Virtual : An Alternative Approach to Means of Egress Design in Airport Pedestrian Tunnel Xiaolei Chen, PhD California State University, Los Angeles, United States Ning (Frank) Wang, P.E. Jensen Hughes, United States INTRODUCTION Airport departure and arrival concourses are either grouped together to form a giant building, or located separately but connected through inter-concourse transit, such as train and bus, or through pedestrian walkway in the form of above-grade bridge or below-grade pedestrian tunnel. When compared with above-grade bridges, pedestrian tunnel is often a preferred and commonly adopted approach since it is able to avoid interference with normal functioning of the airfields. Currently, the typical length of a pedestrian tunnel varies from hundreds to thousands of feet. For instance, the pedestrian tunnels are about 250 meters (800 feet) long at Chicago O Hare International Airport, 280 meters (900 feet) long at Detroit Metropolitan Wayne County Airport, 300 meters (1,000 feet) long at Los Angeles International Airport (LAX), and 460 meters (1,500 feet) long at NYC John F. Kennedy International Airport (JFK). In the design of pedestrian tunnels, one of the key challenges is to ensure the emergency means of egress to comply with applicable code requirements. In accordance with most of current codes and standards, such as IBC [1], UBC [2] and NFPA 101 [3], the maximum allowable travel distance from any point to the nearest exit is limited to 76.2 meters (250 feet) or less, with an automatic sprinkler system provided throughout the tunnel. To comply with such code requirements, most of airport pedestrian tunnels need multiple exits in the middle of as well as at the end of the tunnels. However, it would be extremely difficult to provide such exits in the middle due to security reasons and airport normal operation policies. For example, exit stairs provided in the middle of a tunnel to discharge passengers to the ground of airfield would expose them to an unsafe environment as well as affect the airfield operation, such as airplane departure, landing and maintenance. Therefore, alternative solutions using passive methods are currently adopted to meet the means of egress requirements specified in the relevant codes. Passive methods refer to the approaches that use physical barriers to form physical compartments. Typical examples are area of refuge and horizontal exits. The former is to build a fire-safe area attached immediately to the tunnel to temporarily accommodate passengers during a fire emergency. The latter is to divide a tunnel into numerous physical compartments separated by fire doors (i.e. roll-down fire shutters, Wondoors, etc.) so the travel distance on the egress path is shortened to within the code limit. In some cases, a mechanical smoke exhaust system designed on the basis of air change method is also combined with the passive methods. However, these passive methods rely on distance
requirements only and do not invoke a time factor to reach exits. Furthermore, they do not help with maintaining a tenable environment within the zone of fire origin for safe evacuation or relocation of occupants. In order to maintain a safe egress environment within an airport pedestrian tunnel during a fire emergency, an alternate method of design utilizing engineered performance-based design approach, virtual compartment, is proposed in this paper. Instead of currently adopted physical compartment approach, the proposed virtual compartment approach is to create multiple virtual zones that are isolated through exhausting the virtual zone of fire origin and supplying makeup air from zones immediately adjacent to the fire zone. DESIGN METHODOLOGY Pedestrian Tunnel A pedestrian tunnel with dimensions of 15.24 meters (50 feet) wide by 457.2 meters (1,500 feet) long by 6.1 meters (20 feet) high is modeled for the analysis. The plan and section view of the modeled pedestrian tunnel in Figure 1 shows that a 3.66-meter (12 feet) security screen is provided at the center of the pedestrian tunnel to separate the departure and arrival. Throughout the tunnel is a ceiling height of 4.88-meter (16 feet). The entire pedestrian tunnel is equally divided into three compartments named as West, Central and East s, with a length of 152.4 meters (500 feet) for each. Figure 1. Plan and Section View of Pedestrian Tunnel Occupant loads of the pedestrian tunnel are determined by occupancy classification and air-flight passenger flow. An occupant load of 750 persons is estimated based on the entire tunnel area of 6,967.73 m 2 (75,000 ft 2 ) and an occupant load factor of 9.3 m 2 /P (100 ft 2 /P). An air-flight
passenger flow of 2,400-3,400 persons (605-853 for Airbus A380 [4] and 407-523 for Boeing 747-8 [5]) is projected based on an assumption of 4-flight (2 airplanes landing and 2 take-off) passengers using the tunnel simultaneously. Therefore, approximately 4,150 occupants are assumed to be located throughout the tunnel and an occupant load of 1,384 persons for each physical/virtual compartment is assumed in this analysis. Design Goal and Objective The goal of the virtual compartment approach is to maintain a tenable environment within the zone of fire origin along the egress path for the safe evacuation or relocation of occupants. Accordingly, the performance-based design objective is to slow the smoke layer descent for a period of time sufficient to allow the occupants to safely egress from the zone of fire origin. Evaluation Criteria The evaluation criteria are values with which the performance of the proposed design can be measured, compared and deemed successful (i.e., demonstrate equivalency to the code intent). Numerous tenability criteria have been suggested and used in published fire hazard analyses and test studies over the past two decades. This performance-based analysis only considers the effect of acute (short-term) exposures to toxic products; chronic (long-term) effects are not considered. Three conditions are analyzed to determine whether tenability is maintained: visibility, gas concentration within the breathing zone, and ambient temperature. Table 1 summarizes the evaluation criteria adopted in this analysis. These criteria are utilized to analyze the modeling simulation results for determination of the smoke layer interface (i.e., maintaining the height of the smoke layer at least six feet above the walking surface) in accordance with IBC [1], NFPA 101 [3] and NFPA 92 [6]. Parameter Temperature Visibility Carbon Monoxide (CO) Table 1 Performance-Based Design Evaluation Criteria Description of Criterion Limit temperature to a maximum of 60 C (140 F) at a height of 1.83 meters (6 feet) above the walking surface [7, 8] Maintain visibility of at least 10 meters (33 feet) to an exit sign at a height of 1.83 meters (6 feet) above the walking surface [9] Limit CO concentration to 220 ppm for 20 minutes and 150 ppm for 30 minutes at a height of 1.83 meters (6 feet) above the walking surface [7, 8, 10] Value of Criterion 60 C 10 meters 220-150 ppm Note that the evaluation criteria may vary with projects and authority having jurisdictions (AHJs). The evaluation criteria adopted herein are to validate the proposed virtual compartment approach as an alternate method of design for means of egress in an airport pedestrian tunnel.
Fire Dynamics Simulator (FDS) [11] developed by National Institute of Standards and Technology (NIST) is utilized to evaluate the toxic and thermal threat to which tunnel occupants are exposed. Design Determination Design Fires Smoke production generated by a design fire is the basis for analyzing accumulation and movement of smoke and hot gases. Design fires should consider factors such as characteristics of the fuel, fuel load, fuel spacing, and fuel configuration, fire growth, etc. In addition, the analysis above should consider heat release and sprinkler effectiveness. However, since the intent of this analysis is to demonstrate if the proposed virtual compartment method is an efficient approach as an alternate method of design for the means of egress in an airport pedestrian tunnel, the factors of design fires are not the focus of this analysis. To simplify the analysis, an unsteady sprinkler-controlled design fire of 642 KW (609 Btu/sec) with a fast T- square fire growth rate is used in the FDS simulation based on assumptions of a quick-response sprinkler system provided throughout the tunnel [1, 6, 12]. Key Design Assumptions In addition to the evaluation criteria and design fire size, the following assumptions are included in the analysis. A smoke exhaust airflow capacity of 39.64 m 3 /s (84,000 ft 3 /min) is provided for the virtual compartment in fires and the makeup air supply is sized at approximately 95- percent of the smoke exhaust airflow capacities. All smoke exhaust and makeup air supply points are located at the ceiling of the tunnel. The soot yield (i.e., the fraction of mass converted into smoke particulate) is assumed to be approximately 5% to accommodate mixed combustible materials (wood and plastics) that may be present in the tunnel. The Carbon Monoxide (CO) yield is assumed to be approximately 4% for the mixed combustible materials. A default visibility factor of 3 for non-illuminated objects (i.e., light-reflecting signs) is used in the fire modeling evaluation per Mulholland, SPFE Handbook [9]. Design Fire Scenarios In this analysis, two cases, Central (middle compartment) and West (end compartment), are examined. In each case, we compare the virtual compartment approach with the physical compartment approach, as shown in Table 2.
Case No. Case 1: Central Case 2: West Table 2 Proposed Design Fire Scenarios Fire Location Center of Virtual Center of Physical Center of Virtual Center of Physical Sprinkler Controlled Yes Yes Yes Yes Fire Growth Rate Fast, T-Square Unsteady Fast, T-Square Unsteady Fast, T-Square Unsteady Fast, T-Square Unsteady Max. HRR (kw) Smoke Exhaust 642 Yes 642 No 642 Yes 642 No RESULTS AND DISCUSSION Timed Egress Analysis The most current codes and handbooks require and recommend that a smoke control system operate to maintain a tenable environment for the duration (ASET) longer than the required safe egress time (RSET). The RSET typically consists of the following components [1, 13]. RSET = td + tn + 1.5 (tp-e + te) Where: td = Time from fire ignition to detection (detection time) tn = Time from detection to notification of occupants of a fire emergency (notification time) te = Time from notification until evacuation commences (pre-movement time) tm = Time from start of evacuation movement until safety is reached (movement time) A detection time of 60 seconds is obtained from the FDS modeling analysis based on area smoke detectors placed at a 9.144-meter (30 feet) spacing throughout the modeled pedestrian tunnel. In accordance with 2013 edition NFPA 72 [14], actuation of alarm appliances should occur within 10 seconds after the activation of an initiating device, but can have an alarm investigation phase up to 3 minutes (positive alarm sequence). From airport operation standpoints, the positive alarm sequence is adopted and the notification time is 3 minutes (180 seconds). The SFPE Handbook presents a study conducted by the British Standards Institute in 1997 [15]. This study gives several different times for the pre-movement time to start based on the type of notification system in the building. The first time (W1) is less than 1 minute for buildings with live directives through a voice communication system and trained, uniformed staff. The second time (W2) is 3 minutes for buildings with nondirective voice messages and trained staff. The third time (W3) is greater than 4 minutes for buildings with a fire alarm signal and staff with no
relevant training. Since airports are typically provided with live directives through a voice communication system and trained staff, a pre-movement time of 1 minute (W1) is assumed. The movement time includes travel time to an exit component, flow time through that exit component, time of the last person to egress to safety and all associated queuing times. The traveling speed of occupants is calculated using the equation below [13]: S = k - akd Where: S = Speed along the line of travel (m/s) k = Constant (1.0167 m/s (200 feet/min) for concourses per 2014 NFPA 130 [16]) a = Constant, 0.266 D = Population density in persons per unit area (person/m 2 ) A maximum travel distance from the most remote location of compartments is 76.2 meters (250 feet) [1, 3]. The flow time of occupants passing through an exit component is determined in the following equation [13]: Fc = Fs We Where: Fc = Calculated flow (persons/minute) Fs = Specific flow (82 persons/m/min (25 persons/feet/min) per 2014 NFPA 130 [16]) We = Effective width of the component being traversed (a typical reduction of 0.1524 meters (6 inches) from each side of doors and 0.4572 meters (18 inches) for concourses [13]), four 1.8288-m doors assumed to be provided for each physical compartment, and about 12.8016 meters (42 feet) width of the tunnel assumed to be effective width at nonended sides and two 1.8288-m doors at ended sides for virtual compartments The passage time for the last occupant passing through an exit component is calculated using the occupant load at an exit component divided by the calculated flow. The movement time is the sum of the travel time to an exit component and the time to egress through that exit component. Tables 3 and 4 summarize the calculated movement time and the RSET based on two cases, total 4 scenarios listed in Table 2 above. Case No. Case 1 Case 2 Location Central Virtual Central Physical West Virtual West Physical Table 3 Calculated Movement Time Travel Speed (m/min) Travel Time (s) Calculated Flow (p/min) Passage Time (s) Movement Time (s) 51.3 89 1,800 40 129 51.3 89 200 167 256 51.3 89 1,000 64 153 51.3 89 200 167 256
Case No. Case 1 Case 2 Location Central Virtual Central Physical West Virtual West Physical Table 4 Required Safe Egress Time (RSET) Detection Time (s) Notification Time (s) Pre- Movement Time (s) Movement Time (s) RSET (s) 60 180 60 129 524 60 180 60 256 714 60 180 60 153 560 60 180 60 256 714 FDS Simulation Results Case 1 A design fire in the center of the Central is simulated with provision of physical separation (i.e., roll-down fire shutters) and virtual separation (i.e., exhaust the virtual compartment in alarm and supply from adjacent virtual compartments). The smoke layer interface at 1.83 meters (6 feet) height above the floor are compared in Figure 2 and Figure 3, which present the outcomes of visibility, a dominant criterion among the three design evaluation criteria to justify the smoke layer height. Central Physical at t=288s Central Virtual at t=900s Central Physical at t=549s Central Virtual at t=1,800s Figure 2. Visibility (m) isosurfaces indicating where visibility is 10m (black line shows 1.83m)
Central Physical at t=288s Central Virtual at t=900s Central Physical at t=549s Central Virtual at t=1,800s Figure 3. Visibility (m) horizontal slices indicating where visibility is 10m at 1.83m above floor For the physical compartment, the simulation results show that the smoke layer height starts descending below 1.83 meter (6 feet) around 288 seconds at the physical barrier area and the entire compartment becomes untenable at approximately 549 seconds (all exits blocked by smoke) after ignition of the design fire. For the virtual compartment, the smoke layer height remains above 1.83 meter (6 feet) for the whole simulation duration of 1,800 seconds (30 minutes). As shown in Figure 2, smoke is almost fully constrained within the virtual compartment in fire and does not spread to adjacent non-fire virtual compartments. In addition, the simulation results in Figure 3 for the physical compartment clearly indicate that the smoke layer height descends below 1.83 meter (6 feet) starting from two ends of the compartment.
Case 2 A design fire in the center of the West (end compartment) is simulated with provision of physical and virtual separation. The smoke layer interface at 1.83 meter (6 feet) height above the floor are compared in Figure 4 and Figure 5 that only show the visibility simulation outcomes. West Physical at t=270s West Virtual at t=810s West Physical at t=468s West Virtual at t=1,800s Figure 4. Visibility (m) isosurfaces indicating where visibility is 10m (black line shows 1.83m) The simulation results show that the smoke layer height begins descending below 1.83 meter (6 feet) around 270 seconds at the physical barrier area and the entire compartment becomes untenable at approximately 468 seconds (all exits blocked by smoke) after ignition of the design fire in the physical compartment. In the virtual compartment, the smoke layer height descends below 1.83 meter (6 feet) about 810 seconds at the end of fire-side area but remains above 1.83 meter (6 feet) at all other areas for the whole simulation duration of 1,800 seconds (30 minutes). When compared with Case 1 results, similar outcomes are obtained: smoke is contained within the virtual compartment in fire and the smoke layer height descends below 1.83 meter (6 feet) starting from two ends of the compartment.
West Physical at t=270s West Virtual at t=810s West Physical at t=468s West Virtual at t=1,800s Figure 5. Visibility (m) horizontal slices indicating where visibility is 10m at 1.83m above floor Discussion In comparison of the timed egress analysis and FDS simulation results, the ASET of 288 seconds is less than the RSET of 714 seconds for the Case 1 physical compartment scenario. Similarly, the ASET of 270 seconds is less than the RSET of 714 seconds for the Case 2 physical compartment scenario. Thus, the physical compartment approach cannot ensure occupants to safely evacuate or relocate to adjacent compartments and most of occupants in the fire compartment may be exposed to an unsafe environment. In contrast, in all virtual compartment scenarios, the ASET is greater than the RSET and occupants are able to safely evacuate or relocate from the fire compartment. Typically, the design fire size is one of key factors to impact duration of tenability. For the physical compartment approach, the larger the design fire is, the quicker the compartment
becomes untenable. For the virtual compartment approach, the smoke exhaust fan capacities can be adjusted along with the design fire size change to maintain tenability longer than RSET. Another finding from this study is that an apparent time lag exists between the fire side and nonfire side separated by security screens within the same fire compartment in both physical and virtual scenarios, as shown in Figures 3 and 5. The 3.66-meter (12 feet)-tall security screen is the cause of this difference. Furthermore, the isosurface outcomes presented in Figures 2 and 4 clearly demonstrate the impact of makeup air supply on tenability. When a fire occurs in the central virtual compartment, the smoke layer interface can be maintained steadily because of the equal makeup supply from the two adjacent sides. However, when a fire occurs in an end virtual compartment (west or east compartment), the smoke layer becomes fluctuated and accumulated at the dead-end side due to makeup air coming from one side only. In addition, the simulation results indicate that the smoke layer height starts to descend below 1.83 meter (6 feet) from two ends of the fire compartment in physical compartment scenarios, where the horizontal exit or exit access doors are typically located. This implies that the ASET could be greatly reduced for the physical compartment approach using the horizontal exits method. CONCLUSION This paper performed a preliminary analysis on the virtual compartment design concept. The modeling outcomes indicate that it seems to be a more effective alternate design approach when compared with physical compartment approach, since it is able to achieve the fire life safety code requirements as well as satisfying the airport operation requests. Generally speaking, prescriptive codes have advantages: it is easy to design through following prescriptions and AHJs can easily justify if such design complies with code requirements. However, prescriptive codes may not clearly address and truly implement fire life safety objectives for novel buildings and buildings associated with special operation necessity. The fire modeling analysis in this study demonstrates that the code-compliance physical compartment approach using horizontal exit method does not actually achieve the fire life safety objectives, i.e. safe evacuation or relocation of occupants in a subterranean airport pedestrian tunnel. Furthermore, the code-compliance physical compartment approach may affect TSA s justification and responses to emergency events. In contrast, the virtual compartment approach that exhausts smoke of the fire zone and supplies makeup air from adjacent zone(s) is able to achieve the equivalency of the fire life safety objective specified in building codes and standards. In addition, it does not adversely affect the airport security and normal operation requests. Although this study only analyzed a pedestrian tunnel that is divided into three virtual compartments at a 152.4 meter (500 feet) length of each, this approach can be extended to
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