Jonathan Ochshorn
Then, if a single building has more than one type of occupancy, there are four choices available.
Choice 1: Accessory occupancies
If any building story has primarily one occupancy, but contains not more than 10% of a second (accessory) occupancy, then the building can be designed as if the accessory occupancy were not there, as long as...
The smaller accessory occupancy, all by itself, satisfies the "NS" limits in Chapter 5 for height and area, and...
The accessory occupancy satisfies all other Code requirements that pertain to it. If these conditions are met, then the building can be designed based on the main occupancy and no fire separation is needed between the main and accessory occupancies.
Example: Lecture room (A-3 occupancy) in E. Sibley Hall (whose main occupancy is group B).
Does the lecture room 157 (Group A-3) constitute an accessory occupancy in East Sibley Hall?
Answer: The A-3 lecture room cannot be considered an accessory occupancy since it occupies more than 10% of the first floor area of E. Sibley Hall.
Choice 2: Nonseparated occupancies
In this choice, there is no fire-resistance rated separation required between the mixed occupancies. But, you must satisfy the following conditions:Choice 3: Separated occupancies
If the mixed occupancies are actually separated from each other using approved fire-rated assemblies, then areas and heights are computed differently. Per the 2015 IBC: "In each story, the building area shall be such that the sum of the ratios of the actual building area of each separated occupancy divided by the allowable building area of each separated occupancy shall not exceed 1."
Example: E. Sibley Hall lecture room considered as a separated occupancy:
From appropriate Chapter 5 tables, find allowable areas for Group B and Group A-3 occupancies, assuming Construction Type IV (just for the fun of it — Sibley does not qualify as heavy timber).
The allowable areas per floor are 108,000 and 45,000 s.f. respectively. Assuming If = 0: The allowable areas are 108,000 s.f. and 45,000 s.f.
Checking whether this separation works, we add the ratios of actual to allowable areas:
5,400/108,000 + 1,350/45,000 = 0.05 + 0.03 = 0.08, which is less than 1.0.
Furthermore, each occupancy meets its individual height limit, per Table 503, including the extra height and story for having sprinklers. So, it works as a separated occupancy.
But this requires that the two occupancies are actually separated from each other with fire barriers or horizontal assemblies:
The required separation (1-hour fire-resistance rating required) is tabulated in Table 508.4 of the 2015 IBC (or Table 508.4 of the 2018 NYS Code). In the table, S = Sprinklered and NS = NonSprinklered. And note that the lecture hall also needs to be separated from any Group B occupancies above it and below it.
Choice 4: Incidental uses
Certain small "incidental" occupancies (really, just "uses") can be added to a main occupancy without changing anything as long as such incidental uses are properly separated from the main occupancy. This includes things like furnace rooms, incinerator rooms, laundry rooms, and so forth. The basic information about which spaces can be considered "incidental," and what type of separation is required, is found in Table 509 of the 2015 IBC. Here are some examples from that table:
Section 707 (2015 IBC): Fire Barriers
Definition: A fire-resistance-rated wall assembly of materials designed to restrict the spread of fire in which continuity is maintained.
Used for things like fire areas, occupancy separation, incidental uses, corridors, shaft enclosures, and fire stairs.
Continuity. Fire barriers shall extend from the top of the foundation or floor/ceiling assembly below to the underside of the floor or roof sheathing, slab or deck above and shall be securely attached thereto. Such fire barriers shall be continuous through concealed space, such as the space above a suspended ceiling.
Supporting construction. The supporting construction for a fire barrier shall be protected to afford the required fire-resistance rating of the fire barrier supported.
Openings. Openings in a fire barrier shall be protected… Openings shall be limited to a maximum aggregate width of 25 percent of the length of the wall Other rules and exceptions apply. An example (Sibley-Milstein Halls) can be found here.
Openings have less fire-resistance than the walls they are in. Why?
(from the 2009 IBC Commentary) "The fire protection rating required for an opening protective is generally less than the required fire resistance of the wall… This is based upon the ability of a wall to have material or a fuel package directly against the assembly while fire doors and windows are assumed to have the fuel package remote from the surface of the assembly.
Labels for fire-rated glazing (per 2015 IBC, Table 716.3):
W for glass meeting criteria in ASTM 119 or UL 263: Good enough to be used in a wall assembly
OH for glass meeting criteria in NFPA 257 or UL 9: Good enough to be used in a window assembly including meeting the criteria of the hose stream test (cold water sprayed on hot surface)
D for glass meeting fire door assembly criteria
H for glass meeting fire door assembly "hose stream" test
T for glass meeting 450-degrees temperature rise criteria for 30 minutes
XXX refers to the time (minutes) of the fire resistance or fire protection rating of the glazing assembly.
Fire barrier fire ratings to separate "fire areas" within a single occupancy group (2015 IBC)
Fire areas are areas in buildings bounded by fire walls, fire barriers, or exterior walls. Their size becomes important only in certain occupancy groups in order to avoid the requirement for sprinklers (See chapter 9 of the IBC).
For example, in the A-3 occupancy group, which includes lecture halls and libraries, sprinklers are required if any one of the following conditions is met:
To create a fire barrier within a building or part of a building with a single occupancy in oder to define a fire area, the following table indicates the required fire resistance rating for that fire barrier. Note that this is different from the required fire-resistance ratings for a fire barrier intended to separate different occupancies.
Section 706 (2012 IBC): Fire Walls
Definition: A fire-resistance-rated wall having protected openings, which restricts the spread of fire and extends continuously from the foundation to or through the roof, with sufficient structural stability under fire conditions to allow collapse of construction on either side without collapse of the wall.
General. Each portion of a building separated by one or more fire walls that comply with the provisions of this section shall be considered a separate building. The extent and location of such fire walls shall provide a complete separation.
IBC 2015 Section 706.2 Structural stability. Fire walls shall be designed and constructed to allow collapse of the structure on either side without collapse of the wall under fire conditions. (Also OK to comply with NFPA 221)
Firecut wooden beams in masonry walls:
Gypsum board used as an area separation fire wall: see this PDF.
For fire-resistance ratings of fire walls for various occupancies, see Table 706.4 (2015 IBC) or Table 706.4 of the 2015 NYS Building Code.
Section 706.5 Horizontal continuity: Fire walls shall be continuous from exterior wall to exterior wall and shall extend at least 18 inches (457 mm) beyond the exterior surface of exterior walls.
Exceptions: [many]
Section 706.6 Vertical continuity: Fire walls shall extend from the foundation to a termination point at least 30 inches (762 mm) above both adjacent roofs.
Exceptions: [many]
Section 706.8 Openings: Each opening through a fire wall shall be protected in accordance with Section 716.5 and shall not exceed 156 square feet (15 m2).
The aggregate width of openings at any floor level shall not exceed 25 percent of the length of the wall.
Exceptions?
Opening protectives: fire shutters; UL-rated labeled self-closing doors.
Can glass be used in a firewall?
Review of compartmentation: requires assemblies to achieve fire-resisting ratings.
Review IBC Table 601 "Fire resistive rating requirements for building elements (hours)"
Examine the footnotes to Table 601:
For example, compare Type V-A (using sprinklers to avoid 1-hr construction) versus Type V-B (using sprinklers for bonus area and height): Assume Group B occupancy and no frontage increase:
Note that heavy timber consists of big wooden cross-sections as follows:
Mass timber buildings, e.g., made with cross-laminated timber, offer another possibility for wood-framed tall buildings, typically governed by heavy timber rules.
Examples of various assemblies are given: see chapter 7, IBC, or product literature for gypsum board, masonry units, etc. Typical 1-hour partition assemblies can be fabricated with normal wood or steel studs and 1 layer each side of 5/8" thick "firecode" gypsum board fastened according to the specifications. For a 2-hour rating, one typically uses 2 layers of gypsum board each side, etc. Masonry units (or concrete) is also commonly used for 1-, 2-, 3-, or 4-hour-rated assemblies. For hollow block, manufacturers provide "equivalent" thicknesses so that their ratings can be computed. Sprayed-on or troweled-on cementitious fireproofing is commonly used for steel elements. Shaftwall, where gypsum board can be applied from one side of a shaft (the side with a floor to stand on...) was illustrated. Some images follow:
Changes to the requirements for shaft enclosures (exit stairs, etc.) as a result of recommendations made in the aftermath of the World trade Center collapse:
In the latest versions of the IBC (Chapter 4), enclosures for interior exit stairways and elevator hoistway enclosures shall have impact-resistant construction board, concrete, or masonry materials. This applies to high-rise buildings (buildings with an occupied floor located more than 75 feet above the lowest level of fire department vehicle access) of Risk Category III or IV (see Section 1604.5 of the 2015 IBC), and to all high-rise buildings that are more than 420 feet in building height.
Fire-rated glass types
Fire safing
Firestopping (fire safing) is an increasingly important component of assemblies, to close of gaps and holes, especially at penetrations of conduit, pipes, etc., and at floor-curtain wall intersections.
Lack of proper fire safing was also reported in the Grenfell Tower fire in which 71 people died (London in June, 2017). "The cavity barriers, which are supposed to expand and seal the gap between concrete floors, were the wrong size. The ones installed, according to the investigation, were able to close a gap of 25 millimeters, but the size of the actual gaps in between floors is 50 millimeters. In addition to the size discrepancy, some barriers were installed incorrectly, the report notes." (from Durability + Design, April 18, 2018)
Reasons for exterior cladding fires and their prevention:
Flame spread or surface burning characteristics rating must be considered and tested: it is a ranking derived by laboratory standard test methodology of a material's propensity to burn rapidly and spread flames.
Combustibility is a measure of how easily a substance will set on fire, through fire or combustion. This is an important property to consider when a substance is used for construction or is being stored.
NFPA 285, referenced in the IBC: Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-Load-Bearing Wall Assemblies Containing Combustible Components, provides a standardized fire test procedure for evaluating the suitability of exterior, non-load bearing wall assemblies and panels used as components of curtain wall assemblies that are constructed using combustible materials or that incorporate combustible components for installation on buildings where the exterior walls are required to be non-combustible. Assemblies (not just materials) must be tested to ensure that fire will not spread over the exterior cladding. Some combustible materials — water-resistive barriers or WRBs and foamed insulation — are allowed in exterior wall assemblies where this is otherwise prohibited (Types I – IV construction) but only if they pass NFPA 285.
References (pdf): John Valiulis, Building Exterior Wall Assembly Flammability: Have We Forgotten the Past 40 Years? | Owens-Corning Enclosure Solutions | Joseph Lstiburek, BSI-098: Great Fire of London
Some notes from the John Valiulis reference:
NFPA 285 does not apply to Type V buildings. Therefore, it applies to all other construction types: I, II, III, and IV.
Mostly, these exterior fires have nothing to do with the combustible interior contents of the building. Hence, having interior fire sprinklers is mostly useless in preventing such exterior cladding fires.
IBC generally looks at individual combustible components as well as the total assembly.
For each component, there are typically flame spread limits and cone calorimeter heat release limits. There are also exceptions, e.g., NFPA 285 compliance not required for combustible exterior cladding not higher than 40 feet above grade.[This exception shows up in Chapter 14 - Exterior Walls.]
Special restrictions for Metal Composite Panels (MCP) and fiber-reinforced plastic (FRP) were added to the 2009 IBC. And restrictions on the use of high pressure laminates (HPL) and water-resistive barriers (WRB) were added to the 2012 IBC.
Some testing not required with certain WRBs when they are the only combustible materials in the wall (e.g., where there is no combustible rigid insulation).
Some people and corporations within the building industry have tried to water down these requirements; their so-called Code change proposals for the 2015 and 2018 IBC were rejected. However, they have had limited success within the local Code adoption process, e.g., in the Washington, D.C. 2013 Building Code.
Disclaimer: Building codes (including the International Building Code) are typically structured as a maze of basic statements and qualifying assertions. Often, the qualifiers — sometimes found in other sections, or in footnotes to tables — are more important than the basic statements they modify. For this reason, it is important to scrutinize all relevant sections of the code before drawing any conclusions. The material contained on this page does not contain all qualifying assertions, and provides only a basic overview of fire-safety issues related to occupancy and construction type. These methods are not intended to be used for the design of actual structures, but only for schematic (preliminary) understanding of building code principles. For the design of an actual structure, a competent professional should be consulted.
Disclaimer: Students are responsible for material presented in class, and required material described on course outline. These notes are provided as a tentative outline of material intended to be presented in lectures only; they may not cover all material, and they may contain information not actually presented. Notes may be updated each year, and may or may not apply to non-current versions of course.
first posted Aug. 30, 2012 | last updated: Sept. 4, 2020
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2007–2020 J. Ochshorn. All rights reserved. Republishing material on this web site, whether in print or on another web site, in whole or in part, is not permitted without advance permission of the author.