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j NBS TECHNICAL NOTE 852

"ft-M Of

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

A Study of Wind Pressures on

a Single-Family Dwelling

in Model and Full Scale
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards ' was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs. THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

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THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

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1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

a Part of the Center for Radiation Research.

a Located at Boulder, Colorado 80302.

4 Part of the Center for Building Technology.
Single-Family Dwelling in Model and Full Scale

2 « 1974 A Study of Wind Pressures on a

a_ R. D. Marshall

Institute for Applied Technology

C/ 1 Si National Bureau of Standards

•

'

' Washington, D.C. 20234

Sponsored by:

Tri Service Building Materials

Investigational Program Committee

Department of Defense

Washington, D. C. 20301

Vau <* *

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued October 1974
Library of Congress Catalog Card Number: 74-600152

National Bureau of Standards Technical Note 852

Nat. Bur. Stand. (U.S.), Tech. Note 852, 40 pages (Oct. 1974) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 1974 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:852). Price 85 cents.
TABLE OP CONTENTS

Page

ABSTRACT !

1.0 INTRODUCTION. 1

2.0 THE TEST SITE 2

3.0 FULL-SCALE TEST PROCEDURE 2

4.0 WIND TUNNEL TEST PROCEDURE 3

5.0 DATA REDUCTION AND ANALYSIS *»

6.0 MEASUREMENT RESULTS AND DISCUSSION *

6.1 Simulation of the Atmospheric Boundary Layer 5 6.2 Pressure Measurements 7

7.0 COMPARISON WITH RECOMMENDED DESIGN LOADS 10

8.0 A PROCEDURE FOR THE CALCULATION OF DESIGN PRESSURES 13

9.0 CONCLUSIONS I 1

*

10.0 ACKNOWLEDGEMENTS 15

11.0 BIBLIOGRAPHY '6

ili

A STUDY OF WIND PRESSURES ON A S I NGLE - FAM I LY DWELLING IN MODEL AND FULL SCALE

by

Richard D . Marshal 1

Wind pressures measured on a s ingle -f compared with resul t s ob ta i ned from a 1 : 50

in a turbulent boun da ry layer. It is s h ow

ing components of surface pressures far ex steady pressures an d are well co r re lated areas. The consistently low fluctuating P obtained from the wind tunnel model a re at

s i mu 1 a t i on of the 1 ower port i on of the atm

layer. Comparisons be tween actual loads a loads suggest that ce rta i n current p rov i s i tributary areas and exce s s i ve for 1 oca 1 i z e eaves and corners. A p roced u re for exp res localized and extended roof areas i n te rms coef f icients and a peak factor is d escr i be

am i

1 y dwe 1 1 i ng a re sea 1 e mode 1 p 1 aced

n that the fluctuatceed the mean or

ve r sizeable roof ressure coefficients

tributed to improper

ospheric boundary

nd specified design

ons are marginal for

d areas such as ridges

sing 1 oad s on both

of mean pressure

d.

Key words: Aerodynamics; boundary layers; buildings; codes and

standards; wind loads; wind tunnels.

1.0 INTRODUCTION

Research over the past ten years into the effects of wind on buildings

and other structures is significantly influencing design philosophy and

practice. This is evidenced by recent major revisions of building codes

and standards, both here in the United States and abroad. Perhaps the most significant improvement has been the recognition of wind loading

as a stochastic process and the formulation of design criteria based upon

acceptable levels of risk. Other improvements include provisions for

various classes or categories of terrain roughness and the wind-tunnel

simulation of the atmospheric boundary layer when measuring pressure

coefficients and dynamic response factors.

In spite of these important advances, considerable work remains to

be done. This is particularly true of existing criteria covering the

design of low-rise buildings which have not benefited from this research

In the same proportion as tall structures. It Is Interesting to note

that pressure coefficients in current use are based In large part on wind

tunnel studies carried out in uniform flows of low turbulence, using Instrumentation capable of measuring only mean pressures. It is obvious, therefore,

that a major effort must be made to improve wind-tunnel modeling of low-rise

structures. Once this is accomplished, pressure coefficients and response

factors can be determined in an efficient and systematic manner.

1
Prior to this, however, the validity of modeling techniques must

be established by comparing test results with representative measurements

obtained from full-scale buildings. The investigation described in the

following sections is an attempt to provide a preliminary comparison between

model and full-scale test results for a s i ng 1 e -f am i 1 y dwelling.

2.0 THE TEST SITE

The site selected for this study is located at Malmstrom Air Force

Base, Montana, directly to the east of the city of Great Falls. The region

is noted for its "chinook" winds which regularly blow out of the southwest

during the winter months. The terrain surrounding the test site is markedly

flat and free of significant obstructions. Although the mean hourly speeds

for the area are quite high (6-8 m/s), extreme winds seldom exceed 30 m/s.

The building investigated is a single-family dwelling, one of four

quite similar units located in an area having a clear wind exposure extending

from the west clockwise around to the south. A cluster of two-story housing

units is located approximately 100 m southwest of the test site and extends

in that direction for approximately 550 m (see Fig 1). The test building

and adjacent structures are shown in Fig. 2. The test building has basic

plan dimensions of 6.8 x 23 m with a 5 x 5.8 m wing (Fig. 3). The roof

pitch is 11.5 degrees and the eaves overhang is 0.8 m.

3.0 FULL-SCALE TEST PROCEDURE

Surface pressures were measured at 9 points on the roof (PI-P9) and at one point under the eaves CP 10) as indicated In Fig. 3. The internal pressure (PI2) was measured In the garage which Is located directly under the

instrumented portion of the roof. An additional pressure tap (Pll) was in- stalled under the eaves in the wind-tunnel model. To avoid penetrating

the roof membrane with conventional pressure taps and at the same time

accurately measure pressures oyer the roof surface, the pressure transducers were mounted under low-profile housings having a height of 3.6

cm and a diameter of 61 cm. All transducers were referenced to a vane- mounted pitot static tube located 2.9 m above the ridge line, I.e., 6.1

m aboye ground leyel.

Wind speed and direction were obtained with a propeller-vane anemometer

located 6.6 m aboye ground leyel. A standard National Weather Service

three-cup anemometer was used to trigger the data acquisition system when

wind speeds exceeded a preset level. Positions of the pltot static probe

and anemometers are shown In Fig. 3.

2
A 14-channel analog tape recorder was used to acquire data. In addition

to the eleven pressure signals, and wind speed and direction, a time code

was recorded to identify the data runs or records. Normal operating procedure

was to set the system threshold at 18 m/s and record for 15 minutes. The

system would then enter a 1-hr. hold period before checking the wind speed

against the preset value. While this procedure resulted in a number of

redundant records, it did provide data corresponding to peak winds in winter storms passing through the region. These storms generally had

a duration of from one to three days. Barometric pressure, temperature

and other weather data were obtained from hourly observations made by

the 3rd Weather Wing, USAF4.0 WIND TUNNEL TEST PROCEDURE

In order to increase the value of the full-scale test results, aid

in their interpretation and explore the feasibility of modeling the natural

wind at an unconventional scale, a series of wind tunnel tests were conducted

during the course of the study using a model scale of 1:50.

The tunnel used for these tests is one of several operated by Colorado

State University and has a 1.83-m square by 12-m long working section.

To simulate the natural wind, a thick shear layer was established by use

of a row of spires and a sawtooth fence installed at the working section

entrance, (n addition, all adjacent and upwind structures were modeled

in the tunnel. This approach has been described by Melbourne (1) and

Standen (2) and when compared with results obtained using surface roughness

elements alone, it substantially increased the growth rate of the boundary

layer and increased the scale of turbulence at the position of the model,

1 Q . 3 m downstream from the spires. Spire details and position of the

model are shown in Fig- *» •

Pressures were measured with the same transducers and recording system

used for the fullrscale study. The orifice-tube combinations exhibited

a satisfactory frequency response out to 1 Q Hz. All pressure measurements

were referenced to a pitot static tube located 1.22 m above the tunnel

floor, directly over the building model. Wind speed records and static

and dynamic pressures were obtained by means of a hot<-wire anemometer

and pitot tube located In the same relative positions as the instruments

In the full-scale study. In addition, mean velocity profiles were obtained

at the position of the model for each of the four wind directions studied.
5.0 DATA REDUCTION AND ANALYSIS

Analog tapes containing full-scale and wind tunnel data were processed

using the data system described in Ref. 3. The usual procedure was to plot the full-scale wind speed and direction records on a stripchart recorder

and to then select those records exhibiting a satisfactory degree of stationarity for detailed analysis. Analog to digital conversion was accomplished

at a rate of sixteen samples per second with a total of 12,000 samples

per record or a digital record length of 750 seconds. By using a reduced

playback speed, it was possible to digitize the wind tunnel data at the same effective rate and account for the 50:1 change in time scale. Thus

the time and frequency scales were assumed to be identical in the subsequent

data analysis. With the exception of the hot-wire records, all records were obtained from transducers exhibiting linear characteristics.

A series of computer programs have -been developed at the NBS for

the analysis of random data. These include PROGRAM 2 which formats sequential

channel samples into sequential samples for a given channel : C0RREL which

performs low-pass filtering and contains options for computing the mean,

rms , auto- and cross rcor re 1 a t i on , spectral density and coherence functions;

PDF which computes probability densities and tabulates peak values and

associated zero-crossing rates; and SUMP which generates a new data series

based on the area integration of surface pressures. In addition, subroutines

exist for linearizing hot-wire records and correcting f i xed -d i rect i on propeller anemometer records for departures from the cosine law.

In the study reported herein, no attempt was made to remove trends

from the data series prior to analysis, the records being visually screened

before conversion. However, even with this screening, there were certain

records processed which indicated significant trends as reflected by their

segmental means and auto-correlation functions. Most steps in the analysis

were preceded by lowrpass filtering, each four successive samples being

ayeraged and regulttng in a record size of 3000 samples. Auto-correlations

were calculated for 20Q lags, followed by Q.Q2 Hz fixed-bandwidth spectral

analysis,

6.Q MEASUREMENT RESULTS AND DISCUSSION

Field measurements at the Montana test site were obtained over a 5rroonth period during 1971-72, yielding approximately 15 hours of recordings

(60 records) under strong wtnd conditions. Four different wind directions

4
were selected for subsequent wind tunnel simulation on the basis of differing

obstructions over the wind fetch. Due to demands placed on test equipment,

only a preliminary analysis of the field data was available at the time

the wind tunnel studies were conducted. This proved to be unfortunate

in that a better simulation of the natural wind could have been achieved

with slight additional effort.

In the following sections, run numbers with three digits designate

full-scale data. Wind azimuth angles ((3) are measured clockwise from

north. Five typical full-scale records were selected for detailed analysis

and comparison with the wind tunnel test results.

6.1 Simulation of the Atmospheric Boundary Layer

Because of the model scale used in this study and the physical size

of the wind tunnel facility, a simulation of only the lower 30 m of the

atmospheric boundary layer was attempted. Restrictions placed on the use of the field test site did not allow the installation of a meteorological

tower of sufficient height to establish characteristics of the atmospheric

boundary layer over this range. Field measurements of wind speed were

thus limited to one point, 6.6 m above ground level. As will be discussed

later, this necessitated corrections to the mean reference speeds and pressures during the data analysis.

Typical mean velocity profiles measured at the position occupied

fay the model ClO.3 m downstream) are shown in Fig. 5 as a power law representation. In plotting these profiles, it was assumed that the actual

scale of the shear flow was 1:50 and that the thickness of the atmospheric

boundary layer was. 275 m. The first profile corresponds to the case of

only the spires and sawtooth fence installed and exhibits a satisfactory

velocity distribution only up to a full-scale height of approximately

10 m. The second profile corresponds to an azimuth angle of & = 3**1 degrees,

the direction having the least number of structures upwind. Although

the profile agrees well with the power law up to approximately 27 m , there

is a substantial departure above this height. Only for azimuth angles

of 186, 211 and 256 degrees did the mean velocity profiles correspond

to the power law above 30 m. The departure of the profile for = 211 degrees from the power law for Z/Z < 0.025 is due to the neighboring

house directly upwind.
While the exponents obtained from the plots in Fig. 5 are in good

agreement with recommended values for terrain typical of the test site,

there is nothing to suggest that the scale ratio is in fact 1:50. The

profiles for 3 = 211, 256 and 3^1 degrees are plotted as a log law in Fig. 6. The corresponding roughness heights (Z ), were found to be 0.30,

0.58 and 0.02 cm. He 1 1 i we 1 1 (k) determined a value of 8 cm for open

country at Heathrow and Cardington. This would suggest a scale ratio of

from 1;15 to 1 : A for the atmospheric boundary layer simulation. Only

the very lowest portion of the profile for 3 = 3**1 degrees corresponds

to the log law and it is suggested that the scale ratios associated

with the peaks of the turbulence spectra are more meaningful.

Typical spectral density functions for 3 = 256 degrees (Run Nos.

311 & 22) and Z = 6.6 m are plotted on Monin coordinates in Fig. 7. The

wavelengths associated with spectral peaks are approximately 3.3 and 265

m for model and full-scale respectively, indicating a scale ratio of 1:80

for this wind direction. it is seen that the slopes of the spectra below

and above the peaks agree reasonably well with those of the von Karman

spectral density function, + 1.0 and - 2/3 respectively. The scale ratios

obtained in this manner for 3 = 211 and 3^1 degrees were found to be 1:90

and 1:60, respectively.

Flow properties for four groups of directions are listed in Table

I. Unfortunately, turbulence measurements were obtained only at Z = 6.6

m, these being limited to the longitudinal component. With the exception

3 = 211 degrees, the turbulence intensities measured in the tunnel are approximately 50 percent of the corresponding full-scale values. Run

No. 2k for 3 = 211 degrees was obtained with the spires and upwind models

removed from the tunnel, the boundary layer thickness for this case being

approximately O.'tO m. Also included in Table I are the maximum speeds

observed in the full-scale data over a time interval of 750 seconds, the

average frequency of occurrence of peak values (commonly referred to as the zero crossing rate), and the mean velocity at Z = 10 m. Full-scale

values of U, n are based upon the ratio U. n /U which was measured in the

1 u I U R wind tunnel.

The use of spires and roughness elements In tunnels with short working

sections results In mean velocity profiles that are quite acceptable when

compared with either the log law or power law representations. However,

it Is obvious that the turbulence characteristics are not always in satisfactory

agreement with full-scale values.
While the shapes of the spectral density functions agree fairly well

with the full-scale functions, both the scale and intensity of turbulence

are low. This suggests that the surface roughness elements must be modeled

at a distorted scale and that a minimum roughness must be maintained in the tunnel when modeling relatively smooth prototype surface conditions.

6.2 Pressure Measurements

It was anticipated, and later confirmed by the wind tunnel tests,

that corrections would have to be applied to the full-scale dynamic and

static reference pressures due to the close proximity of the anemometer

and pressure probe to the test building. The procedure was to determine

a static pressure correction coefficient

P - P

R 10 SP 1/2 p C£

in the tunnel and to then apply this correction as an offset to all fullscale pressure records Csee page 17 for notation). The dynamic pressure

correction coefficient

'dp

1/2 P U 1Q

'

1/2 P U„

2

was applied in a similar manner. Both corrections were d i rect i on -dependen t and values of C and C, ranged from -0.10 to -0.21 and from 1.02 to 1.39

sp dp

respect I ve 1 y .

The results of pressure measurements are summarized in Tables I I - V and are compared graphically In Figs. 8 and 9- The mean pressure coefficient

C , and the fluctuating pressure coefficient, C r , correspond to the usual

P pf

definitions and are referenced to the free-stream dynamic pressure at Z = 10 m The peak factor, g, IS defined as the number of standard deviations included

in the maximum peak departure from the mean, i.e.

9 = max or mm

rms

Pi

The zero crossing rates (Table V) are as previously defined.
The mean pressure coefficients, C , plotted in Fig. 8, indicate considerab

disagreement between model and full-scale measurements. Equipment used

to measure and record the field data was somewhat prone to zero drift

with the extreme range of temperatures experienced at the test site (+30 C).

The field test equipment has since been modified to provide transducer

zero readings and recorder calibrations prior to each data run. Corrections

for this drift could not be accurately determined and any resulting errors

are directly reflected by Fig. 8. With the exception of Run No. 310,

the addition of a fixed value for a given full-scale run would improve

the agreement between the two sets of coefficients.

le

Changes in equipment sensitivity due to temperature variations were

found to be small and, therefore, greater confidence can be put in the

measurements of pressure fluctuations. Fluctuating pressure coefficients,

C ,, plotted in Fig. 9 indicate a fairly good correlation between model

pf

and full-scale, the former averaging approximately one-half of the latter.

This discrepancy is believed to be due primarily to the low turbulence

intensities observed in the model studies, although scale effects cannot

be completely ruled out. There is no obvious explanation for the good

agreement between Run Nos. 207 and 21, and the relatively poor agreement

between Run Nos. 311 and 22. It may be that the pressure fluctuations

are quite sensitive to wind direction and that directions were not properly

matched in the model studies.

It is expected that a new series of tunnel tests in which the turbulence

characteristics are more accurately simulated will bring the results into

much better agreement.

Spectral densities for tap positions P3 and P10 (Run Nos. 311 and

22) are compared In Figs. 10 and 11, respectively. The shapes of the

spectra compare qutte favorably and suggest a slope of -4/3 over the higher

frequency range. As Indicated previously, the wind data were normalized

on the assumption that the scale ratio was 1:50. Although the peaks

are not well defined, they suggest a scale ratio tn line with that obtained

by coropartng the velocity spectra.

Spectral densities for the full-scale pressure fluctuations above

the spectral peak usually exceeded those obtained from the wind tunnel

model (see Ftg. 11). Thts ts qutte ltkely due to the fact that the analog
filters used in the full-scale studies had a cutoff frequency of 10 Hz,

resulting in aliasing errors in the spectral analysis. The wind tunnel

pressure signals, on the other hand, were subjected to pronounced attenuation

above 100 Hz which would appear as a 2 Hz cutoff in the analysis. Thus

the aliasing errors can be expected to be considerably smaller for the

wind tunnel data.

Another indication of similarity between model and full-scale pressure

fluctuations is the coherence function. The coherence function, or more properly (coherence) , is the normalized modulus of the cross -s pec t rum

and is a measure of the correlation between fluctuations at two points

over the frequency range for a given separation distance. This function

for P1-P6 and Run Nos. 310 and 23 is plotted in Fig. 12.

The peak factors, which are listed in Table IV, are fairly consistent

and suggest an average value of *t . 6 . It has been shown, both theoretically

and experimentally (5 , 6), that the peak factor increases with length

of record and a value of g = *» . 3 has been suggested for calculating design

loads on cladding elements for those situations where resonant response

is insignificant. The peak factors in Table IV are based upon single

records of 750 seconds and represent a worst-case excursion from the mean, i.e. negative excursions for pressure taps 1-9 and positive excursions

for taps 10rl2. tt should be pointed out that the peak factors will vary

from record to record for a given wind speed and direction and that several

records would be required to establish their mean and variance.

Fluctuating pressure coefficients and peak factors determined from

the a rea<- i n teg ra 1 1 on of pressure records are presented in Table VI. The

procedure was to construct a new pressure time series by multiplying the

samples of each record to be summed by a weighting factor. The weighting

factor was proportional to the area attributed to each pressure tap which

implies that all pressure fluctuations over that area are perfectly correlated

The resulting record was then analyzed In the usual manner. As expected,

the fluctuating pressure coefficients show a decrease with increasing

area, but there is no indication of a similar reduction in the peak factor.

Although the transducer separations used In this study do not permit a detailed assessment of the correlation of peak pressures acting over extended

roof areas, some indication of the area reduction of peak pressures can

be derived from Table VI. Comparing the product of the fluctuating pressure

coefficient and peak factor for transducer combinations 1, 2, 3 for Run

No. 332 with the product of the averaged fluctuating pressure coefficients

and peak factors for the individual transducers (Table III and IV), the

9
corresponding reductions in peak pressure fluctuations are approximately

27 and 40 percent, respectively. A similar comparison for the combination

1, 2, 3, 6, 7, 10, 12 (Run No. 310) indicates a reduction of 42 percent.

In averaging the pressure records, it was assumed that the pressure fluctuations under the eaves overhang were perfectly correlated and equal to the

fluctuations measured by transducer No. 10. An increase of 42 percent

was obtained from the combination 1, 2, 3, '0 for Run No. 310. It should

be noted that the records for transducer Nos. 10 and 12 were inverted

so that positive fluctuations acted in the same sense as negative fluctuations

on the roof

.

7.0 COMPARISON WITH RECOMMENDED DESIGN LOADS

In addition to providing a check on wind tunnel test results, the

full-scale data reported herein allow some direct comparisons with current

recommended design wind loads. American National Standard Building Code

Requirements for Minimum Design Loads in Buildings and Other Structures,

A58. 1-1972, states a procedure for calculating wind loads on roofs and

considers both tributary (parts and portions) and total roof areas (7).

The basic wind speeds used in this document are the fastest-mile speeds

for a 50-year mean recurrence interval and flat, open country (Exposure

C) at a height of 30 feet above ground.

Because they are defined differently, the pressure coefficients measured

in full scale and those specified in the A58.1 Standard cannot be compared

directly. However, the pressures can be compared for a given wind speed

and category of exposure. Assuming flat, open country and a basic wind

speed of 40.2 m/s, the corresponding effective velocity pressures for

heights less than 10 m are as follows:

2

q f = 960 N/m ( A 5 8 . 1 - Table 5 - Ordinary buildings and structures)

q = 1480 N/m 2 (A58.1 - Table 6 - Parts and portions)

For buildings with a ratio of wall height to least width of less than

2.5, the A58.1 Standard specifies a general external pressure coefficient

of -0.7 for roofs. For gabled roofs with the wind direction perpendicular

to the ridge and the height-width ratio and roof slope being considered

here, a pressure coefficient of -1.0 is specified for the windward slope.

The Standard also allows for local peak pressures which are assumed to act at 90-degree corners and on strips running along the ridge and eaves.

10
The width of these strips is taken as 10 percent of the least width of

the building normal to the ridge. The specified pressure coefficients

are -2.4 for ridges and eaves and -3-9 for 90-degree corners.

No specific provision is made for pressures on the underside of eaves, but this can be taken as the pressure acting on the windward wall

for which a coefficient of 0.8 is specified. Internal pressures are based

on the fastest-mile speed at 30 feet above ground for the appropriate

terrain cateqory 3 and an internal pressure coefficient, C ., which is related

pi to the distribution of wall openings and the ratio of open to solid wall

area. The design pressures for the assumed wind speed and exposure are

as foil ows :

(a) Leeward slope, total area = (-0.7)(960) = -670 N/m

(b) Windward slope, total area = (-1.0)(960) = -960

(c) Leeward slope, tributary area = (-0 . 7 ) ( 1 480) = -1040

(d) Windward slope, tributary area = ( - 1 . ) ( I 480 ) = -1^80

(e) Ridges and eaves

(f) 90-degree corners

(g) Underside of eaves

(h) Internal pressure

= (-2.4) (1480)= -3550

= (-3.9) (1480)= -5780

= (0.8) (1480)= 1190

= (0.3) (1000)= 300

At Z = 10 m, the corresponding mean speed averaged over 750 seconds (the

record length used in this study) is approximately 33.5 m/s (see Ref.

8). For standard atmospheric conditions the effective velocity pressure

2 is 670 N/m . Using the full-scale pressure coefficients listed in Tables

II, III and IV, the mean and peak pressures were calculated for 10 transducer

locations and four wind directions. The results are presented in Fig.

13 along with the design pressures specified by the A 5 8 . 1 Standard (case

f excepted)

.

It is seen from Fig. 13 that the negative design pressures for the

total leeward and windward roof areas (lines a and b) exceed the observed

mean pressures for all transducer locations. However, the observed peak

pressures dominate and exceed the design pressures for tributary areas

(lines c and d) at several locations. This may or may not be significant,

depending on the degree to which pressure fluctuations are correlated

over the roof area. The design pressures for ridges and eaves (line e) substantially exceed the observed peak pressures and the design pressure

1 1
for 90-degree corners is approximately 2.5 times the maximum observed

negative pressure. The positive design pressure for the underside of

the eaves (line g) is exceeded by the observed peak pressure for two wind

directions and the internal design pressure (line h) is less than the

measured peak internal pressure for all four wind directions.

As discussed previously, the fluctuating pressure coefficients determined

from the a rea - i n teg ra t i on of pressure records decrease with increasing

area while the peak factors remain about the same. Although reductions of

from 20 to 40 percent in the peak pressures (peak departures from the

2 mean) are indicated for roof areas of up to 5 m , the data presented in Fig. 13 suggest that the provisions of the A 5 8 . 1 Standard for tributary

roof areas are marginal.

The provisions for ridges and eaves and for 90-degree corners, on the other hand, appear to be overly conservative. The maximum negative

2 pressure, based on the measured coefficients, was -2390 N/m for transducer

No. 1 at 256 while the design pressures are -3550 N/m for ridges and

2 eaves and -5780 N/m for 90-degree corners. It is recognized that extreme

negative pressures are associated with vortices generated along the edges

of the roof and that these vortices are extremely sensitive to wind direction

and roof geometry. However, the wind tunnel studies described previously

did not reveal any critical wind directions not covered by the full-scale

2 data. If the effective velocity pressure of 960 N/m (q f

) is used in

2 2 place of 1 480 N/m (q ), the resulting design pressures are -2300 N/m

P 2 for ridges and eaves and -3730 N/m for 90-degree corners.

As with the negative pressures acting over the roof area, the peak

pressures under the eaves (transducer No. 10) and in the garage area (transducer

No. 12) far exceed the corresponding mean pressures averaged over the

record length of 750 seconds. Again referring to Table VI and Fig. 13,

the average maximum pressure (peak plus mean) acting upward on the eaves

2 is 1680 N/m for Run No. 310 and combination 1, 2, 3, 10. The corresponding

2 design uplift pressure is 3540 + 1200 = 4740 N/m . For combination 1, 2> 3, 6, 7, 10, the average maximum pressure acting over a roof area of

24 m is 140 + 620 = 766 N/ffi 2 as compared with 1040 + 300 = 1340 N/m 2 (tributary area plus internal pressure) as specified by the A58.I Standard.

The maximum internal pressure (based on measured coefficients) in the garage area was approximately five times the corresponding design

pressure. The ratio of effective open area to solid area is difficult

to determine stnce all doors and windows were closed during the recording

12
intervals reported herein. One door and one window of approximately 1.8

2 and \.k m , respectively, are located on the S.E. wall, one window of

2 2

1 . k m on the S.W. wall, and an overhead garage doof of 5.1 m is situated

on the N.W. wall. Because of the extremely cold winters in Montana, great

care is usually taken to provide doors and windows with adequate seals

or wea the r s t r i pp i ng . It is probable therefore, that infiltration rates

for this garage area would compare with those for living quarters in regions

having a mild climate.

8.0 A PROCEDURE FOR THE CALCULATION OF DESIGN PRESSURES

The 1970 edition of the National Building Code of Canada (NBC) (9)

provides for risk of occurrence, terrain roughness, height above ground

and building geometry in calculating design wind pressures

p = q C C C . (1)

e g p

In this expression, q is a reference mean velocity pressure for a qiven 3 mean recurrence interval, C is an exposure factor which varies

e with surface roughness and height above ground, C is a gust effect factor

to provide for the dynamic response characteristics of the structure and

surface pressure fluctuations caused by turbulence and localized flow

phenomena, and C is the conventional mean pressure coefficient determined

P from wind tunnel tests. For the design of cladding, it is assumed that

dynamic response can be neglected and a factor of 2.5 is used for C . 9

The form of Eq. (1) is particularly convenient in that it allows

a complex process to be treated as a combination of independent elements

and provides for the separate treatment of mean and fluctuating components

of pressure. The peak design pressure at any point on a roof area can

be expressed as p=C p=p+gp g r — Jr rms

Or in terms of d i mens i on 1 ess pressure coefficients based upon a suitable

reference pressure, such as the free-stream dynamic pressure at Z = 10 m,

+ 9 (2)

13
While Eq. (2) is a practical means of expressing C , full scale and wind

tunnel measurements (Figs. ]k and 15) suggest an empirical relationship

between C r and C . If the envelope of full-scale C , values is expressed

pf p

K pf

C , = 0.3(1 + |C pf p

and it is assumed that g = 5.0, Eq. (2) becomes

(1 + |C |)

C = 1 + 1.5 E— 9 lr I (3)

As indicated earlier, the peak factor g does not appear to change with

surface area. Therefore, it may be possible to determine a gust effect

factor, C , for extended roof areas simply by reducing C , by means of

g pf

a factor, R , roughly analogous to the size reduction factor used in calcup'

lating dynamic response. The gust effect factor would then be defined by

(1 +

+ 1 .5

C | )R

_eJ e. (A)

where C is the corresponding mean pressure coefficient for the extended

P roof area.

9.0 CONCLUSIONS

The use of spires at the entrance of a wind-tunnel working section

substantially increases the growth rate of rough wall boundary layers,

thereby placing the study of building aerodynamics within the capability

of many conventional tunnels. However, the use of spires and scaled upwind

roughness elements does not alone ensure the establishment of flows with

proper turbulence characteristics. A minimum degree of surface roughness

is required to establish suitable scales and intensities of turbulence.

With some modification of the roughness elements, it is believed that

close simulation of the lowest 30 m of the atmospheric boundary layer

can be achieved at a scale ratio of 1:50.

The agreement between model and full-scale spectra for both velocity

and pressure fluctuations is encouraging. Measurements of coherence suggest

that the spatial extent of surface pressure fluctuations can be modeled

to an acceptable degree of accuracy.
It is believed that the consistently low values of C , determined

pf

from the model result primarily from improper simulation of the turbulence.

However, scale effects cannot be ruled out at this time. Peak factors

were found to agree quite closely with previous measurements, the overall

average for model end full-scale results being 4.6. Based on the preliminary

results reported herein, it appears that a gust effect factor can be expressed in terms of a peak factor, mean pressure coefficients and a size reduction factor.

Wind pressures based on measured coefficients and an assumed wind

speed suggest that certain provisions of the current A58.1 Standard deserve

additional study. For the building investigated, design pressures for

tributary areas and interiors appear to be marginal while those for localized

areas such as ridges, eaves and corners appear to be excessive.

10.0 ACKNOWLEDGEMENTS

Acknowledgement is made to the Tri Service Building Materials Investigational Program Committee, Department of Defense, for financial support of the

field studies reported herein. The writer wishes to express his thanks to

Lt. Col. C. Y. Holland, Jr., USAF, for his assistance at the field test site

and to Col. R. A. Nelson and Family whose home was instrumented during the course of the study. Special thanks are extended to Dr. H. I. Laursen who

developed much of the computer software used in the data analysis during his term as a Visiting Researcher at the NBS.
11.0 Bl BLIOGRAPHY

1. Melbourne, W. H., "Comparison of Pressure Measurements

Made on a Large Isolated Building in Tull and Model Scale."

Proceedings, Third International Conference on Wind Effects

on Buildings and Structures, Tokyo, 1971, pp 253"262.

2. Standen, N. M., "A Spire Array for Generating Thick Tur- bulent Shear Layers for Natural Wind Simulation in Wind

Tunnels." National Aeronautical Establishment Report

LTR-LA-94, Ottawa, May 1972.

3. Marshall, R. D. and Hsi, G., "Techniques for Measuring

Wind Loads on Full-Scale Buildings." Proceedings, Re- search Seminar on Wind Loads on Structures, University

of Hawaii, October 1970, pp 133-148.

4. Helliwell, N. C., "Wind Over London." Proceedings,

Third International Conference on Wind Effects on Buildings and Structures, Tokyo, 1971, pp 23 _ 32.

5. Dalgliesh, W. A., "Statistical Treatment of Peak Gusts on Cladding." Proceedings of the Structural Division,

ASCE, Sept. 1971 , pp 2173-2187.-

6. Davenport, A. G., "Note on the Distribution of the Largest

Value of a Random Function with Application to Gust Loading."

Paper no. 6739, Proceedings, Institution of Civil Engineers,

Vol. 28, June 1964, pp 187-196.

7. American National Standard Building Code Requirements for

Minimum Design Loads in Buildings and Other Structures, A 5 8 . 1 - - 1972. Issued by American National Standards Institute, New York, New York.

8. Deacon, E. L., "Wind Gust Speed; Averaging Time Relationships."

Australian Met. Mag. No. 51, 1965.

9. National Building Code of Canada 1970. Issued by the Assoc.

Comm. on the NBC, National Research Council, Ottawa, Canada.

16
NOTATION

The following symbols are used in this paper:

dp

'pf

sp

10

10

dynamic pressure correction coefficient

exposure factor

gust effect factor

mean pressure coefficient

rms pressure coefficient

static pressure correction coefficient

intensity of turbulence, in percent

mean velocity at standard 10-meter height, in m/s

reference mean velocity at 6.6 m

height above ground, in m gradient height

surface roughness height, in cm

peak factor

mean pressure, in N/m

rms pressure

reference dynamic pressure at 6.6 m reference dynamic pressure at 10 m reference dynamic pressure for open country

size reduction factor

wind direction measured clockwise from north, in degrees

mass density of air

spectral density

17
TABLE I

FLOW PROPERTIES

Run* D i rect ion °R f? 1 Max i mum Zero

°,o

Numbe r (deg rees ) (m/s) (m/s) (pe rcen t

)

Peak

(m/s)

Crossing Rate

(Hz)

(m/s)

207

21 186 186

11 .37

1 1 .89 3.38

2.54

30

21 19.93 .267 12.07

12.62

310

23

24

217

21 1 21 1

7.77

13.17

18.71

2.93

4.08

1.10

38

31

6

18.41 .158 9. 14 15.51

18.78

31 1 22 256

256

8.38

14.69

2.62

2.59

3118 16.25 .169 8.47

15.06

322

1 1 101

309

3^1 347

9.91

13.78

9.24

2.35

1.43

2.50

24

1027

15.64

14.81

.166

.21 1

10.36

14.14

9.45

*Run numbers with 3 digits denote full scale data

TABLE 1 1

MEAN PRESSURE COEFFICIENTS

C P Run Numbe r

D i rec t i on (degrees)

Pressure Tap Number

1 2 3 4 5 6 7 8 9 10 1 1 12

207

21 186

186 -.38

-.49

-.43

-.59

-.59

-.76

-.45

-.68

-.64

-.62

-.44

-.22

-.29

-.24

-.32

-.06

-.56

-.14

-.08

.45 -.21

-.20

310

23

24

217

21 1 21 1

.26 -.09

-1 .00

.23 -.11

-.60

-.18

-.14

-.53

-.18

-.13

-.55

-.38

-.18

-.63

-.05

-.05

-.22

-.05

-.03

-. 12

-.41

-.04

-.16

-.04

-.02

.22 .08

.67

.07 -.19

.24

31122 256

256

-. 12 .10

. 12

-. 1 1 -.36

-.36

-.45

-.55

-.61

-.59

-.84

-.41

-.21

. 10 -.70

-.54 -.18

.46

.30 .37

.35

322

11 309

341 -.52

-.32

-.05

-.14

-.22

-.15

-.20

-.23

-.24

-. 12 -.78

-.50

-.47

-.14

-.47

-.37 -.10

-.09

-. 12 • 30

.15

18
TABLE I I I FLUCTUATING PRESSURE COEFFICIENTS

Run Number

Direction

(degrees )

P essure Ta p Number

1 2 3

Ij 5 6 7 8 9 10 " »

207 186 .2k .28 .32 .32 • 31 .21 .19 .18 .21 .27 _ .19

21 186 .25 .28 .31 • 33 .38 .08 .09 . 10 .08 .26 .08 "

310 217 .38 • 33 .28 .28 .32 .32 .25 .22 _ .29 - .29 23 21 1 .\k .15 . 15 .15 .17 .09 .07 .07 .Ok .]k .1 1 2k 21 1 .22 . 10 .09 .09 . 10 .09 .05 .07 .03 .16 .07

21 1 256 .70 .29 • 32 .ko • A3 .62 .31 .30 - .kk A3

22 256 . 12 .19 .18 .25 .20 .18 .09 .2k .05 .13 .16 -

322 309 .30 .15 .15 .17 .19 .3k .21 .18 - .15 - .18

1 1 341 .13 .05 .05 .07 .07 .19 . 10 . 1 I .05 .Ok . 12 - 101 3*7 .31 . 10 .2* . 12 .09 .38 .25 .20 . 10 .25

- :_TM_ .kS

TABLE IV

PEAK FACTORS

9

Run Numbe r Direction

( deg rees )

1

P res su re Ta a Num be r

2 3 k 5 6 7 8 9 10 1 1 12

207 3.7

21 186 186 5.9

5.2

6.8

7.0

k.k

k.S

5.1 6.0k.S

4.1 3.7

3.8

2.6

3.9

3. 1 6.9

1 .5 6.8

k.7

k.3 k.S

310

23

2k

217

21 1 21 1

k.S

k.Q

2.9

k.S

6.5

6.2

6.6

5.7

k.S

7.1 5.8

5.7

e.k

5.1 k.3

S.k

5.3

3.k

3.8

k.3

3.7

k.3

k.S

3.5

3.9

k.2

5.8

6.6

5.9

3.3

3.1

k.e

311

22 256

256

k.S

5.7

5.5

S.k

5.0

3.9

5.0

7.0

3.9

6.3

k.3

k.S

k.S

k.3

3.k

3.3 3.k

5.0

3.5 3.9

k.7

322

1 1 101

309

3k\

Zkl

k.2

3.6

6. k

3.8

k.k

2.7

5.0

3.6

2.3

3.7

k. 2 2.8

7.3

6.6

2.6

k.7

5.0

k.S

6.5

3.7

(,.k

3-7

6.3

2.9

3.5

3.1 k.2

2.k

3.6

3.1 2.0

19
TABLE V ZERO CROSSING RATES

(C ross i ngs/Sec .

)

Run

1 Number

D I rect i on (deg rees )

P ressu re Ta d N umbe r

1 2 .3 4 5 6 7 8 9 10 1 1 12

207

2) 186 186

.37

.39 .34

.43 .34.31 .35 .35 .36.33 .36 .30

.37

• 36

.39 .34

.31 .15 .37

.21 .26

.25

3 1?

24

217

21 I 21 1

.23 .29

.47

.26

.33

.32

.32

.29

.30

.31 .32 .33

.32 .35 .28.29

.29 .48

.27

.31 .45

.31 .34

.52 .16.17

.31

.21 .33 .16

.31

. 1 1

31122 256

256

.20

.30

.28

.33 .26

.33

.21 .36

,21 .28.24 .24

.34

.29

.20 .20 . 12 .26

.19 .18 .17

322

} \ 101

309

341 3^7

.40 .34

.47

.47

.49

.46

.45

.32

.48

.36

.35 .34

.37

.35

.42

.32 .25

.33

.41 .27

.38

.30 .23

.47

.1 1 .44

.31 .22 .26

.25

.21

.22

TABLE VI AREA-AVERAGED COEFFICIENTS

Run Number1 rect ion

(degrees)

Tap

Comb 1 na 1 1 on

C P f g Area

(m

2

) 310

n

n

322

ii

217

ii

ii

309

iin

1 ,2,3

1 ,2,3,10

1,2,3,6,7,10,12

1 ,2,3

1,6

4,5,10

.28

.40

.21

. 12 .27

.12

4.6

6.2

4.3

4.3

5.3

8.8

(.76) (4.27) - 3.25

(.76) (4.27) = 3.25

(5.61) (4.27) = 23-97

(.76) (4.27) = 3.25

(.82) (5.61) = 4.65

(.76) (3.05) = 2.32

20
LUI- oo

en

LU

tu

21
o

<

UJ

LU

CSI

22
ozQ

_J=> PQ

I-ozo<>QZ<z<

_l Q_O

23
0)o

cozO

c

c

c

co

(X Q UJ

3*2

N 7!

Q. CO\

24
1 1 i i i i—i—

r

1 1 1 i i i—i—

r

o

<

UJco

en

a.s3

o

CO

o

o

in

10

' ' ' ' ' ' L '''' I L I ' ' » I ' ' ' L i

N

25
0.8

0.7

0.6

Uz /Ug 0.4

T

1 1 1—I I I I I

1 1 1—I—I I I I

/3 = 341 degrees Z = 0.2 mm

$* 256 degrees Z * 5.8 mm

J I I

I I i i i I

I I I l I I I I .01 0.1

Z (METERS)

Fig, 6 Mean Velocity Profiles - Log Law Representation

i.o

26
i i i i—i—

r

TT i r

I I I I i—

i

i i I i i I i l| | L

o dD

CNI

CNJ

00o

<

o

o

oo

c

27
O Run Nos. 207, 21 D 310, 23

A 311, 22

322, II

FULL-SCALE

A A

+ +

-I- 0.2 •

/

/

/ / D

/

/

/

/ / 4

-0.8 -0.6 -£.4 q -0.2 B

o m

o /i

0.2 a n°

0.2

K/

/

/

// / o

a /a

9V o

/ o

o

0.4

LU

I- -0.6 q

Z

0.8 /

VFig. 8 Mean Pressure Coefficients

28
0.8

_, 0.6

IdZ

Z

z>

I-q 0.4

z

0.2

1 1 1 1 /

Run Nos. 207, 21 /

/

a 310, 23 /

/

A 311, 22 /

/ — 322,11 / —

/

• 101,11

/

/

/

/

/

/

/

/

/

/

/

/

/

- /

a-^A / a — / a qH • * A —

4 /

/ • A'"

A

/ 1 S 1

s 0.2 0.4 0.6 0.8

FULL-SCALE

Fig. 9 Fluctuating Pressure Coefficients

.0

29
CM

CM

od

toO<

0. CO

LU

in

LU

a: Q30
TT

CN1

COO

CD

i—

I

5 c|d I

o

Hi

0. GO

IX. =>

CO

CO

LU

tx Q_

Mill L o

c

o

o _o

o

d

Tk

31
0.8

lu 0.6 o

z

l±J trUJ |

04

• FULL-SCALE

A WIND TUNNEL

• A

A A A

± A i

0.2

0.0 1

0.0 0.2 0.4 0.6 0.8

n (H z )

Fig, 12 Coherence (P1-P6, Run Nos, 310 S 23)

32
o9€Z 2 w/n - 3dnSS3dd OO

<M ooo

roi 1

co

LU

ce=5CO

CO

LU

ct CUz

CDCO

LU QQ

LU

i—*

LU.

C_> LU

Q_ <T go

III m

u 5 < j7 Q

LU cr 3 Ul CO o < 3 LU o Sil CO

?• LL. < O

CE T. H O

CO

cr<

Q.

•z. o

CJ

N~\

33
\\

\

\\ 00

\ ~o

\

1 A

/ \ u> -^ \ ~d

jj. \ + \ *

\ d

Q. < \ o

v

\Sag CJ

\ d

\ * \ < CM * U>

ID * ' CM d d \ d d d d

1

1 v 1 1 1 1

1 1 1 * d o dd30o ss3dd QNiivniomj

1 \ 1 1 on

° \

\

CM •s

<

\

Q. o -

u.UJ

~d

i d » 8 o „\

o

o \ o \ f^ O _ CJ O — — CM

c/j v> -

UJ

(0d « < X cj to ro po

a.

i ° \ 6z

c

3

or

z

<

UJ2 - _0D \ ~d \ . OD< 1

i \ <

\

\

\

<

o

oo

a.

CO>

C_)

34
a.A\ o

to \ O

o \ + \

CM6 \

II \

a. o \\

\ u> f

o o

+

00 "d

ID "d

CMd d

u>d

* d o dd30o *ss3yd 9Niivniomj

- io evi -

CM CM CM —

3

o <

o

o

CO

00

UJ

ce

a.

*k>

(0d

i

00 "d

\ < \ >

<

o

\

35
NBS-1UA (REV. 7-731

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. IH.'M.K ATION OR REPORT NO. NBS TN-8522. Gov't Ac

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE A Study of Wind Pressures on A Single-Family Dwelling

in Model and Full Scale

5. Publication Date- October 1974

6. Performing Organization Code

7. AL'TMOR(S) R. D. Marshall

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE

WASHINGTON. D.C. 20234

10. Proicct/Task/Worlc Unit No. 4fil5132

II. Contract/Grant No.

12. Sponsoring Organization Name and Comp AJdr Tri Service Building Materials

Investigational Program Committee

Department of Defense

Washington, D.C.

(Street, City, State, ZIP) 13. Type of Report & Period Covered Final 1971-1973

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES Library of Congress Catalog Card Number: 74-600152

16. ABSTRACT (A 200-word or [ess factual summary of most significant information. It document includes a significant bibliography or literature survey, mention it here.)

Wind pressures measured on a single-family dwelling are compared with

results obtained from a 1:80 scale model placed in a turbulent boundary

layer. It is shown that the fluctuating components of surface pressures

far exceed the mean or steady pressures and are well correlated over sizeable roof areas. The consistently low fluctuating pressure coefficients

obtained from the wind tunnel model are attributed to improper simulation

of the lower portion of the atmospheric boundary layer. Comparisons

between actual loads and specified design loads suggest that certain

current provisions are marginal for tributary areas and excessive for localized areas such as ridges, eaves and corners. A procedure for expressing loads on both localized and extended roof areas in terms of mean pressure coefficients and a peak, factor is described.

17. KEY WORDS fst'x to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Aerodynamics; boundary layers; buildings; codes and standards; wind loads wind tunnels.

18. AVAILABILITY [^Unlimited

For Official Distribution. Do Not Release to NTIS

r~/» Order From Sup. of Doc, U.S. Government Princint! Office Washington, D.C. 20-102, SD Cat. No. CI 3 .46:852

Order From National Technical Information Service fNTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

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(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

40

22. Price

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