Back to Some Basics
Differences in Design Pressure and
by Ken Brenden
Fenestration products are undeniably becoming more complex
as performance expectations diversify and tighten. The same is true of
the standards that guide those who design and specify these products.
The lynchpin of these standards is the AAMA/WDMA/CSA 101/I.S.2/A440, The
North American Fenestration Standard/Specification [NAFS] for Windows,
Doors and Skylights, the latest version of which is 2008 – although the
2011 version is waiting in the wings for adoption into the 2012 I-codes.
There is apparent widespread misuse of certain NAFS performance terms
within the industry, especially design pressure performance class and
performance grade. At times, marketplace confusion has led to specifying
the wrong product for a given building application.
Recall that wind force is the primary source of structural loading on
an installed door or window. Wind exerts pressure directly proportional
to the square of the wind velocity. Design pressure thus serves to “localize”
a design for the wind loading expected at the installation site, based
on maximum expected wind speeds per the wind speed contour map in ASCE-7.
Design pressure is the anchor from which the key attributes of structural
strength and resistance to air and water penetration flow. It is a starting
point for defining the performance of a given fenestration product, yet
it is not a performance rating itself.
Performance class roughly describes the likely target application for
a door or window. Like design pressure, it is not in itself a specific
performance rating. The four classes defined in NAFS are:
R: commonly used in one- and two-family dwellings;
LC: commonly used in low-rise and mid-rise multi-family dwellings
and other buildings where larger sizes and higher loading requirements
CW: commonly used in low-rise and mid-rise buildings where larger
sizes, higher loading requirements, limits on frame member deflection
and heavy use are expected; and
AW: commonly used in high-rise and mid-rise buildings to meet extreme
loading requirements and limits on deflection.
The specific performance level of a fenestration product that falls within
one or more performance classes is given by the performance grade. While
this is related to the design pressure, that is not of itself the performance
A product only achieves a performance grade rating if that product complies
with all requirements of the NAFS standard for a certain design pressure
corresponding to a maximum expected wind velocity regime at the given
geographical location. This is the link between design pressure and performance
grade – but the latter goes further by encompassing:
a) Structural performance: The pressure applied for structural performance
verification is 150 percent of design pressure for doors and windows and
200 percent for unit skylights and tubular daylight devices. This is a
test that assesses any permanent damage associated with the assembly;
b) Water penetration resistance: The water penetration resistance test
pressure, which simulates the force behind wind-driven rain, is generally
is based on 15 percent of the design pressure for R, LC, and CW performance
classes and 20 percent of the design pressure for AW performance classes;
c) Air infiltration resistance;
d)Uniform load-deflection test;
e)Forced-entry resistance (if applicable);
f) Operating force (if applicable); and
g)Product type- or class-specific durability tests.
"There is a difference
between design pressure and performance grade. Some manufacturers have
erroneously marketed products solely based on the design pressure rating."
Each performance class has a least permissible threshold
performance grade based on a minimum design pressure for the structural
capacity, air infiltration resistance or water penetration resistance
that a product must have for it to qualify for that class. This lowest
level is known as the entry level or “gateway” performance requirement.
Gateway performance grades for the four performance classes are:
15 psf (720 Pa) for R class;
25 psf (1,200 Pa) for LC class;
30 psf (1,440 Pa) for CW class;
40 psf (1,920 Pa) for AW class.
Note that a single product may qualify for multiple performance classes.
The essential takeaway from this abbreviated “seminar” is that there is
a difference between design pressure and performance grade. Some manufacturers
have erroneously marketed products solely based on the design pressure
rating. Performance grade means that the product was tested at the applicable
design pressure for all performance requirements identified by NAFS.
Minimum performance grade and design pressure and the corresponding uniform
load structural test pressures and water penetration resistance test pressures
for the four NAFS performance classes are indicated in the table below.
Optional higher performance grade may be specified in each performance
class (except AW) above the minimum gateway requirement in increments
of 5 psf (240 Pa), up to a maximum cap of 100 psf (4,800 Pa). For Classes
R through CW, optional performance classes are capped at 60 psf (2,880
Pa) above the minimum gateway design pressure. There is no maximum performance
grade limit for AW products.
Per NAFS, unit skylights are specified as either R or CW class, with the
minimum performance grades shown. Optional performance grades within these
classes for unit skylights are the same as those for doors and windows.
AW and LC classifications are not available for unit skylights.
Ken Brenden serves as technical services manager for the American
Architectural Manufacturers Association in Schaumburg, Ill. He may be
reached at email@example.com.
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