Planificación

The Available Safe Egress Time (ASET) is the amount of time that elapses between fire ignition and the development of untenable conditions within a given room or space. The Required Safe Egress Time (RSET) is the amount of time, which is also measured from fire ignition, that is required for all occupants to evacuate a building or given space in order to reach either the building exterior or some dedicated, protected exit enclosure. RSET is defined by the general expression below and includes the following time subsets:

RSET = Detection Time + Notification Time + Pre-Movement Time + Movement Time Detector Activation Time (td)

Detector Activation time is the time that elapses from ignition of the fuel source until activation of the first detector device. The detector device could be either automatic or manual, but is usually based on any automatic detection systems in place.

Notification Time (tn)

The time until notification is the time that elapses between detector activation and notification appliance activation. This is usually assumed to be almost instantaneous in modern systems.

Occupant Pre-Movement Time (tp-m)

Page | 49 The pre-movement period is the time from when occupants receive notification until they begin to evacuate, and includes perception, interpretation, and any delays before movement. No situation will arise in which occupants within a building will react to evacuation alarms in the same way, and individual people will react differently based on setting and location. There are several factors that influence the pre-movement times of occupants within buildings after they have been alerted to a fire or other life safety emergency. Occupant reaction and movement times are influenced by a combination of audio, visual, and physical factors which include the following:

Audio Factors:

 Fire alarm

 Sprinkler system water alarm

 Occupant to occupant notification

 Audio cues of an impending fire situation Visual Factors:

 Seeing the fire

 Seeing combustion products (smoke)

 Secondary impacts from fire (shattering glass) Physical Factors:

 Smelling smoke

 Feeling heat

 Other physical cues stemming from a fire situation

Bryan’s chapter in the SFPE handbook outlines several psychological and physical processes that are related to pre-movement time:

 Recognition: The initial stages of the occupant’s awareness that a fire situation is developing. Audio and/or visual cues will help differentiate situations.

 Validation: This occurs when the occupant defines whether a fire situation has developed or not. More information is usually sought after.

 Definition: In this stage, the occupant defines the fire situation and processes available information relating to the situation.

 Evaluation: Occupant fully realizes the extent of the situation, acknowledges the presence of a potential life threatening situation, and comes to the conclusion that egress is necessary, and goes about determining the best way to do so.

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 Commitment: Some situations arise in which the occupant may not want to leave, and during this stage the occupant decides whether to evacuate or to stay in place.

 Reassessment: Stress and anxiety levels of the occupant are at its highest levels at this point. Continual reassessment of whether to evacuate or not is at the forefront of the occupants mind.

It is hard to try and model the pre-evacuation time for a given scenario due to the many factors involved, so typically data is used that has been collected on delay times regarding typical occupancies. One such source of data is Table 4.2.1 from the NFPA Handbook which characterizes delay times for hotels, offices, and apartment buildings of varying heights.

Furthermore, there are six building characteristics that can help reduce the pre-movement time of building occupants. Proulx outlines these characteristics in the SFPE handbook:

 Types of warning systems: Advance and appropriate warning systems, such as fire alarms, smoke alarms, or voice evacuation systems, will have a positive effect in reducing the pre-movement times of occupants. Live messages meeting NFPA audibility and intelligibility have been shown to be the best way to move occupants from unsafe locations to safe areas.

 Building layout and way finding: Appropriate building layouts and exit signage is important for helping occupants decide where to go and how to go about exiting.

 Visual Access: Clear visual access of alarms and other occupants helps decrease pre-movement times by speeding up the decision of occupants to leave. Clear visual paths of exit signage is important as well.

 Training: Training is one of the most important aspects of decreasing pre-movement times of occupants, although it is not always possible (for example transient

occupancies like malls). Regular fire and evacuation drills, either scheduled or unscheduled, help occupants be aware of proper procedures and pathways.

 Frequency of false alarms: To many false alarms create a complacent attitude and environment for occupants, so when a real life threatening fire situation arises, important seconds can be lost before occupants realize the severity of the situation.

Occupant Movement Time (tm)

Movement time, or occupant travel time, is the time period from when occupants decide to evacuate until they safely egress the building. Pathfinder was used to model the building and occupant loads using the occupant load factors discussed previously. Both the Steering and SFPE models were utilized in determining occupant travel time, but the results from the Steering model were used in computing the RSET value because of its more complex and accurate nature. The SFPE method calculates travel time based on flow rates through egress

Page | 51 components (doors and stairways) but allows occupants to occupy the same space. The

Steering method takes into account agent collision, inertia and momentum, and other factors during queuing and travel.

A Pathfinder simulation was modeled of egress under normal conditions (no fire, all exits available to occupants) in order to gain a baseline egress time for later comparison. The Steering and SFPE values for that simulation are as follows:

 Steering: 70.5 seconds

 SFPE: 80 seconds

A successful analysis is completed if ASET > RSET + Factor of Safety. The performance based design of this report will utilize the computer fire modeling software discussed previously (Fire Dynamics Simulator and Pathfinder) in order to complete an ASET/RSET analysis. Pathfinder will be utilized in order to calculate the movement time of building occupants in order to ultimately determine the Required Safe Egress Time. Fire Dynamics Simulator will then be used in order to model various reasonable but challenging fire scenarios in order to approximate the time needed to reach the predetermined tenability limits (ASET). Once both values are

obtained, a comparison can be made, and depending upon the relationship between the ASET and RSET values, it can be reasonably determined whether or not the building meets the performance criteria set forth for each individual fire scenario.