IRC: Floor assemblies can achieve improved fire resistance
“Floor assemblies in multi-family dwellings can be constructed to achieve both improved acoustic isolation and fire resistance,” according to the Institute for Research in Construction (IRC), Ottawa, Canada.
The IRC is investigating the acoustic and fire resistance performance of hundreds of floor assemblies to reaffirm the existing fire resistance and acoustic ratings in the current National Building Code of Canada (NBC) and to develop ratings for cost-effective and innovative floor assemblies that would achieve these improvements.
Under study are lightweight floor assemblies protected with gypsum board ceilings. A large variety of construction parameters, such as type of joist, joist depth and spacing, type of subfloor, type and thickness of sound-absorbing material, methods of attaching the ceiling to the joists, and structural load, are being evaluated.
Interim results indicate that increasing the number of gypsum board layers “significantly” increases the floor`s fire resistance as well as sound isolation.
The researchers also found that adding sound-absorbing material to the cavity of a floor with resilient metal channels increases the sound isolation considerably but that the effect on fire resistance depends on the protective value of the insulation. Insulating a floor cavity reduces the heat transmission from the gypsum board to the cavity; the retained heat causes the board to calcinate and crack faster than in an empty cavity. Once the board cracks and fails, the insulation and joists are exposed to heat.
In assemblies with solid wood joists and a single layer of gypsum board, glass fiber insulation reduced the fire resistance because it melted quickly when exposed directly to heat and could not compensate for the board`s early failure. In similar assemblies with “rock- and cellulose-fibre insulations,” fire resistance increased because the insulations stayed in place longer.
Moving the gypsum board screws from 10 mm to 38 mm from board edges and using additional pieces of resilient channel to better support the butt ends of the gypsum board significantly improved fire resistance, but this reduced sound isolation slightly when only a single layer of gypsum board was used.
The results of this research are being incorporated into the appendix of the NBC in the form of an expanded table of fire resistance and sound transmission loss ratings that include more than 150 floor assemblies.
A second phase of the study is about to start. For additional information, contact Dr. Mohamed Sultan, Fire Risk Management Program, at (613) 993-9771 or by e-mail: mohamed.sultan@nrc.ca.
(Source: Construction Innovation, Institute for Research in Construction, 4:1, Fall 1998, 6,7.) n
