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.