Comparison of Available Safe Egress Time (ASET)
For the performance-based design of smoke control systems, and atrium smoke control systems, in particular, an ASET/RSET analysis is usually conducted. The Available Safe Egress Time (ASET) is the amount of time that elapses between fire ignition and the development of untenable conditions. The Required Safe Egress Time (RSET) is the amount of time (also measured from fire ignition) that required for occupants to evacuate a building or space and reach the building exterior or a protected exit enclosure. A candidate design is acceptable if the ASET is greater than the RSET, after applying an appropriate safety factor.
ASET is determined by applying empirical correlations or fire modeling. First, a design fire (heat release rate history) is established by considering the types of combustibles present and their associated product yields (primarily soot and carbon monoxide). Next, this design fire is provided as input to a calculation tool such as a fire model to determine the time after ignition at which space through which occupants must pass becomes untenable due to the presence of smoke or heat. The time at which conditions become untenable (ASET) is the time at which the amount of smoke or heat, as calculated by a fire model or similar tool, first exceeds pre-established tenability criteria. Sample tenability criteria for determining ASET might be:
- Visibility must remain above 10 m.
- The temperature must remain below 65° C.
- Carbon monoxide concentration must remain below 1,400 ppm.
RSET is the sum of the alarm time, the evacuation delay time (sometimes called the pre-movement time), and the movement time. Alarm time is the time at which occupants first become aware of a fire through a building’s automatic or manual fire alarm system (occupant notification). The evacuation delay time, or pre-movement time, is the time that elapses between activation of the occupant notification system and the time at which occupants make the decision to begin evacuating. Pre-movement activities might include an investigation to determine if the fire is &real&, gathering belongings, searching for friends and family, etc. Depending on the type of occupancy, the pre-movement time maybe a few seconds or a few minutes. Finally, the movement time is the time required for occupants to reach a protected exit enclosure or the exterior of the building once the decision to evacuate has been made and occupants begin moving toward exits. The movement time is calculated by applying empirical relations for walking speed and occupant flow rates through egress elements such as doors, stairs, and corridors, or by applying evacuation modeling such as FDS-EVAC. Due to uncertainties associated with human behavior, a factor of safety is generally applied to the movement time (and occasionally the alarm time, pre-movement time, and evacuation time) before the RSET is calculated.
In the context of an ASET/RSET analysis, there are several tools at the designers’ disposal that can be used to develop customized fire protection and life safety systems for the building under consideration. The ASET can be increased by limiting combustibles, providing adequate separation distances between fuel packages, providing customized fire suppression systems to suppress incipient fires or limit peak heat release rates or provide active or passive smoke control systems. On the RSET side, strategic placement of smoke detectors (spot-type, aspirated, or projected beam) or UV/IR flame detectors can be used to reduce the detection time. Pre-movement time can be reduced by specifying an occupant notification system equipped with voice occupant notification, particularly one that can communicate live voice messages to building occupants. Movement time can be reduced by strategically placing exit signage and arranging the means of egress in an intelligent way to prevent pinch points, excessive queuing, etc.