Wisconsin Department of Commerce Newsletter
How is that Aircraft Hangar Regulated Anyway?
Referencing NFPA 409 within the Wisconsin Commercial Building Code
Some suggest that because of the wording of IBC section 412.2.6, all aircraft hangars must be protected by some type of fire suppression system. That is not accurate. All hangars do not have to be protected by sprinklers. In reality the majority of the small hangars that we review do not require protection by a suppression system. This article will help explain how to use the reference to NFPA 409 from within the Wisconsin Commercial Building Code (WCBC).
To determine if sprinkler protection is required for a specific aircraft hangar, one must first go to NFPA 409 to establish the classification of the hangar [Group I, II, III or IV], an exercise that includes the concept of fire areas.
Note that Commerce's Safety & Buildings (S & B) division staff still use IBC chapter 5 and specifically Table 503 to determine maximum building size based on construction class, use, open space increases and sprinkler increases. Do not use chapter 5 of NFPA 409.
The majority of the hangars in question are classified as Group III. A smaller percentage of hangars fall into the Group II classification, due to the size of the fire areas within the hangar. The requirement for sprinkler protection is put into place first and foremost by the hangar classification (Group I, II, III, or IV). Once it has been determined that the hangar fits in one of the groups that require a suppression system, the type of suppression (water or foam) is determined. The type of protection is called out in Chapters 6, 7, 8 & 9 of the NFPA standard.
Hangers classified Group I & II require some type of automatic suppression [sprinkler, foam, etc. - refer to NFPA 409 Chapters 6 & 7 for details] system. Group III hangars do not require sprinkler protection until the fire area or group of hangars (referred to as a cluster) exceeds the triggers spelled out in Chapters 4 and 8 of NFPA 409. When single hangar buildings (smaller than the maximum fire areas specified in Table 4.1.3 of NFPA 409) are created by separating one from another by fire walls, they are not considered a hangar building cluster. It is important to remember that S & B does not use the other parts of Chapter 8.
A hangar that has an aircraft access door opening that is 28 ft. or less in height is classified as a Group III as long as it is one-story and does not exceed the given size limitations. Group III hangars are considered to be a Group II when over one-story or when exceeding the areas provided for in those triggers [refer to NFPA 409 sections 8.1.2 & 4.1.3] and once classified as a Group II, the hangar must be sprinklered as called for in Chapter 7 of the standard.
As an example, if you have a single-story hangar that is of type VB construction and has an aircraft access door opening that is 28 ft. or less in height, it is classified as a Group III hangar until it has an undivided area exceeding 5,000 SF. A Group III hangar does not require sprinkler protection.
If that same single-story building is larger, but is subdivided into 5,000 SF fire areas by the 3 HR fire barriers required by the WCBC [refer to Table Comm 62.0706], it is still considered a Group III hangar and need not be protected by a sprinkler system until it exceeds the area limitations governed by IBC Chapter 5. Using IBC Chapter 5, if that building is provided with open sides as specified in section 506.2, the largest the single story hangar building could be without sprinkler protection is 15,750 SF.
A different example using the Type II-B construction classification that is very common is as follows. A single-story hangar that is of type IIB construction and has an aircraft access door opening that is 28 ft. or less in height it is classified as a Group III hangar until it has an undivided area exceeding 12,000 SF. As such it would not require sprinkler protection. As in the first example, if that same single-story building is larger, but is subdivided into 12, 000 SF fire areas by 3 HR fire barriers, it is still considered a Group III hangar and need not be protected by a sprinkler system until it exceeds the area limitations governed by IBC Chapter 5. Using IBC Chapter 5, if that building is provided with open sides as specified in section 506.2, the largest the single-story hangar building could be without sprinkler protection is 30, 625 SF.
A far less common occurrence and for that reason not being presented in detail within this article is hangar building clustering. In such a cluster concept, a group III hangar may also be selected to become a group II hangar. When a hangar building cluster exceeds the cluster fire area limits [refer to NFPA 409 Section 8.3], individual group III hangars are selected and considered as Group II hangars and sprinkled accordingly. The selection is made such that the remaining unsprinkled hangars in the cluster will not exceed the aforementioned trigger limits. Because the IBC controls, most designers chose to separate their buildings in accordance with IBC section 704 or by the use of fire walls in accordance with IBC section 705.
NOTE: It is very important to remember that although the classes of construction referenced from within NFPA 409 [refer to Tables 4.1.2, 4.1.3, & 8.3.1] are not an exact match to the IBC, they do not have to be. To apply the various tables in NFPA 409 to the IBC construction classifications, use the following crosswalk.
Last but not least, it is important to remind readers that S & B does not layer on the subject matters found in NFPA 409 that deal with other things like setbacks, curbs or roof coverings. Use the IBC for regulating all those types of tangential provisions.
Should you have any questions on this article or would like to learn more about the subject, feel free to contact either Henry Kosarzycki or myself. Contact Henry Kosarzycki by email at firstname.lastname@example.org or by phone at 608-212-0138. Contact James B. Smith by email at email@example.com or by phone at 608-266-0251.
-- J.B. Smith
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Please send comments or questions to Barbro McGinn, editor.