Recent world events have demonstrated the need for increased security measures and the ability to evacuate large numbers of people efficiently. Long lines and handheld metal detectors have become familiar sights at the entrances to stadiums, arenas and other large places of assembly.
In most buildings the security and life safety systems work toward opposite goals. The goal of the security systems is typically to limit access to areas of a facility, while the goal of the life safety systems is to allow people to evacuate the building as quickly as possible via multiple remote exits. In few occupancies is this apparent dichotomy of goals more evident than in large-assembly occupancies such as stadiums and arenas.
Event security at large venues such as stadiums and arenas is extremely complicated. Security personnel and systems must maintain controlled access to sensitive areas while allowing thousands of people to enter and use the facility. Simultaneously, the security systems and personnel must be able to evacuate the occupants in a quick and orderly manner. This article will serve as an introduction to egress design for security professionals, to allow them to better understand the baseline requirements for the design of fire protection and life safety systems.
Both the model codes and NFPA 101 ? Life Safety Code require that areas or levels serving more than 1,000 people be equipped with at least four remote exits. This is required to minimize the impact of one or more exits becoming obstructed in the event of a fire emergency.
Occupants are not likely to be familiar with their surroundings when visiting a large-assembly occupancy. It is normal human behavior for them to attempt to exit the building via the same route by which they entered it. Thus, an emergency could potentially lead to queuing at egress choke points if proper precautions are not taken. To address this situation, the model building codes and the Life Safety Code require all assembly occupancies with an occupant load greater than 300 persons to have a main entrance/exit that provides sufficient capacity for at least 50 percent of the building's occupant load. The remainder of the required exits should then be evenly distributed throughout the facility.
Generally, the sheer size of stadiums and arenas makes it impractical to have a single, defined main exit that is sized for half of the occupant load. To meet the requirements, the main exit of a stadium would have to be large enough to accommodate more than 35,000 occupants. Using standard requirements for the provision of exits, this would require approximately 160, 44-inch-wide doors for the main exit alone.
In deference to this impracticality, the codes include an exception that states when there is not a well-defined main exit, the exits may be distributed around the perimeter of the building provided that they are sized to accommodate the total occupant load that they serve. This design approach helps to minimize the number of choke points in the egress system and reduces the impact of an exit being unavailable in an emergency condition.
The model building codes and the Life Safety Code prescribe an exit width of 0.2 inches per building occupant for horizontal exit components (i.e., doors, corridors, and ramps) and 0.3 inches per building occupant for vertical exit components (i.e., stairs). These measurements stem from a performance criterion that requires that occupants be able to exit the building or floor within 3.5 minutes (SFPE Handbook, Sec. 3, Ch. 13, ?Movement of People? by Jake Pauls). While these requirements serve most occupancies well, use of the prescribed 0.2- and 0.3-inch egress width factors in stadiums and arenas can result in enormous stairs, ramps, and banks of doors that might not have an appreciable effect on the overall life safety rating of the facilities. Also, with the occupant loads expected in these types of buildings, a 3.5-minute exiting time is an unrealistic benchmark.
The code development groups have recognized that large groups of people cannot exit a stadium or arena in the time frames laid out for more standard occupancies, and that the standard exit width factors are not practical for these larger facilities. Both the model building codes and the Life Safety Code address these concerns by incorporating more performance-oriented design options. One such option allows the building design team to take advantage of a design concept referred to as smoke-protected assembly. This approach takes into account that by nature of design, large-assembly occupancies tend to have extensive unenclosed spaces that can create a reservoir for smoke and hot gases to fill, thus extending the available time to exit the facility before conditions in the space become untenable.
In order to fulfill the smoke-protected assembly provisions, the building design must first undergo a life-safety evaluation and then meet specific criteria for automatic sprinkler protection, smoke control and travel distance. Outdoor or retractable-roof facilities do not necessarily fulfill the smoke-protected assembly criteria. They, too, must be evaluated to confirm they meet the requirements.
The benefit of smoke-protected assembly design is that the required exit-width factors may be significantly reduced for large occupant loads. This reduction recognizes the large volumes of these spaces and associated greater available safe egress times than for typical occupancies. In lieu of using the standard 0.2 inches per person and 0.3 inches per person exit width factors for horizontal and vertical exit components, a smoke-protected assembly may use the values in the chart below, which is a recreation of Table 188.8.131.52 in the Life Safety Code.
The Life Safety Code and the International Building Code require that a life-safety evaluation be performed by a person acceptable to the authority having jurisdiction prior to the institution of the smoke-protected assembly concept. According to the Life Safety Code, the fire protection engineer's life-safety evaluation should include at a minimum an assessment of the following conditions and related appropriate safety measures.
1. Nature of the events and the participants and attendees
2. Access and egress movement, including crowd density
3. Medical emergencies
4. Fire hazards
5. Permanent and temporary structural systems
6. Severe weather conditions
8. Civil or other disturbances
9. Hazardous materials incidents within and near the facility
10. Relationships among facility management, event participants, emergency response agencies and others
A complete listing of items that should be included in this evaluation is included in Appendix A of the Life Safety Code. If the evaluation indicates that an acceptable level of risk is present, egress design using the smoke-protected assembly concept may proceed with the AHJ's approval.
Since the nature and types of performances taking place in stadiums and arenas constantly change, the life-safety evaluation should be periodically reviewed to ensure that all aspects of these dynamic occupancies are addressed.
Automatic Sprinkler Protection
Automatic sprinkler protection must be provided throughout the facility to fulfill the smoke-protected assembly criteria. The codes generally recognize that due to the heights of the bowl and seating areas of such facilities, a fire may not actuate the automatic sprinklers. To address this, designers may omit the installation of sprinklers in select areas where the ceiling height over the performance area is greater than 50 feet above the floor. Exceptions are also permitted in performance areas where the ceiling height is 50 feet or less and above the seating in the bowl, provided that an engineering analysis demonstrates that sprinklers would be ineffective.
One of the requirements of the smoke-protected assembly analysis is to evaluate movement of smoke through fire scenarios. The model building codes and Life Safety Code require that the building be designed such that exit paths remain tenable during the required emergency exiting period. (The Life Safety Code requires that the smoke layer be maintained a minimum of six feet above the highest floor of the means of egress, whereas the model building codes require that the smoke layer be maintained a minimum of 10 feet above the highest occupied level.)
To maintain the smoke layer height above the exiting occupants, a mechanical smoke-control system is typically required for indoor facilities. (Note: Based on the analysis of smoke production in the fire scenarios, retractable-roof facilities may be required to use smoke-control systems. Additionally, because retractable roofs are generally not operated in all weather conditions, retractable-roof facilities must often be reviewed for smoke-control purposes as fixed-roof structures.) The design intent of the smoke-control system is to exhaust the volume of smoke produced so as to maintain the smoke layer at the minimum specified height above the occupants for at least the time required to completely evacuate the building.
Designers may choose between two methods of designing the smoke-control system. The more common method is to design the smoke-control system exhaust rate to match the smoke-production rate of the fire at the prescribed height. In theory, this method would allow the system to maintain the smoke layer at the design height indefinitely. An alternative method is to calculate the egress time for the entire facility and design the smoke-control system to maintain the design height for a period of time not less than the calculated egress time. To address the possibility of exits being obstructed and other unknown factors, a factor of safety of at least two should be used for these types of calculations.
Fire Alarm System
The codes require that large-assembly occupancies be provided with a fire detection and alarm system. If a mechanical smoke-control system is provided for the facility, it must be actuated by the fire alarm system upon sprinkler system initiation, activation of smoke or heat detectors or other approved means.
Evacuating tens of thousands of occupants in an alarm condition presents a hazard in itself. Recognizing this, the Life Safety Code permits a positive alarm-sequence response in accordance with NFPA 72 ? National Fire Alarm Code requirements. This type of system transmits an alarm signal to a constantly attended location?typically the security control room in the stadium or arena. Upon receipt of the alarm, security staff has 15 seconds to acknowledge it, which then initiates a 180-second alarm investigation period. This gives the security personnel an opportunity to evaluate the alarm. If the alarm is not acknowledged within the initial 15-seconds or the system is not reset within the 180-second investigation period, a general alarm to the occupants is initiated.
An approved voice alarm system and visual alarms must notify occupants of the hazard. Normally, the alarm system uses an approved public address system to provide audible notification to the bowl area and on the concourses. The use of the P.A. system allows the security staff to provide specific evacuation information, such as which exits may be obstructed as a result of the emergency situation. Visual alarms are typically interfaced with the facility scoreboards.
While the items noted above detail many of the general requirements relating to stadium and arena egress design, they are not all-inclusive. Each of the model codes includes provisions to permit the development of alternative methods as long as they achieve an equivalent level of safety. If alternate materials or methods are to be used, the security-design professional and the fire-protection and life-safety consultant must coordinate early in the design process.
Corey C. Weldon is a fire-protection engineering consultant for Rolf Jensen & Associates Inc. (www.rjagroup.com). W. Phillip Guy, P.E. is a licensed fire-protection engineer for Mustang Engineering (www.mustangeng.com).