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In the AEC world, the terms “mixed use” and “mixed use building” are commonplace. Developers refer to new buildings that contain both office and retail spaces as mixed use projects. Architects and Engineers refer to a floor containing multiple occupancy types as mixed use. You’ll even hear AHJs drop the term mixed use in plan review meetings on occasion. Colloquially, “mixed use” is meant to refer to a building or space that contains multiple occupancy types. But did you know that other than parking garages, the building code has essentially no requirements pertaining to a mixed use building? In fact, if you search for “mixed use” in the International Building Code (IBC), you won’t find any requirements related to building height, building area, construction type, fire-rated construction or means of egress. * So why is “mixed use” a common term in the AEC world but not addressed in the codes? Long story short: there is a difference between use and occupancy in the IBC and people frequently confuse the two. We’ll do a full analysis of the differences between use and occupancy in a later article, but here is a simple way to understand the difference.  This is both a mixed used and mixed occupancy building (image courtesy of Brett VA). Mixed Use Vs. Mixed OccupancyThe use of a space is a description of how the space will actually be used. This could be a broad, general description such as “office” or “conference room”, or it could be more specific, such as “visiting team locker room and shower.” The use of a space is helpful in determining (1) the occupancy classification and (2) the appropriate occupant load for a space. On the other hand, the occupancy classification of a space is 1 of 10 categories (plus sub categories) in Chapter 3 of the IBC. These occupancies are:
For example, most offices and places of business have conference rooms, where multiple people come together for meetings, presentations or collaboration. The use of such spaces might lead you to think that the conference room is an assembly occupancy. But per IBC 303.1, if your office has a conference room with 49 people, the conference room is considered a business occupancy. There are many similar examples where the use of a space does not necessarily align with the occupancy, so always refer to the definitions in Chapter 3 when determining the occupancy classification. Now getting back to the original discussion, “mixed use buildings” do not have specific requirements because nearly every building contains multiple uses. Does your office suite have a storage area? You have at least two uses there. Does your retail store have an office in the back? Multiple uses. Does your apartment building contain an amenity space? You get the point. So when people use the term “mixed-use,” it doesn’t mean much from a code standpoint, and practically, almost all buildings contain multiple uses. But what most folks intend to convey by this term is that the building has at least two distinct occupancy types, which the IBC would consider “mixed occupancy.” And unlike “mixed-use,” there are numerous requirements for “mixed occupancy buildings” in the IBC. In fact, IBC Chapter 5 has sections for allowable height, allowable area, allowable number of stories and required separation of occupancy, all of which are impacted when a building is mixed occupancy. ConclusionConversationally, “mixed use building” is generally understood to mean a building containing multiple occupancy types. Architects, engineers, contractors and code officials all use this term and it typically does not create a misunderstanding. But as far as the code language goes, mixed use doesn’t mean anything for the requirements for a building. Mixed occupancy, on the other hand, carries many code requirements that must be understood for a code-compliant building design.
*While IBC 508 is entitled "Mixed Use and Occupancy,' it does not actually have specific requirements for mixed use buildings, only mixed occupancy buildings.
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If you are located in any major city, it’s likely that you can take a short walk down the street and find an instance of two adjacent buildings built up next to each other. If you’re out in the suburbs, you have probably seen this situation in the form of a row of townhouses. In the U.S., and other countries that adopt the International Building Code (IBC), these abutting buildings likely fall into one of the following cases:
In either case, the IBC recognizes three distinct approaches for the wall(s) located between the abutting buildings. All references are to the 2015 IBC. Walls Between Abutting Buildings Abutting buidlings (image courtesty of agracier). Abutting Exterior Walls In both cases described above, the IBC allows for two abutting exterior walls to separate the two buildings. Since the two buildings have a zero fire separation distance, IBC 602 requires both walls to have a 1-hour fire-resistance rating for most occupancies (the requirement is higher for Groups M, F-1, S-1 and H). Similarly, IBC 705.8 prohibits openings in either of these walls. Since both structures are considered separate and distinct buildings, structural independence is required. Both exterior walls are prohibited from bearing on each other, and the walls must be supported and braced by their respective buildings. This strategy is common when the two buildings have tenants or occupants who are unrelated to one another. Since openings between the buildings are not permitted, this approach is not practical when doors between buildings are required. Once common exception is a concrete parking structure that is surrounded on all sides by a residential building. This type of project, often referred to as a “wrapper building” or “Texas Donut," utilizes a newly-added provision in the 2015 IBC 705.3, which allows protected openings between the buildings, as long as the opening in the parking garage building has a 1.5-hour fire resistance rating. The opening in the residential building does not require an opening protective in this case.  A "Texas Donut" building under construction with the garage wall directly abutting the residential building wall (image courtesy of Payton Chung). Fire Walls Fire walls are required to be structurally independent from the building and must be continuous to the foundation. For two abutting buildings located on the same lot, this means that neither of the buildings can bear on the fire wall, which often results in the construction of three walls: the fire wall plus a separate exterior wall for each building. Floor assemblies can be connected to the fire wall using breakaway clips for continuity purposes, but they cannot be supported by the fire wall. While the structural requirements for a fire wall are more restrictive than for exterior walls located near a lot line, fire walls are permitted to have protected openings. The openings in fire wall are limited to 156 SF, unless both buildings on either side of the fire wall are fully sprinkler-protected, in which case there is no limit to the area of openings (IBC 706.8). The required fire resistance rating of a fire wall is dependent on the occupancy classifications involved (IBC Table 706.4).  A fire wall creating two separate building (image courtesy of Ben Schumin). Beginning in the 2015 IBC, code language was added to allow for compliance with NFPA 221 to satisfy the requirements of IBC 706. NFPA 221 contains provisions to allow double fire walls (two rated walls built next to each other) in lieu of one structurally independent fire wall. As shown in NFPA 221 Table 4.5 (copied below), two walls, each with a 2-hour fire resistance rating are deemed equivalent to a single 3-hour fire wall.
This is similar to the abutting exterior wall strategy described previously, but the required wall rating is increased in order to meet the NFPA 221 requirements. Though this approach involves two separate walls, the entire assembly is considered a fire wall by the IBC. The fire wall strategy is best suited when there is a single owner or tenant in both buildings and openings between the two buildings are necessary.  Detail of a double fire wall (image courtesy of John Ivison). Party Walls The final option described in the IBC for the wall between two abutting buildings is a party wall. A party wall is described in the IBC as “Any wall located on a lot line between adjacent buildings, which is used or adapted for joint service between the two buildings…” (IBC 706.1.1). This description indicates that party walls are specific to abutting buildings located on separate lots; a wall between buildings on the same lot would not be considered a party wall. Party walls must meet all of the requirements for fire walls, except that no openings are permitted. Generally, party walls require both owners to agree on how the wall will be used and any future design changes will be handled. ConclusionAbutting buildings can be found on the same lot or on two adjacent lots. Architects and engineers can choose between: (1) two abutting exterior walls, (2) a fire wall, or (3) a party wall when designing the wall between the two buildings. How have you approached designs in this situation? Comment and let us know!
When evaluating a building design for egress compliance, knowing the difference between the required and accessible means of egress is important. In this post, we’ll explore the difference between the required means of egress and the accessible means of egress and review the building design requirements for both.  Means of Egress: Required vs. AccessibleAll definitions and requirements are from the 2015 International Building Code (IBC). Note that the IBC does not always include the word “required” when referring to a means of egress. Generally, when the code references either “required means of egress” or “means of egress,” it’s referring to the first definition below. The IBC always uses the full term “accessible means of egress” when referring to the second definition. Definitions (IBC 202): Means of Egress: “A continuous and unobstructed path of vertical and horizontal egress travel from any occupied portion of a building or structure to a public way. A means of egress consists of three separate and distinct parts: the exit access, the exit and the exit discharge.” Accessible Means of Egress: “A continuous and unobstructed way of egress travel from any accessible point in a building or facility to a public way.” From these definitions, notice that both involve “a continuous and unobstructed…” path or way of egress, and that both will lead you “…to a public way.” But the definitions differ in starting point. For a means of egress, the starting point is “from any occupied portion of a building or structure,” whereas for an accessible means of egress, the starting point is “an accessible point in a building or facility.” So simply from these definitions, we can gather an important distinction: an accessible means of egress only originates from an accessible point in a building or facility. If a building or portion of a building is not required to be accessible, it is not required to have an accessible means of egress (IBC 1009.1). Means of Egress Components A required means of egress consists of 3 distinct components: the exit access, the exit and the exit discharge.  Image courtesy of Access-Board.gov Exit access is the first part of a means of egress and is the path between an occupied portion of the building and the exit. Typically, this consists of level walking surfaces such as corridors, and changes in level, such as exit access stairs and ramps. Any doorways along this path would be considered exit access doorways. Next is the exit, which can be any of the following components: an exterior door at the level of exit discharge, interior exit stairways, interior exit ramps, exterior exit stairways, exterior exit ramps, exit passageways, and horizontal exits. Finally, the exit discharge is the portion of a building or site between an exit and the public way. This could be both areas inside the building, such as a stairway that discharges through a lobby or vestibule, or outside the building, such as a courtyard located between an exterior exit door and the public way. Accessible Means of Egress Components An accessible means of egress consists of one or more of the following components (IBC 1009.2):
*serving levels other than the level of exit discharge **serving exits at the level of exit discharge You’ll notice that some of these components can also be part of a required means of egress – interior and exterior exit stairways, exit access stairways and ramps. But an accessible means of egress can include specific components for occupants who need accessible accommodations, such as elevators, lifts and exterior areas for rescue assistance, that are not part of the required means of egress. Another important component is the accessible route, which is always required as part of an accessible means of egress. Since all portions of a building that are accessible require both an accessible route an accessible means of egress, you can’t provide one without the other. An accessible route could also be part of the required means of egress, but it’s not necessarily required. Key Differences Unlike the required means of egress, elevators can be used as a part of an accessible means of egress. And for buildings with an accessible floor located four or more stories above the level of exit discharge, at least one elevator is required as part of the accessible means of egress (IBC 1009.2.1). Another important distinction exists for exit access stairs, which are permitted as part of the required means of egress and in certain cases as part of an accessible means of egress. However, exit access stairs that connect levels in the same story are not permitted as part of an accessible means of egress (IBC 1009.3). This means that stairs connecting levels within a single story, even if a just single step, are not considered part of an accessible means of egress. Accessible Means of Egress: Where Required The criteria for where an accessible means of egress is required are simple. Any accessible space within a building requires at least one accessible means of egress. Where two or more means of egress are required for any space, at least two accessible means of egress are required for that space (IBC 1009.1). There are several exceptions for existing buildings, mezzanines and assembly areas with seating areas served by aisles. There is no code requirement for more than two accessible means of egress to serve any space. Required Means of Egress: Where Required The number of means of egress required from any space is governed by one of the following sections: 1006.2. If the number of occupants in a room or space or the common path of egress travel exceeds the value listed in Table 1006.2.1, two means of egress are required. 1006.3. Generally, all stories require at least two means of egress. If the story contains more than 500 occupants, the requirements increases to 3 means of egress, and increases again to a minimum of 4 when the story has an occupant load of more than 1,000. The above code sections contain numerous exceptions, so be sure to review for your specific situation. ConclusionThe IBC provide distinct criteria that defines both the means of egress and accessible means of egress, and where both are required. An accessible means of egress can include vertical transportation elements such as elevators and lifts that are not typically part of a required means of egress, and it will always include an accessible route from the area served.
All accessible portions in a building require at least one accessible means of egress, and where two or more means of egress are required, two accessible means of egress are required. What questions to you have about accessible means of egress? Let us know in the comment section! |
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