FIRE SAFETY DISIGN 2000

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Fire Science & Technology Vol.20 No. 1 (13~25) 2000 FIRE SAFETY DESIGN BASED ON RISK ASSESSMENT Robert Jönsson Johan Lundin Dept. of Fire Safety Engineering Lund University, P.O. Box 118, SE-221 00 Lund, SWEDEN http://www.brand.lth.se/english/ ABSTRACT The present paper concerns a performance-based fire safety analysis and design of a high-rise building. The resulting fire safety recommendations are compared with those speci- fied by acceptable solutions in terms of the earlier prescriptive building code. The respective risk to life resulting from the analyses is compared and discussed. The benefit and need of us- ing a risk based verification method as a complement to a deterministic fire safety engineering method is demonstrated. The cost-effectiveness of different solutions is also demonstrated. This paper is based on the results from a case-study presented at the International Con- ference on Performance-Based Codes and Fire Safety Design Methods in Maui 1998. 1. INTRODUCTION dance with traditional acceptable solutions, de- The present report concerns a performance- noted here as the standard method. The proce- based fire safety analysis and design for a high- dures used in the standard method are described rise building, the resulting fire safety recommen- in chapter 2.1, the report being based on a design dations being compared with those specified by prepared in accordance with traditional accept- acceptable solutions in terms of the earlier pre- able solutions, here the solution for the building scriptive building code. The respective risk to life described in chapter 3.2. With this as a starting resulting from the analyses is compared and dis- point, alternative designs are investigated. cussed. The benefit and need of using a risk based verification method as a complement to a deter- 2. DESIGN METHODS ministic engineering method is demonstrated. In 2.1 Standard Method this executive summary, the fire safety goal is In the former prescriptive Swedish building limited to that of safeguarding occupants. code [1], detailed solutions to most fire safety problems for simple buildings were given. The Since it is not customary to erect 40-story standard method, which is the traditional ap- buildings in Sweden, the solutions acceptable in proach, was developed on the basis of these de- terms of the building code are probably not valid tailed solutions together with detailed instructions for that height. Thus, the building used in this from different handbooks. No engineering calcu- case study is one 20 stories in height. lations are used and the knowledge required is minor. Technical systems are not introduced The solutions and engineering calculations unless specified in the acceptable solutions. The presented are not complete. They simply exem- standard method is still used today in the majority plify how some of the important fire prevention of design applications, and most of the earlier steps that could be taken. accepted solutions meet the performance re- The report is based on a design in accor- quirements in the new code [2]. 13R.Jönsson, J.Lundin The standard method is based on the follow- on a long tradition in Sweden, might appear to ing assumptions: be low. If an automatic sprinkler system is in- stalled or if the fire brigade can put out the fire The action of the fire brigade would generally within 60 minutes, the highest requirement is be expected within the normal attendance time 90 minutes (R90) independent of the fire load (10 minutes). If not, improvements in fire density. safety are sometimes needed. 2.2 Alternative standard method The threat of fire spreading to neighbouring These alternatives can be found in hand- buildings is taken care of by safe distances, books and are widely accepted. In this method fire brigade interaction and specified load alternative solutions involving use of various bearing capacity. technical systems are presented and are applied. If a technical system is introduced as a trade-off, it Safe evacuation is primarily achieved by re- should have the same safety goal, since the origi- quiring at least two independent escape routes, nal solution and the overall safety level need to be and by fire compartmentation limiting the maintained. In some trade-offs seen as acceptable, travel distances to and within the escape routes. however, this is not always the case. Some trade- Sometimes, one escape route is enough, al- offs, in fact, are made without any justification though the escape route must then be con- being given. Another matter to consider is that the structed with higher fire safety. In buildings two systems compared should have the same or- where the fire brigade's ladder equipment can der of reliability. Normally, no consideration is be used, windows can sometimes be consid- given to this fact. This leads to there in effect be- ered escape routes. ing different safety levels. Sometimes, simple engineering calculations are used to demonstrate The design of the required exit widths is done the relative safety in comparing different solu- according to prescribed occupant loads and tions. from the assumption that 150 persons calls for 1 meter clear door width. The case in which 2.3 Engineering methods one escape route is blocked should also be 2.3.1 General conditions and assumptions considered. A 1 meter width is needed then for The model used to describe the conse- 300 persons. quences of efforts to exit from rooms in which there is fire involves comparing the time available Buildings taller than 20 to 25 floors are usu- for evacuation with the time evacuation itself ally not built in Sweden, not being a part of would take. This model is widely used and can be our building tradition. This is an important found in many handbooks. The following rela- limitation to the use of the standard method. tionship can be said to hold: Sprinkler systems are traditionally not used, M = tcritical - tevacuation but are gradually being used more often. The alternative standard method can then be em- where M denotes the evacuation margin, ployed. tcritical denotes the time available, i.e. the time elapsing until the critical conditions have been In an international comparison, the load-bearing reached, and tevacuation evacuation time, i.e. the and fire compartmentation requirements, based time evacuation would require. 14Fire Science & Technology Vol. 20 No. 2000 The evacuation time can be described in 2.3.3 Risk based verification method term of three separate variables which describe To compare the safety level achieved by the the evacuation process. different designs, a simple form of risk analysis was performed. The buildings contained safety = tdetection treaction devices of various kinds, such as a sprinkler sys- tem, an evacuation alarm and an automatic fire The detection time is either computed or as- alarm. Safety planning consultants had indicated sumed, depending largely on whether or not there the probable course of events in case of fire, un- is an automatic detection system which activates. der the assumption that everything in the building functioned as expected. In order to assess the The reaction time involves all actions that safety level in the buildings adequately, however, people carry out before beginning to move to a account needed to be taken of the fact that the safe location. It is not at all unusual for some time safety devices might fail to function. What con- to elapse before the evacuation itself begins. sequences would it have if the sprinkler system Other activities that may occur include those of failed to work or the smoke detector failed to re- attempts being made to extinguish the fire or to spond? What would happen if a fire broke out and find out what has happened, as well people first both devices failed to function? completing something they are in the process of doing. In some handbooks reaction time is di- The risk analysis performed is very simple in vided into response time and recognition time. its conception. A number of initial events that could cause a fire are identified. These scenarios The travel time is estimated on the basis of vary, depending on such factors as the location of such matters as the number of persons involved, the fire, access to the site, and access to inflam- the width of doors, passageways and the like and mable material. The attempt is made to choose the distance to be covered to be in a safe place. It scenarios that differ in both their structure and in is normally assumed that people can be at many their consequences. For each scenario, different different locations in the building, not simply at events are considered, defined for the safety de- the place where the fire breaks out. It is relevant vices as the device's functioning properly or fail- to consider how evacuation can be affected by the ing to do so. To facilitate work with the different presence of other persons not directly affected by subscenarios, use is made of an event-tree tech- the fire. nique. In an event-tree, the traditional engineering method is only described as a single event and in 2.3.2 Fire safety engineering method terms of everything functioning properly. In this method [3, 4] deterministic values are used in the design equation. No consideration is The risk analysis involves studying the dif- taken of system reliability. It is assumed to be a ferent scenarios and examining the various possi- safe design if the evacuation margin exceeds zero. ble consequences and the probability of occur- Normally, only a few different scenarios are stud- rence of each. On the basis of the information this ied. The design parameters used here, such as the provides, two simple measures of risk are derived, design fire growth rate, are chosen in a somewhat producing a means of comparison. These two conservative way. The choice made is a likely measures are a risk profile and a measure of aver- value chosen as being somewhat on the safe side. age risk. For each subscenario, both the consequences 15R.Jönsson, J.Lundin and the probabilities are known. The question is of the probabilities of those subscenarios that 'How can this information be of any use when causes a consequence of greater than 0. If each of making decisions related to fire safety engineer- the alternative subscenarios in the event tree re- ing?' There are different ways of describing risk sults in one or more persons failing to evacuate [5, 6, 7, 8, 9]. The most usual way is to present the rooms in time, the intersection will be at 1.0. the probabilities and the consequences graphi- Since what is of interest is simply to compare the cally in a so-called F N diagram. An F N curve effects of different designs, the probability of a (Frequency Number Curve) indicates at each fire is coming about is not dealt with here. That point on the curve the probability that the conse- probability is assumed to be the same in all cases. quences will be worse than a certain designated Otherwise, one would need to multiply the values level, the latter normally defined in terms of a on the vertical axis by the probability that the certain number of deaths. Since in the report here scenario of fire itself will occur. A risk profile is what is involved is not deaths but is rather simply interpreted in such a way that those estimates lo- the state of being a victim of the fire (being ex- cated closer to the lower left-hand corner of the posed to critical conditions), the term "risk pro- profile are regarded as being more certain. file" instead of "F N curve" is employed. "Risk profile" is a more general term, one that encom- It is by no means obvious in some cases, passes the F N curve. which of two risk profiles is better than the other. If the two curves cross each other, the matter can- The risk profile is a step-wise function, not be determined in any simple, direct way, showing an increase left-to-right in the serious- other means being called for. The contents of an ness of the consequences. Figure 1 indicates entire risk profile can be used to obtain a single schematically how a simple scenario can be struc- figure, that of average risk, providing a means of tured with the help of an event tree and the corre- risk assessment parallel to that of the risk profile. sponding results of a risk profile. Note that the For each alternative scenario, the product of the probability of each alternative subscenario is rep- probability and the consequence is obtained, the resented by the height of the curve. sum of these products representing the average risk. The average risk indicates how many per- The intersection of the curve with the verti- sons on the average can be expected to be ex- cal axis is located at a point representing the sum posed to critical conditions. 0.7 no failure 60% 0.48 0 0.6 no failure 80% exit available 0.5 failure 40% 0.32 7 probability (Ci >= nbr) design criteria 0.4 fire starts alarm bell 0.3 risk profile no failure 60% 0.12 2 0.2 failure 20% exit available 0.1 40% 0.08 20 0 failure 0 5 10 15 20 25 consequence (nbr) Figure 1. Example of an event-tree and a risk profile. The dotted line is the limit to acceptable design. 16Fire Science & Technology Vol. 20 No. 1 2000 If there are no rules or regulations concern- above the other, measures to reduce risk being ing acceptable risk, and if neither the risk profile generally seen as appropriate for results that fall nor the average risk can provide a satisfactory between these two lines. This is to provide a cer- means of comparing the risk, a qualitative analy- tain degree of flexibility, since some measures to sis of risk can serve as a last resort. The appear- reduce risk can be more expensive than others. ance of different risk profiles can be studied and a Thus, one may regard it as temporarily acceptable general assessment be made of their acceptability. for a risk profile to partly lie within this interme- Problems can be encountered when one or more diate zone. of the alternative scenarios involves very serious consequences or when the probability is very high 2.3.4 Risk based method of at least some non-zero consequence occurring. A few methods exist, none of them ready at Graphically, this appears as a tendency of the risk the moment for use in fire safety design. profile to extend far along either of the two axes Frantzich [9] provides a survey of methods avail- in the diagram, a tendency which, if pronounced, able. These methods provide a better basis for can be regarded as unacceptable. decisions than a risk profile, but there is still a lack of rules and regulations concerning accept- able risk. Access to data allowing one perform a solid analysis needs to be improved, however. probability (Ci >= nbr) probability (CI >= nbr) One method is derived from The First Order Second Moment (FOSM) reliability index method [5, 6]. The index is used to determine the prob- consequence (nbr) consequence (nbr) ability that the escape time margin will be nega- tive. Consideration is taken of variation in the Figure 2. Risk profiles with the same average risk but variables studied (i.e uncertainties). The case differing in their characterisation of the risk. study does not involve use of any of these meth- ods. From the standpoint of society, it is normally 3. DESCRIPTION OF THE BUILDING easier to accept small negative consequences that AND FIRE SAFETY DESIGN ALTER- occur frequently than very serious ones that occur NATIVES seldom, even if the average loss is the same. In 3.1 Architectural design cases in which the criteria for acceptable risk are The building in question is a double ellipti- specified, such as in the building of large chemi- cally formed 20-storey office building with a cal factories in Holland, the line of reasoning just basement. The basement includes various ser- presented can be expressed as a risk profile. A vices rooms for the building as well as parking line running left to right indicating the maximal spaces. The ventilation equipment is located in level of risk that can be accepted is incorporated the attic. into a risk profile, figure 1. Any risk profile that at no point exceeds this level is regarded as ac- Floors 1 and 2 are assembly rooms including ceptable. If, in contrast, a curve exceeds this level a foyer, a cafeteria, an insurance company, office at some point, it is considered to not be accept- services, building maintenance, etc. The first two able and appropriate measures are taken. Often, floors are rectangular in shape. On the ground two separate lines of this sort are drawn, one floor there are two arcade entrances. 17R.Jönsson, J.Lundin 10 m 20 m 30 m Figure 3. Floor plan layout of the building. Floors 3-20 contain office premises with 3.2 Design according to the standard method modular office rooms. It is intended that these Depending on their function, elements of floors be able to provide flexible tenant accom- structure are assigned to classes R (load bearing), modation, with 1-2 companies per floor. E (integrity) and I (insulation). The classification for doors could perfectly well be combined with All workplaces have direct or indirect the designation C (for doors with an automatic sunlight, which is required by regulations. The closing device). The symbols are those used in top floor has a restaurant and can also be used for the Swedish building code [2]. The fire resistance conferences. classes used in accordance with the code are based on fire load intensities lower than 200 A maximum of six staircases passes through MJ/m² (surrounding area). The classes could per- each floor of the building. The number of stair- fectly well be applied, without special examina- cases depends on what design alternative is used. tion, for dwellings, offices, schools, hotels, ga- rages for cars, food-selling shops, for store rooms In the middle of the building there are four for residents, and comparable fire compartments. sets of elevators, a total of 18 altogether. Storage, bathrooms and kitchens are located in the vicinity 3.2.1 Fire resistance classification of the elevators. The office building is classified An of- fice building is assessed as contain a fire loading Each floor has an area of 2000 m², and is in- of less than 200 MJ/m², and should therefore be tended to accommodate up to 200 persons on fire resistant for at least 90 minutes (R90) in the each floor. The top floor is designed to accom- case of structural and 60 minutes (EI60) in the modate a maximum of 800 persons. case of partitioning material. 18Fire Science & Technology Vol. No. 1 2000 The fire compartment separation partitions staircase are both EI-C30. The door to the stair- are to consist of steel studs and 2 X 13 mm gyp- case, Tr2 is of class EI-C60. sum plaster sheets on each side (EI 60). The in- door windows and doors in the fire compartment The escape width required is generally 0.9 walls are to be made of class EI 60 (60 minutes). metre (0.8 for door openings). In premises de- signed for more than 150 persons, it is recom- The surface layer must be of the highest mended that the width be 1.2 metre. The total class (Class I), in this case at least 9 mm gypsum width of emergency exits should be equal to 1.0 plaster sheets. The same requirements apply to metre per 150 persons. The building owner has walls in staircases and assembly rooms (may be chosen all staircases to be 1.2 m and the effective painted but not wallpapered). Other walls can be door width to be 1.0 m, which fulfils the require- made of class II, which permits thin wallpaper but ments above. not unprotected wood as the surface layer mate- rial. The equipment required for escape routes are exit signs, and emergency lighting in meeting Facades and roofs are to be made of non- rooms, basement corridors and staircases. combustible material, except for the facade sur- face of the ground floor, which can be combusti- 3.2.3 Fire compartment subdivision ble. The roof surface can be combustible, but Each staircase is a separate fire compartment. should contain material that is difficult to ignite, Each group of elevators is likewise a separate fire on top of non-combustible material. compartment. The areas for kitchens, bathrooms, etc. are also separate fire compartments. Each The components of the HVAC-systems floor is separated into two fire compartments. The should in principle be made of non-combustible floors, in turn are separated from each other. material and should not contribute to the spread of fire. Unless there is a sprinkler system, no fire compartment, apart from the staircases, is al- 3.2.2 Escape routes lowed to comprise more than two floor levels. Escape routes are defined as either doors to the open air or other fire compartments in the 3.2.4 Installations and HVAC-systems (not building (staircases and corridors) which lead to complete) the open air. Occupants are assumed to evacuate The four staircases (Tr2) have smoke and without the help of the Rescue Services. The heat ventilation to facilitate extinguishing of fire maximum permitted walking distance to the near- as well as rescue activities. Hatches or fans at the est escape route is 45 metres for offices, where top of staircases are opened/started manually the coincident distance to another escape route is from the entrance. to be multiplied by 1.5. Six staircases are in- cluded, due to the requirements concerning All premises have access to fire extinguish- maximum allowable travel distances. The two ing equipment in the form of hand-held fire ex- centre staircases are designed to be staircases Trl tinguishers, or internal fire hydrants. The extin- and the other four to be staircase Tr2. A staircase, guishing equipment is supposed to be used by Trl is in communication with other spaces persons in the building in case of fire. All stair- through a protected lobby, which is open to the cases are equipped with rising mains for the fire external open air. The doors to the lobby and the department. 19R.Jönsson, J.Lundin No alarm having direct communication with rection factor has been used in accordance with the Rescue Services is installed. In order to fulfil the reference. The sprinkler activation times and the general clause concerning escape in the event smoke detector activation times are calculated of fire a manual evacuation alarm is installed on using the computer model DETACT-T2 [12] and each floor. When someone has pushed the alarm are presented in the table. On the basis of an ex- button, the evacuation alarm starts, on that floor tensive sensitivity analysis [13], the time to criti- and on the two floors above and below, a signal cal conditions was chosen as being 660 S. also going to the lobby personnel. The critical levels for untenable conditions 3.3 Design according to the alternative stan- were chosen according to the recommendations in dard method the Swedish building regulations as being 1.9 m. A sprinkler system is installed as a solution (room height 3m), which is a rather conservative of the form described in the alternative method. value. This allows the required travelling distance to an escape route to be increased by 33%. It allows the 4.2 Evacuation time external staircases on the long side of the building 4.2.1 Detection and reaction times to be omitted Scientific knowledge at the moment is not sufficient to model the behaviour of the different 3.4 Design according to the fire safety engi- persons in the building with any precision. Rather, neering method a rough assessment of the course of events over In this solution engineering equations are time needs to be made. used to justify the tradeoffs. These are omitted in the two staircases on the long side of the building, Table 1. Activation and reaction times used in the analysis without a sprinkler system being installed, and Time Reference four staircases combined with a sprinkler system are omitted. Sprinkler activation time 230 S calculated Smoke detector 185 S calculated 4. RESULTS activation time Manuel activation time: 300 S assumed 4.1 Design fire scenarios The design fire used is a "medium" t- Reaction time, no alarm 300 S BSI guide [3] squared fire according to the BSI guide [3]. The Reaction time, alarm bell 240 S BSI guide [3] heat release rate per unit area is 250 the maximum rate of heat release being assumed to be 4 MW. It is possible for the fire to continue to 4.2.2 Travel time grow, since more fuel is available, but this is not These calculations were performed using the likely to occur during the time period of interest. computer program SIMULEX 2.0 [14] and simple The BSI guide does not recommend any particu- hand-calculation methods. The hand-calculation lar smoke spread model, which indicates that the methods use a flow of 1.2 person/s in the stairs, designer has to deal with the uncertainty con- and of 1.2 person/s through a door (effective tained in the model employed. In this case width 1.0 m). The walking speed taken is 1.3 m/s CFAST 2.01 was used [10]. Bragason [11] sug- for the office people [4]. The critical factor in the gest that the time up to the critical conditions can hand calculation is the flow through the door be extended by 35% when smoke layer height is opening. Two different general scenarios were used as the basis for measurement. A model cor- studied: 20Fire Science & Technology Vol. 20 No. 1 2000 1) A detailed study of the evacuation of a floor. ences in reaction times that are found between the fire floor and the other floors. In the evacuation of a floor the simulated scenarios assume that a fire starts in one part of 2) A study of phased evacuation according to the the floor and that the whole floor is evacuated. In evacuation strategy of the building. the scenarios, different doors are blocked in the part of the floor where were the fire starts In these calculations, the restaurant floor is not taken into account. The scenarios are chosen For phased evacuation, only one staircase is as above. The fire floor and two floors above and studied, using the computer model SIMULEX. below it are evacuated. An approximation of the This study was made to determine whether it is number of people entering the staircase from each reasonable to evacuate five floors at the same floor is made. If consideration were taken of the time and whether or not this is the critical evacua- fact that the reaction times on the fire floor and tion scenario. In using the standard method, this the other floors would differ this would reduce evacuation plan is assumed, but for the other the evacuation time. A difference of 120 S has methods more sophisticated means of evacuation been used. The values in parentheses do not take can be used. Consideration is taken of the differ- account of this difference. 6 5 4 3 1 10 m 20 m 30 m 2 Figure 4. Exit numbers and room configuration for modelling the travel time. Table 2. Number of staircases and anticipated travel time for each scenario. Scenario Available staircases (exits) Travel 1 2 3 4 5 6 time Standard method X 70 Alt. standard method 80 Fire safety engineering method a 80 Fire safety engineering method b X 100 Fire safety engineering method c 100 21R.Jönsson, J.Lundin It is shown here that the design values conditions. should be chosen on the basis of the phased evacuation results. In the present study, values 4.4 Reliability of technical systems with different reaction times are chosen. The possibilities for assessing the reliability of a technical system today are rather poor. In 4.3 Verification of design alternatives most fire safety planning it is simply assumed The following scenarios were studied using that a system will function 100 %. Such is not the the risk based verification method and the fire case in reality. The reliability is dependent, of safety engineering method principles. course, on how appropriately the program of con- trols is carried out. To an untrained eye, it looks as though all the alternatives would be appropriate, and they In some alternatives the fire alarm can be also are according to the fire safety engineering started either manually by pressing a button or method, if used in a simplified way. The longer automatically by means of a signal from the evacuation times for phased evacuation could sprinkler or from the smoke detectors. It is as- also be used if the alarm bell was changed to use sumed that, if the sprinkler or the smoke detector of a recorded message. Viewed in the same way, functions properly, the fire alarm will be set off a sensitivity analysis would indicate acceptance. automatically with the degree of reliability listed Take case c; change it to use of a recorded mes- in the table. If neither the sprinkler nor the smoke sage and a "fast" fire growth rate. The result then detector sets off the alarm, it must be started would be: 465-185-180-135=-35s Not too bad, it manually. The reliability of manual activation of would appear (not taking account of the faster the fire alarm's succeeding, listed in the table as response of the smoke detector). This corresponds 0.50, represents the probability that someone will to consequence of 42 persons exposed to critical push the alarm button. Table 3. Simulation results for phased evacuation. Number of persons from each floor Travel time Standard method 80 persons (180 s) 120 S Alternative standard method 90 persons (220 s) 125 S Fire safety engineering method 100 persons (250 s) 135 S Table 4. Summary of escape margin for the design scenarios in the different alternatives tc td = M Standard method, 660 300 240 120 0 Manual detection and alarm bell Alternative standard method, N.A. 230 240 125 not critical Sprinkler system and alarm bell Fire safety engineering method a, 660 185 240 125 110 no sprinkler or smoke detectors Fire safety engineering method b, N.A. 230 240 135 not critical Sprinkler but no smoke detectors, only exit 3 and 4 Fire safety engineering method c, 660 185 240 135 100 smoke detectors and no sprinkler, only exit 3 and 4 22Fire Science & Technology Vol.20 No. 1 2000 Table 5. Probability of failure used in the event trees. Probability of failure Reference Automatic sprinkler system 0.05 BSI guide [3] Smoke detection system 0.1 BSI guide [3] Alarm bell 0.15 BSI guide [3] Manual detection 0.5 Estimate 4.5 Summary and discussion should be made of the information in an event In figure 5 the event-tree from the fire safety tree, as displayed in a risk profile. The risk pro- engineering design b is shown as an example. In files corresponding to the event-trees are shown this design what is found are a sprinkler system, in figure 6. no smoke detectors and only two escape routes (stairs 3 and 4). The different times in the equa- Table 6. Average risk for the different alternatives. tion for the evacuation margin M are given. Design according to Average risk One simple way to compare the different de- signs would be to compare the average risks, cal- Standard method 41 culated by Cᵢ). Alt standard method 2.4 Fire safety engineering method a 15 As discussed in chapter 2.3.3, the average risk is not reliable as a basis for the decision of Fire safety engineering method b 3.0 whether a design should be accepted. Instead, use Fire safety engineering method c 18 C₁ tkrit td M(x) 230 240 135 N.A. no failure 85% 0.81 0 N.A. no failure 95% alarm bell failure 15% 0.14 0 N.A. 300 300 135 N.A. FSEM b sprinkler no failure 85% 0.021 18 660 300 240 135 -15 no failure 50% alarm bell failure 15% 0.004 90 660 300 300 135 -75 5% failure manual detection failure 50% 0.025 90 660 300 300 135 -75 Figure 5. Event tree evaluation of the design alternative FSEM b. 23R.Jönsson, J.Lundin The building code does not provide any cri- building code, its risk profile should be close to teria that could be translated into a risk measure, the curve for the alternative methods design curve not yet at least. Due to this lack of information, or lie under that curve. If the standard method the earlier prescriptive solution is used as a com- design had been done in a modern way, it would parative design criterion. In this case study, one at least have included a smoke detector system. In of the standard methods was used. that case, the risk profile and the average risk would have been closer to those given by the al- Although the standard design should provide ternative standard method design. the acceptable limit for all other designs, the level of risk varies considerably among different build- The fire safety engineering designs have ings that all have been erected in accordance with marginally greater consequences. The alternative the standard method [1]. Kristiánsson [15] re- b has the risk profile which is closest to the "ac- ported on the fire hazards persons were exposed ceptable" curve and has a low average risk value to in different buildings, all of them constructed as well. The solution with only two centre stair- in accordance with building regulations. The al- cases (with protected lobbies open to the outside) ternative design has a much lower probability for and a sprinkler system is thus recommended. The a given consequence, due mainly to the sprinkler solution should be improved so that the F/N- system. curve moves more to the lower left corner and also checked against the other fire safety goals If a design is to be compared with the design stated in the regulations. The cost effectiveness acceptable in terms of the earlier prescriptive should be examined as well. 0.7 0.6 0.5 Probability (Ci >= nbr) Standard 0.4 alt standard FSEM a FSEM b 0.3 FSEM C 0.2 0.1 0 0 10 20 30 40 50 60 70 80 90 100 Consequence (nbr) Figure 6. Risk profiles for the different design alternatives evaluated 24Fire Science & Technology Vol. 20 No. 1 2000 REFERENCES Fire Safety Engineering, Lund University, 1, Nybyggnadsregler (NR), BFS 1988:18, Lund, 1994. 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