FIRE SAFETY: Modelling the Future
August 1, 2002
By Randall Kovacs, P.Eng
Fire protection engineers have a variety of engineering tools they can use to develop accurate predictions of the consequences of a fire in a building. By modelling a fire and its growth or suppressio...
Fire protection engineers have a variety of engineering tools they can use to develop accurate predictions of the consequences of a fire in a building. By modelling a fire and its growth or suppression, they can design an effective fire protection strategy for a building. These tools help the engineers meet the fire and life safety building code objectives, without undue restrictions on the architectural or functional objectives of the design.
Recent research in fire growth, fire dynamics, smoke spread and human behaviour has fostered the development of highly specialized dynamic models. The models are used to predict the outcome of fire scenarios chosen to represent credible, worst-case occurrences. Once the outcomes are known, the engineer can design systems such as automatic sprinklers, fire alarms, smoke controls and passive forms of fire protection to mitigate the most likely risks.
When the next edition of the national and provincial building codes are introduced they are to be in an objective-based format. The requirements governing fire and life safety, structural safety, health, plumbing and accessibility will be contained in broad objective statements. The basis for compliance with the objectives will be included in a second part to the codes containing a set of “acceptable solutions.” The acceptable solutions will be a revised version of the prescriptive requirements of current building codes in use in Canada.
Modelling will play a major part in fire protection once objective-based codes are adopted. However, there are a number of issues surrounding modelling predictions that still need to be resolved. This article will look at how engineering models are key to designing for objective based codes, and it will consider some of the major types of modelling.
More freedom for designs
A fundamental assumption in the existing, prescriptive building codes is that a fire can start anywhere in a building. This comprehensiveness may no longer be necessary with objective-based codes, because in principle they should allow a more deterministic approach. By evaluating the hazards attributed to the design, construction and eventual use of a building, a designer should be able to identify the most probable fire scenarios that would occur. He or she can then use engineering models to simulate fire and smoke spread through a building and how the fire safety features will react. Provided that the models are based on realistic representations of the anticipated size, rate of growth and location of the fire, and provided that valid assumptions are used in applying the models, the designer can use the simulation to show that a design will achieve code objectives.
Dynamic smoke spread models can be used to demonstrate, for example, that effective sprinkler protection, coupled with a smoke exhaust system may be just as effective at providing an acceptable period of time for occupants to safely evacuate a building as a passive fire separation that otherwise would be prescribed by the existing building code. Innovative building designs such as large, open atria can be modelled to show that the same level of safety prescribed by the “acceptable solutions” can be provided for the occupants and fire fighters through the judicious selection of fire protection features.
Modeller’s tool chest
The fire protection engineer’s tool chest contains an array of specialty type models. The models fall into the following categories.
Hydraulic Models — Primarily used in the design of automatic sprinkler and standpipe systems, the models are based on the Hazen Williams friction loss formula. The hydraulic models simulate water flow in pipes, and the discharge of water through sprinkler heads, nozzles and hydrants. This is probably the only type of fire protection model that is in widespread, everyday use.
Zone Models — These are relatively simplistic models that simulate how smoke and hot gases behave in one or multiple fire enclosures. Many of the models can simulate the effects of natural or forced ventilation. They are known as zone models because they separate an enclosure into two distinct zones: an upper, hot layer of smoke, hot gases and the products of combustion, and a lower, cool layer. The zone models solve mass and energy conservation equations for pressure, the temperature of each of the layers, and the layer interface height above the floor. They simulate how the hot layer descends down from the ceiling of a room enclosure as a function of time. This is a very useful piece of information, because the height of the smoke layer is directly related to the time an occupant of a building has to evacuate. The models can be run on a personal computer, but because they are relatively simplistic, their results are limited. They treat each zone as a homogenous mixture, having the same physical properties throughout, and can only evaluate square or rectangular rooms. If other configurations are employed, they must be approximated as a rectangle with appropriate shape factors. The fire size and growth rate are input parameters, as is the room configuration. Fires are treated as point sources, and multiple fires in the same compartment are generally not permissible yet.
Field Models — These are the next generation of dynamic models that predict fire and smoke spread in buildings. Rather than two simple zones, the field models are complex fluid mechanical models that solve mass, energy and momentum equations for very small elements, the size of which is set by the user. Nearly any fire compartment configuration can be simulated, as can the location of a fire within the compartment. The models generally include elaborate simulation aids, and they yield pressure, temperature and species concentrations at any point. The graphic packages give a visual representation of the smoke spread through the building or room. Because field models are complex, they need significant computer power to run them. However, they give a much more realistic representation of how a fire and the resultant smoke and products of combustion spread through a building.
Special Purpose Models — These models are specific routines developed to simulate key elements of fire protection. Examples include:
CONTAM — a smoke control model, used to simulate zoned smoke control or stairwell pressurization using natural or forced ventilation.
DECTACT/QS — a single compartment model used to determine activation times for heat detectors, smoke detectors or automatic sprinkler heads. The model is set up based on ceiling height and fire size.
FIRES-T3 — a structural heat transfer model used in failure analysis.
EVACNET — predicts the time required for the evacuation of a building or portion thereof, based on the number of occupants and the configuration of the egress routes. It accounts for the width of the means of egress including corridors and stairs, and constrictions or bottlenecks in exit paths such as doorways. This is a key tool for determining the time required to evacuate a building, and it can be used to compare the evacuation times for configurations that require a departure from the standard travel distances prescribed by the existing codes.
Proceed with caution
The introduction of the new objective-based codes together with the use of the latest fire protection engineering tools to demonstrate compliance raise a number of questions. Most of these issues are already being addressed but they need to be fully resolved and the solutions adopted as part of the design process for building construction under the new code. Here is a summary of key issues.
Validation of models. While excellent progress is being made in developing new programs for modelling fire behaviour, each model must be rigorously validated by comparing the predictions to controlled test results. If the model is successfully validated, it can be accepted as having a reasonable level of reliability.
Operating within limitations. Each model has fundamental assumptions and limitations, which, if not adhered to, will yield misleading results. Skillful engineering j
udgement and experience is needed to establish the applicability of a model to an architectural configuration.
Uncertainty. The selection of the likely size and location of the fire is based on a probabilistic determination, but the assessment will often require the use of multiple “design fire” scenarios, usually indicative of the “worst case” scenario. In addition, while the computer models may provide valid predictions of the outcomes of a design fire, the probability that the fire will start in a location other than as designed still exists. Other events, such as shut-downs or the failure of fire safety systems, contribute to the inherent uncertainty in the outcome of a real fire in a real building. The tolerable degree of uncertainty must be acceptable to building code officials, the building’s owners and users, and to society as a whole (manifested in the objectives of the building codes).
Third party review. Use of the models requires the designer to have extensive knowledge of fire protection principles, an understanding of the limitations of the models and the ability to properly interpret the results in the context of the fire and life safety objectives of the codes. Current designs are evaluated against a prescriptive set of guidelines by independent third parties, usually the municipal building officials. Qualified individuals will have to be properly trained to be able to evaluate building plans based on a fire protection program designed to meet the objectives of the building codes rather than the prescriptive requirements. This issue has been raised by the governing code bodies and will have to be resolved before the new objective based codes can work to promote innovative designs.
Building Construction. Many of the existing prescriptive building details will continue to be built in the field. Stair tread and riser dimensions, handrails and guards, for example, will likely continue to be designed and built according to the provisions in the existing prescriptive codes. The fully objective based design will be useful primarily for larger buildings and those having unique designs that offer innovative solutions for the end user’s needs. If a truly objective-based design is to be implemented as a code compliant design, the specified fire protection components need to be properly and reliably installed in the field.
Changing end use. A building designed on the basis of meeting broad objective statements for fire and life safety will be formulated to suit a specific hazard or particular use. If the building use or operation changes over time, however, it will be necessary to ensure that the design parameters and assumptions used in the development of the fire protection program remain valid. The owners and users must take responsibility in ensuring that the use of the building doesn’t outgrow the original fire protective design that was built. The same is true for maintaining the fire protection features that were originally installed.
The objective based building codes could be implemented in the provinces as early as 2004. In the meantime, fire protection technology will continue to grow as more efficient and accurate computer modelling tools become available. Fire protection engineers must be aware of the above issues as they prepare to work with the models and the new codes, and make the transition into what may prove to be a new era in fire protection engineering.
Randall Kovacs, P.Eng. is the president of Gage-Babcock & Associates, of Vancouver, a fire protection, life safety and security design consulting engineering firm. He can be reached at: firstname.lastname@example.org.