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|Trends in Exterior Curtain Walls|
Issue 24: Trends in Exterior Curtain Walls
By William E. Fitch, P.E.
A "curtain wall" is a wall that encloses the space within a building
but does not support the roof or other floors, typically on a modern
high-rise. One of the most significant developments in modern high-rise
architectural design was the curtain wall. A curtain wall cladding
system is a continuous vertical "curtain" of glass, stone or other
material that does not provide structural support, and is supported by
the floor slab. A curtain wall is, however, required to keep out the
wind, rain, heat and cold.
By the very nature of materials used, curtain wall systems typically
do not have any level of fire resistance rating and are generally
utilized on buildings where there is no requirement for such. The major
fire safety issue that must be addressed with curtain wall systems is
fire spread up the building from floor to floor. This can take two
paths: spreading up the inside of the wall due to inadequate fire
stopping at the floor-wall interface; and spreading up the exterior
surface on combustible facing materials.
Interior Fire Spread
The primary evaluation method is an ASTM E23071 test. This test utilizes NFPA 285's2
two-story apparatus with a wall representative of that used in common
construction practice. In this test, the front is closed and the
perimeter joint system to be evaluated is placed between the wall and
the second story floor. Burners expose both the exterior face of the
wall and the underside of the joint to the fire.
Buildings expand, contract and move in response to live loads, wind, earthquakes, temperature, freeze-thaw cycles and other forces. Accordingly, this test standard also contains a provision for cycling the joint system prior to fire testing.
Exterior Fire Spread
To control fire spread up the exterior face of a curtain wall, the code
addresses two factors: fire separation distance as a function of ease
of ignition of the wall covering; and flame spread up the face from a
fire breaking out a window.
Combustible exterior wall coverings are required to be tested for ignition resistance per NFPA 268.3 This test standard exposes a 4 ft. x 8 ft. (1.2 m x 2.4 m) test specimen to a 12.5 kW/m2
heat flux from a large (3 ft. x 3 ft. – 0.9 m x 0.9 m) radiant panel
for 20 minutes to see if the material ignites. If the material passes -
i.e., does not ignite - it can be used on the face of a building with a
fire separation distance of 5 feet (1.5 m) or less.
If it does ignite, the material can be tested at a reduced heat flux
until a passing level is determined. That heat flux level then
determines how far the fire separation distance must be.
Metal Composite Materials
If the MCM has a flame spread index of 25 or less, a smoke developed
index of 450 or less, and a thermal barrier is placed between the MCM
and the interior of the building, it can be used without limits. Flame
spread and smoke developed indices are determined per ASTM E84,4 NFPA 2555 or UL 723.6 If the MCM passes large scale tests, such as UL 10407 or UL 1715,8 and it passes NFPA 2852 for fire propagation, the thermal barrier can be eliminated.
If the MCM has a flame spread index of 75 or less and a smoke
developed index of 450 or less, then further restrictions apply. On a
building up to 40 ft. (12 m) high and with more than 5 feet (1.5 m) fire
separation distance, there is no limit. If the fire separation distance
is less than five feet (1.5 m) the material is limited to 10 percent of
the wall area. On a building up to 50 feet (15 m) in height, the MCM
must have an ignition temperature of 650°F (340°C) or greater and is
limited to sections not more than 300 square feet (28 m2) with 4 foot (1.2 m) separation between sections.
Foam Plastic Insulation
Finally, unless the system incorporates a thermal barrier on the exterior face, it must also pass the NFPA 268 radiant panel ignition test.
The combination of testing requirements detailed above provides a
reasonable means of evaluating existing and new materials and systems.
The key is that they have to be evaluated.
The question is what constitutes a new material that requires
testing? Numerous EIFS systems have been tested, some including large
decorative sections such as cornices. Yet an apparently untested
polyurethane resin coated expanded polystyrene (EPS) foam was used
around the top of the Monte Carlo hotel and casino in Las Vegas leading
to $100 million in damages when the material was ignited by workmen.9
A number of manufacturers are working on EIFS-type systems that use a
noncombustible insulation instead of EPS. Others are developing thermal
barrier coating systems.
A unique material, Greenpix, has been developed which incorporates
photovoltaic cells into the glass curtain wall. Greenpix behaves like an
organic system, absorbing solar energy during the day and then
generating light from the same power that evening. See Figure 1.
The polycrystalline photovoltaic cells are laminated within the glass of the curtain wall and placed with changing density on the entire building's skin. The density pattern increases the building's performance, allowing natural light when required by interior program, while reducing heat gain and transforming excessive solar radiation into energy for the media wall. But, the question to ask is how, if at all, does this affect the fire performance?
As with any new development in building materials or systems, it is
critical to consider not only the specific code requirements but the
as-built, in-use performance. This is the advantage of the two-story
firestopping and flame spread tests. They evaluate the materials and
systems in a realistic arrangement that, while more costly, provide much
greater assurance of safer performance in the field.
William Fitch is with Phyrefish Enterprises.
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