Timed egress requirements for transit and passenger rail station evacuation as described in NFPA 130.

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ABSTRACT

This paper provides an overview of the timed egress model for fixed guideway transit and passenger rail stations described in NFPA 130. First, the paper discusses the method for determining evacuations of station populations. It then discusses how the egress capacity for the station is determined. Finally, the method used to determine the time it takes to evacuate the station is discussed. The reader is referred to NFPA 130 2003, Appendix C, for specific guidance as to how to perform these calculations.

INTRODUCTION

NFPA 130, Standard for Fixed Guideway Transit and Passenger Rail Systems (NFPA 2003) specifically states that in the event of a fire, passengers must be evacuated through a tenable environment. The standard defines a tenable environment as "an environment that permits the self rescue of occupants for a specific period of time." An environment is considered to permit self rescue if it meets requirements for visibility in smoke, exposure to heat, toxic gas, and other criteria, all commonly referred to as tenability criteria. Computational fluid dynamics (CFD) is generally used to determine if the tenability criteria are being met for the passenger egress path during evacuation. Passenger evacuation in a tenable environment is a particular concern in enclosed stations, but it may also be a concern in unenclosed stations depending on prevailing winds and exit locations.

This paper specifically addresses the "period of time" issue as defined in the tenable environment definition. The simple passenger flow model used to determine egress time that is established within the standard, as well as its limitations, are discussed. This includes the determination of passenger population in public areas and the determination of egress capacity as outlined in NFPA 130 and its appendices so that an overall station egress time can be determined.

Normal system operations present a different set of requirements for passenger movement than the emergency condition and are beyond the scope of this paper. Tunnel egress requirements, provisions for the evacuation of the disabled (which vary by international jurisdiction), and requirements for evacuation from "back of house" spaces are also not addressed. Finally, the tenability conditions along the egress path will get worse as a fire burns. The use of different vehicles on different systems, the burning rate of these vehicles, and the consequential influence on tenability along the path of egress are also beyond the scope of this paper.

NFPA 130 is updated on a three-year cycle using the NFPA consensus process. If specific questions arise concerning the intent or meaning of the standard, the committee should be asked for its interpretation through the staff liaison. Any opinions expressed on the intent of the standard or the

standard itself are the author's own and should not be considered definitive.

STATION EVACUATION

Transit stations are constructed so that passengers can enter and exit transit vehicles, usually via an adjacent platform. Stairs extending above or below usually provide passenger access to the platform itself. In the event of an emergency, the requirements of NFPA 130 state that all passengers from vehicles on both sides of a platform--as well as passengers waiting for vehicles to arrive during the emergency condition--must be evacuated.

The major restriction to the evacuation of passengers is almost always the capacity of the platform or mezzanine stairs, often referred to as vertical circulation elements (VCEs). The model for passenger evacuation described within NFPA 130 is a first-order model that takes into account the restrictions to passenger egress provided by the VCEs between the platform and any mezzanines, as well as between the mezzanine(s) and the street. The simple model described within the standard is usually implemented with a spreadsheet. It is a good model for evaluating conceptual station designs where many changes will be made over a short period. More advanced multi-dimensional models such as Steps (1) or Simulex (2) take into account VCE location along the platform, as well as a specific requirement of NFPA 130 that if escalators are used in a station design, the worst-case escalator location should be considered out of service.

To use the model described in NFPA 130 to determine an overall time to evacuation, a Station Occupant Load and the total VCE capacity must be determined as described below.

Determination of Station Occupant Load

The total passenger load on the platform is made up of the people waiting to board the incoming vehicles (entraining load) and the people who will be on the incoming vehicles (link load), as shown in the equation below.

Total Station Occupant Load = Platform Entraining Load + Vehicle Link Load

The NFPA 130 requirements provide for the simultaneous evacuation of vehicles on each side of a platform for a center platform station or from both platforms simultaneously for a side platform design. Any platform load accumulated during the emergency must also be accounted for.

The passengers waiting for a vehicle to arrive are known as the platform entraining load. The standard states that the number of passengers should be determined based on a minimum of two headways in the peak direction, a headway being the time between scheduled vehicle arrivals. Peak direction is the main direction of movement, usually toward a city or concentration of business in the morning and away from the area in evening. Off-peak traffic travels in the opposite direction.

The reason that two headways are used is that it is assumed that a worst-case scenario is occurring and that a vehicle was not added to the system at the appropriate time. In this case passengers in

the peak direction must wait twice as long for the next vehicle. The platform will therefore have abnormally high passenger loads, as passengers will accumulate on the platform for this extended time.

It is assumed that the fire incident vehicle will not only be in the peak direction of travel but will have twice its normal passenger load as determined by the passenger demand model or will be at maximum vehicle capacity. This condition is also due to the "missed headway," as this vehicle will be picking up twice the normal passenger load at each station preceding the incident station.

A "normal" passenger load is assumed to simultaneously need to egress from the vehicle on a non-incident track, traveling in the off-peak direction.

The platform entraining load and vehicle link loads are determined based on the output of a travel demand model. This model describes the number of passengers in a system at a given time and is usually developed for new and existing transit systems. Often the travel demand model was a main component in the justification for the construction of a system. The specific requirements for a travel demand model are not covered by the standard. However, based on this model, the number of passengers waiting on a platform for the vehicle to arrive and the number of passengers on vehicles within the system are predicted. This model is crucial to the development of station egress, as it provides direct input into the number of station occupants that must be evacuated in the event of an emergency.

Other Factors in Determining Station Occupant Load

Maximum Vehicle Passenger Load. Determination of how many passengers a vehicle contains when it is considered "full" can be difficult. A number of specific loadings (A0, A1, A2) are provided by manufacturers for transit vehicles. However, each of these numbers has its own meaning for vehicle test purposes during production and testing, and none reliably answers the question of how many passengers this vehicle can accommodate. For example, vehicle capacity is often quoted at the crush loading level. However, such loading levels are rarely achieved in practice in the US and Canada. Crush loading represents the load for which a vehicle's structure, propulsion, and braking systems are designed. This is not necessarily the maximum number of people the vehicle can hold. Data are available on the maximum passenger density in vehicles in various cities, and more realistic passenger occupancies may be obtained using a layout of the vehicle, dividing by this number, and adding a safety factor.

However,

the only true means of measuring achievable car capacity is on those

systems where pass-ups occur. That is where passengers wait for the

next vehicle rather than crowd onto the one in their station.

Determining full car capacity and pass-up capacity depends on interior

arrangements, type of system, old or new, and time of peak loading

(TCRP 1999).

Maximum Platform Load. While not specifically addressed within the standard, the maximum capacity of a rail platform is important. Designers are frequently pressured to make stations smaller to save on the cost of construction, specifically the excavation and mining of underground stations. It is important that the platforms be designed to accommodate a reasonable number of people. Fruin (1971) provides passenger densities that can be used in this determination so that the "maximum reasonable occupancy" of a platform can be determined.

Surge Factor. NFPA 130 requires that the number of platform occupants be determined for the peak 15 minutes of system time. Most newer simulation programs developed provide peak-15-minute-period data, in which case a surge factor is not necessary. However, models used by some systems only provide a peak hourly load. In this case, a surge factor is necessary. The surge factor compensates for the fact that passengers will not arrive at a constant rate over the peak hour.

The surge factor may have a significant influence on the number of occupants that are calculated in the total passenger load. Different subway systems will experience different loading trends; therefore, it is reasonable to use information specific to the local system when available. Schachenmayr (1998) recommends a value of 1.5 for cases when there are no ridership data available.

DETERMINATION OF STATION EGRESS CAPACITY

In normal operation, when a vehicle arrives in a station, most passengers remain on the vehicle, as they are usually traveling farther along a transit line. The number of passengers moving through a station during an emergency is clearly much larger than the number of passengers moving through a station during normal operation, as all of the passengers on vehicles from both platforms will be making use of the exit stairs, not just the normal entraining and detraining passengers.

The passenger speed and VCE capacity are summarized below. The speeds and capacities used are based on the work of Fruin's (1971) research, which established the concept of level of service (LOS) for pedestrian flow on walkways and stairs. Levels of service related the area per pedestrian (what Fruin called the module) to both the passenger capacity of a given walkway width and the passenger speed on that walkway. The units of capacity used by Fruin were people per inch per minute (pim) or people per millimeter per minute (p/mm x min). The length dimension referred to the units of walkway width. The units of speed along the walkway used by Fruin were feet per second or meters per second.

Fruin defined six different LOS, A through F. LOS A provided the least amount of pedestrian congestion and LOS F the most. At the low end of Fruin's expressed module range for LOS F, the module approached zero and Fruin determined that the effective capacity of the walkway also approached zero. In this case he assumed that pedestrians were so crowded together that they

could not move.

The passenger capacities reached their peak in a higher module range of LOS F, and then they started to descend. This peaking was also due to the crowding. In this case a large quantity of moving passengers resulted in a large expressed capacity of the walkway width. The capacities kept descending in the higher LOS letter grades (Fruin 1971).

Passenger Speed on Platforms and VCEs

At this time a tentative interim amendment (TIA 2004) to NFPA 130 dictates the passenger speeds on VCEs, platforms, and corridors. These requirements are outlined in Table 1.

Platform and VCE Capacity

The TIA also requires a 1.31 p/in./min (0.0516 p/mm x min) capacity upstairs and a 1.41 p/in./min (0.0555 p/mm x min) capacity downstairs. This is summarized in Table 2, along with the capacity requirements for platforms and doors. It was proposed in the 2006 version of NFPA 130 to harmonize the number to 1.41 p/in./min (0.0555 p/mm xmin) in both directions to correspond to an LOS E/F on VCEs, platforms, and corridors.

Escalator/Stair Parity

NFPA 130 requires that if the egress capacity of a stopped escalator, or an escalator running in the direction of evacuation, is included in the total egress capacity of a level, an equal corresponding width of stair must exist on that level. This provision has been part of the standard since it was first adopted; however, it continues to be controversial. Some within the NFPA community argue that all stopped escalators should be used as egress, while others argue that none should be allowed. The frequent use of escalators in newer systems, particularly deeper systems, will continue to make this an important issue.

Worst Case, Escalator Impassable

The standard requires that the escalator in the "worst case" position be considered impassable. That is, the escalator is not just stopped but has had all of its steps removed or is in some other way unusable as a stair. The simple model described within the standard can deduct the width of this escalator but, as it is a one-dimensional model, cannot account for the location of the escalator. Generally the worst-case location of the escalator can add 15% to 20% to the egress time. The location of the escalator can be accounted for by using more advanced multidimensional computer models; NFPA 130 does not advocate the use of any particular model.

DETERMINATION OF TOTAL EGRESS TIME

To determine the total time for all passengers to evacuate a station to a point of safety as described within the standard, the path of the "worst case" passenger is evaluated. This passenger is located as far as physically possible from the VCEs at platform level and is therefore the last passenger on any

queue that forms at any VCE, either platform to mezzanine or mezzanine to street. The walking time on the platform and mezzanine as well as the passenger waiting time are both incorporated into the total evacuation time.

In most designs, the "worst case" passenger walking speed on the platform is insignificant, since most time will be spent waiting behind other passengers to get onto the stairs. This waiting time is a function of both the station occupant load as well as the total VCE capacity.

It becomes very clear that the crucial component is the VCE capacity, in other words, the number of people/minute that the VCE is capable of servicing. This is a direct function of the capacity dictated by NFPA 130.

Because stair capacity is developed in terms of people per minute, the total platform population can be divided by the total egress time to determine the amount of stair capacity necessary off a platform or mezzanine. The number of circulation elements can be determined roughly by dividing the desired width of each of the elements by this aggregate width.

The "Four and Six Minutes" Requirement

NFPA 130 specifically states that passengers should be cleared from the platform in four minutes and at a point of safety in six minutes, unless tenability along the egress path can be established for a longer duration. In the author's experience, these are excellent criteria for early design development. However, for enclosed stations, the standard does not specifically exempt the designer from establishing tenability before the four- and six-minute times are reached. As a station design progresses, these times may become less relevant as a determination is made as to the actual time the egress path can be kept clear of smoke by using designed smoke control.

Passenger Pre-Movement Time and Time to Fire Detection

Different authorities have different requirements that address issues such as passenger pre-movement time before evacuation commences, time to fire detection, and other issues that may influence the passenger evacuation time, all of which the standard does not address directly. It is clear that if a full simulation of how a fire scenario may occur is to be developed, these factors need to be accounted for, in addition to those factors described within the standard.

SUMMARY

NFPA 130 has a specific requirement that passengers be evacuated in a tenable environment. The method of determining station population as well as the egress capacity from platforms and mezzanines have been described. A short description was also provided as to how a final determination of the evacuation time can be calculated. The benefits of using multidimensional egress programs such as Simulex and Steps were also mentioned.

REFERENCES

Fruin, J. 1971. Pedestrian Planning and Design. New York: Metropolitan Association of Urban Designers and Environmental Planners, Inc.

NFPA. 2003. NFPA 130, Standard for Fixed Guideway Transit and Passenger Rail Systems. Quincy, Mass.: National Fire Protection Association.

Schachenmayr, M. 1998. Application Guidelines for the Egress Element of the Fire Protection Standard for Fixed Guideway Transit Systems. Parsons Brinckerhoff Quade & Douglas, Inc.

TCRP. 1999. TCRP Web document 6 (Project A-15), Contractor's final report transit capacity and quality of service manual. Prepared for Transit Cooperative Research Program Transportation Research Board, National Research Council, Submitted by Kittelson & Associates, Inc. in association with Texas Transportation Institute Transport Consulting Limited, January 1999.

TIA. 2004. NFPA TIA 03-1 (NFPA 130) (SC-04-07-17/Log 797). Dated July 15, 2004.

DISCUSSION

Wan K. Chow, Chair, Architectural Science and Fire Engineering Department, The Hong Kong Polytechnic University, Hunghom, Kowloon: How realistic are the specified data, i.e., four minutes off the platform and six minutes to a safe place, bearing in mind that some crowded subway stations in the Far East might accept evacuation time up to 30 minutes?

Robert C. Till: The standard states that passengers need to be evacuated in a tenable environment. The four and six minute criteria may be difficult to achieve, especially in existing stations where full evacuation of vehicles was not considered in the original design. NFPA Standard 130 allows for extended evacuation times beyond four and six minutes if it can be established that the evacuation route will be tenable in the event of a fire. This is usually done using a CFD model that incorporates an appropriate design fire.

In stations where a relatively small design fire is deemed appropriate, it may be determined that tenability can be achieved for extended periods, perhaps indefinitely with a properly designed smoke control system. Due to the increased volumes of smoke, larger design fires will require a much larger smoke control system to achieve extended periods of tenability.

It should be noted that fully sprinklering stations might not be useful, since the water from sprinklers is unlikely to have a significant influence on fires inside of covered vehicles. Fires inside of vehicles are almost always the largest fires that need to be considered. The early suppression and resulting reduced smoke conditions normally provided by sprinklers are unlikely to be realized in rail vehicle fires.

A fire-hardened vehicle design can provide superior passenger safety in new and existing systems. Unfortunately in many systems around the world vehicle fire hardening isn't considered until after an event or multiple events make problems apparent.

Robert C. Till, PhD, PE

Robert C. Till is an associate professor of fire science in the Department of Public Management, John Jay College of Criminal Justice, The City University of New York.

(1). http://www.mottmac.com/skillsandservices/software/stepssoftware/

(2). http://www.crowddynamics.com/Egress/simulex.html