Appeal 33479

2022 OSSC 602 – Construction Classification; 2022 OSSC 510.2 – Horizontal building separation allowance

602.1 General.
Buildings and structures erected or to be erected, altered or extended in height or area shall be classified in one of the five construction types defined in Sections 602.2 through 602.5. The building elements shall have a fire-resistance rating not less than that specified in Table 601 and
exterior walls shall have a fire-resistance rating not less than that specified in Table 602. Where required to have a fire-resistance rating by Table 601, building elements shall comply with the applicable provisions of Section 703.2. The protection of openings, ducts and air transfer openings in building elements shall not be required unless required by other provisions of this code.

Per Table 601, interior bearing walls of Type IA construction require 3-hr fire resistance rating.
Per Table 601 footnote “a”, Roof supports: Fire-resistance ratings of primary structural frame and bearing walls are permitted to be reduced by 1 hour where supporting a roof only. This reduction is not applicable to occupied roofs or to roofs supporting rooftop structures regulated by Section 1511, vegetative roofs and landscaped roofs.

510.2 Horizontal building separation allowance
A building shall be considered as separate and distinct buildings for the purpose of determining area limitations, continuity of fire walls, limitation of number of stories and type of construction
where all of the following conditions are met:

1. The buildings are separated with a horizontal assembly having a fire-resistance rating of not less than 3 hours. Where vertical offsets are provided as part of a horizontal assembly, the vertical offset and the structure supporting the vertical offset shall have a fire-resistance rating of not less than 3 hours.
2. The building below, including the horizontal assembly, is of Type IA construction.
3. Shaft, stairway, ramp and escalator enclosures through the horizontal assembly shall have not less than a 2-hour fire-resistance rating with opening protectives in accordance with Section 716.

Allow the use of Footnote 'a' in Table 601 to reduce the rating of the bearing walls and related headers from 3 hours to 2 hours in Type IA construction, where those bearing walls are supporting only one floor and no roof.

The Pepsi B apartment building located at the corner of Oregon St and NE 25th Ave in Portland has 5 stories of Type IIIA construction over 3 stories of Type IA construction and will be equipped throughout with an automatic sprinkler system in accordance with Section 903.3.1.1 (NFPA 13).
There are 2-level multi-story units located on the ground floor under the 3-hr horizontal separation, where we are using special provisions of section 510.2. Per Table 601, primary structural frame and interior bearing walls must have a 3-hr fire-resistance rating in Type IA construction. For interior bearing walls, Table 601 footnote “a” allows this fire-resistance rating to be reduced by 1 hour where supporting a roof only. In our case, the bearing walls in question support only the upper floor of the multi-story units and no roof, as the concrete structural columns and shear walls of the building support all levels above. See Exhibit A for more information.

Our proposed design is to use the footnote “a” approach below the podium level to allow the fire-resistance rating reduction of the multi-story units load bearing walls & related headers from 3 hours to 2 hours since only supporting one floor above (a 2-hr fire resistance rated horizontal assembly) within the Type IA construction. To offset the reduction in fire-resistance rating of those bearing walls in these multi-story units, we propose to use commercial sprinkler heads that are 5.6k factor to offer additional protection, in lieu of residential sprinkler heads which are 4.4 to 4.9k factor. Sprinkler system density to be 0.10 light hazard.

Although the bearing walls in question are only supporting the upper floor of the multi-story units and no roof, there is no language which directly permits the use of Footnote 'a' in Table 601 to reduce the rating of the bearing walls from 3 hours to 2 hours for this specific condition. It appears that the code was not anticipating this situation when the rating reduction was developed for Type IA construction. The rationale is that the interior bearing wall and related headers ratings would match the supported floors rating and have no effect on the rating of all supported floors above. Upgrading the sprinkler heads from residential to commercial in those spaces would offer additional protection.