Codes: Fire Protected Openings
Section 716.6 of the 2013 California Building Code (CBC) addresses the requirements for fire protected-rated glazed opening. The desire for to accommodate glazed opening within fire rated partitions has always been in the code, ever since the original edition of the Uniform Building Code (UBC) in 1927. The technology for fire resistive glazing has come a long way. With the lack of true fire resistive glazing materials, the earlier code editions permitted the use of ordinary glass in a somewhat rated assembly by limiting the size of glazed openings and requiring the frames to be metal/steel. Glass has a reasonable resistant to heat, about 600-700 degrees F, depending on the type of glass, before it begins to deform and lose its structural integrity. The biggest major failure of glazed openings is the dissimilar expansion between the glazing materials and frames. This difference in expansion, glass expanding faster than steel, causes failure of the glass (breakage).
The basic intent of any fire resistive assembly is to provide protection. In most cases we are talking about protecting a means of egress (MOE) from a potential hazardous adjacent use. This would include corridors and exit enclosures. The protection needs to maintain a tenable environment for the occupants. This would include not only a physical separation from a fire, by limiting products of combustion (smoke and heat, including radiant heat) from rendering the MOE unusable.
Wired Glass – The use of wired glass has been historically allowed in the code until recent times. It was the only material that met the requirements as a fire resistive element that was transparent. The wires held the glass in place even when broken. This was also thought to be a form of safety glass. The downfall of wired glass has been based on one incident. A university student put his fist through a pane of wired glass and became trapped. The wired held his limb in place and could not be withdrawn, causing serious injury. This has been presented as a major safety concern (there may have been other similar incidents) and from this wired glass has all but vanished from the code. Note: Section 715.5.4 of the previous code (2010 edition) on wired glass has been deleted.
So why does the code have different required ratings for openings in Table 716.6? When one looks at the table and sees fire window rating ranging from 1/3-hour to ¾-hour for a 1-hour rated wall, why is that? Why not just 1-hour within a 1-hour rated wall? Historically, the code has limited the size of glazed openings within a rated wall assembly. As a controlled opening, it was considered reasonable by the authors of the code (with some lobbying by the glazing industry) to have a lower/different standard.
Recent changes in the code reflect the higher level of standard required for all rated glazing materials used within rated assemblies. With the advent of newer and better glazing materials, the design approaches for glazed opening have become almost limitless. We have laminated glass and ceramic glass that can create unlimited glazed openings. Add to this the continued use of sprinkler protected glass, which is still acceptable by most jurisdictions under the Alternative Methods and Materials approach. Technology has served the design industry well in this area.
The major change in the new code is the inclusion of new marking requirements and criteria for fire-rated glazing. See Table 716.3. Note that this is a new table, with a change bar in the margin in the 2012 IBC, but with the bar omitted in the 2013 CBC. There are now broad categories of fire-rated glazing and the windows with those characteristics must have indelible marks on the glazing so that the design n team and the inspection team can identify that the proper glazing is in place. The term “glazing” is more appropriate than “glass”, since there are not transparent ceramic materials that can obtain a “W” or “wall” fire resistance rating and should be thought of as transparent walls rather than as windows. The materials are costly, but can allow designers much greater flexibility in providing vision lites in rated wall assemblies. The categories in the table are as follows, with the rating in minutes appended to the descriptors. Thus a transparent wall assembly rated for 60 minutes would bear a W-60 descriptor:
W-xxx Fire rated glazing assembly meeting wall assembly criteria for xxx minutes
OH-xxx Fire rated fire window meeting fire resistance criteria for xxx minutes, including a hose stream test
D-xxx Fire rated door assembly meeting fire resistance criteria for xxx minutes
H-xxx Fire rated door assembly meeting fire resistance and hose stream criteria for xxx minutes
T-xxx Assembly meets 450 degree temperature rise maximum for 30 minutes