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 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.
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).
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.
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.
Abutting 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!