Características generales: Performance Based Design (PBD) 11 agosto 2010Posted by pbdfire in performance based, simulacion de incendios.
Tags: Fire Modelling, performance based design, SFPE, simulacion de incendios
Os incluimos este artículo, que nos ha parecido más que interesante, sobre las características generales del Performance Based Design, con algunos ejemplos sobre su aplicación, su implicación en el diseño de instalaciones de protección contra incendios, incluso con la implicación del coste del mismo en el proyecto.
El artículo original lo hemos extraído de la revista oficial de la SFPE.
Lo dejamos en el idioma original. Si alguno tiene alguna consulta o aclaración, no tiene más que pedirla por este medio.
GENERAL FEATURES OF PERFORMANCE-BASED DESIGN
As the name implies, performance-based design is design that meets a specified performance level. The structural and mechanical engineering disciplines have been using performance- based design for many years.
For example, structural engineers size the structural members of a bridge to support the weight of the bridge plus expected live loads, dead loads and loads from snow, wind earthquake and other sources. They demonstrate that the bridge can support these loads with structural calculations.
Mechanical engineers design HVACR systems to cool spaces by so many degrees in a certain amount of time based on the expected heat loads. They demonstrate that the systems can provide this cooling with heat exchange calculations.
Performance-based design is a newer development in fire protection engineering. One type of performance-based fire protection design ensures that smoke will not reach occupants before they can safely evacuate the building. Fire protection engineers demonstrate this with calculations of smoke development and occupant egress based on an evaluation of the types of fires that may be expected to occur in the facility.
Another type of performance-based fire protection design ensures that fire in a compartment with a given amount and type of combustibles will not proceed to flashover. Fire protection engineers demonstrate this with calculations of layer temperatures or heat release rate as the fire burns. Determining when flashover will occur then helps determine whether the fire will spread beyond the room of origin and what effect it will have on the rest of the facility and its occupants.
These types of designs are becoming more popular because they allow maximum flexibility in meeting the fire-protection needs of buildings. However, the decision to use performance-based design in fire protection engineering is not as cut and dry as it is in the other engineering disciplines.The main reason is that assumptions about expected fire loads are more likely to change than assumptions about expected structural and heat loads. A second reason is that structural loads are standardized, codified and accepted by society. Fire loads are not. Another reason that performance-based design is not as common in fire protection engineering as it is in the other engineering disciplines is because these design methodologies, along with an understanding of what design bases are acceptable, are still evolving. Predicting fire behavior is comparable in complexity to modeling weather. However, weather modeling has many more years of research to support it than fire modeling does. Furthermore, the accuracy of weather forecasts is not as critical as accuracy in prediction of fire behavior.
Modeling the effect of suppression systems on fire is similarly complex. In addition, fire models encompass substantial limitations, and these models can be difficult to understand and use appropriately.
EXAMPLES OF PERFORMANCE-BASED DESIGN
Prescriptive codes apply to facilities with widely varying characteristics. For example, the same prescriptive codes apply to an office building whether it has one or several stories, small or large building footprint, or low or high combustible loading. Different provisions in these codes might apply to different buildings, so the prescriptive protective scheme can vary from building to building. But some features are unlikely to change from one office building to another.
A performance-based design creates a unique set of requirements for each building based on specific assumptions derived from the project design goals. The sidebar on the right describes three possible examples of such designs.
These three examples of performance- based design are hypothetical and simplified. Performance-based designs would normally use a bounding scenario (worst case) to the building. However, the design assumptions can still restrict the future use of a facility. This is why prescriptive designs are not always less flexible than performance-based designs, as is often believed.
Implementing performance-based design requires close cooperation with the authority having jurisdiction (AHJ) throughout the design and construction process. AHJs usually want experienced fire protection engineers to review these designs.
USING FIRE MODELS IN PERFORMANCE-BASED DESIGN
Computer fire models are increasingly being used to substantiate performance-based design of fire protection systems. Most computer fire models don’t model fire so much as the effect of fire on the compartment where it is burning. For example, the models estimate temperature, smoke descent, flame spread, time to flashover and many other effects. (See sidebar right.)
The fire models simulate these effects in many ways. Because accurately modeling fire at the molecular level is beyond the capability of today’s computers, fire models consider only the large-scale features of flames and rooms, and they use many approximations. They also put limits on such things as compartment and flame geometry.
Using these models appropriately requires knowing which effects they estimate, what approximations they make, what limitations apply and how closely the results can be expected to reflect the risk in the facility being modeled. This level of knowledge comes from having a background in fire protection engineering and experience with using the models.
Fire modeling software can be difficult to use. Furthermore, the output from fire models can be difficult to review. The pretty picture is not necessarily what would actually happen. Close review of the input and detailed understanding of how the model processes the input are necessary to judge whether the model was used appropriately.
Some models use results from fire tests to characterize the computerized fires. These models are good for analyzing fires similar to those in the tests (same commodity burning in the same configuration and the same size fire). Using the results of a test to model different conditions must be done with a good understanding of what effect the differences could have.
Computer fire models are becoming more popular, but they are still not the answer to every fire protection design problem. These models are still evolving and must be applied with care. Even experienced fire protection engineers still struggle with some fire modeling applications.
USING NEW TECHNOLOGIES IN PERFORMANCE-BASED DESIGN
In addition to computer fire models, performance-based design of fire protection can also use new fire protection hardware technologies. Well-known devices for which new technologies are being developed include sprinklers and fire alarm systems. Other new hardware is also being developed.
New technologies for both sprinklers and fire alarm systems have prescriptive rules. The prescriptive rules usually lag the introduction of new technologies. Performance-based design can be used to implement new technologies before they are formally recognized by prescriptive codes.
The standard spray sprinkler flows water through a standard orifice against a deflector that produces a particular spray pattern. This spray sprinkler was used almost exclusively in North America until 1981, when the residential sprinkler was developed. The engineering required to develop the residential sprinkler was quickly applied to sprinklers for nonresidential areas of certain geometry or containing particular hazards.
Many sprinklers are now available for special geometries. Other sprinklers are available for protecting particular hazards. (See sidebar right.)
In the realm of prescriptive codes, special-geometry sprinklers must be installed according to the rules for location and spacing that are cited in their product listings. Likewise, sprinklers for protecting particular hazards must be installed in accordance with very strict rules. For example, ESFR sprinklers can protect limited types of storage commodities to maximum heights, with an allowable range of clearance from the top of storage to the roof. ESFR sprinklers must conform to strict rules for location and spacing, and they must meet strict hydraulic criteria.
Using sprinklers designed for particular geometries and hazards requires extensive knowledge about their designs and limitations. The more specialized a sprinkler, the more likely a design change would be needed if the occupancy changes.
Choosing a sprinkler is now more complex than ever.
Fire Alarm System Technologies
Fire alarm systems monitor buildings for adverse conditions. Conditions include alarms (operation of a sprinkler, manual pull station, smoke detector or heat detector), supervisory conditions (loss of heat or closure of a sprinkler system control valve) and trouble (loss of power or an open circuit).
Older fire alarm systems send generic signals to the alarm receiving station but do not provide responding plant or fire department personnel with detailed information on the alarm or its location. For example, the systems would report a fire alarm, but not the device that actuated or its location. Responders would have to investigate for themselves.
Today’s electronic alarm systems are far more advanced. Their circuits are much simpler, and they can monitor many more devices. These systems interrogate, or poll, each connected device to always know its status. The devices themselves are smart and can tell the panel when something is wrong. For example, a smoke detector knows when it is dirty, adjusts its own sensitivity and reports this to the panel.
These alarm systems can be programmed to display any information desired to responders. For example, the system can tell a responder where every device is located and what type of device it is. It can report that if a particular zone is in alarm then certain additional actions should be taken.
Graphic annunciators are the norm. Overlaid on a plan of the building, they visually display the exact location of the device that actuated, the alarm the alarm type and the action required. These annunciators are installed where the fire department would be expected to enter the building. They can also be installed at central alarm-monitoring locations.
These advanced fire alarm systems put digital technology to work to enable the fastest and most-effective response to fire alarms. Today’s fire alarm systems also integrate many nonfire protection features. Examples of these features could include paging and security systems. The systems are designed so that failure of any nonfire protection function cannot disable the alarm-monitoring function.
New features are constantly being designed into fire alarm systems. These features can take years to be addressed by code. Whenever the codes lag development of new fire alarm technology, performance-based design can be used to fill the gap.
Another example of a newer fire protection technology that is being worked into performance-based design is the very early warning fire detection system.1
Jane I. Lataille is with the Los Alamos National Laboratory.