Prescriptive design

Prescriptive design is fire safety design that follows explicit rules in regulations, standards or codes, which state the measures to be provided and their quantities instead of the outcome to be achieved. The International Fire Engineering Guidelines (IFEG, 2005) define prescriptive provisions as provisions that are expressed explicitly in quantitative form. Typical rules limit the travel distance to an exit in metres, the fire resistance of separations in minutes and the floor area of a fire compartment in square metres.

A prescriptive design is simple to apply and to check, because compliance can be confirmed by comparing the drawings with the rules. In Australia such a design is called a deemed-to-satisfy (DTS) Solution and is described as following a set recipe. The rules are generic, however. They are only partly tailored to the building, its people and its surroundings, and they are often based on consensus rather than on a quantified goal.

Prescriptive design (left) is checked by comparing the drawings with the rules; performance-based design (right) by analysing a trial design against agreed criteria. Based on Australian Building Codes Board (2020b), pp. 12–13 and Van Rossum (2027), §1 and §2.1.

Prescriptive codes also cannot cover emerging technologies or every combination of circumstances. Performance-based codes such as the Australian National Construction Code therefore allow departures from the rules. The design must then be shown to meet the performance requirements or to be at least equivalent to the DTS provisions.

Definition and terminology

Prescriptive rules are found in many forms, and the names differ between countries. In the performance-based code of Australia, the prescriptive part consists of the DTS provisions. The IFEG define these as the prescribed provisions of a code that are deemed to satisfy its performance requirements. A DTS Solution uses materials, components, design factors and construction methods that are deemed to meet the performance requirements if they are used. In New Zealand, simple buildings are covered by the Acceptable Solutions C/AS1 to C/AS7, while Verification Method C/VM2 applies to complex buildings.

In the United States, the IFEG describe the codes in primary use as prescriptive codes, although they contain some elements of performance. Model codes such as the International Building Code (IBC) are written by not-for-profit organisations and adopted by state and local governments as their building regulations. At the time of the IFEG, most US jurisdictions used a prescriptive building code, and the IBC was the predominant one.

Introduction to Fire Safety Engineering uses the term rule-based for this approach. Rule-based means that predetermined frameworks and limit values are met in order to achieve the fire safety goals. The book gives three examples of such rules: legislation such as the Dutch building regulations (Besluit bouwwerken leefomgeving, Bbl), the conditions of an insurer, and requirements that a large company imposes on all its sites. Its opposite is the risk-based approach, which looks at fire safety project by project and with more variables.

What prescriptive rules regulate

Prescriptive rules set measurable limits on the means of fire safety. Introduction to Fire Safety Engineering names metres of travel distance, minutes of fire resistance against fire spread, and square metres of fire compartment as typical examples. Escape provisions are a clear case. The Australian handbook for the Fire Safety Verification Method (FSVM) notes that it is not practical to provide multiple escape paths from every point in a building. The DTS provisions therefore prescribe a maximum distance of travel to an exit, or to a point from which travel in two directions to two exits is possible. They also set minimum and maximum distances between exits and a minimum separation between paths of travel.

Typical quantities controlled by prescriptive escape and compartment rules. Based on Australian Building Codes Board (2020b), p. 66 and Van Rossum (2027), §1.2.

The numbers can be small and specific. In one handbook example, the DTS provisions limit the travel distance from an apartment door to a fire-isolated stair to 6 m. In New Zealand, a single escape route has long been permitted to serve up to 50 people. The commentary to C/VM2 calls this limit largely historical and consistent with the earlier Acceptable Solution C/AS1 (2011).

The World Trade Center towers in New York show how a building was designed to such rules. Each office floor had three stairs, two 44 inches (1.12 m) wide and one 56 inches (1.42 m) wide. Together they provided exactly the capacity required by the building code for 390 occupants per floor, based on 100 ft² (9.29 m²) of floor area per person. The layout met the code limits on travel distance of 200 ft (61 m) without sprinklers and 300 ft (91 m) with sprinklers. The code also traded one measure against another: dead ends could extend to 100 ft (30 m) instead of 50 ft (15 m) where corridor partitions were 2-hour rather than 1-hour fire rated.

Code limits that applied to the World Trade Center towers: the maximum travel distance depended on sprinklers and the maximum dead end on the fire rating of corridor partitions. Based on Lew et al. (2005), p. xlviii.

Fire compartmentation is often regulated through limits on compartment size. In the Netherlands, models for large fire compartments are often used to show that a large compartment performs as well as a compartment of, for example, 1,000 m². In New Zealand, the previous Acceptable Solution C/AS1 (2011) set firecell floor area limits using a limiting fire load of 2 million MJ. Other rules prescribe the construction itself. For example, the Building Code of Australia of 2005 required a stair to be located in a fire-isolated shaft that discharges directly to the outside. In the United States, IBC section 803.5 limits the flame spread index of interior wall and ceiling finishes according to the occupancy group and location.

Origins of prescriptive rules

Prescriptive rules grew out of experience. Watts and Hall describe narratives, series of recommendations on what to do and not do, as probably the earliest approach to fire risk. Over the years this narrative approach developed into the multi-volume set of NFPA National Fire Codes, which contain the bulk of present-day knowledge on fire safety. Checklists, which list hazards and recommended practices, are a related tool.

Introduction to Fire Safety Engineering notes that rules sometimes originate in a risk assessment but are often the result of consensus. The book explains that Dutch regulations arose partly from consensus and partly from history, and that no quantified goal was pursued when they were written. The commentary to New Zealand's C/VM2 makes a similar observation: international codes contain a number of historic values that are commonly accepted but have no technical basis.

Prescriptive provisions are nevertheless refined over time. In Australia, the DTS provisions have been continuously improved since the first Building Code of Australia of 1988. Most technical changes went through extensive public consultation, often supported by detailed fire safety analyses and cost-benefit analyses. The 1988 code stated that its requirements should extend no further than necessary in the public interest. They should also be cost-effective, not needlessly onerous and easily understood.

Advantages

The main advantage of prescriptive design is simplicity. The Australian handbook describes the DTS route as generally having a simpler assessment process than a Performance Solution. Introduction to Fire Safety Engineering points out that in a rule-based approach, the approving authority can check the relevant aspects much more easily than in a fire safety engineering approach. A fire engineering justification, by contrast, can be so complex that the authority finds it hard or impossible to verify.

Prescriptive rules also give a common benchmark. The Australian FSVM assumes that, in the absence of quantified performance requirements, acceptance criteria reflecting community expectations can be derived by comparison with the current DTS provisions. The DTS provisions thus define the level of safety that a performance-based design must at least match.

Generic rules are also less sensitive to change during the life of a building. The IFEG note that a fire engineered design is specific to its building, occupants and site, so later changes may require a new evaluation. Such a re-evaluation might not be needed if the broader DTS provisions had been adopted.

Limitations

The main limitation is that prescriptive rules regulate means rather than outcomes. The IFEG explain that a prescriptive code simply prescribes the required performance, without a method to determine or quantify the level of performance of a building. Requirements on heights and areas, sprinklers and structure address the hazards and tolerable losses only indirectly, so the level of safety they provide is hard to measure. For the same reason it is difficult to say what "equivalent" means when an alternative is proposed.

Because the rules are generic, they fit some buildings poorly. Introduction to Fire Safety Engineering describes them as very generic and inflexible measures that are only partly tailored to the building, the people and the environment. The IFEG add that a prescriptive code has only one value that is deemed reasonable. Communities must amend the code to reflect local needs, and such amendments are hard to justify when the code offers a single solution. Prescriptive codes also cannot cover emerging technologies and every combination of circumstances.

The resulting level of safety can be inconsistent. Introduction to Fire Safety Engineering notes large differences between occupancy types in actual performance on casualties and fire spread. Per hour of stay, the chance of dying in a fire is much greater in a residential building than in an assembly building, yet the Dutch detection requirements are stricter for assembly buildings. The rule-based limits also cannot be translated unambiguously into risk-based limits.

Watts and Hall point to a further weakness of checklists and narratives. A risk is judged acceptable if it is addressed in line with the published recommendations, so the test is pass or fail and the residual risk is never quantified. Checklists also do not capture the interaction between fire safety factors. The relative value of hydrants, sprinklers and extinguishers, for example, depends on the form and use of the building.

Relationship with performance-based design

Most building codes combine prescriptive rules with a way to depart from them. In the United States, section 104.11 of the IBC states that the code does not prohibit a design that it does not specifically prescribe, provided the alternative is approved. Approval requires that the alternative is at least equivalent in quality, strength, effectiveness, fire resistance, durability and safety. In the Netherlands, article 4.7 of the Environment and Planning Act (Omgevingswet) allows a departure from the prescribed measures of the Bbl on the basis of equivalency. The alternative must reach at least the same level of fire safety as the original measure.

A design that meets all prescriptive provisions is deemed to comply; departures are analysed for equivalence or against the performance requirements. Based on National Research Council of Canada et al. (2005), pp. 0.2-5 (Part 0, United States) and 1.2-12, and Australian Building Codes Board (2020b), p. 16.

In Australia, a design can use a DTS Solution, a Performance Solution, or a combination of both. The FSVM requires a Performance Solution to be compared with a reference building that complies fully with the DTS provisions. A design may mix complying and non-complying features. As the degree of compliance with the prescriptive provisions increases, the scope of the fire engineering evaluation generally decreases, because only the non-compliant features need to be analysed.

The two approaches therefore work together rather than as strict alternatives. Prescriptive rules cover most ordinary buildings, and performance-based design is used where the rules do not fit. In the Netherlands, fire safety engineering is applied mainly where the prescribed measures are not effective, or where a building falls outside the scope of the regulations. The IFEG caution that a single prescriptive requirement often serves several intents. A change made to the fire safety strategy may therefore affect other aspects of the building, such as acoustics or structural integrity.

Common misconceptions

Compliance with prescriptive rules does not mean that a building is free of risk. The IFEG explain that the fire safety benchmark of a performance code cannot guarantee absolute safety or zero risk, because these are not possible. The benchmark must reflect what a community expects and what the protection costs, and the IFEG add that the same is true of prescriptive codes.

Compliance also does not mean that the building itself is protected. Meacham notes that some owners may not realise that code compliance can be very different from protecting their building, processes, contents or other assets. In the Netherlands, preserving the building is explicitly not a goal of the fire safety rules of the Bbl. In Australia, the NCC addresses the protection of the subject building and its contents only to a limited extent for some building classes.

Prescriptive rules are not simply less refined than engineered solutions. The IFEG record a concern that performance designs may remove redundancies that prescriptive codes contain. An example is relying on a single active fire protection system instead of a combination of passive compartmentation and active systems. Nor do performance-based frameworks avoid prescriptive elements altogether. The commentary to C/VM2 describes its firefighting operations scenario as prescriptive in nature, simply because no design methodology is available for these issues.

See also

• Fire safety strategy
• Fire scenario
• Ethics in fire safety engineering
• Fire safety objectives

References

• Australian Building Codes Board. (2020b). Handbook: Fire Safety Verification Method (Print version 1.1).
• Lew, H. S., Bukowski, R. W., & Carino, N. J. (2005). Design, construction, and maintenance of structural and life safety systems (NIST NCSTAR 1-1). National Institute of Standards and Technology.
• Meacham, B. J. (2002). Building fire risk analysis. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 5-153–5-175). National Fire Protection Association; Society of Fire Protection Engineers.
• Ministry of Business, Innovation and Employment. (2013). Commentary for Building Code clauses C1–C6 and Verification Method C/VM2.
• National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
• Watts, J. M., Jr., & Hall, J. R., Jr. (2002). Introduction to fire risk analysis. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 5-1–5-7). National Fire Protection Association; Society of Fire Protection Engineers.