Structures Congress 2015
2735
Wind Performance and Evaluation Methods of
Multi-Layered Wall Assemblies
Murray J. Morrison1 and Anne D. Cope2
1
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Insurance Institute for Business & Home Safety, 5335 Richburg Rd., Richburg, SC
29729. E-mail: mmorrison@ibhs.org
2
Insurance Institute for Business & Home Safety, 5335 Richburg Rd., Richburg, SC
29729. E-mail: acope@ibhs.org
Abstract
Wind loads on different layers of a multi-layer wall assembly were
determined under realistic loading conditions in the IBHS test chamber. The current
results indicated the PEF of 0.36 currently used in the ASTM D3679-13 for vinyl
siding is likely too low and a more suitable PEF would be in the range of 0.4 to 0.7,
with an optimal value likely between 0.55 and 0.6.. In contrast, the results from the
present study on foam backed vinyl siding indicated that the PEF in ASTM D7445-09
of 0.7 are too high and may be overly conservative. The current results indicate most
PEF values for foam backed vinyl fall between 0.2 and 0.55.
INTRODUCTION
The determination of design wind loads for multilayer walls systems is
challenging since it is difficult to determining how much of the external wind load
acts on each layer of the wall. Relatively large cavities that extend over large
portions of the wall, such as those between typical residential sheathing and siding are
a particular challenge. The pressure and pressure variation along these cavities is
dependent on both the air paths and the temporal and spatial variation of the external
pressures on the wall. It is extremely difficult using traditional air boxes to test siding
under conditions it is likely to experience in the field due to the inherent limitations of
the conventional airbox technique where a static or spatially uniform time varying
pressure is applied over the entire surface of the specimen. For example Kopp and
Gavanski (2011) have shown that the net load on vinyl siding is nearly zero when a
uniform external pressure is applied to a wall using an airbox. That being said several
standards such as ASTM D3679-13 (2013) to test vinyl siding and ASTM D7445-09
(2009) to test foam backed vinyl have been developed. The ASTM D3679-13 (2013)
allows use of a pressure equalization factor (PEF) of 0.36 applied to the external
design pressure on the wall to arrive at the siding design pressure, while ASTM
D7445-09 (2009) allows a PEF of 0.7. In other words vinyl siding needs to only
resist 36% of the external design pressure, due to the equalization of pressures across
the siding. Recent results by Cope et al. (2012) at the Insurance Institute for Business
& Home Safety (IBHS) have shown that the siding can experience up to 75% of the
external wind load or a PEF=0.75, which is significantly larger than the PEF for vinyl
siding provided in ASTM D3679-13 (2013). The current investigation expands on
the previous study of Cope et al. (2012) and aims to quantify the wind loads on
several different siding products.
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2736
EXPERIMENTAL SETUP
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Test Specimen and Instrumentation
In order to test different siding products a single story building was built and
tested in the IBHS test chamber. The building had plan dimensions of 9.1 m (30 ft.)
by 12.2 m (40 ft.), and eaves height of 2.7 m (9 ft.). The roof was a half hip, half
gable roof with a roof slope of 4 on 12 and 0.3 m (1 ft.) overhangs. The four walls of
the test building were divided into 8 different walls sections for siding installation and
testing purposes. Each wall was divided into two sections which were isolated from
each other structurally and aerodynamically by using an air seal at the dividing line.
Significant effort was made during the construction of the walls to match typical wall
construction as closely as possible. The walls of the test building were typical of
residential construction with studs at 0.4 m (16 in.) on center and 11 mm (7/16 in.)
OSB sheathing on the exterior. The OSB was then covered with standard building
wrap. Insulation was installed in the stud bays and sheetrock installed on the interior
of the walls (including taping and mudding) to match typical construction.
A total of four different residential siding products were tested. These include
vinyl, foam backed vinyl, wood and cement siding products. Each product was
installed with strict adherence to the manufactures installation guidelines. Figure 1
presents a plan view of the test building outlining the location of the 8 test wall
sections, details of the product installed on each wall is provided in Table 1.
In order to test all 8 walls shown in Figure 1 the building was tested at 36
wind angles over a full 360° rotation at 10° increments. Pressures on the surface of
the walls and between the different layers of the wall assembly were measured in a
similar manner to that described in Cope et al. (2012). The pressures on each wall
were measured at 12 locations on walls 2, 3, 5 and 7, and at 16 locations for walls 1,
4, 6, 8. Figure 2 shows the pressure tap locations for walls 1, 4, 6, 8. Regardless of
the orientation of the wall on the building the coordinate system for each wall remains
similar to that shown in Figure 2, with the origin at the bottom corner of the wall
closest to the corner of the building. As outlined in Table 1 each wall used either a 2
or 3 pressure tap configuration. A two pressure tap configuration uses two
independent differential pressure transducers at each tap location; the first to measure
the net pressure across the siding (“siding”) and the second to measure the net
pressure across the OSB sheathing (“sheathing”). A 3 pressure tap configuration adds
a third differential pressure transducer to measure the external pressure on the wall
(“external”) relative to the static reference pressure in the IBHS test chamber. The
pressures on all walls are measured simultaneously at a rate of 100 Hz.
Wind Flow details
General details of the IBHS large wind tunnel can be found in Liu et al.
(2011) and Morrison et al. (2012). The flow simulation used for the current
experiments were designed to model an open country terrain (zo=0.01). Specific
details the flow simulation in the IBHS test chamber, including turbulence and
spectral characteristics can be found in Morrison et al., (2012). In order to examine
the effect, if any, of scaling wind speeds, 4 different wind speeds were tested in the
current study. The 3s gust wind speeds (V3s,H) at roof height used in the current study
were 18.8, 22.0, 28.5 and 31.5 m/s.
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Datta Reductio
on
The currrent study presents
p
all pressures
p
ass GCp coeffi
ficients that are directly
com
mparable to ASCE7-10 (2010). Whhen reportingg peak GCp values, as in
i Figure 3,
Figgure 5 and Table 2 the peeak GCp coeefficients prresented hereein are statisstical peaks.
Thee time history is divided into 3 secctions, a gum
mbel distribbution is theen fit to the
peaak value from
m each secttion using a Liebline BL
LUE formullation (Lieblline, 1974).
Thee median vaalue from thhis distributiion is then reported as the peak foor this time
histtory. The Pressure
P
equualization facctor (PEF), which is thee ratio betw
ween the net
loadd across the siding and the
t external pressure
p
is calculated
c
ussing Eq. 1.
(1)
It should
s
be noted
n
that when
w
examinning PEF as
a a functionn of externnal GCp no
stattistical proceedure is appllied to eitherr the externaal or Siding GCp
G values.
Figuree 1 Plan view of the tesst building in
ncluding waall location and wind
direction
n nomenclaature
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Structures Congress 2015
2738
Figure 2 Pressure Tap locations for 6.1 m (20 ft.) walls
Table 1 Product and pressure tap details for walls shown in Figure 1
Wall
Product
1
2
3
4
5
6
7
8
Vinyl
Vinyl (Stiffened)
Vinyl
Cement board
Cement borad
Foam backed vinyl
Foam backed vinyl
Wood
#
Pressure Tap Configuration
(2 or 3)
3
3
2
2
2
3
3
2
RESULTS AND DISCUSSION
Effect of Wind angle on external and siding GCp Coefficients
Figure 3 presents the peak minimum external GCp for each tap locations on
wall 1 at all wind speeds and wind angles tested. For wall1 the highest suctions
occurs at wind angles between 140° and 190°. While there is some variability
between results obtained using different scaling wind speeds, this is likely natural
variability in peak values from test to test. The mean external GCp coefficients
shown in Figure 4 indicate good agreement with between results obtained using
different scaling. Figure 5 and Figure 6 show the peak minimum and mean siding
GCp coefficients respectively for all tap location on wall 1. Similar to the peak
external GCp the highest pressure coefficients on the siding occur at wind angles
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between 140° and 190°. However, the siding coefficients also exhibit high peak
values between wind angles of 40° and 100°. Interestingly, over the wind angles of
40° to 100° the mean siding GCp values shown in Figure 6 seem to separate into two
groups. One group of taps over this wind angle range exhibit relatively low
magnitude pressure coefficients while a second group exhibit the largest negative
mean siding GCp values over all wind angles. Over the wind angles of 40° to 100°
wall 1 experienced largely positive external pressures as shown in Figure 4. The
positive external pressures likely have a high spatial correlation over the wall and
thus likely result in a high positive pressures between the siding and the OSB
sheathing. In areas where the external pressure are substantially lower i.e. near the
corner of the wall, this would result in a relative high outward load on the siding.
Similarly where the external pressure is high the load on the siding is relatively small.
Based on the variation of external and siding GCp values presented in Figure 3
through Figure 6 wind angles ranging from 40° to 190° will be examined in detail in
further analysis. Similarly, analysis on other walls will follow the same relative wind
angle swath (i.e. wind angles for wall 6 would range from 220° to 10°).
Figure 3 Peak external Pressures for all 16 taps on Wall 1. Black Squares
V3s,H=18.8m/s., Red circles V3s,H=22.0 m/s, Green Triangles V3s,H=28.5 m/s, Blue
Asterisk V3s,H =31 m/s
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Siding Pressure Equalization Factors
Figure 7 and Figure 8 presents the PEF versus external GCp for the largest
0.7% of the entire external GCp pressure time histories for each tap over all wind
angles for wall 1 and wall 6. In general, pressure taps closer to the edge of the wall,
close to the corner have both the highest external pressure and PEF values, in some
cases exceeding 1. The external GCp values for walls 1 and 6 are similar, however,
wall 6 exhibits lower PEF values than wall 1. This finding is interesting, since
ASTM D3579-13 (2013) to which vinyl siding is tested allows a PEF of 0.36, while
ASTM D7445-09 (2009) to which foam backed vinyl is tested allows a PEF of only
0.7. The current results suggest that foam backed vinyl should have a lower PEF than
vinyl siding and that possibly that these test standard PEF values should be switched.
Figure 4 Mean external Pressures for all 16 taps on Wall 1. Black
Squares V3s,H=18.8m/s., Red circles V3s,H=22.0 m/s, Green Triangles V3s,H=28.5
m/s, Blue Asterisk V3s,H=31 m/s
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Figure 5 Peak Pressures across vinyl siding for all 16 taps on Wall 1.
Black Squares V3s,H=18.8m/s., Red circles V3s,H=22.0 m/s, Green Triangles
V3s,H=28.5 m/s, Blue Asterisk V3s,H=31 m/s
Figure 6 Mean Pressures across vinyl siding for all 16 taps on Wall 1.
Black Squares V3s,H=18.8m/s., Red circles V3s,H=22.0 m/s, Green Triangles
V3s,H=28.5 m/s, Blue Asterisk V3s,H=31 m/s
The PEFs calculated in Figure 7 and Figure 8 are for single taps and are thus
point measurements. To examine the spatial correlation of PEFs area average
external and siding GCp time histories were computed for 24 area of different sizes
and locations on wall 1 and 6. Resulting PEF time histories were then computed for
each area. Table 2 shows the location and dimensions of the 24 areas following the
coordinate system shown in Figure 2. Figure 9 and Figure 10 show the PEF values
plotted versus corresponding external GCp values for the highest 0.7% of external
GCp time history values for each Area defined in Table 2 over all wind angles for
wall 1 and wall 6 respectively. The trends of the PEF values versus external GCp
values for the different areas is similar to that for the individual taps. In general the
PEFs decrease with increasing peak external GCp. The PEF for foam backed vinyl
(wall 6) continue to be lower than for vinyl (wall 1). For wall 1 areas 1, 7 and 13 all
have PEFs greater than 1. The common link between these three areas is that the
tributary area for Tap 1 (see Figure 2) accounts for over 33% percent of the overall
area. This represents a far greater percentage than for any other area which includes
Tap 1.
Figure 11 shows the PEFs for all areas for wall 1 with the exception of areas
1, 7 and 13. The PEF values correspond to the largest 10 peak external GCp
coefficients for each area are shown as black circles in Figure 11. While there are
some outliers the PEFs for most of the largest external GCp coefficients are between
0.4 and 0.7. However, determining a precise recommendation of an exact PEF for
vinyl siding is challenging, requiring some degree of engineering judgment. Figure
12 uses the largest PEF values corresponding to a certain number of the external GCp
coefficients for each area, shown in black in Figure 11, on wall 1 and ranks them
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from largest to smallest and presents a cumulative average of the PEFs for the sorted
external GCp coefficients. For example for a horizontal axis value (referred to as n in
the text) of 2, the corresponding PEFmean is the average of the PEFs from the largest
two external GCp, while when n=3, the corresponding PEFmean is the average of the
PEFs from the largest three external GCp. For areas with PEFmean at n=1, greater
than 0.7, the PEF values reduce to less than 0.7 once n>6. For areas with PEFmean
less than 0.7, the PEFmean are relatively constant over all n. Figure 13 presents
PEFmean versus n for wall 6, while Figure 14 presents PEFmean versus n for wall 1 for
V3s,H=28.5 m/s. Figure 13 clearly shows consistently lower PEF for foam backed
vinyl than for vinyl siding. Figure 14 shows less scatter than Figure 12. Nevertheless,
the range of PEF values for wall 1 seem largely independent of the wind speeds tested
in the current study.
Figure 7 PEF versus external GCp for all Taps on Wall 1 for V3s,H=18.8
m/s
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Figure 8 PEF versus external GCp for all Taps on Wall 6 for V3s,H=18.8
m/s
Figure 9 PEF versus external GCp for Different Areas on Wall 1 as
Defined in Table 2 for V3s,H=18.8 m/s
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Figure 10PEF versus external GCp for Different Areas on Wall 6 as
Defined in Table 2 for V3s,H=18.8 m/s
Figure 11 PEF versus external GCp for all Areas on Wall 1 as Defined in
Table 2 for V3s,H=18.8 m/s. Highest 10 external GCp coefficients for each area
are shown in black.
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Figure 12 PEFmean calculated using different number of peak external
GCp coefficients for each Area on Wall 1 for V3s,H=18.8 m/s.
Figure 13 PEFmean calculated using different number of peak external
GCp coefficients for each Area on Wall 6 for V3s,H=18.8 m/s.
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Structures Congress 2015
2746
Figure 14 PEFmean calculated using different number of peak external
GCp coefficients for each Area on Wall 1 (for V3s,H=28.5 m/s.
Comparison of loads on siding with ASTM D3679-13
Table 2 presents a design example for the 24 area averages for both Wall 1
and 6. Column 3 presents the ASCE7-10 (2010) GCp coefficients calculated for each
area using Figure 30.4-1 in ASCE7-10 (2010). Column 4 shows the factored GCp
value (GCpfactored) by applying a PEF of 0.36 from ASTM D3679-13 (2013). It
should be noted that ASTM D3679-13 (2013) uses the maximum GCp value for walls
found in Figure 30.4-1 of ASCE7-10 (2010) of -1.4 which leads to a GCPfactored of 0.5
for all areas. Columns 5 and 6 of Table 2 present the peak external GCp and peak
siding GCp for wall 1, while columns 7 and 8 present the peak external GCp and peak
siding GCp for wall 6. All results in columns 5-8 in Table 2 are for V3s,H=18.8 m/s.
The external GCp valuesbetween Wall 1 and Wall 6 match relatively well
with all values from the two walls being within 10% of each other and most values
being within 5%. When compared to the ASCE7-10 (2010) the external pressure
coefficients in the current study are approximately 20% lower, indicating, at least for
the current study, that the ASCE7-10 (2010) peak wall coefficients are conservative.
When comparing the siding GCp between the two walls, wall 1 has larger loads on
the siding than wall 6. This result is consistent with the previous findings in the
current study that the PEFs for foam back vinyl are lower than those for vinyl. siding
GCp values that exceed the GCpfactored used in ASTM D3679-13 (2013) (i.e. -0.5) by
more than 10% are highlighted in yellow in Table 2. For wall 1 siding GCp exceeds
GCpfactored for approximately 40% of the areas, while the load on the siding for wall 6
never exceeds GCpfactored.
Table 2 Area Average Load locations on the walls. ASCE7-10 GCp and
factored GCp coefficients along with measured peak GCp coefficients
(x,y,Δx,Δy,) m
1*
2
(0,0,0.61,2.4)
(0.61,0,0.61,2.4)
ASCE7
GCp
-1.33
-1.2
GCpfactored
-0.48
-0.43
external
GCp
(Wall 1)
-1.05
-0.93
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Structures Congress 2015
siding
GCp
(Wall 1)
-0.85
-0.55
external
GCp
(Wall 6)
-0.94
-0.89
siding
GCp
(Wall 6)
-0.45
-0.42
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3
4
5
6
7*
8
9
10
11
12
13*
14
15
16
17
18
19
20
21
22
23
24
(1.2,0,0.61,2.4)
(1.8,0,0.61,2.4)
(2.4,0,0.61,2.4)
(3.0,0,0.61,2.4)
(0,0,1.2,1.2)
(0,0,1.8,1.8)
(0,0,2.4,2.4)
(0.61,0,1.2,1.2)
(0.61,0,1.8,1.8)
(0.61,0,2.4,2.4)
(0,0,1.2,0.61)
(0,0,1.8,0.61)
(0,0,2.4,0.61)
(0.61,0,1.2,0.61)
(0.61,0,1.8,0.61)
(0.61,0,2.4,0.61)
(0.61,0,1.2,1.2)
(0.61,0,1.8,1.2)
(0.61,0,2.4,1.2)
(0.61,0,1.2,1.8)
(0.61,0,1.8,1.8)
(0.61,0,2.4,1.8)
2747
-1.06
-1.06
-1.06
-1.06
-1.26
-1.1
-1.02
-1.13
-1.04
-0.98
-1.35
-1.23
-1.16
-1.2
-1.13
-1.1
-1.13
-1.07
-1.04
-1.09
-1.04
-1
-0.38
-0.38
-0.38
-0.38
-0.45
-0.4
-0.37
-0.41
-0.37
-0.35
-0.49
-0.44
-0.42
-0.43
-0.41
-0.39
-0.41
-0.39
-0.37
-0.39
-0.37
-0.36
-0.82
-0.9
-0.89
-0.68
-0.93
-0.9
-0.87
-0.88
-0.83
-0.77
-1
-0.89
-0.84
-0.94
-0.86
-0.77
-0.88
-0.82
-0.76
-0.88
-0.83
-0.76
-0.51
-0.58
-0.64
-0.42
-0.82
-0.5
-0.42
-0.61
-0.44
-0.45
-0.88
-0.65
-0.49
-0.71
-0.54
-0.44
-0.61
-0.48
-0.42
-0.54
-0.44
-0.44
-0.9
-0.89
-0.81
-0.75
-0.84
-0.85
-0.82
-0.89
-0.84
-0.76
-0.84
-0.86
-0.84
-0.91
-0.87
-0.79
-0.89
-0.85
-0.76
-0.9
-0.84
-0.76
-0.4
-0.47
-0.46
-0.33
-0.33
-0.29
-0.28
-0.45
-0.31
-0.26
-0.37
-0.37
-0.34
-0.48
-0.42
-0.32
-0.45
-0.37
-0.28
-0.38
-0.31
-0.25
*Tap 1 accounts for over 33% of the total area for these areas
CONCLUSIONS
The current study examined the wind loads on several common residential
siding products. Loads on both vinyl and foam back vinyl are examined in the
current analysis. Load reduction factors in the form of pressure equalization factors
(PEF) from the external pressure are warranted for both products. However, the
current results indicated the PEF of 0.36 currently used in the ASTM D3679-13 for
vinyl siding is likely too low and a more suitable PEF would be in the range of 0.4 to
0.7, with an optimal value likely between 0.55 to 0.6. In contrast, the results from the
present study on foam backed vinyl siding indicated that the PEF in ASTM D7445-09
(2009) of 0.7 is too high and may be overly conservative. The current results indicate
most PEF values for foam backed vinyl fall between 0.2 and 0.55. The current results
indicate no significant wind speeds dependence on PEF values.
REFERENCES
American Society of Civil Engineers (ASCE 7-10) (2010). Minimum Design
Loads for Buildings and Other Structures, American Society of Civil Engineers,
Reston, Virginia.
ASTM D3679-13 (2013), Standard Specification for Rigid Poly (Vinyl
Chloride) (PVC) Siding, ASTM International, West Conshohocken, PA.
ASTM D7445-09 (2009), Standard Specification for Rigid Poly(Vinyl
Chloride) (PVC) Siding with Foam Plastic Backing (Backed Vinyl Siding), ASTM
International, West Conshohocken, PA.
Cope, A.D., Crandell, J. H., Johnston, D., Kochkin, V, Liu, Z., Stevig, L.,
Reinhold, T.A. (2012). “Wind Loads On Components Of Multi-Layer Wall Systems
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With Air-Permeable Exterior Cladding”, Applied Technology Council, Miami, FL,
USA.
Kopp, G.A., Gavanski E. (2011). “Effects Of Pressure Equalization On The
Performance Of Residential Wall Systems Under Extreme Wind Loads”, Journal of
Structural Engineering, 138 (4), 526-538.
Lieblein J. (1974). Efficient Methods of Extreme-Value Methodology.
National Bureau of Standards, NBSIR 74-602.
Liu, Z., Brown, T.M., Cope, A.D., Reinhold, T.A. (2011). “Simulation Wind
Conditions/Events in the IBHS Research Center Full-Scale Test Facility”, 13th
International Conference on Wind Engineering, Amsterdam, Netherlands.
Morrison, M.J., Brown, T.M. Liu, Z., (2012). “Comparison of Field and FullScale Laboratory Peak Pressures at the IBHS Research Center”, Applied Technology
Council, Miami, FL, USA, 2012.
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