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May 2000: Performance Based Design vs. Prescriptive Design

Performance Based Design (PBD) and performance based codes are buzz words in fire protection engineering today…but what exactly is PBD and how does it differ from Prescriptive Design?

The idea behind PBD is that the designers can use any solution they like, as long as it meets the goal stated in the performance based code. The goals of a performance based code are usually in very broad terms. Prescriptive codes specify exactly what steps need to be taken to achieve the end goal.

There are pros and cons to both design methods. The key point is that one should not replace the other. Firstly, there has been misconception that prescriptive codes and standards have not achieved their fire safety objectives. Everyone must keep in mind that prescriptive codes have not failed. We would not have been the perception of failure if all prescriptive requirements were followed in each application.

Secondly, there is a notion that performance based codes are implemented solely to reduce cost. PBD adds flexibility to existing prescriptive requirements. Performance based codes are not intended to replace existing prescriptive codes. PBD is very useful when designing unique structures since prescriptive codes often do not allow for innovation.

PBD is still a relatively new and unproven field in the fire protection arena. Fire modeling is constantly evolving and becoming more popular. However the fullest range of data does not currently exist that would allow a performance based approach to be used for fire protection purposes. There are other modeling programs such as structural modeling that have proven to closely approximate real world conditions. Fire models though are still inconclusive and inconsistent. They appear to be gaining in integrity but have not reached the point of simulating a real word condition like with structural modeling.

What of safety factors? Prescriptive codes usually allow for a minimum 10% safety factor. Under PBD each situation is based on the potential uncertainties and is evaluated on a case by case basis. There appear to be too many assumptions that are used to determine a PDB safety factor.

Fire is often an unpredictable phenomenon. The use of PBD is becoming more and more popular. If used correctly the use of PBD can be an excellent enhancement to Prescriptive based codes. However at this time PBD is still at a starting point and should be viewed with healthy skepticism.