Prescriptive Fire Design
Prescriptive fire design is what is designed as fire safety for building, which is, based on national and international standards and codes and that building comes under “code compliance”, so it is acceptable by legal authorities/authorities having jurisdiction and also from other aspects that is: as this building is designed as per codes and standards, so it is a safe building, having less fire risk.
So prescriptive fire design is as such easy to adopt while giving fire safety to buildings, because directly it can be said whatever asks in codes, is complying in terms of fire design, so designer apply clause of codes/standards while designing fire system. However, sometimes designer is following the codes partially may be due to constraint of the cost or may be due to inadequate understanding of the codal provisions.
For example, let us talk about table 7 of National Building Code 2016 (NBC). Table 7 of NBC defines that fire safety equipment are to be provided in building and with that provision, building falls under less fire risk. At the same time NBC 2016-table 7 does not includes system like smoke evacuation, fire evacuation route, fire door etc. The same are defined in the other part of the code. If the designer is exclusively following the table-7 and the other important parameters like fire door, smoke evacuation system, etc. are not considered then the building would not have adequate fire safety as per the codal requirements. Hence, in the prescriptive design sometimes the buildings may not be truly safe from the perspective of fire design.
Performance based fire design approach
Performance based design approach works with objectives, instead of pre-set rules. It is necessary to show that a certain deviations from the codes are acceptable that the performance of the proposed design is as safe as code solution would have been. A performance based design is an engineering approach to fire protection design, this includes Quantitative assessment of the design against fire safety objectives using acceptable engineering tools, methodology , Qualitative design review (QDR), Assessment against criteria.
Best example of performance based fire design is for airport terminal. Airports, due to their design, necessitate the adoption of a fire protection engineered approach as opposed to following the recommendations of prescriptive guidance where it would be difficult to apply prescriptive fire in terms of emergency exit, evacuation, smoke control, external fire hydrant (2 to 15 mtrs from building face), so here rather than applying prescriptive based fire design, performance based fire design to be applied to make building more and more safe as per actual practical requirements from potential fire.
Putting performance based fire design in actual, couple of examples are as below:-
- Increase sprinkler densities to limit fire sizes or provide additional fire-resistance to building structural members. Increased sprinkler densities have also been used in lieu of a required fire resistance rating. A typical application of this concept is an approved water curtain that provides a one hour fire resistance rating
- Conduct timed egress analysis. A timed egress analysis may result in to additional measures in lieu of standard fire protection features to decrease travel distance or to increase number of exits. Timed egress accounts for the fact that not all building occupants will reach an exit door simultaneously, allowing a design team to analyse specific architectural feature. Additional doors or different width components that exceed the prescriptive requirements of the code may be used in these analyses to provide sufficient time for evacuation.
However beside these two approaches, one major point to note here is the competency of designer, who should have clear fundamentals of the fire engineering, principles of the fire system and specific skill set to apply performance based design.
The competency level of the designer is the important criteria when it comes to performance based fire design. Before applying performance based fire concept along with prescriptive fire design, fire designer should understand fire safety goals and objectives and must discuss with factory owner and legal authority.
Performance based design allows for significant design flexibility: however therein lies a great responsibility to maintain fire protection features that might be beyond those normally required by code. Without knowledge of the mitigating features, a well-intentioned contractor or designer could compromise the building’s safety.
So by going back to the title again, Fire Engineering: Prescriptive Vs performance based design, it seems like there is a clear picture of how and when to use these two fire safety concepts to make building as fire safe as possible.
– Nancy Macwan Sr. Associate (Mechanical)