Validation of the "FRAME" -method.   FRAME appeals to fire safety professionals as it "feels good", i.e. the results of the calculations fit the intuitive evaluation, based on knowledge and experience. But as for all empirical methods, the question is often raised where does it stand scientifically? The distrust for empirical methods is understandable, yet such calculation rules have been established and used with confidence in many fields and for many years before there was any scientific support for them. Men did have some empirical rules to build ships before Archimedes discovered the physical law for buoyancy. It is fairly clear that organising a research program to check the validity of all the parameters by fire tests is an impossible task, which goes beyond the need for accuracy. However, FRAME-users are entitled to know on which scientific knowledge and proof the method has been based. The basis and development of the method is described in the “Theoretical basis and technical reference guide.”  , and a selection of calculations made with the first versions is given in the Calculation examples book. Assessing Hazards and risks Risk assessment has become a common growing practice in all safety related disciplines, and various methods are used to assist the decision making processes, not only to know what to do, but also to have a reasonable insight in the cost/benefit balance of a proposed set of provisions. Fire risk assessment has been practiced for many years with variable success. The development of performance based fire codes and fire safety engineering has increased the interest for risk assessment tools suitable for fire safety issues. Some methods exist already for more than 40 years, others were developed and have been abandoned again, some in-house methods are only accessible for a few, but others are easily available at limited or even at no cost. All currently available methods use some kind of software to make calculations and to issue reports. The WG6 workgroup of the EC-funded FiRE-tECH research programme, Fire Risk Evaluation to European Cultural Heritage, issued in 2004 an overview report describing some 12 fire risk assessment methods and their main characteristics. The reports of the 8 working groups issued reports were for some time available at the Internet, but unfortunately the website is closed and the reports are no more readily accessible. An overview of the project and some documents are available on the FRAME - website under “FiRE-Tech project ". Acceptable risk arguments. All methods for risk assessment give some way of quantification of hazards, and in most of them there are also guidelines to compare the risk with a benchmark which is considered to be an "acceptable level" of risk. The "acceptable risk" idea has been debated for a long time. One of the first publications on the subject was William W. Lowrances' book "Of Acceptable risk, Science and the determination of safety." published in 1976. Lowrance already wrote that people accept risks, even with deadly consequences, if the combination of probability, exposure and severity is low enough. He also indicates a number of factors that influence the way risk are accepted or tolerated.   Sometimes people are bound to accept a risk because they do not have the means to protect themselves or because they consider these risks as inherent to life. In this way, people accept in some countries floods or earthquakes as part of their living conditions. Risks are less acceptable when the consequences are more severe, when more people are exposed to the risk at the same time, and when the duration of exposure is lengthy. A risk is considered less acceptable when the consequences are readily visible.   A risk is more easily accepted when the consequences are reversible, of short duration or repairable, when there is a visible benefit in taking the risk, and when one thinks to have control of the causes of the undesirable event. An unknown or hidden risk will be less acceptable. In this way, people have more fear of a fire during the night than during daytime, although the probability of starting a fire is much higher when they do all kind of things, then when they are at rest. The basic acceptable level of risk in a situation of permanent exposure to a hazard is a risk level that is lower than the natural death level. That level is usually defined as the risk of one death with a probability of less than 1 per million of persons per year, or 1.10-6 / person*year. For accidents with a potential of multiple deaths, it appears that the acceptance of risk is reduced by the square of the number of possible victims: for 3 deaths, the acceptability is 9 x less, for 10 victims, it is 100 x less, and for a 100 deaths, it is 10.000 x less. When the exposure to the hazard is not permanent, a higher level of risk will be accepted. NFPA 551 makes no clear distinction between continuous and discontinuous exposure, unlike the CFPA Guideline n° 4 which uses the following approach: RISK is defined as a function of EXPOSURE x SEVERITY, where EXPOSURE can have a range from 0 to 1 and SEVERITY can have a range from 1 to 3. For life threatening hazards such as aids, smoking cigarettes and also fire, it appears that the acceptable risk level is not defined by the frequency of occurrence but by the level of exposure. As the outcome is always death, the recommendations are meant to reduce the exposure to the hazard, not to reduce the effect of it. In fire protection, the common practice to impose higher requirements on high rise buildings compared to low or medium height buildings with the same occupancy, cannot be explained by a difference in fire probability, but is fits perfectly with the higher exposure in high rise buildings, where the time required to evacuate the occupants is much higher. Richard Bukowski defined this approach in one of his papers as "hazard based" regulation but I dare say that "exposure based" is a more appropriate definition.   There a several approaches possible to make a (fire) risk assessment ( see also : "risk evaluation" ), but the most elaborate methods use mathematical expressions, based on a combination of values representing the probability of fire, the severity of the fire scenario and the level of exposure. Probability based risk quantification methods.   Probability based methods start from the observation that unwanted vents like fire have a variety of outcomes. Most methods are not specifically developed for fire risks and need some kind of interpretation to be used for fire risk assessments. The existing methods can be classified as two-dimensional or three-dimensional evaluation of risks.   The "American school" of fire protection engineers uses mostly two-dimensional risk quantification schemes, based on the technique described in NFPA 551, Guide for the Evaluation of Fire Risk Assessments. Two-dimensional methods were developed by the nuclear, chemical and aviation industry where the hazard exposure is continuous or exists over long periods. They express the risk as a combination of a probability of occurrence or frequency and a magnitude of the consequent loss or severity. These two-dimensional schemes tend to create some confusion as they do not fit the usual practice for life safety, where a combination of severity and exposure is more often used in the risk assessments. The most commonly used approach is the ASET/ RSET evaluation: A worst case RESET, Required safe egress time, calculated with an egress model is compared with a worst case ASET, the Available safe egress time, obtained with a zone or field model. Once the RSET is higher than the ASET, the occupants are considered to be exposed to a life threatening and unacceptable risk. In reality the probability of such a simultaneous occurrence of both scenarios is very low, and in many situations there will be still a comforting safety margin. NFPA 101 "Life safety code" chapter 5.5 defines 8 design fire scenarios for approvingperformance based designs. Except for scenario 1, none of these scenarios can be found back in the fire statistics among the most probable fire cases. This means that the probability component of the risk is disregarded and that the focus lays on the exposure component, when life safety is at stake. Methods based on a combined evaluation of exposure, probability and severity offer a better approach for fire, which is a non- continuous hazard and basically a rare but unwanted event. This approach fits better to the intuitive way of making fire risk assessments and to the reasoning underneath code requirements. Three-dimensional methods use Exposure, Probability and Severity for risk quantification. This type of risk evaluation is practised in those areas where the exposure to the hazard is not continuous, such as workplace risks, machinery defects, and fire. One of the oldest and widespread methods for workplace risk assessment was developed back in 1976 by Kinney, Wiruth e.a. and is widely used for the analysis of workplace hazards. The approach is known as the Kinney or ESP method. A similar approach can be found in the standards for the safety of machinery, such as EN-ISO 14121-1:2007 and the older EN 1050 and EN 954-1 standards, and this three-dimensional evaluation is also included in "FRAME".  Three-dimensional risk expressions.   For the safety of machinery, the way to combine severity and probability of occurrence into an acceptable risk is clearly defined and explained in EN1050 and EN954-1. The severity of a risk is evaluated as "worst case consequence" without consideration for the effectiveness of the protection or for the duration of exposure.  Such "worst case" becomes acceptable when the combination of exposure and low probability balance the severity of the case. A generally used mathematical expression developed by KINNEY e.a. For such an acceptable situation is the formula: Sev * Poc * Exp <= C whereby : Sev = measure for severity Poc = measure of probability of occurrence Exp = measure of exposure C(constant) = measure of acceptable risk level   It should be noted that the severity, probability and exposure are linked to the same undesirable event.  The similarity between the "FRAME" basic formulas and the expression used in the KINEY method clearly shows that "FRAME" is not a "point-system" like some checklist based other methods ( e.g. NFPA 101A FSES method) but a probability - exposure - severity based method.  In the FRAME expression, the potential risk P is the severity dimension, the protection degree D is the probability dimension and the acceptable risk A is the exposure dimension. PRINT  THIS SECTION  (pdf)