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Fire and life safety challenges in convention centers
Convention centers are growing and along with them is the need to provide the best approaches to life safety. NFPA’s codes and standards offer designers the most effective guidelines to these types of assembly occupancies.
NFPA Journal ®, March/April 2007
By Geza Szakats, P.E.
The convention center business has mushroomed in the United States. In 1970, there were just 6.5 million square feet 1.9 million square meters) of convention space. Today that figure stands at close to 70 million (21.3 million square meters) and continues to grow as more cities throw their hats into the convention ring. In 1995, nearly 50 cities were building or planning new or expanding centers. As the convention industry continues to add more space, the design of convention centers grows in complexity. Once resembling little more than oversized warehouses, convention centers today feature increasingly elaborate displays, buildings within buildings, sophisticated technical equipment, and deluxe corporate meeting rooms.
As convention centers grow larger, the fire and life safety designs of these buildings become more challenging. Convention centers are now all-purpose exhibition centers—the bigger, the better. The growth in the size of exposition centers means there are frequently in excess of 25,000 occupants in a single hall. To save on land costs, two- or three-level expo centers are not uncommon, presenting significant new challenges for design teams. The huge numbers of visitors and the potentially significant range of fuel loads make fire and life safety a top design concern.
Since combustible materials are quite often displayed in substantial quantities, exhibit halls can have unusually large fuel loads, such as boats, trucks, recreational vehicles (RVs), and manufactured homes. The most common primary means of controlling the fire hazard is either by passive (compartmentalization) or by active (automatic sprinkler) systems. Controlling fuel load and fire spread by compartmentalizing exposition facilities is rarely a feasible option because of the required flexibility of the space. Exposition and trade shows also require the flexibility to permit the display of almost any product or merchandise, and thus, limiting the fuel load is seldom an option. However, NFPA 101®, Life Safety Code® addresses the construction of exhibit booths themselves, display boards, signs, and the materials of curtains, draperies, and other similar decorations. The management can relatively easily control these elements of an exhibit. In order to lower the combustible load of convention centers to tolerable levels, the Life Safety Code also requires excess combustible materials that are not in use (e.g., shipping materials, more than a one-day supply of brochures) be kept in a separate storage area with a 1-hour fire-resistive separation from the expo areas. In addition, the Life Safety Code provides guidance on ignition source control by addressing the fundamental dangers of cooking equipment.
Sprinkler System Design Considerations
The use of large and multilevel display booths is increasingly widespread, presenting one of the most severe fire protection challenges in exhibition halls. Although virtually all new convention centers are protected by automatic sprinkler systems throughout, there are many cases when the building’s automatic sprinkler system may be tested. An incipient fire, if shielded from the discharge of the ceiling automatic sprinkler system, could significantly tax a building’s automatic sprinkler system. For this reason, special attention should be given to providing automatic sprinkler protection for sizable shielded areas, such as large covered or multistory exhibition booths, manufactured homes, boats, and similar display products. One potential solution includes strategically located taps to the building’s automatic sprinkler system. Unusually large exhibit items and display booths can be built with special connections in order to be outfitted with a temporary automatic sprinkler system. The Life Safety Code requires that most multistory booths and single-level covered exhibit booths over 300 ft2 (28 m2) be protected with an automatic fire extinguishing system. Similar protection is required for smaller covered booths if they are not separated by a distance of at least 10 ft (3 m) and the combined ceiling area is more than 300 ft2 (28 m2). Additionally, a close review of NFPA 13, Installation of Sprinkler Systems should be considered, as improper installation of large banners and signs might reduce the effectiveness of the ceiling automatic sprinkler system.
Sometimes providing temporary automatic sprinkler protection for goods on exhibit is not always a feasible option. For example, a fire within a large yacht could present a severe fire scenario, but installing automatic sprinklers inside a large boat or an RV is not practical. As one alternative, providing single-station smoke detectors within large covered exhibit items might be an inexpensive and nondestructive approach. The smoke detectors can offer early warning of a developing or smoldering fire. The early warning, in turn, can significantly reduce the fire hazard by providing an opportunity for intervention in the initial stages of a fire. This is the rationale behind the Life Safety Code requirement to install single-station smoke detectors in unsprinklered display items that have a roofed area over 100 ft2 (9.3 m2).
All of these fire and life safety measures supplement the ceiling automatic sprinkler systems, so whenever they are activated, the ceiling automatic sprinklers have a better chance to control a shielded fire. Two other building characteristics typical of large convention centers that can impact the timely activation of the building’s automatic sprinkler system include high ceilings and potentially high ambient ceiling temperatures. In order to provide the greatest flexibility for a convention center, new exposition halls with 35- to 40-foot (10 to 12 meters) ceiling heights are not unusual. Excessive vertical clearances can delay the activation times of standard automatic sprinklers and reduce their effectiveness. In addition, because the upper portions of halls with large ceiling heights are usually not air conditioned for energy efficiency reasons, and high temperatures may exist under normal circumstances, NFPA 13 may not permit the use of ordinary temperature-rated automatic sprinklers. On cooler days, the higher than ordinary temperature rating will further delay the activation of the automatic sprinklers. In order to counter the negative effects of high ceiling elevations on the automatic sprinklers, special design considerations are necessary. Increased sprinkler discharge density and design area of operation and quick-response sprinklers are recommended, based on FM Global research and the results of the full-scale tests conducted by UL after the
Potentially an even greater challenge than the control of the fuel load or a fire in large expo centers is the safe and timely evacuation of tens of thousands of visitors. When the vast majority of exhibition halls were single story with plenty of exits on grade, providing a safe egress system for large crowds was not difficult. Today most new large convention centers operate as multistory facilities, and for the newest centers, it is common to have a net, combined exhibit area well in excess of 1 million square feet (100,000 m2). Events in these large centers can draw huge numbers of guests, frequently in the range of 60,000 to 70,000 people. Crowds of this size present significant challenges in providing adequate exit widths and acceptable exit travel distances. Sufficient exit width and exit travel distance are important to make possible a timely evacuation. Several large stair enclosures in the middle of the exposition halls are not a desirable option but are sometimes unavoidable. Such designs can significantly impact the owner’s goals for the greatest possible flexibility. In order to mitigate these issues, the use of horizontal exits can be an attractive option. Although movable temporary partitions, frequently used to divide large exhibition halls into smaller halls, cannot be used as horizontal exits, the large prefunction areas adjacent to the halls can help in forming horizontal exits. Since natural light is usually not a design feature for expo halls, there is generally a solid wall between the exhibition halls and their prefunction areas without windows or glazed doors. This separation can provide an ideal arrangement for horizontal exiting, often permitting the reduction of the necessary stairway width by nearly 50 percent. The large prefunction areas can generally accommodate the refuge area requirements for horizontal exits. Additionally, since the visitors enter the expo halls through the prefunction areas, this horizontal exit arrangement can also fulfill the main exit requirements.
Meeting the required egress door width, maximum exit travel distance, and minimum exit separation by themselves will not guarantee the safe evacuation of large crowds. It is imperative that exhibits and displays not impede the evacuation of the visitors by blocking access to exits or by concealing exit signs. Similar issues need to be addressed for the multipurpose, flexible meeting rooms that are usually provided in, or adjacent to, convention centers. The meeting rooms can often be configured in dozens of different ways with the help of movable partitions. The number and remoteness of exits and the proper door swing for all possible meeting room configurations have to be ensured. The easy recognition and accessibility of all exit doors should also be considered. Doors cannot be hidden behind catering tables or blocked by the speaker’s podium. These items are mainly enforcement concerns, and the meeting room operators and trade show designers shoulder a great responsibility.
Due to the huge volume and tall ceiling heights of the newer facilities, even a substantial fire would not rapidly fill large exposition halls with smoke. These inherent features provide support for the possible application of a performance-based design approach to convention center egress systems. Such an approach should be taken with the utmost of care. Although the expo halls are enormous in scale compared to most buildings, a potential fire in an expo center can be sizable. An analysis of the fire growth and smoke generation of the virtually boundless potential combination of combustibles on display can call for design fires with a heat output in the range of 15,000 to 20,000 Btu/sec (15 to 20 MW). This fire size does not take into account the operation of the automatic sprinklers, because their activation and effectiveness cannot always be assured due to the nature and arrangement of the goods on display. However, a careful analysis might show that smoke and heat from massive fires would not severely impact egress if the volume of the hall is sufficiently large.
As part of the performance-based design of the egress system of large convention centers, one may choose to provide a smoke control system to enhance tenability during egress. If the convention center is a high-rise structure, or part of one, a smoke control system might be a requirement (depending on the applicable building code). Design of smoke control systems for large exhibition halls can present unique challenges as well. An exhaust-type smoke control system is frequently the best choice because this type of system can manage smoke within large-volume spaces where the fire is located. By keeping smoke from descending from the ceiling level, a smoke exhaust system can help to maintain a tenable environment in the means of egress for a time period necessary for evacuation. The necessary exhaust airflow rates, even for the expected sizable fires, would likely be in the range of the airflow rates necessary for the proper ventilation of high-volume expo halls with high occupant loads. This means that the building’s HVAC fans might be used for smoke control and additional dedicated, separate smoke exhaust fans may be unnecessary.
A secondary but significant challenge for such smoke control systems is providing the makeup air in the necessary quantity and at low enough velocity. NFPA 92B, Guide for Smoke Management Systems in Malls, Atria, and Large Areas recommends limiting makeup air velocity to a maximum of 200 feet per minute (1 m/s) near potential fire locations because higher velocities are believed to disturb the fire plume, possibly causing more air entrainment and smoke development. Since virtually the entire area of an exhibition hall is a potential fire location, this velocity limitation may result in very large supply air grilles or very large louvers to the outside around the perimeter of the hall. One possible way to provide sufficient makeup air with adequately low velocity is to divide large expo halls into several smoke zones. Because smoke exhaust systems are suitable to limit smoke migration among spaces not separated by smoke barriers, only a portion of a hall, where the fire is located, has to be exhausted. Although dividing the halls into several smoke zones will not result in reduced smoke exhaust airflow rates, the normal outside air supply systems of the adjacent smoke zones can be used to provide the required makeup airflow. This approach not only reduces the amount of necessary separate mechanical equipment and number of shafts, but also increases the overall reliability of the smoke control system by using equipment whose daily operation provides ample supervision.
Even if safe and adequate smoke-protected exiting is provided, an evacuation can only be successful if the occupants can be notified of an emergency in a timely manner. The size and often chaotic nature of large trade shows make it difficult for visitors to become aware of a fire event without a properly designed occupant notification system. Generally, smoke or flame detection can provide the fastest automatic fire recognition. However, besides the often prohibitive cost of smoke and flame detection systems in very large spaces, these systems are rarely suitable for the protection of the great variety of trade shows the exhibit halls may host. There is a concern that the various demonstration activities related to the shows could lead to frequent unwanted alarms. The automatic sprinkler systems will initiate the occupant notification system, but the time delay related to sprinkler activation might not be acceptable for evacuation purposes. Therefore, some form of manual activation of the occupant notification system might be necessary. Since trained staff is usually present when large crowds are in attendance, manual fire alarm pull stations located at guarded locations can be a preferred option to initiate the evacuation process.
The next challenge is providing notification signals with adequate audibility, intelligibility, and visibility. Since exhibition halls are large-volume spaces that are usually built without any acoustic consideration, a special audio system might be necessary to meet the intelligibility requirements of NFPA 72®, National Fire Alarm Code® . With the permission of the authorities, the public address system can potentially be designed to double as an emergency voice alarm system. The obvious advantage is that the public address system is used frequently during exhibits and trade shows. If the audibility or intelligibility of this system is not adequate, the problem receives immediate attention and is remedied. However, during some trade shows (for example, consumer electronics or fashion shows) the ambient noise level might be so high that relying on audible notification only may not be prudent. Accessibility requirements would also mandate the provision of visual notification appliances. Some form of visual notification is necessary to make certain that the evacuation signals will be recognized by all occupants. Because of large, often multilevel exhibit booths, displays, drapery, and signs, visible notification appliances (commonly strobes) that are mounted only on walls and columns cannot ensure complete coverage. Ceiling-mounted strobes would likely face fewer obstructions. However, the National Fire Alarm Code does not currently provide guidance for ceiling-mounted visible notification appliances in spaces with ceiling height in excess of 30 feet (9 meters). There is language proposed for the Annex of the 2007 Edition of NFPA 72 that will provide guidance to designers for the placement of strobes in high ceiling spaces that could be used as a basis for an engineered approach. The designer can also explore performance-based alternatives, a new option permitted by the 2002 edition of the National Fire Alarm Code. Additional alternatives, outside the scope of the National Fire Alarm Code, could also be the subject of discussions with the authorities (e.g., flashing the building lights, high-intensity revolving signals, etc.). Nevertheless, the proper operation of the emergency occupant notification system should be tested under circumstances as close to the anticipated conditions as possible.
Structural Fire Engineering
Since convention centers are often characterized by tall, large open spaces, the structure is frequently a steel frame, with slender 60-to 90-foot-(18 to 27 m) long spans. This structural layout and diverse, large fire load lends itself to performance-based design and a structural fire engineering assessment. This type of analysis can bring value by potentially reducing fire proofing where it is not required and can be used to check the long span beams against the forces induced by thermal expansion.
Fire resistance design traditionally relies on very simple, single element tests to calculate insulating material thickness for a frame, in order to limit its temperature increase for specific fire resistance ratings. Thermal induced forces, as a consequence of restrained thermal expansion, are generally not calculated or designed for. This is in contrast to wind, seismic, and live loads that structural engineers account for in the design of the structural frame by appropriate sizing and placement of steel members. In particular, long span beams or trusses that are common in large exhibit halls cannot be tested in a standard furnace; therefore, their true performance in a real building fire is not known. The true response may be better or worse than the standard furnace test predicts. However, by designing the structure to resist worst case credible fires, safety can be achieved by re-sizing critical structural members if required and applying passive fire protection where it is needed rather than to the whole frame.
The aim of structural fire engineering on a large convention center project is to explore better solutions for passive fire protection, by taking into consideration the inherent design strengths of the structure and exposing any intrinsic design weakness. The result is a robust structure that is designed for the forces induced by fire.
The unusually massive occupant and fuel loads in exhibition halls can present complex challenges for the design team. Fire and life safety in large convention centers relies on various passive and active systems that require careful examination as part of a holistic fire strategy. A comprehensive approach should be taken to maximize the level of safety and to create the most cost-effective solutions that can provide the desired flexibility.
In this Section:
|Life Safety in Tall Buildings
Changes to the 2009 editions of NFPA 1, NFPA 101, and NFPA 5000 include proposals focused on improving safety in high-rise buildings.
|Fire and Life Safety Challenges in Convention Centers
As convention centers grow larger, the fire and life safety designs of these buildings become more challenging.
Nuisance fire alarms have long been the bane of the commercial high-rise environment. Resolving the problem requires careful study and action.
|Inside Propane Tanks
NFPA’s Technical Committee is recommending changes to NFPA 58 including provisions regarding composite cylinders.