The International Building Code^®1 (IBC^®) incorporates significant changes from the previous editions including limiting mercantile, light- and moderate-hazard storage occupancy buildings of unprotected, noncombustible construction (IBC Type IIB) to a maximum of two stories high (three stories when protected throughout by automatic sprinklers). This is a reduction from the previously permitted four stories in height (five stories with automatic sprinklers).^2,3 The code change is not in response to a life loss or property damage experience; rather, it's an attempt to make the IBC 2009 consistent with the most restrictive requirements of the three legacy model building codes from which the IBC was formed.

When considering the merits of a code change, there are three questions that should be asked:

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• What is the desired end result?
• How will success be measured?
• What possible ways might success be achieved?

The International Code Council was formed by the merger of three predecessor organizations: Building Officials and Code Administrators International, Inc.; International Conference of Building Officials; and Southern Building Code Congress International, Inc. The origin of the IBC height and area table is the Building Officials and Code Administrators (BOCA) Basic Building Code 1950 edition,⁴ and is a function of the formula:

Allowable Area = U x C x Base Building Area

Where:
U = Relative Risk to Fire (use factor)
C = Relative Construction Value to Fire (construction factor)
Base Building Area = Assumed allowed areas under critical conditions, i.e., highest risk and lowest construction type.

The Relative Risk to Fire use factor, U, was determined by a study of fire experience data reported in the National Fire Protection Association Quarterly for the years 1930 to 1942. The analysis tested the number of fires in each occupancy use group, the number of lives lost per fire and the amount of property damage per fire. A construction factor, C, was developed for each construction type based upon fire resistance rating and combustibility of the floor construction and exterior walls. The Base Building Area was originally a constant 1,000 square feet (93 m²) and was increased to 1,200 square feet (110 m²) in 1970.

Increases in the base building area for excess street frontage (openness of the other building sides) were permitted. The number of stories permitted was an arbitrary value based upon limited substantiation.⁵ The allowable areas in the early editions of Southern Building Code Congress International's Standard Building Code and International Conference of Building Officials' Uniform Building Code possibly were derived from a similar analytical method.

Revisions to the code over the years have altered whatever relationship on which the original analysis was based. In the late 1970s, the model building codes recognized the effectiveness of an automatic fire sprinkler system in controlling fire. Thus, a doubling of the maximum allowable building area was permitted and the maximum number of stories permitted increased by one.

There are many possibilities of the desired end result of limiting a building's height and area as a function of its structural fire resistance. The most obvious are:

• Achieve safe evacuation of all building occupants in a fire condition;
• Maintain structural integrity in a fire condition to facilitate manual firefighting operations; and
• Limit the value of the fire loss. As with any code provision, there are likely other methods of achieving the goals and objectives of the restriction of maximum height and area of a building as a function of building construction type.

To answer this question, it is necessary to take a quantitative risk-based approach.

QUANTITATIVE RISK-BASED APPROACH

The IBC^®, the Life Safety Code^®6 and the NFPA Building Construction and Safety Code^®7 all identify an acceptable level of risk. For example, each of these codes accepts the risk associated with permitting a maximum of 50 persons in a room/space served by a single exit. The current process for determining height and area is also risk-based, albeit not quantitative by engineering standard.

Quantitative risk-based approaches to fire safety are not a new concept. The Goal-Oriented Systems Approach to Building Fire Safety introduced in the early 1970s was one of the first risk-based fire protection engineering approaches employed.⁸

A quantitative risk-based approach measures the total risk as function of the probability of an event and the consequences, and is expressed as:⁹

Risk =ΣP_i x C_i

Where:
Risk = Calculated Risk
P_i = Probability of Event i
C_i= Consequence of Event i

"Event trees" can be used to determine the risk associated with a initiation event. (See Figure 1.) The total risk is the sum of the probabilities and consequences of each branch of the event tree.

Fault trees can be used to determine the failure probabilities used in the event tree. (See Figure 2.) The overall probability of failure or success can be determined by examining the possible failure modes using "and" and "or" logic gates.

Determining whether the calculated total risk determined for the event tree results in an acceptable design solution depends upon the acceptable risk of the project stakeholders.

The Guide to the Fire Safety Concepts Tree¹⁰ provides an effective framework for identifying the various fire safety subsystems that can be used to achieve success. Figure 3 on page 38 illustrates the "top gates" of NFPA 550 with selected lower-tiered gates. Controlling the fire by construction is just one of three possibilities to manage the fire. Building fire safety objectives identified by NFPA 550 are achieved through various fire safety subsystems that reduce the likelihood or consequence of a fire. These subsystems can be classified according to their functions:¹¹

• Control of fire ignition and development;
• Control flame spread;
• Control spread of smoke and toxic products;
• Provisions of means to allow occupant avoidance; and
• Provisions of sufficient structural integrity.

QUANTIFYING ENHANCEMENTS

Quantitative risk-based approaches afford the ability to holistically identify building fire safety subsystem enhancements that improve overall building fire safety while also allowing for cost-benefit comparison. Adding additional branches - additional or redundant subsystems - increases the probability of success. For example, fire sprinkler system reliability can be greatly increased by simply supplying the system from two separate risers - risers already required by the building code for fire standpipes - on each floor. Recognizing that all Class A fire department pumping apparatus is capable of providing 100 psi (6.9 bar) to standpipe outlets on floors up to 150 ft. (45.7 m) above the fire department connection and thus affords a level of redundancy to the fire sprinkler/standpipe booster pump system.

The effect of an extraordinary event such as an earthquake and resulting fire can be evaluated to determine whether or not the building fire safety objective is achieved and what enhancements, if any, are needed.

The recognition of performance of automatic sprinklers, more specifically their reliability, to achieve the fire safety objectives in lieu of passive fire protection remains a contentious issue. A quantitative risk-based approach identifies the various subsystem failures that must occur before a building's structural frame is subjected to excessive thermal exposure.

A fundamental fire safety goal of a building's structural frame is to maintain structural integrity in order to facilitate safe evacuation of the building occupants and permit interior manual firefighting operation.

For buildings up to about 15 stories and 150 feet (45.7 m) tall - and of conventional structural framing techniques - it's possible that no adverse effects may occur as a result of exposing the structural framing system to elevated fire temperatures for the short duration associated with initiating manual fire suppression. Beam failure/yield in a fire condition might not adversely affect the integrity for various types of conventional steel framing. Column failure/yield inherently is of greater concern than beams and their submembers. Accordingly, a quantitative risk-based approach can focus attention on those building's elements deemed critical to achieve the overall fire safety goal if the various other fire safety systems were to fail.

A quantitative risk-based approach provides a more holistic understanding and quantifiable outcome of the interaction of various building fire safety systems.

THE FUTURE

Numerous technical papers have been written over the past decade on the virtues of a risk-based approach to building fire safety design. Some building professionals remain skeptical that such an approach could be effectively applied, in part, due to the lack of understanding and technical competency. Building regulatory authorities in Australia and New Zealand, where quantitative risk-based approaches are used, seem to have addressed this concern. The quantitative risk-based analysis and fire testing associated with 140 Williams Street Building, Melbourne, Australia, serves as reference guide for such analyses.¹²

Recent publications of textbooks and standards covering the topic of performance-based design for building fire safety based upon risk assessment, coupled with the various document publications by the SFPE on performance-based fire protection, have filled the previous void of technical documentation.^{9, 11, 13}

Perhaps the greatest challenge currently facing the fire protection engineer in the United States building regulatory arena is the acceptance of a risk-based approach. Replacing the current basis for determining building height and area with a quantitative risk-based approach will provide more flexibility, while ensuring an adequate level of safety. Quantifying building enhancements affords the ability for cost-benefit analysis for potentially more economical building construction.

John F. Devlin, P.E., is with Schirmer Engineering Corporation.

References:

1. International Building Code^®, International Code Council, Falls Church, VA, 2009.
2. International Building Code^®, International Code Council, Falls Church, VA, 2006.
3. 2008 ICC Final Action, G117-07/08; G119-07/08; G120-07/08; G121-07/09, International Code Council, Inc., Minneapolis, MN, September 17-23, 2008.
4. Basic Building Code, Building Officials and Code Administrators, Country Club Hills, IL, 1950.
5. George E. Strehan, "The BOCA Plan for Better Codes," Engineering News Record, Vol.143, No. 3, January 20, 1949.
6. NFPA 101®, Life Safety Code^®, National Fire Protection Association, Quincy, MA, 2009.
7. NFPA 5000^®, Building Construction and Safety Code^®, National Fire Protection Association, Quincy, MA, 2009.
8. Goal-Oriented Systems Approach to Building Fire Safety, PBS P 5920.9, Building Fire Safety Criteria: Appendix D, U.S. General Services Administration, Washington, DC, 1972.
9. SFPE Engineering Guide to Performance-Based Fire Protection, National Fire Protection Association, Quincy, MA, 2007.
10. NFPA 550, Guide to the Fire Safety Concepts Tree, National Fire Protection Association, Quincy, MA, 2007.
11. Hasofer, A.M., Beck, V.R., Bennetts, I.D., Risk Analysis in Building Fire Safety Engineering, Elsevier, First Edition, Oxford, UK, 2007.
12. Thomas, I.R., Bennetts, I.D., Poon, S.L., Sims, J.A., The Effect of Fire in the Building at 140 William Street, BHP/ENG/R/92/044/SG2C, Broken Hill Proprietary Company, Ltd., February 1992.
13. PD 7974-7: 2003, Application of Fire Safety Engineering Principles to the Design of Buildings - Part 7: Probabilistic Risk Assessment, British Standards Institute, London, UK, 2003.