Part 2 – Horizontal Exit Code Requirements
By Steven Dannaway, PE, DBIA – Fire Protection Engineer | Los Angeles Office
Part 1 of this series described the general concept of the Horizontal Exit. In Part 2, several of the key requirements for Horizontal Exits will be discussed. Code references are to the 2021 Edition of the International Building Code (IBC). Horizontal Exits are regulated by IBC Section 1026.
Basic elements of Horizontal Exit construction include:
- A minimum two-hour fire barrier separation or firewall separation that creates two fire compartments on a story. The Horizontal Exit fire separation must extend from exterior wall to exterior wall so as to completely sub-divide the story.
- The Horizontal Exit fire separation must extend vertically through all stories of the building, unless the floor assemblies (above and below) are two-hour fire-resistance rated and have no unprotected vertical openings.
Horizontal Exits may be easier to implement in certain buildings than in others. In Type II – Type V construction, IBC Table 601 permits the floor assemblies to be either non-rated or 1-hour fire-resistance rated. Introduction of a Horizontal Exit into these buildings requires the 2-hour fire barrier to be extended vertically through all stories or requires a 2-hour fire-resistance rating for the floor assemblies that bound the story with the Horizontal Exit.
IBC 707.5.1 and 711.2.3 require the supporting construction of fire barriers and horizontal assemblies to have the same fire-resistance rating as the fire barrier or horizontal assembly supported; therefore, this results in a trickle-down effect to require the columns and all supporting structures of the 2-hour floor assemblies to also have a 2-hour fire-resistance rating. For this reason, in buildings of Type II – Type V construction it is less common to encounter Horizontal Exits that are not firewalls.
Buildings of Type IA or Type IB construction will already have 2-hour fire-resistance rated floor assemblies; therefore, introducing a Horizontal Exit on one story does not trigger additional structural fire-resistance ratings. A firewall can be used as a Horizontal Exit and because a firewall is already required to extend vertically through all levels of the building, the issue of vertical continuity of the Horizontal Exit is inherently addressed.
- The Horizontal Exit is not allowed to serve as the only exit from any portion of a building, except in Group I-3 occupancies. The compartment on each side of the Horizontal Exit must be provided with at least one exit. A “dead-end” compartment condition must be avoided.
- Horizontal Exits may provide up to 50% of the number of exits from a given compartment as well as 50% of the exit capacity from a given compartment. There are exceptions for Group I-2 and Group I-3 occupancies that allow increased percentages.
- Travel Distance: As an “Exit” component, exit access travel distance measurements (IBC 1017) can terminate at a Horizontal Exit This is one common reason a designer may elect to introduce a Horizontal Exit into a project. In lieu of incorporating additional stairways into the building to satisfy an excessive travel distance, a Horizontal Exit can be introduced into the design for this purpose.
It is not a correct assertion that a Horizontal Exit merely restarts the travel distance measurement. The only code requirement to be satisfied is that all portions of a story must be within their maximum exit access travel distance limitation of at least one exit, whether that exit is a Horizontal Exit, exterior exit door, or interior exit stairway. Once the travel distance measurement terminates at a Horizontal Exit, there is no code requirement to achieve a certain travel distance measurement downstream of the Horizontal Exit to another stairway or exit door.
Horizontal Exits contribute to the exit capacity of the floor in the same manner that an interior exit stairway or exterior exit door would. When conducting an occupant load and exit capacity analysis, the compartment on each side of the Horizontal Exit is analyzed separately.
Example: A story served by two, 60-inch stairways would have an exit capacity of 400 occupants (200 occupants per stairway based on a factor of 0.3 inches/occ.). If the building was sub-divided by a Horizontal Exit with a dual-egress door with two door leaves with a clear width of 44-inches each, the exit capacity of the building increases to 800 occupants.
In this example, the compartment on each side of the Horizontal Exit has an exit capacity of 400 occupants, provided by the stairway (exit capacity of 200) and Horizontal Exit door (exit capacity of 220). The exit capacity is 400 and not 440 based on the egress distribution restrictions of IBC 1005.5.
IBC 1026.4.2 states that the exits in the refuge area/fire compartment downstream of a Horizontal Exit are not required to accommodate the occupant load imposed by persons entering the refuge area, across the Horizontal Exit. For the purposes of occupant load and exit capacity calculations, occupants “disappear” from the calculation once they pass through the Horizontal Exit; however, adequate refuge area size must still be demonstrated per IBC 1026.4.
Once occupants egress through the Horizontal Exit, they must be provided with sufficient space, “refuge area”, to gather while they continue their egress to exits downstream of the Horizontal Exit such as an exit stairway or exterior exit door. IBC 1026.4 prescribes requirements for calculating the required size of the refuge area necessary to accommodate the occupants already within the refuge area and those occupants exiting into the refuge area from the Horizontal Exit. These calculations should be depicted on the life safety drawings for the project, along with the other egress calculations.
Standpipe Hose Connections
IBC Section 905.4 – Item 2 requires a standpipe hose connection on each side of the exit opening of a Horizontal Exit. There is an exception to this requirement for situations where the floor area adjacent to the Horizontal Exit is reachable from an interior exit stairway hose connection by a 30 ft hose stream from a nozzle connected to 100 ft of hose. In this situation, the hose connection may be removed from that side of the Horizontal Exit (not both sides).
Note: If a building does not have a standpipe system to begin within and a standpipe system was not otherwise required by the building code or the local jurisdiction, it is not the intent of the code to require a standpipe system to be installed within the building simply because the design has implemented a Horizontal Exit. An example might be a one- or two-story building that has used horizontal exits to increase exit capacity or address exit access travel distance requirements.
▲ In Part 1 of this article, several of the key requirements for horizontal exits are discussed.