The Wind Investigator: How To Approximate Wind Velocities At Roof Level

May 15, 2005

In the wake of major hurricanes such
as Katrina that recently devastated
much of the Gulf Coast area near
New Orleans, roofing professionals
are often asked to investigate the
cause of roof wind damages. A key to
many damage investigations is approximating
the maximum gust wind velocity experienced
at roof level. Although wind velocity
data are usually available from the National
Weather Service (NWS), the
National Oceanic and Atmospheric
Administration
(NOAA), and other sources,
the data are usually for terrain
exposures, heights
above ground, and averaging
times different than what is
needed. This article discusses
how to adjust wind velocity
data for site-specific terrain
exposures, heights, and
averaging times.
Wind Warranty Claim
To illustrate how these
adjustments work, we will
use the following hypothetical
example scenario:
On August 29, 2005,
a 2-year-old, built-up
roof covering on a 45-
foot tall building in a
mostly suburban and
wooded area southeast
of New Orleans,
Louisiana, lifted near one corner
and peeled back (see Figure 1). The
building owner submitted a warranty
claim, but it was denied because,
according to the manufacturer, wind
gusts in the greater New Orleans
area exceeded those covered by the
warranty. The warranty excludes
coverage for damages experienced
by “wind gusts greater than 100
mph at the roof.” The building owner
hires you to provide an approximation
of the maximum wind gust
velocity experienced at roof level on
his building. No nearby wind velocity
data are available, but the site is
within the geographic region covered
by “wind swath” data published by
NOAA shortly after Hurricane
Katrina made landfall.
Figure 1: A built-up roof covering lifted and peeled back near one corner.
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NOAA H*Wind
Figure 2 shows an excerpt from wind
swath data published by NOAA shortly after
Hurricane Katrina made landfall on August
29, 2005. The graph shows the estimated
maximum sustained winds in miles per
hour along the hurricane path at a height of
33′ (10 meters) in open terrain (exposure C).
Sustained winds are defined as the maximum
velocity averaged over a 1-minute
interval. See the NOAA Hurricane Research
Division, Surface Wind Analysis website
(www.aoml.noaa.gov/hrd/data_sub/wind
.html) for information about how the data
were gathered and reduced to graphical
form.
Because many roofs are not positioned
at a height of 33′ in open terrain, and
because current wind design guides (and
many wind warranties) are based on threesecond
gusts, not one-minute winds, the
NOAA wind swath data often need to be
adjusted for height, exposure, and/or averaging
time before they can be used.
Approximate Sustained Winds
Approximating the maximum sustained
winds experienced at a building site is simply
a matter of interpolation between the
wind isotach lines on the NOAA H*Wind
map. Assuming the example building site is
located at the asterisks on Figure 2 yields a
maximum sustained wind velocity of 85
mph.
Adjust For Averaging Time
The H*Wind data are wind velocities
averaged over one minute. Wind velocities
referred to in most building codes and most
roof manufacturers’ warranties are wind
gusts. Wind gusts are typically wind velocity
data averaged over three seconds. The
“Durst Curve” (Durst 1960 and ASCE 7-02)
can be used to adjust one-minute wind
velocities to equivalent wind gust velocities.
The Durst Curve (Figure 3) shows how
wind velocities averaged over “t” seconds
compare to wind velocities from the same
windstorm averaged over one hour (3600
seconds). The ratio is 1.0 for an averaging
time of one hour and something higher for
shorter averaging times.
For our example, the Durst Curve indicates
a 103 mph 3-second gust wind velocity
is considered equivalent to an 85 mph
1-minute wind velocity. The math for this is
found in Formula 1.
Adjust for Height
Winds in a wind stream are assumed to
be moving at a constant velocity above a
certain height (i.e., the gradient height, Zg).
Winds in this same wind stream but below
the gradient height are moving at a slower
velocity. They are slowed by friction
between the wind stream and the ground.
Figure 4 illustrates how the rate of wind
stream slowing increases as the terrain gets
“rougher” [source: Texas Tech University].
The wind velocities discussed in our
example so far are applicable to a roof
height of 33′ (10 meters). However, while the
wind is blowing at 103 mph at a height of
33′, it is blowing faster at greater heights.
The Power Law can be used to adjust
wind velocity data from one height to another.
The Power Law equation shows the relationship
between the wind velocity at a
given height (Vz) and the gradient velocity
(Vg). (See Formula 2.)
Figure 5 provides numeric values for
gradient heights (Zg) and the alpha exponents
for different terrain exposure categories
[Davenport, 1960].
For our example, the Power Law is used
in a two-step manner to indicate that a 107-
mph gust measured at a height of 45′ is
considered equivalent to a 103-mph gust
measured at a height of 33′. The math for
this is found in Formula 3.
Adjust for Terrain
The wind velocities discussed so far are
applicable to open terrain (Exposure C), a
terrain similar to that surrounding most
airports. Since the rate at which a wind
stream is slowed by “friction” depends on
Figure 3: The Durst Curve [Durst 1960 and ASCE 7-02] shows how wind velocities
averaged over “t” seconds compare to wind velocities from the same wind storm averaged
over 1-hour (3600 seconds).
Formula 2
Figure 2: Excerpt from Hurricane Katrina
wind swath data published by NOAA.
42 • I N T E R FA C E OC T O B E R 2005
Formula 1
ground roughness, wind velocities need to
be adjusted if the building is in a terrain
exposure other than C. See the commentary
of ASCE 7, “Minimum Design Loads for
Buildings and Other Structures” for
descriptions and photo examples of different
terrain exposures.
The gradient velocity of a given wind
stream is the same no matter what the terrain
exposure is. The Power Law can be used
to adjust wind velocities from one exposure
condition to another. Referring again to
Figure 4, one could say terrain-adjusted
wind velocities are approximated by “sliding
up” to the gradient velocity along one terrain
curve and “sliding down” another.
The “suburban and wooded” terrain
surrounding our example building is classified
as terrain Exposure B. Using the previous
approximation that the wind stream at
this site has a gradient wind velocity of 146
mph, the Power Law (with exposure B values),
indicates the wind gust velocity at 45′
in Exposure B would be 92 mph, which is
less than the 107 mph approximated for
Exposure C. The math for this final step is
shown below in Formula 4.
Since the final approximated wind gust
velocity of 92 mph at roof level in our example
is less than 100 mph, the building
owner thanks you and submits your information,
along with his warranty claim for
reconsideration.
Be Careful
Warranties are legal documents and
require legal assistance to interpret. Roofing
professionals are encouraged
to limit the information
they offer as part of warranty
claims to technical considerations.
The procedures discussed
provide approximations
of site-specific wind
velocities based on generalized
wind field type data.
Wind velocities at specific
sites can vary significantly
from these types of generalized
data. Investigators are
encouraged to supplement
and to corroborate NWS and
NOAA data with wind velocity
data obtained from other
sources. More sophisticated
analytical procedures, including
wind tunnel modeling
and/or site instrumentation,
are recommended
when data beyond approximations
are desired.
This article focuses on the velocity of the
wind stream as it approaches a building. It
does not address how wind velocities are
affected as the wind stream is diverted up,
over, and around a building and it does not
address how wind streams are affected by
upstream terrain features such as escarpments
and valleys. Readers are directed to
ASCE 7 for more information.
This article also does not address how
roof wind damages can start and progress,
yet these considerations are important in
terms of what may or may not be covered by
roof warranties.
Summary
A key to many wind damage investigations
is approximating the maximum gust
wind velocity at roof level. Wind velocity
data available to investigators, however, are
often for heights, terrain exposures, or averaging
times different than what is needed.
For example, the NOAA H*Wind data represents
one-minute sustained wind velocities
at a height of 33′ in open terrain (Exposure
C). The Durst Curve and the Power Law can
be used to adjust NOAA data, as well as
data from other sources, for desired averaging
times, roof heights, and terrain exposures.
Acknowledgments
The author wishes to thank Jim
McDonald, PhD, PE, with McDonald, Mehta
& Yin for useful comments and suggestions
offered during preparation of this article.
Figure 4: Illustration of how the rate of wind stream
slowing increases as the terrain gets “rougher” [source:
Texas Tech University].
OC T O B E R 2005 I N T E R FA C E • 4 3
Formula 4
Figure 5: Numeric values for gradient heights (Zg) and the alpha
exponents for different terrain exposure categories [Davenport,
1960].
Formula 3
References
Davenport, A.G., “A Rationale for the
Determination of Design Wind
Velocities,” Proceedings ASCE
Structural Division, 86: 39-66, 1960.
Durst, C. S., “Wind Speed Over Short
Periods of Time,” Meteorological
Magazine, 89, 181-187, 1960.
SEI/ASCE 7-02, “Minimum Design
Loads For Buildings and Other
Structures,” ASCE/SEI, 298, 2003.
Philip Dregger, PE, RRC, FRCI
Philip Dregger, PE, RRC, FRCI, is president of Technical Roof
Services, Inc., Concord, CA. He is a past region director of
RCI, a former faculty member of RIEI, and serves as RCI’s
representative to RICOWI. Mr. Dregger has designed roof and
waterproofing systems to meet some unusual and challenging
requirements of clients, including the University of California,
Pacific Bell (now SBC Communications), Kaiser Hospitals,
Mervyn’s, and Disney. He has investigated numerous roof and
waterproofing problem conditions, including damages sustained
from major hurricanes and earthquakes. Dregger is the author of several articles
on roof technology, including “The Role of Air Retarders Deserves Closer Scrutiny,” referenced
in the National Wind Design Standard ANSI/SPRI/RP-4.
If your home or business was affected by Hurricane Katrina and
you would like to share your experiences with members in RCItems,
please e-mail Kristen Ammerman (kammerman@rci-online.org) or
Catherine Moon (cmoon@rci-online.org), or call 1-800-828-1902
44 • I N T E R FA C E OC T O B E R 2005