem arise when pre-movement, recognition time, and travel time are calculated because this will vary significantly in real-life fire occurrence. For instance, not all alarm is effective while evacuation depends on who is evacuating and the number of people evacuating at the same time. It is somewhat difficult to predict occurrences using human behaviour because there may be different kind of people occupying a certain building, different culture, different level of recognition, different approaches to danger, and others. In contrast, it is much more logical to increase the margin of safety through alternative escape routes, relative place of safety, fire-resisting construction, and adequately dimensioned escape routes and exit as ADB generally recommend. This is because safety is ensured regardless of pre-movement and travel behaviour. 8. Conclusion/Summary Although making some reference to BS 7974 and BS 9999 for a more technical approach to fire safety Approved Document B is generally prescriptive and focusing on the advantage of fire-resistant escape routes. In contrast, both BS 9999 and BS 7974 are dependent on the principle of performance-based design of buildings where margin of safety for occupants during a fire is determined by the availability of time to escape (ASET) or ASET and required time to escape (RSET). Critical review and analysis of ASET and RSET shows that they are generally dependent on occupancy characteristics that in real-life situation varies significantly. Although the assumption that occupants should have safe escape time, the dependency in human behaviour makes prediction of required escape time difficult particularly when pre-movement and recognition time is involved. In general, it will be very helpful if ASET and RSET consider the possibility of alternative escape route, the impact of other human behaviour in fires, and realistic scenarios that can greatly reduce the margin of safety. 9. Reference list Approved Document B, (2007), Volume 2 – Building Other than Dwellinghouses, HM Government, UK BS 7974, (2004), The Application of Fire Safety Engineering Principles to Fire Safety Design of Building-: Part 6: Human Factors: Life Safety strategies- Occupant Evacuation Behaviour and Condition , BSI British Standards, UK BS 9999, (2008), Code of Practice for Fire Safety in the Design, Management, and Use of Buildings, BSI British Standards, UK CFPA-E, (2009), European Guideline: Fire Safety Engineering Concerning Evacuation from Buildings, CFAPE Europe, UK Read More