Computer Fire and Egress Modeling

Sept. 1, 2009
Use computer fire and egress modeling to save lives and protect your buildings  

Building owners appreciate designers and facility managers who find innovative ways to make efficient use of company funds without compromising safety. If you view fire protection as no more than sprinklers and fire-alarm systems, consider the contributions that modern fire-protection engineering can make to your facility or project.

From analyzing the response of a unique structure during a fire condition to evaluating the time needed for occupants to safely exit a building, advances in computer resources have provided new tools for the design and analysis of fire protection and life safety for buildings. Today, computer analysis of fire development, fire-protection system response, and occupant evacuation is often applied to the design of buildings. Could your buildings or projects benefit from modern engineering analysis of fire and egress?

Computer Fire Modeling
Approximately 30 years ago, scientists and engineers began implementing computer models to calculate the effects of fire. Early models were limited by available computer resources. A significant increase in available computer power during the 1990s provided the resources needed for complex calculations of the effects of fire and the ability to graphically illustrate the results.

Modern computer fire models include computational fluid dynamics (CFD) analysis that’s used to provide detailed calculations of the movement, concentrations, and temperature of smoke in a building. This is the same type of model used to design jet airplanes, but applied in a different context to calculate the movement of smoke and hot gases in a building.

Heat transfer models are also available to analyze the increase in temperatures caused by a fire condition. This might include analyzing the heating of sprinklers to determine appropriate locations or the heating of a structure to calculate how long the structure will withstand a fire condition. Special-purpose CFD models are available that include heat transfer models designed specifically for the analysis of fire conditions.

Computer fire models are available that use a 3-D model of a building or portion of a building for accurate calculations. The results of the calculations, which might include temperatures of a structure or locations where smoke moves in a building, can be visually illustrated in pictures or video animations of the fire condition. This provides information not only for engineers, but also helps those with a limited technical background to understand the results of very complex calculations.

Computer Egress Modeling
Computer egress models vary in complexity, with some able to analyze the movement of thousands of occupants through complex buildings and structures. More advanced computer egress models have the ability to import the actual floorplans for a building and account for the movement of each occupant through the building’s floors, stairs, and ramps.

Computer egress models are available that account for the movement of various types of occupants with different levels of mobility based on studies of people movement. These models calculate the speed at which occupants move toward and through exits during egress, and account for the interaction amongst occupants as they leave the building. The detailed information provided by computer egress models can be used to create video animations depicting egress from a building or structure.

A common application of computer fire modeling is the design and analysis of smoke-control systems. Mechanical or other systems used to protect occupants from smoke may be required in atriums; large assembly buildings, such as arenas; and high-rise buildings. Similarly, smoke and heat vents may be required in warehouses and factories.

The basic calculations for the design of smoke-control systems that are included in building codes often result in smoke-control-system designs that are inefficient and sometimes unrealistic. More efficient and effective smoke-control-system designs can be developed by using computer fire models to calculate the movement of smoke in a building and to design an appropriate smoke-control system based on an analysis of the actual conditions expected during a fire. This often results in a significant savings in construction cost while improving safety by more accurately accounting for the movement and concentrations of smoke in the event of a fire.

Structural fire resistance is an important component of fire-protection design for buildings. Fire-rated construction can be used to contain a fire condition within a portion of a building, protect occupants as they exit from a building, and ensure that a building remains standing as the temperature of the structure increases. Most commonly, fire-rated construction is specified based on fire-resistance-rated assemblies that are tested and listed by approved fire-testing laboratories. As an alternative, the unique configuration of a building can be analyzed using computer fire modeling to calculate the response of the structure to a fire condition. Buildings with very tall ceilings, areas that are open to the outside, and very large structural columns and beams can often be designed without the need for applied fireproofing based on an analysis of the actual exposure expected during a fire condition.

In the unfortunate event of a fire in a building, computer fire modeling has been used to help understand events that occurred during the fire. Experts who investigate and analyze fire conditions may use computer fire models to support theories on what happened during the fire and what might’ve happened under an alternative theory of the events. The National Institute of Standards and Technology (NIST) has made extensive use of computer fire models in investigating the 9/11 World Trade Center disaster, the Cook County Administration Building fire, the Station Nightclub fire, and several other multiple-life-loss fire incidents.

The design and evaluation of fire-protection systems for unique structures can benefit from the use of computer fire models. For example, sprinkler protection may become less effective in spaces with very tall ceilings. Computer fire models can be used to analyze the activation of the sprinkler system based on the actual dimensions of the building and to develop alternative protection systems for the space.

Similarly, the response of smoke detectors can be estimated to design efficient systems for early warning of a fire condition. In some cases, sensitive equipment or processes may require designing a system to detect very light concentrations of smoke; in other instances, a limited number of smoke detectors can be justified as meeting the requirements for the facility based on the results of a computer fire modeling analysis.

Building owners who are concerned about security often use computer egress modeling as part of a risk-assessment process. Computer egress models can be used to calculate egress times for buildings based on various scenarios of exit elements being blocked or rendered unavailable by malicious acts. Understanding the amount of time required for occupants to exit during different scenarios can help building owners and security personnel make informed decisions regarding protection of a structure and its occupants.

Similarly, computer fire modeling can be used to assess the impact of fire on a building and the threat to the building’s occupants. Whether protecting against malicious acts or understanding the impact of an accidental fire, computer fire modeling can be used as part of an owner’s risk-management program to help understand how a fire might threaten people and property.

Large or unique structures may require egress systems that differ from the specific requirements outlined in building and life-safety codes. For example, building and life-safety codes often limit the travel distance permitted from any point in a building to reach an exit. The results of computer egress modeling can be used as justification for extended travel distances beyond the limits typically required by the applicable building and life-safety code. The analysis might justify the extended travel distances by demonstrating through other features, such as increased door and stair widths, that the additional travel distance does not impact the amount of time required for occupants to reach an exit. Often, the results of a computer egress analysis are compared with the results of a computer fire modeling analysis to demonstrate that occupants are able to exit from a building prior to the onset of untenable conditions.

These examples illustrate several ways in which modern engineering analysis using computer fire and egress modeling can result in more efficient building design and improved operations while protecting a building, its contents, and, most importantly, its occupants and visitors.

Steven Wolin is technical director at St. Louis-based Code Consultants Inc.

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