Code strategies for mixed occupancy buildings
The code application options for mixed occupancy buildings, when applied strategically, can minimize the amount of fire-resistance-rated construction required by code.
John Mahoney, PE, LEED AP, Arup, Washington, D.C.
The code allows the option of applying various design approaches to a mixed occupancy building. The strategy can drive the requirements for the fire-resistance of assemblies. For a mixed occupancy building, the options identified in the 2012 International Building Code, Section 508: Mixed use and occupancy, include:
- Accessory occupancies
- Separated occupancies
- Nonseparated occupancies.
The intent of the accessory occupancy provisions recognizes that buildings often have rooms or spaces different from but accessory to the main occupancy. When such areas are limited in size, they do not represent a significantly different hazard or risk as compared to that of the main occupancy. As such, the code allows the accessory occupancy to be nonseparated from the main occupancy and does not require the accessory occupancy to be included in allowable area determination for building construction type.
The accessory occupancy provisions require that the space be accessory or ancillary to the main occupancy and that it does not exceed 10% of the area of the story in which it is located. The accessory occupancy is also not permitted to exceed the tabular values in the 2012 IBC Table 503: Allowable heights and building areas, without increases such as those for sprinkler protection to the tabular values. The space is individually classified in accordance with Chapter 3: Use and occupancy classification. For most occupancy classifications, no separation is required between the main occupancy and accessory occupancy. High-hazard occupancy classification Groups H-2, H-3, H-4, and H-5 are exempt from the separation allowance and must comply with the separation requirements for separated occupancies. Consider an office building with an accessory storage area. The storage area is classified as a Group S occupancy but is not required to be separated from the main Group B occupancy when applying the accessory occupancy provisions.
The accessory occupancy does not need to be accounted for in construction type determination and related height/area determination for a building. The allowable building area and allowable building height are permitted to be based solely on the main occupancy classification(s). The accessory occupancy must comply with the tabular values of Table 503 for such occupancies, but it does not need to be accounted for as a separate occupancy when determining the minimum type of construction and limits on height and area.
For an office building with a storage area, the Group S classification would not be included for the purpose of determining the minimum required type of construction and the maximum permitted allowable area/height of the building. The construction type and height/area can be based solely on the Group B classification. Even though the building allowable height and area are not impacted, the accessory storage occupancy must be located in the building to meet the allowable area and height provisions for a storage occupancy for the type of construction. As an example, a business occupancy of Type II-B construction would not be allowed to have an accessory Group S-1 storage area on the fourth story because storage occupancies of Type II-B construction are only permitted up to three stories in height.
The nonseparated occupancy provisions allow multiple occupancies without a physical separation between them. Occupancies are individually classified in accordance with IBC Chapter 3 but are not subject to the 10% area limit applicable to accessory occupancies. The requirements of the code are based on the occupancy classification of the space, except that the most restrictive requirements of Chapter 9: Fire protection systems, are applied to the total nonseparated occupancy fire area. If the building is a high-rise, the most restrictive requirements of the high-rise provisions also apply throughout the building.
Area limits, height limits, and construction type requirements are based on the most restrictive occupancy groups involved. For an office building with a storage area, the construction type and limits for height/area can be based solely on the Group S values in the code. Essentially this means that the construction type would be determined as if the entire building were a Group S occupancy.
The accessory and nonseparated provisions can be advantageous to avoid requirements for fire-resistance ratings between occupancies. In some instances, however, consequences may outweigh the advantages. For example, a nonseparated strategy could result in a need for a greater type of construction when compared to a separated occupancy or accessory occupancy approach. This would occur if the size of a nonseparated occupancy fire area exceeded the tabular values for area/height (with appropriate increases) for a type of construction that would otherwise be permitted when determined using a separated or accessory approach.
If the most restrictive occupancy in a nonseparated occupancy fire area exceeds 10% of the floor area, or it is not accessory to the main occupancy, the accessory provisions are not a viable option. In such cases the engineer has the option of using the nonseparated occupancies provisions or using the separated occupancy provisions with fire-resistance-rated separations as required between occupancies. The minimum type of construction required to for each strategy may differ.
The separated occupancy provisions require physical separation by fire barrier walls and/or horizontal assemblies with a fire-resistance rating between occupancies. Occupancies are located in separate fire areas, and each fire area is required to comply with the code based on the occupancy classification of that portion of the building. The engineer has the option applying any of the options available for mixed occupancies to a given fire area.
Construction type requirements are based on the height of each fire area and the sum of ratios of the actual area divided by the allowable area for each separated occupancy. Area limits require the sum of the ratios to not exceed one. Additionally, for separated occupancy buildings with more than three stories above grade plane, the aggregate sum of the ratios for each story is not permitted to exceed three.
When applying code provisions to a fire area that utilizes the separated occupancies approach, each fire area is considered separately. The provisions of the code are applied only to that specific fire area unless the code indicates otherwise. The concept to apply the code to each fire area separately is a significant difference from the nonseparated occupancies provisions. The nonseparated occupancies provisions require the most restrictive provisions of the high-rise and fire protection systems requirements to apply throughout the nonseparated areas.
If a nonseparated occupancy fire area is of a size that would require a greater type of construction when compared to a separated occupancy approach, the separated occupancy approach may often be more desirable. That is, it may be more desirable to provide fire-resistance rated separations between occupancies than provide fire-resistance ratings for the structural frame, floors, and roof of the building.
Incidental use areas are spaces with ancillary functions associated with a given occupancy that generally pose a greater level of risk compared to the main occupancy group classification in which it is located. These types of spaces require protection features, such as physical separation, which are intended to address the higher relative fire hazard of these areas.
Prior to the 2012 edition of the IBC, the incidental use provisions could be strategically applied at the option of the engineer. The engineer had the option of applying the incidental use provisions or applying the separated or nonseparated occupancies provisions of the code. In the 2012 IBC, the incidental use provisions were moved to Section 509: Incidental uses, through which the incidental use requirements apply regardless of the code strategy for mixed occupancies. Therefore, application of the incidental use provisions is no longer optional in the 2012 IBC. However, some of the rooms or areas that were historically identified as incidental uses have been revised in recent editions of the code. For example, prior to the 2009 IBC, storage rooms greater than 100 sq ft in area had been included in the incidental use provisions of the code. With the 2009 and 2012 editions, storage rooms are no longer considered incidental uses.
These strategies can be used in combination with each other. For example, a building may contain multiple fire areas, and each fire area can utilize a different strategy for a mixed occupancy building. Some fire areas within a building may utilize the accessory provisions, while other fire areas in the same building can utilize a different strategy to achieve a particular goal or to optimize the design.
A comprehensive review should always be done when selecting a code strategy. In doing a comprehensive review, the advantages and disadvantages of each approach will be identified and used as the basis for selecting the desired approach. Regardless of the strategy used, it should be documented and included in the building permit application submittal. It should also be kept on file by the building owner or manager. The original code application strategy when a building is permitted is a fundamental piece of information that is critical to evaluating future changes and alterations to that building. If an alteration project is not coordinated with the original code application concepts, it could compromise the original approach and create compliance issues.
Mahoney is an associate in Arup’s Washington, D.C., office. He has extensive experience in applying the fire protection and life safety aspects of the codes and standards to various building types and occupancies.
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