Performance-based structural fire design
In the codes and standards reviewed, there is general agreement that the performance goals of structural design against fire are to limit risks to the individual and society, to directly exposed or neighboring property, and to the environment. Satisfactory performance can be demonstrated by engineering analysis or qualification testing. To achieve these overall goals, many countries are currently developing performance-based standards that would allow designers flexibility in the use of new materials and technology, while possibly reducing cost. According to Buchanan (2001), performance-based design starts with the setting of general, high-level goals, and then gets more specific with the definition of functional objectives and performance requirements that guide the designers to meet these goals. At the design level, performance-based standards recommend acceptable solutions and approved calculation methods, but leave open the possibility of alternative designs, provided these can be proven to meet the performance goals. Compliance with performance-based codes can be attained by using either prescriptive methods (sometimes called acceptable solutions or approved calculation methods), or performance-based design (PBD).
Society for Fire Protection Engineering (SFPE)
Engineering Guide to Performance-based Fire Protection, 2nd edition, 2007
The SFPE Guide provides a general framework for performance-based design against fire.
The First step is to define performance objectives, which are to mitigate the consequences of fire in buildings in terms of loss of life, financial cost on property, impact on operations and the environment, or maximum allowable conditions. These conditions include stability of structure, integrity of partitions, maximum temperature, extent of fire and smoke spread, and spread of combustion products.
The second step is to develop performance criteria, i.e., assign threshold values for temperature of materials and gases, toxic gas emission, thermal effects on structures, fire spread, fire barrier damage, structural integrity, damage to exposed properties and the environment, etc. These can be stated as either deterministic criteria, e.g., preventing flashover in the room of fire origin, or probabilistic criteria, e.g., reducing the probability of flashover below a threshold value.
The third step is to develop design fire scenarios
The fourth step is to develop trial designs for fire protection systems, construction features such as fire barriers, and operational procedures that meet the specified performance criteria for the design fire scenarios.
The fifth and final step is to evaluate the trial designs and select the final design based on effectiveness, reliability, availability and cost.
NFPA 5000 Building Construction and Safety Code Handbook (2003)
According to the NFPA 5000 Building Code, at a minimum, a fire scenario consists of the following:
· Ignition factors (source, location and material);
· At least one heat release rate curve;
· Occupant locations;
· Occupant characteristics;
· Special factors (shielded, systems unreliable, open door).
NFPA 5000 recommends the following methods to select fire scenarios:
· statistical analysis of fire experience of similar buildings, e.g., fast fire growth in room contents, to select common scenarios
· refine common scenarios, e.g., flammable liquids in means of egress, to select high challenge scenarios;
· Special problems, e.g., sprinklers or fire detector out of commission, to select special scenarios. More specifically, NFPA recommends eight fire scenarios:
- Occupancy-specific and representative of typical fire for that occupancy. This first scenario must explicitly specify the following: occupant activities; number and location of occupants; room size; furnishings and contents; fuel properties and ignition sources; ventilation conditions; first item ignited and its location. Example: a hospital room with two occupied beds, fire initially involving one bed and room door open.
- Ultrafast developing fire in the primary means of egress, with interior doors open at the start of the fire. This scenario is intended to address reduction in the number of available means of egress. Example: fire in clothing rack in corridor.
- Fire starts in a normally unoccupied room that can endanger a large number of occupants in other areas. Example: fire starts in a storage room adjacent to the largest occupiable room in the building.
- Fire originates in a concealed wall space or ceiling space adjacent to a large, occupied room.
- A slow developing fire, shielded from fire protection systems, close to a high occupancy area. Example: cigarette in a trash can.
- Most severe fire resulting from the largest possible fuel load characteristic of normal operation.
- Outside fire exposure.
- Fire originates in ordinary combustibles in a room where each active or passive fire protection feature is rendered ineffective.
All prescriptive building codes and standards surveyed specify fire rating depending on occupancy, type of structure and structural component, and based on criteria of stability, integrity (ignition of material separated from fire by barrier with required integrity, propagation of smoke) and insulation (maximum and average temperatures on surfaces not directly exposed to fire). Of the testing methods used to establish fire rating, ASTME 119 puts particular emphasis on restrained versus unrestrained end conditions of beams and floor and roof assemblies.
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