Large arena designers must address many considerations when designing fire/life safety systems due to the unique challenges presented by these large spaces with large occupant loads. Like any assembly occupancy, the sheer number of expected occupants presents an egress challenge during a fire event. This challenge is compounded by the human tendency to consider leaving only the way they came in, even when their safety demands they leave a space as quickly as possible. Those of us in the life safety industry may make a habit of familiarizing ourselves with the many available exits as soon as we enter a new environment, but few in the general population do so. Under panic conditions, most of the general population can be expected to leave the same way they came in, reflexively, especially under panic conditions. As a result, the International Building Code requires rather particular egress challenges be considered in arena design.
Performance-based design is used often to address the challenge through a thorough life safety analysis.
Fire modeling combined with egress modeling can achieve elegant solutions that bring not only equal or greater life safety performance but often at considerable cost savings and improved design freedom related to aisle width, number of occupants, automatic sprinkler and smoke control systems, and even the type of construction. Who knows better than the specialty fire and life safety firms who are known for their work on these kinds of projects? A quick poll yields similar lists with a consistent focus on the importance of the computer modeling tools to maximize the efficiency and efficacy of their life safety evaluations. Jeremy Mason, P.E., associate principal of Howe Engineers lists his top five life safety considerations for their arena project designs as such:
- Understanding use and designing the fire protection systems accordingly;
- Means of egress design and understanding operations for various events to ensure required widths are maintained;
- Type of construction and fireproofing requirements and the ability to use performance based design to limit fireproofing or other forms of protection;
- Site constraints; and
- Consider computer modeling vs. boilerplate one size fits all design for smoke protected seating approach.
When the computer modeling defines the required capacity of a smoke exhaust system, to maintain the smoke layer safely above egressing occupants, the results provide project specific design parameters for that exhaust system. An experienced fire/egress modeling team can often balance the egress elements with smoke exhaust system’s capacity in ways not available with basic algebraic calculations. Once a smoke exhaust system and its specific capacities are finalized, the heavy lifting may seem to be done. Here’s where it's important for one not to drop his or her guard.
Any part of these solutions that is not prescriptively code-conforming will demand submission to the AHJ for approval as an alternate method, which must demonstrate equivalent fire resistance, for one. Fire resistance is part of the long list of specific features of performance that are supposed to be included in such a report. The smoke control system’s infrastructure is an important part of the system’s minimum capabilities. Achieving results found by the computer modeling must now be translated into a detailed design that, once constructed, will achieve the agreed upon level of performance.
One of the most common mistakes is overlooking the importance of specifying the appropriate level of design for the smoke exhaust system’s infrastructure in addition to its volumetric capacity. This makes the ductwork a required element of discussion for an Alternative Means/Methods Report (AMMR). Fire/egress modeling are commonly understood justify prescriptive reductions covered in the scope of an AMMR’s demonstration of equivalency. Lesser understood is the extent to which the associated ductwork and related system infrastructure are affected.
Sprinkler systems may modified or even omitted in circumstances where design fires are shown to negate any benefit that might be provided. Smoke control infrastructure may be located in the hot smoke layer or likely even subject to more direct effects of the fire plume or jets. Smoke control fans, exhaust, or makeup air might not be easily arranged for direct roof/exterior placement. Where probable fire exposures affect any required interior ductwork, fans, power, or controls, the life safety analysis must address these smoke control system elements. Otherwise, how is the smoke control system expected to survive that fire exposure so the many associated design reductions are legitimately realized. Clearly, the knee-jerk decision to harden none of this smoke control system infrastructure is rarely justified. Sprinkler-controlled hot layer temperatures cannot be applied where plume or jet exposures are possible. The ductwork is not ‘always’ on the other side, away from the fire origin. And what about when sprinklers are omitted or hot layer stratification occurs? Where is the ductwork? Tread carefully with knee-jerk justifications that tend to be more rationalized than rational. The smoke control system is a keystone life safety design feature when designers are dealing with smoke-protected assembly seating.
Fire-rated duct-product designs are tested and approved by the IAS/NRTL laboratories that use quite a bit of industry jargon in their listings/approvals. One must understand that jargon to adequately define a smoke control system design in consideration of all relevant considerations.
Many designers look to fire-rated ductwork to address not only the survivability features this kind of smoke exhaust system must have but to do so in an economical way. All too often, though, a detailed understanding of the available products capabilities is not translated into the AMMR. This is a critical error, because all fire-rated duct products currently available in the US market are tested to ASTM E2816 (ASTM) or ISO 6944 (ISO) for horizontal fire-rated duct applications. These are not test standards referenced by the International Building or Mechanical Codes nor any of their referenced NFPA standards. Absent wholly direct roof exhaust, an adequate smoke control design likely is going to involve horizontal fire rated ductwork. If so, a designer will be applying product designs tested to these alternative test standards. Applying designs from these nonprescriptive test standards is exactly one of the issues that needs to be covered within an AMMR along relevant aspects of the appropriate fire and egress modeling.
ICC’s Acceptance Criteria AC179 addresses how select fire-rated duct designs achieve equivalent fire-rated protection. In other words, the evaluation service provides a report (ESR) that tells a designer exactly what duct can be considered equivalent using the jargon on the listing/approval card from the test laboratory. This is exactly how specific one will need to be as he or she covers the basis of equivalency, because a designer cannot just include the ESR by itself in the AMMR or he or she will likely be fending off inferior duct configurations, if the defect is noticed at all.
The ESR itself tells the designer to include the actual listed/approved designs that he or she will be providing on the project. The ESR does so because that is the only way to demonstrate conformance with the terms of the ESR. Every ESR describes exactly the same thing for smoke exhaust ductwork, in this regard. Unfortunately, most of the available product designs found in a test laboratory's fire resistance directories are limited to Duct A (ISO) or Conditions A & B (ASTM), which are not allowed by any version of AC179 or any other evaluation service from the U.S. market.
Thus, fire-rated duct designs that actually demonstrate they do meet the AC179 criteria for the smoke exhaust ductwork must be included in an AMMR. That is Duct B (ISO) or Conditions C & D (ASTM). Only these duct designs are tested to handle the products of combustion directly inside the ductwork as it is being exhausted by your smoke control system. Upon reviewing the available Duct B designs, one will discover the real world often involves duct sizes, penetrated assemblies, and similar elements of those designs for which a project's conditions are out of range. Some of those design variables may be show stoppers for achieving equivalent fire resistance, while others can be addressed through legitimate engineering judgement activity.
Failure to identify alternative test standards and design limitations will lead to avoidable problems. At best, these difficulties can include requests for information (RFIs), if recognized by the contract team. Better than being left unresolved, this RFI is nonetheless an avoidable zero-budget activity and could possibly lead to a change order. Near the other end of the spectrum, the situation can be much worse. What if Duct A slips through and is installed by the contract team but eventually is identified during field inspections? The AHJ isn’t likely to accept an excuse: The schedule is adversely affected or the change order cost is unpleasant. These are not justifications for approving a defective installation. It doesn’t cost more to do it right up front and the schedule would not have been impacted had the Design been clear in the first place.
The omission of clarity on the issue from the AMMR eventually cascades to a vague smoke control system design, which, if left to the contract team to sort out, can lead to late stage project delays and avoidable change order costs. So, it isn’t merely minding protocol on what should be covered, technically, within an AMMR but it's essential to sound and economical design.
If reductions in egress, sprinklers, or fireproofing are predicated on the smoke control system performance, then it is critical that the ductwork be able to transport the actual products of combustion while possibly being exposed to direct fire effects at the same time. If the exhaust system doesn’t actually perform, then all the computer modeling is just paperwork, and related reductions in other areas of design may come at higher costs than that of the construction. The chance of a catastrophic loss of life under such circumstances is very real. And that’s the worst possible outcome for a life safety design.
Arena life safety design can be challenging, but the tools are improving every day. In expert hands, the performance of the life safety systems can be evaluated with greater accuracy than ever. Just be sure to translate the computer modeling results into the plans and specs with enough specificity to achieve the predicted results when the smoke exhaust system is ever actually brought to bear.
Vetting the available fire-rated duct designs is challenging — more so than may at first seem apparent. Starting with weeding out Duct A (ISO) and Conditions A & B (ASTM) is just the beginning. To get a clearer understanding of how specific a designer need to be, the right questions must be asked, because the work is still not done. The contract team’s submittals must match the AMMR.
Proper design to achieve adequate fire/egress modeling results is achievable, but that means properly documenting (AMMR), specifically specifying (Duct B), thoroughly submittal a review, and, finally, following through on inspections of the smoke control ductwork. It’s probably more complicated than one has been led to believe.
Top Five Life Safety Considerations a Consulting/Specifying Engineer Should Consider When Designing an Arena.
Looking for a quick list of life safety design considerations within large arena settings? Engineered Systems asked Ed LaPine, P.E., director of the White Plains office, Jensen & Hughes, and Rachel Bourassa, staff consultant, Jensen & Hughes, to provide their insight. Here is a list of their top five concerns.
Egress & Occupant Loading/Smoke Protected Seating – Arenas are some of the most densely populated buildings that are designed. Very large occupant loads can require complex exiting systems. Smoke control systems may be incorporated as a tool to help design such systems and maintain life safety. Smoke protected seating allows different egress widths and will require both fire modeling and egress modeling.
Flexibility of the Space – Arenas are often used for a variety of different types of events and owners want to be able to have flexibility with these events. For an arena, you may need to design for a hockey game, concert (with different stage set ups), a graduation, conferences, and even monster truck shows. Each one of these events could have a different occupant load, types of occupants, lighting, sound levels, and fire loads. Agreeing on the uses of the arena with the owner and designing to some limitations proves difficult.
Sprinkler Design – Sprinklers are often deemed ineffective in certain areas of an arena based on the large size of the bowl space and the height of the space. Consultants will use smoke control modeling to determine if sprinklers will not activate and therefore remove the sprinklers. Sprinklers can be difficult to maintain in many of the high places and this is often a concern for owners and designers. Omission of these sprinklers in most jurisdictions may require a request for variance through the alternate methods approach in the building code.
Audio/Visual Notification – Designing a fire alarm and emergency notification system for an arena often requires an alternate method to meet the code requirements. Due to the size of the bowl, requirements for fire alarm cannot be met using conventional devices and layouts. The alternate method will include the use of arena infrastructure that is already there to give the occupants notifications. For example, score boards and ribbon boards may be used for visual messaging and a PA system may be used for audible notification in order to ensure intelligibility.
- Response and Preparedness for Fires and other Emergencies – Arenas typically require a life safety evaluation that includes an assessment of a variety of different emergency conditions. The evaluation must include assessments of both building systems and management features that ensure the safety of the building occupants and consider scenarios that are appropriate to the specific facility. These assessments span across multiple stakeholders and team members. An important aspect of this is coordination with the responding emergency agencies and capabilities of these individuals. Most jurisdictions are not used to buildings holding events with tens of thousands of people. Their capabilities, equipment, and experience should all be considered in the design, depending on how they will respond to various emergency events, not just fire.