Appeal 8906

2010 Oregon Mechanical Specialty Code (OMSC) Section 602.2

OMSC 602.2 requires that plenums be constructed of materials permitted for the type of construction classification of the building, and limits exposed materials within plenums to noncombustible materials or materials with a flame spread index of not more than 25 and a smoke developed index of not more than 50.

The project involves the demolition of a 2-story-above-plaza retail portion of the existing Pioneer Place Tower and its replacement with a 1-story retail store. The public portion of the store will be located in this new construction. The new building areas will be of Type II-13 construction per the current OSSC. The new and existing portions of the building are sprinkler protected throughout.

The public portion of the store consists of a large open retail area, with no enclosed areas. The design proposes to have a stretch ceiling system constructed with Newmat fabric materials within this public retail area. The area above the fabric ceiling will serve as a return air plenum for the retail space. This return air plenum only serves the open retail area and does not serve other building areas. The stretch fabric ceiling system materials have a flame spread rating of 5 and a smoke developed index of 55 when tested in accordance with ASTM E84, and a melting temperature in excess of 330 degrees Fahrenheit.

The team is proposing a stretch ceiling system within the public retail areas as part of the interior design and unique architectural design of the building. The stretch fabric ceiling will function similarly to a t-bar type dropped ceiling element, The building will include sprinklers below the fabric ceiling, and the stretch ceiling system has a melting temperature far in excess of the sprinkler activation temperatures, such that this will not adversely impact the performance of the fire sprinkler system within the building. The stretch fabric material meets the NFPA 13 definition of a limited combustible material as it relates to combustible concealed spaces, such that NFPA 13 does not require sprinkler protection above the stretch fabric ceiling.

The retail space is permitted to have Class C interior finish materials. This would permit materials with a flame spread Index up to 200 and a smoke developed index up to 450. The stretch fabric ceiling material has flame spread and smoke developed index values far below these limits.
The space above the stretch fabric ceiling system will serve as a return air plenum for the open retail areas. OMSC typically limits exposed materials within plenums to non-combustible materials or materials with a flame spread index of not more than 25 and a smoke developed Index of not more than 50 when tested in accordance with ASTM E 84. The proposed stretch fabric ceiling material has a flame spread rating significantly lower than that required by the code (rating of 5) but a smoke developed index of 55. However, the area above the stretch fabric ceiling only serves as a return air plenum for the open retail space. The space above the stretch fabric ceiling does not serve as an air movement path for other building areas, and the fan system for this space does not serve any other building areas. Therefore, even if the fabric material were to ignite, the higher smoke developed index would not adversely impact building areas remote from the fire location and its impact would be no greater than if it were applied as an interior finish anywhere else in the space. Therefore, the slightly increased smoke developed index of the stretch fabric material relative to the typical plenum requirements will not adversely impact the performance of the mechanical system or the protection within the building.

Based on the fact that the space above the stretched fabric ceiling only serves the open retail areas directly below, the design team requests approval of this alternate materials request in accordance with OSSC Section 104.11.