possible outcomes and compare against the performance criteria. The basic process is described in NFPA 101: 5.2, with additional information in the SFPE Handbook and in the NFPA Handbook.
From the NFPA 101 Handbook, the flow chart in Figure 24 is a suggested method to use for a performance based approach.
FIGURE 28. PERFORMANCE BASED LIFE SAFETY CODE COMPLIANCE FROM NFPA 101 A-5.1.1
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This method was followed based on the NFPA 101 flowchart in Figure 28.
1. The building design is retaining the prescriptive elements as described earlier in the report per NFPA 101 5.3. This includes the fire protection, building features, means of egress, and applicable NFPA codes and standards.
2. The design fire scenarios were developed based on NPFA 101 5.5 requirements and are as follows:
a. Design fire scenario one is an analysis of the South Tower based on NFPA 5.5.3.1 and a typical fire for the occupancy accounting for occupants, number and location, room sizes, contents, fuel properties, ventilations, and identifying the location of the item ignited. This is a chair fire that is ignited by a smoldering source in the common area on the lower floor of the two-story space.
b. Design fire scenario two is an analysis of the Ground Floor Cafeteria and
Assembly Area based on NFPA 5.5.3.2 and an ultra-fast fire in the primary means of egress reducing the overall means of egress by two double door exits. This is an ultrafast fire based on a Douglas fir tree used as a Christmas tree.
3. Per NFPA 5.4, there were no changes to the construction, fire protection features, or egress for this analysis. Occupant characteristics have not changed from what has been described previously.
The time of day for each scenario is a specification to evaluate the design at the highest occupant loads, or during the least occupied times to achieve the worst potential conditions for the evaluation.
a. Design fire scenario one was analyzed based on a fire occurring during the night, or a time when the common area was not fully occupied.
b. Design fire scenario two occurs during the day when the space is occupied and the occupant load is close to the calculated values.
4. The goal is established per NFPA 101, section 4.1 to protect the occupants and provide safe egress.
5. The objectives per NFPA 101 section 4.2 are to protect the occupants with a structure designed to allow safe egress and protect occupants who are not intimate with the initial fire time to evacuate.
6. The performance criteria established for the scenarios per NFPA 101 section 5.2 is
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based on tenability criteria to determine if with the given the design fire, all occupants can exit safely using an analysis of Acceptable Safe Egress Time (ASET) and the Required Safe Egress Time (RSET). The tenability criteria are visibility, carbon monoxide, and temperature (hyperthermia).
7. The prescriptive elements, design fire scenarios, and specifications with the performance criteria was presented to the Authority Having Jurisdiction (AHJ) for review and comment. After discussing the performance based approach, the AHJ approved the scenarios and performance criteria.
8. The design fire scenarios were evaluated using Fire Dynamics Simulator (FDS) versions 6.0 with models created from the building design. The results of the FDS simulation provided tenability results that were coupled with the egress analysis to determine the Acceptable Safe Egress Time (ASET) and the Required Safe Egress Time (RSET).
9. The results were compared to performance criteria, and an iterative process takes place to revise the trial designs and if needed the objectives can be redefined if it is not possible to find a viable design.
10. If the trial designs meet the performance criteria, the final design is selected.
The following method is discussed in detail in the following sections. The scenarios were evaluated multiple times each until an acceptable outcome was found where the ASET exceeded the RSET with an acceptable margin of safety.
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Tenability criteria was selected from The Society of Fire Protection Engineers (SFPE) Handbook 4th Edition which serves as a design guide to Fire Protection Engineers. The performance criteria of visibility, carbon monoxide poisoning, and temperature exposure are common limits to assist in defining when a space is not tenable.
1. Visibility at height of 1.8m above floor per SFPE Handbook Table 2-6.11, s required to be maintained at an optical density of OD/m 0.08 (10m visibility) for a large enclosure, and an OD/m 0.2 (5m visibility) for a small enclosure. Testing and analysis has shown that 30% of occupants turn back rather than enter smoke with a visibility at 4m typically leading to death.
2. Carbon Monoxide concentrations at height of 1.8m above floor per SFPE Handbook Table 2-6B are required to be maintained at a maximum of 1,700 ppm leads to incapacitation in 30 minutes.
3. Temperature at height of 1.8m above floor per SFPE Handbook Table 2-6.17 and Figure 29 is required to be maintained at 110 °C for 25 minutes, or 70 °C for 60 minutes to avoid the effects of hyperthermia. The goal is to maintain upright egress if possible.
FIGURE 29. THERMAL TOLERANCE FROM SFPE FIGURE 2-6.27
USE AND LIMITATIONS
This analysis is an estimate of the time to evacuate the building based on engineering and accepted design methods. It is not a precise number as any number of factors from time of day to weather can have an effect on the overall time. This is a student dorm and the day after a popular football game could leave a large population of the tenants reacting slowly to any emergency situations.
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The analysis is limited to being as estimation of the best possible conditions. It does not directly address the rescue of a mobility impaired individual on the fire floor. This would require a fire fighter to ascend the staircase against the flow, rescue the individual, and descend the stairwell.
SMOKE DETECTOR ACTUATION AND DETACT
Smoke detection performance was analyzed to assist in the calculation of the available and required egress times for a few scenarios consistent with the egress analysis. The scenarios presented are a dormitory room fire alerting the occupants, a chair fire in the common area with one based on the initial design fire scenario.
The DETACT model, or DETector ACTuation Time Squared model is used to estimate the actuation of detection devices based on a t-squared fire. The model will estimate the device actuation using transport equations, temperature, and the response time index (RTI). In our models for smoke detectors, criteria are used relating the activation of smoke detectors with heat detectors. For the smoke detection models, Actuation temperatures of 30 degrees Celsius and an RTI of 5 m-s1/2 were based on NFPA 72 Annex B and Table
B.4.7.5.3.
SMOKE DETECTION PERFORMANCE - ROOM SCENARIO
In the SFPE Handbook Chapter 3 provides heat release rates (HRR) for mattress fires. There are a number of variables involved including mattress composition and thickness and no clear standard heat release rate. Peak heat release rates for these fires can vary from 19kW to 2,550kW.
The DETACT model calculation input parameters are shown in Table 23 for the room fire scenario. Radial distance in Table below is based on the size of the dormitory room.
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TABLE 23. DETACT INPUT PARAMETERS AND RESULTS – DORMITORY ROOM
For a typical fire in a dorm room, the time for smoke detector activation at 30°C is approx.
25 seconds with a HRR of approx. 32kW. Tracing the detector activation on Figure 30 provides similar results. Activation is fast as expected, with the gas temperature and detector temperature closely following each other as there is minimal thermal lag in the detector.
FIGURE 30. DETACT CURVE FOR CHAIR IN DORM ROOM
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SMOKE DETECTION PERFORMANCE – POLYUERETHANE CHAIR FIRE IN COMMON AREA The SFPE Handbook Chapter 3 provides information on furniture fire heat release rates (HRR). Test specimen 21 is a chair with flexible polyurethane foam. Peak heat release rates for this fire were 2,000 kW per Figure 3-1.52. Other input parameters are in Table 24.
TABLE 24. DETACT INPUT PARAMETERS AND RESULTS – CHAIR FIRE IN