OSSC Appendix P604.2.2.3

P602.4.2.2.2 Protected area. All interior faces of all mass timber elements shall be protected in accordance with Section P602.4.2.2.1, including the inside face of exterior mass timber walls and mass timber roofs.

Exceptions: Unprotected portions of mass timber ceilings and walls complying with Section P602.4.2.2.4 and the following:

1. Unprotected portions of mass timber ceilings, including attached beams, shall be permitted and shall be limited to an area equal to 20 percent of the floor area in any dwelling unit or fire area.
2. Unprotected portions of mass timber walls, including attached columns, shall be permitted and shall be limited to an area equal to 40 percent of the floor area in any dwelling unit or fire area.

The proposed 8 story Type IV-B building will include exposed mass timber ceiling including beams based on enhancements to sprinkler density, coverage area, and water supply reliability to satisfy the intent of the code by alternate means.

The proposed 8-story 103-ft mass timber building will meet all of the requirements for Type IV-B construction except that the exposed surface area of the ceilings and beams will exceed the 20 percent allowed by Section P604.2.2.3. The mass timber ceiling, including the beams, is proposed to be fully exposed.

Equivalent protection will be provided by the following:
• Columns and load-bearing walls – reinforced concrete columns and load-bearing walls with 3-hour fire-resistance ratings are provided to support the glulam beams.
• Beams – 12.25 × 25.5-inch for 33-ft span and 12.25 x 33-inch for 41-ft span glulam beams, spaced 10 feet on center, extend east-west from wall-to-wall except for a section identified for mechanical system laterals running north-south from the core.
• Floor – A 4?-inch thick CLT panel with at least 3-inch minimum concrete topping slab is designed to span between the glulam beams using the reduced 1.2DL+0.5LL load combination per ASCE 7-10 Section 2.5 overlay.
• The walls, including columns, will be of non-combustible construction.
• No floor openings such as atria, exit access stairs, communicating stairs, escalators, or other unprotected vertical openings are planned.
• The interior partitions of this building beyond the common core areas will be undefined in the Shell-Core design permit package but the structural design, including beam depth and spacing, and the provision of other draft stops will be known and materially unchanged during tenant improvement.
• Draft stops will be provided about mid-building, which will separate the beam bays into two compartments to restrict lateral spread of the ceiling jet produced by a fire and limit smoke transport.
• Enhanced sprinkler coverage with 120 sf per sprinkler coverage, 0.125 gpm/sf density and 1,500 sf remote demand area.
• Two fire pumps taking suction from the fire water storage tank, following the requirements of Portland Fire and Rescue Design Manual for buildings more than 17 stories.
• Two services from different streets (SE Ash and SE Pine) provided with automatic valves to refill the fire water storage tank.
• A bypass around the fire water tank and directly to the fire pumps that would be normally closed.

Refer to the attached report, stamped by Oregon Registered Fire Protection Engineer, for the justification for the alternate proposal. The additional automatic sprinkler protection and other features proposed in this report are at least equivalent in safety and reliability to the limitations on exposed ceiling area for Type IV-B construction and as such satisfy the intent of the code.

The design team has also engaged a peer reviewer as recommended by the city during the pre-app meeting. WJE, the peer reviewer selected for this project, has provided their evaluation of the design strategy and reviewed the report. Find their letter of support for the proposed elements of the design.

Therefore, based on the non-combustible vertical structural members, non-combustible walls including attached columns, enhancements to sprinkler density, coverage area, and water supply reliability, we seek approval of the proposed conditions consistent with the authority granted to the building official by OSSC Section 104.11.