mai Buraav. or standards

Library, N.W. Bldg

APR 2 9 1959

Wind Pressures in Various Areas

of the United States

United States Department of Commerce

National Bureau of Standards

Building Materials and Structures Report 152
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UNITED STATES DEPARTMENT OF COMMERCE • Lewis L. Strauss, Secretary NATIONAL BUREAU OF STANDARDS . A. V. Astin, Director

Wind Pressures in Various Areas

of the United States

Guttorm N. Brekke

Building Materials and Structures Report 152

Issued April 24, 1959

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington 25. D.C

Price 15 cents

Contents

Page

1. Introduction 1 2. Principal sources of strong winds 1 2.1. Cyclonic storms 1 2.2. Frontal regions

2.3. Tropical cyclones (hurricanes)

2.4. Severe thunderstorms and squalls 2

2.5. Tornadoes _ 2

2.6. Special winds 2

3. Definitions 2

4. Letter symbols and abbreviations 3

5. Computation of minimum allowable wind pressures 3

5.1. Minimum allowable wind pressures at 30 feet 3

5.2. Wind pressures at height H 4

6. Adjusted wind-pressure values for 15 stations- 6

6.1. Variation with altitude 6

6.2. Tornado-like winds 6

6.3. Extreme winds in coastal areas 7

7. Wind pressures for certain counties 7

8. Discussion 8

9. References S

m

to

to

Wind Pressures in Various Areas of the United States

Guttorm N. Brekke 1

A procedure is described for developing a wind-pressure map that shows minimum

resultant wind pressures (30 feet above ground) for design purposes throughout the continental United States. This map was used in the 1955 revision of the American Standards

-Association’s standard A58.1, American Standard Building Code Requirements for Minimum.

Design Loads in Buildings and Other Structures.2 It is based on records of annual fastest- single-mile wind speeds at 155 U. S. Weather Bureau stations. In computing resultant wind pressures front these data, allowance was made for gusts and for building shape.

Values for 15 of the stations were also adjusted for altitude or for unusual local wind conditions. A table of design wind pressures at various heights from less than 30 to more than

1,200 feet above ground is derived for use with the map. The principal sources of strong

winds affecting building constructions are briefly reviewed.

1. Introduction

Experience has shown that the derivation of

i wind pressures used hi designing structures and

! writing building codes is freqentlv misunderstood.

At the same time, a need has been observed for

i greater emphasis on a wind-pressure map as a primary tool for determining minimum design wind

! pressures at any particular location within the

United States. This paper contains a step-by-step description of

: procedures used to develop a design wind-pressure

map for the United States and to construct a table

: of wind pressures for different heights above

I ground. Major emphasis is placed on the method

j used to calculate wind pressures from recorded

wind speeds. However, a number of other factors

that affect the final value of minimum design wind

pressure are also considered. These include the

; effect of gusts, variation of wind speed with height

; above ground, altitude above sea level, distance

I from the seacoast, topographical configuration, and

the frequencies of various types of storms. In revising the 1945 standard, American Stand-

| ard Building Code Requirements for Minimum DeI sign Loads in Buildings and Other Structures, A58. 1

,

I a new wind-pressure map (fig. 1) was developed.

It was originally prepared by the National Bureau

j of Standards, with the assistance of the U. S. Weather Bureau, for use by American Standards

! Association Sectional Committee A58. After careful study, the new map was accepted by the

i Committee and now* appears in the revised

standard, dated 1955.

In the 1945 edition of A58.1, a table of design

wind loads had a prominent place in the standard,

whereas the wind-pressure map appeared in the

l appendix. The table gave the wind-pressure

values for different heights starting with a value of

j 20 lb/ft2 at 30 ft above the ground. However, it

; did not show the increase in wind pressure with

height above ground for those areas in which the

! Dr. Brekke was formerly with the National Bureau of Standards, now

retired.

- Obtainable from the American Standards Association, 70 East 45thStreet, Xew York 17, X. Y., for $1.50 a copy.

wind pressures differed from 20 lb/ft2 at 30 ft. To

remedy this situation, the wind-pressure map in the 1955 revision showing design wind loads at 30

ft above ground throughout the United States was

included in the standard, together with a table

showing the increase in wind pressure with height

for each wind-pressure area on the map.

It should lie noted that minimum wind loads

recommended by existing building codes are often

quite different from those given on the windpressure map in A58. 1-1955. These differences

may be due to the use of different sources of data

or a different method of determining wind pres- sures from recorded wind speeds. The values given on the map are based on the fastest-single-mile

wind speeds reported during the 39 vears prior to 1952.

2. Principal Sources of Strong Winds

The discussion (sec. 5) of the determination of minimum resultant wind pressures and of the

development of the map may be more readily

understood if, first, the types and sources of the

strong winds involved are briefly reviewed.

According to the U. S. Weather Bureau, the

principal sources of strong winds are cyclonic

storms, frontal regions, tropical cyclones (hurricanes), severe thunderstorms, squalls, tornadoes,

and special winds. The driving force of winds

depends upon the horizontal pressure gradient

(closeness of isobars); the steeper the pressure

gradient, the greater the tendency for strong winds.

2.1. Cyclonic Storms

Cyclonic, storms, or extratropical cyclones, are the low-pressure areas in the middle latitudes, particularly those with steep horizontal-pressure gradients. These storms cover circular or nearly

circular areas with an average diameter of about

1,500 miles. Unlike tropical cyclones, which

affect only small areas, a cyclonic storm may cause

precipitation and cloudiness over an area of l million or more square miles.

404126—59 2 1
2.2. Frontal Regions 2.5. Tornadoes

A frontal region is a region of transition between

two air masses of different temperatures, densities, or wind speeds, in which relatively steep gradients

characterize the horizontal variations of such

elements. Strong, gusty winds are usually asso- ciated with frontal regions when the horizontal

pressure gradient is very steep.

A frontal region may contain either a cold front or a warm front. A cold front is a line at the

earth’s surface along which the forward edge of relatively cool air acts like a wedge in underrunning and displacing a warmer air mass. A

warm front is a line at the earth’s surface along

which the forward edge of an advancing current

of relatively warm air is replacing a retreating

colder air mass.

2.3.

Tropical Cyclones (Hurricanes)

Tropical cyclones are severe storms accompanied

by strong, squally winds ranging from 75 to 150

mph. Those of concern to the United States are the hurricanes which develop over the Caribbean

Sea, Gulf of Mexico, or neighboring Atlantic

waters, and which affect the southern, southeastern, and eastern coasts of the United States. When they move northward, their characteristics

become somewhat modified. Thus, when such

storms move inland they may tend to dissipate,

especially if their moisture supplies are cut off as they move over a homogeneous land environment

or impinge against mountains, producing heavy

rain. However, when these storms move along a path that permits a continuous influx of moisture

from the neighboring ocean or a lake, they may

display considerable energy and do much damage

from strong winds, water action, or precipitation.

If conditions are such that the circulation about

the storm draws in cold air as it moves into middle

latitudes, it may develop into a large extratropical cyclone of great intensity.

2.4.

Severe Thunderstorms and Squalls

Severe thunderstorms and squalls are often

observed in conjunction with fronts, frequently

with prefrontal squall lines and at other times as severe local storms. A squall is a wind of considerable intensity caused by atmospheric instability; it is characterized by gusts that rise quickly

and fall gradually. A squall is sometimes accompanied by thunder, lightning, and precipitation,

and is often named after the special weather phe- nomenon that accompanies it; e.g., rain, snow,

or bail squall [1, 2],

3

3 Figures in brackets indicate the literature references at the end of this paper.

Tornadoes are violent whirlwinds producing ; narrow vortices 4 with cores of low atmospheric

pressure. The wind pressures and suctions in the

immediate path of tornadoes are considered to be

so great that construction strong enough to i withstand them is not economically feasible. Therefore, in recommending design wind pressures, building codes do not take into consideration the

j violent forces that can be expected within the narrow path of a tornado.

It is well known that there is usually a signifi- cant difference between the pressure of the core

of a tornado and the pressure of the surrounding- area and that this is believed to be the cause of j the so-called “explosion” of tightly closed buildings over which the vortex passes. In the U. S. Weather Bureau’s publication on tornado safety

rules, 5 there are recommended, among other

things, certain measures believed to be helpful

in relieving interior pressures that might occur j

1 at the time of a tornado passage [3, 4, 5],

2.6. Special Winds

Special winds are associated with mountainous

or other topographical configurations where steep

pressure gradients and strong downward accelerations occur along steep slopes. Such winds are the Santa Ana w-inds in southern California;

the gorge winds in the Columbia River Valley of Washington and Oregon; the Wasatch Mountain

winds in Utah; and the Chinook winds along the eastern slope of the Rockies in Montana, Wyoming, and Colorado [6]. No specific minimum

allowable wind pressures are assigned to the re- gions of these winds for design purposes. However, | attention is called to these regions on the windpressure map in order that special precautions

may be taken against unshielded exposures when

important wind-sensitive buildings are to be

erected.

3. Definitions

For the purpose of this discussion, the followfing

definitions are submitted:

Design wind pressure. The gust-velocity pres- sure times a shape factor.

Effective height. The theoretical distance above

ground at which a given anemometer w

T ould ! register a mean wind speed equal to that which

would be observed if there wrere no obstructions

such as buildings or trees in the vicinity.

Fastest-single-mile wind speed. The wind speed

in miles per hour based on the shortest recorded

time interval in which the wind traveled a horizontal distance of one mile relative to the earth.

* By vortex is meant a column of air whirling around a core of low pressure. :

s See U. S. W'eather Bureau Tornado Safety Rules, January 1952. Means | for ventilation can be installed to obviate or mitigate the destructive effects of unequal pressure (see references 3, 4, and 5). 2
Gust. A sudden brief increase in the speed of the wind due to turbulence set up by some surface

obstacle, by eddies, or by thrusts of more rapidly

moving air from the free atmosphere.

Gust factor. The ratio between the true gust

speed and the fastest-single-mile speed.

Gust-velocity pressure. The velocity pressure

multiplied by the square of the gust factor. Modified shape factor. The shape factor appropriate for a given structure relative to the shape

factor for rectangular buildings (1.3) which is included in the values on the map and in the table

of A58.1-1955 and of this paper.

Normal sea-level conditions. Barometric pressure

(B)=29.92 in. mercury; temperature (0 =59° F;

and acceleration of gravity (<?) =32.2 ft/sec 2

. Resultant wind pressure. For the purpose of

this paper, the gust-velocity pressure at a given

height multiplied by the shape factor for rec- tangular buildings, 1.3. The value 1.3 is the

resultant of the pressure and suction coefficients on the exterior walls (0.8 for the windward and — 0.5 for the leeward wall); it thus represents the

total force exerted per square foot of exposed

surface.

Shape factor. The ratio of the actual wind pres- sure on a building or structure to the theoretical

wind pressure or velocity pressure.

Velocity pressure. The pressure developed when

air of a given velocity is brought to rest by impact

(variously called dynamic pressure, impact pressure. stagnation pressure).

Wind-sensitive buildings. For the purpose of

this paper, buildings for which wind loads are the

major external loads.

4. Letter Symbols and Abbreviations

Letter symbols and abbreviations used in this paper are defined below:

.£>= barometric pressure, inches of mercury,

CL=shape factor for rectangular buildings

(see Definitions),

C^G3o=gust factor at 30 ft above ground,

=gust factor at Hft above ground,

g =acceleration of gravity, ft/sec 2, H=a specified height above ground, ft, ^=height of anemometer above ground, ft, -Pi30 =minimum allowable resultant wind pres- sure at 30 ft above ground, lb/ft 2

, PiH =minimum allowable resultant wind pres- sure at H ft above ground, lb/ft 2

,

g 0 =velocity pressure=- pv

2

, lb/ft 2,

?3o=velocity pressure at 30 ft above ground,

lb/ft 2

, ?!3o=gust velocity pressure at 30 ft above

ground, lb/ft 2

, p = density of air (mass of unit volume),

slugs per ft 3 or (lb sec

2/ft4), t= temperature, ° F,

r=wind speed, ft/sec,

U=wind speed (fastest-single-mile speed),

mph,

Vh = recorded wind speed at anemometer

height h, mph,

U30 =wind speed at 30 ft above ground, mph,

F,3o=gust wind speed at 30 ft above ground,

mph,

Fff =wind speed at height II, mph,

ViB —gust wind speed at If ft above ground,

mph.

5. Computation of Minimum Allowable Wind Pressures

To provide a basis for the wind-pressure map,

minimum allowable wind-pressure values at 30-ft heights were computed for representative locations

throughout the United States. The computations

involved the previously mentioned conditions that

affect wind pressures; however, the basic formula

used was eq (1) which gives velocitv pressure in

lb/ft2:

<lo=\ pv

2

, (1)

where p is the density of air, 0.00238 lb sec2/ft 4 (at

normal sea-level conditions), and v is the wind

speed expressed in feet per second. When the

wind speed is given in mph, V,

<Zo=^X0.00238x(||^)

2 V2 =0.00256 V2

. (2)

The original computations were made on the basis

of normal sea-level conditions, realizing that

modifications would be necessary for those stations

at high elevations where the decrease in air density

and temperature was sufficient to have an appreciable effect on the result ing wind pressure (see

sec. 6.1). For the greater part of the country,

the height above sea level is not enough to cause any significant differences.

The next step was to obtain appropriate values

for V and to substitute them in eq (2) along with

the other factors that would affect the minimum

allowable wind pressures.

5.1. Minimum Allowable Wind Pressures at

30 Feet

Through the courtesy of Herbert C. Tliom. Chief

Climatologist, U. S. Weather Bureau, the latest data were obtained for the maximum recorded

single-mile wind speeds at 155 well-distributed

U. S. Weather Bureau stations. These data are given as monthly maximums and, in most eases, covered a period of about 39 years prior to 1952.

The maximum single-mile wind speed ever re- corded at a station was used to calculate the wind

pressure for that station.
Information on the height of the anemometer

above ground at the time the maximum wind

speed occurred was also necessary, as the wind

speeds had to be adjusted to the previously selected

height of 30 ft. This height corresponds approximately to the 10 m (about 33 ft) which is the

international standard for effective height. For

149 of the 155 stations, the local climatological

summary for each station included this information. For the remaining 6 stations, at which the

anemometer height was unknown, a height of 30 ft was used.

To adjust the 149 fastest-single-mile wind-speed

values Vh , observed at anemometer height, h, to the wind speed at a height of 30 ft, T

7

30 , the 1/7-

power relationship was used

:

r,o-r„(ff. (3)

Then, in order that the final resultant wind

pressure represent the maximum to be expected

at any time during the life of a building, it was

necessary to take into account the gusts that occur during the time interval for the passage of a mile of air. To do this, the wind speed, V30 , Avas multiplied by 1.3, which is the gust factor

at 30 ft (see page 6 for discussion of gust factors). Denoting the gust factor at 30 ft as CG30 , and the corresponding gust wind speed as Vm, we have

walls, 0.8 for the Avindward and —0.5 for the

leeAvard wall. By substituting for qm as given I by eq (5), Pm= 1.3X0.01143 (F*) 2 X/r2/7

I = 0.01486 (F*) 2 X/r2/7

. (7) | By substituting the Weather Bureau values for the fastest-single-mile wind speed, Vh, and the

height of anemometer, h, in eq (7), the minimum

allowable resultant wind pressure at 30 ft is readily calculated.

It was found that the distribution of wind-pressure values was such that by rounding off these

values to the nearest numbers divisible by 5, the ! entire country could be divided into sizable areas representing 5-lb/ft.2 intervals betAveen 20 and 50

lb/ft2

. In plotting wind pressures near the sea- coast on the map, it was necessary to take into

account the gradual decrease in wind pressure

with distance from the coast. In the absence of precise experimental data on this effect, the highpressure winds along the seaboard due to hurri- canes and other severe storms were reduced stepAvise to the loAver pressures prevailing inland. This

accounts for the three or four zones along the

coast, each approximately 30 miles wide (totaling

90 to 120 miles) and each representing a 5-lb/ft2 decrease in wind pressure. This procedure seemed

reasonable considering the following statement by

W. T. Van Orman [7]:

I z'30 — 0(730 F30 — 1-3 T 7, (ir

= 1.3 30l ' 7 Vh Xh- 1/7

. (4)

This value for ITo may iaoav be substituted

for V in eq (1) and the corresponding velocity

pressure at 30 ft, q m, then becomes

We know definitely from temperature studies, from bal- loon flights, and similar phenomena, that the direct in- fluence of the sea breezes extends at least 30 miles inland. Furthermore, it is also reasonable that the complete damping influence of the surface friction of the land area is not

fully effective until a distance of about 100 miles from the | Atlantic seaboard has been reached.

5.2. Wind Pressures at Height H

q m= 0.00256 (W30 )

2 = 0. 00256 X1.32 X302/7 X(F7,)2 X/r2/7 = 0.01143 TO2 X/r2/7

. (5) On the map prepared for A58. 1 — 1955, it was

decided to present the wind pressures as minimum alloAvable pressures for direct use in design

of ordinary rectangular buildings rather than as velocity pressures as Avas clone in the 1945 standard. This required one additional step to take

into consideration the shape factor, CR, for rectangular buildings.

The resultant wind pressure, Pm, at the 30-ft height is then given by

Pm~ GrX ? so; (6)

Avhere CR is equal to 1.3, the resultant of the

pressure and suction coefficients on the exterior

Table 1 gives the design wind pressures at vari- ous heights above ground for each of the AvinclJ pressure areas indicated on the map. These |

Table 1. Design wind pressures for various height zones

above ground a

Height zone

AAr ind-pressure-map areas (lb/ft2

)

20 25 30 35 40 45 50

Design wind pressures, lb/ft 2 ft

Less than 30. _ _ ___ . 15 20 25 25 30 35 40 30 to 49 20 25 30 35 40 45 50

50 to 99 25 30 40 45 50 55 60

100 to 499 30 40 45 55 60 70 75 500 to 1,199 35 45 55 60 70 80 90

1,200 and over 40 50 60 70 80 90 100

a This table is designed tor use with the wind-pressure map (fig. 1). The ! wind pressure at any height is found in the body of the table under the column | heading giving the minimum resultant wind pressure for any particular

locality on the map. 4
D

Figure

I.

Minimum

allowable

resultant

wind

pressures

at

SO

feet.
values were obtained by using the wind speeds at 30 ft and taking into account: (a) increase in wind

speed with height; and (b) decrease in the gust

factor with height.

Inasmuch as both the wind speed at height H,

VH, and the wind speed at a height of 30 ft, V30 , are given by the 1/7-power formula, these two

wind speeds are related as follows:

Then if the gust factor, CGH , which decreases

with height above ground, is taken into account

as was done in eq (4), the gust wind speed, ViH , at height H can be obtained. The values for the

gust factor used in these computations are based on rather limited data and are those recommended

by aerodynamic authorities at the National

Bureau of Standards and the U. S. Weather

Bureau. They are the values 1.3 at 30 ft (as was used in eq (4)), 1.14 at 500 ft, and 1.08 at 1,200 ft. Thus, the gust wind speed, ViH, at height II is given by

I iH = ^GH^ 30 \^3Q )

‘ (9)

The values for CGH between 30, 500, and 1,200 it were interpolated as follows: Between 30 and 500

ft, CG*=1.3-(0.16/470)(tf-30); and between

500 and 1,200 ft, CGH= 1.14- (0.06/700) (#— 500),

where II is t lie specified height above ground,

in feet. By taking into account the shape factor for rectangular buildings, CR , the resultant wind

pressure, PiH , at any height II is Pti=CR X0.00256 l^)2

= 1.3X0.00256(CG„)

2 (U3O ) 2 H2n 302/7 -0.00126 {CoH?(y^ Hw. (10)

Substitution of values for II, and the corresponding values for CGH as interpolated, and the value

for V30 which was used in calculating the wind

pressure at 30 ft, in eq (10) gave the wind pressures

for the various heights in table 1.

6. Adjusted Wind-Pressure Values for

15 Stations

For 140 of the 155 stations, the design wind

loads calculated from eq (7) were such that areas

of approximately the same wind load could be

fairly well defined on the map. However, for each of the remaining 15 stations, this equation

gave a result that was much greater than the

average of the other stations in the same area. In each case, there appeared to be special conditions which caused the computed value for each

of these 15 stations to be greater than that of the area surrounding it. These special conditions

and the reasons for reducing the computed windpressure values are discussed in the following sections. Table 2 shows the computed values

together with the wind pressure shown on the map

for the surrounding area.

Table 2. Adjusted minimum allowable resultant wind

pressures

Stations

Minimum allow- able resultant wind pressures

From

eq (7) Mapvalue

Albuquerque, Bernalillo County, X. Mex. ..

ib/fr- 40 Mr30 Block Island, Newport County, R. 1. 40 35 Cape Ilenrv, Norfolk County, Va. . 85 40 Evansville. Ind.». ...... _ ... 45 30 Green Bay, Brown County, Wis. 05 35

Hatteras, Dare Countv, N. C. 60 45 Indianapolis, lnd.» . 40 30 Marquette, Marquette County, Mich. .. 40 30 Miami, Dade Countv, Fla. . . 80 50 North Head, Pacific Countv, Wash. . _ 50 30 Omaha, Nebr.» 50 30 Pueblo, Pueblo County, Colo. 35 30 Sheridan, Sheridan Countv, Wyo. . 35 30 Tatoosh Island, Clallam Countv, Wash. 40 30 Wichita, Kans.» . . .... ... . 35 30

\

}

In

» Counties are not mentioned because the area of the whole State is in- volved.

6.1. Variation With Altitude

Table 3 gives the variation of the coefficient of I

72 in eq (2) as p varies with temperature and alti- tude. Up to this point, this coefficient has been

considered constant at 0.00256, the value for dry

air at normal sea-level conditions. However, at i higher altitudes this assumption is not justified. \ For three entries in table 2 (Albuquerque, N. Mex. ; j Sheridan, Wyo.; and Pueblo, Colo.), a temperature of 59° F was assumed and correction for 1 barometric-pressure change with elevation was

made.

Albuquerque, for example, has an elevation of 5,000 ft. The minimum allowable resultant wind

pressure corrected for the air density at 5,000 ft would be as follows: ;

I Pl'30 0.00213

0.00256

X40=33 lb/ft 2 -

1

1

'

n 6.2. Tornado-Like Winds

For Wichita, Ivans., table 2 shows a reduction in minimum allowable wind pressure from 35 to 30 |

lb/ft 2. Although Wichita is 1 ,372 ft above sea

level, its altitude is not sufficient to account for the 1 difference of 5 lb/ft 2

. However, the unusually high

wind speeds recorded at Wichita and other parts of the Mississippi Valley basin can be attributed to an unusually high frequency of tornadoes and a high frequency of severe thunderstorms. Winds I following tornadoes are similar to tornadoes in that
Table 3. I nines lor '£XpX (5,280/8,b‘00)~ in eq (2) as p vanes with changes in temperature and deration above sea, level [Standard value is for 0 feet and 59° F.J

Elevation above sea level (feet) Air tem- I perature

0 1.000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000

0 F

-50 0. 00325 0. 00313 0. 00301 0. 00290 0. 00280 0. 00270 0. 00261 0. 00251 0. 00242 0. 00234 0. 00225 -40 . 00317 . 00305 . 00294 . 00283 . 00273 . 00264 . 00254 . 00245 . 00237 . 00228 . 00220 -30 . 00310 . 00298 .00287 .00277 . 00267 . 00257 . 00248 . 00240 . 00231 . 00223 . 00215 -20 .00303 .00292 . 00281 .00271 00261 . 00252 . 00243 , 00234 . 00226 .00218 . 00210 -10 .00296 . 00285 .00275 . 00265 . 00255 . 00246 . 00237 . 00229 . 00221 . 00213 . 00205

0 .00289 .00279 .00268 . 00259 .00249 . 00241 . 00232 . 00224 . 00216 . 00208 .00201 +10 .00283 . 00273 . 00263 . 00253 . 00244 . 00236 . 00227 . 00219 .00211 . 00204 . 00196

20 .00277 . 00267 . 00257 . 00248 . 00239 . 00231 . 00223 .00215 . 00207 . 00200 . 00192

30 .00272 . 00262 . 00252 . 00243 . 00234 . 00226 . 00218 . 00210 . 00203 . 00106 . 00188

40 .00266 . 00257 . 00247 .00238 . 00230 . 00221 . 00214 . 00206 . 00199 .00192 .00185

50 .00261 . 00252 . 00242 . 00233 . 00225 . 00217 . 00209 . 00202 . 00195 . 00188 .00181

60 . 00256 . 00247 .00237 . 00229 . 00221 . 00213 . 00205 . 00198 . 00191 . 00184 . 00178

70 . 00251 . 00242 .00233 . 00225 . 00216 00209 . 00202 . 00194 . 00187 . 00181 . 00174

80 . 00246 . 00238 . 00229 . 00220 . 00213 . 00205 . 00198 . 00191 . 00184 . 00177 . 00171

90 .00242 .00233 .00225 . 00216 . 00209 . 00201 . 00194 . 00187 .00181 . 00174 .00168

100 .00238 .00229 . 00221 . 00213 . 00205 . 00198 . 00191 . 00184 .00177 . 00171 . 00165

110 .00234 . 00225 .00217 . 00209 .00201 . 00194 . 00187 . 00181 .00174 . 00168 . 00162

they generally strike in short, narrow paths. Thus,

the probability that any particular building or structure will be hit is comparatively small, and

these winds may be partially discounted. This

I type of reduction is not unreasonable for Wichita,

j Kans. The same conditions apply to Evansville,

Ind.; Indianapolis, Ind.; and Omaha, Nebr.

6.3. Extreme Winds in Coastal Areas

Abnormal distortions and funneling effects at- tend the passage of cyclones near coastal areas. Also, when the wind passes from sea to land, even when the land is low and flat, surface friction

markedly increases. The consequent retardation

| of the lower air causes thickening of the flow sheet,

in effect simulating a small hill. This increases

wind speeds in the upper air. For offshore wind,

! the situation is reversed and wind speeds increase

; at the lower levels because of falling air. Down

currents may be found even at a height of ] ,500 ft. Conditions such as these may be the cause of

| higher winds not only along the ocean coasts but

also at Green Bay, Wis., and Marquette, Mich.

Exposed locations along the seaboard are occasionally hit by severe storms, squalls, or hurricanes

; with winds of exceptionally high speeds. Squalls

develop because of differences in air temperature

over land and sea. They are often accompanied

by gusts and are characterized by splashes of cold

air that fall from heights and disperse horizontally.

Squalls and hurricanes are the probable causes of the unusually high winds recorded at Weather

Bureau stations such as Block Island, R. I.; Cape

Henry, Ya.; Hatteras, N. C.; and Miami, Fla. Likewise, North Head, Wash., and Tatoosh Island,

Wash., are in exposed locations and subject to severe cyclonic storms and a long fetch of wind

over the ocean. These winds can hardly be considered typical of the adjacent areas; therefore, in each case the same wind pressures were used for these localities as for the surrounding area.

7. Wind Pressures for Certain Counties

On the wind-pressure map, the boundary lines between pressure zones are generally the State or county lines. However, the markings for some

counties are not clearly distinguishable because of the small scale of the map. For this reason, the

minimum allowable resultant wind pressures for

certain counties along the Atlantic seaboard, as well as for two counties on the Pacific, are given

below

:

Connecticut

New London

Delaware

Sussex - - Kent -

.

New Castle.

_

lb ft 2 30

35

30

On

Georgia

Chatham 35

Maryland

Worcester 40

Somerset and Wicomico 35

Caroline, Dorchester, and Saint Marys 30

Massachusetts

Barnstable, Dukes, and Nantucket

Bristol, Essex, parts of Norfolk and Middlesex facing Massachusetts Bay, Plymouth.

and Suffolk 35

New Jersey

Cape May 35

Atlantic, Cumberland, Monmouth, and

Ocean 30

New York

Cayuga (part north of 43d parallel L Chautauqua, Erie, Jefferson, Kings, Monroe.

Nassau, Niagara, Orleans, Oswego.

Queens, Richmond, Suffolk, and Wayne 30
North Carolina lb/ft2 Dare and Hyde 45

Beaufort, Camden, Carteret, Currituck,

Pamlico, Pasquotank, Tyrrell, and Washington 40

Pennsylvania

Crawford, Erie, and Warren 30

Phode Island

Bristol, Newport, and Washington 35

Kent and Providence 30

South Carolina

Beaufort, Charleston, Georgetown, and

Horry 35

Virginia

Accomack, Northampton, and Princess

Anne 40

Elizabeth City, Gloucester, Isle of Wight,

James City, Lancaster, Mathews, Middlesex, Nansemond, Northumberland, Surrey, Warwick, and York 35

Washington

Clallam and Pacific 30

8. Discussion

The maximum wind-speed value for each station was used in determining the pressures given in this paper and in the ASA standard A58.1-1955; however, during the study of wind problems by the

National Bureau of Standards and the U. S. Weather Bureau, investigations were made of several methods of treating the problem statisti- cally. The latter investigation was only of a preliminary nature because funds were not available for conducting the extensive analysis of the

Weather Bureau data that would be required for computing the statistical quantities.

It is generally realized that, in time, the highest

recorded wind speed for any station will be ex- ceeded and that the frequency of this occurrence

will diminish as the value of the highest wind speed

increases. By treating statistically the frequency

of occurrence of wind speeds and the frequency at which the maximum wind speed is exceeded, it would be possible to determine for a station a windspeed value corresponding to a selected probability

of occurrence. Some of the methods investigated

in this study were those developed by E. J. Gumbel

[S] and those of Fisher and Tippett [9], Gumbel

perfected a chart on which the frequency

curves for the occurrence of extreme values can be

shown as straight lines. The chart has been applied successfully to flood problems and to gust

problems in aeronautics for gust data accumulated

on flights above 1,500 ft. The LT. S. Weather Bureau, in cooperation with

the American Society of Civil Engineers, has been

working for years to fit recorded data into a pattern

to which mathematical statistics might be applied,

and has perfected a chart that also shows the fre- quency curve for extreme values as a straight line. The Weather Bureau chart, which is based on

Fisher and Tippett’s second method with additional consideration for likelihood of occurrence, ! is said to be applicable to conditions below 1,50C ft.

[ Study of wind speeds is complicated by the

difference in recordings made at stations in certain

cities and adjacent airports. On the average, the

recordings at the airports show higher speeds than 1 could be expected if the recorded values at the

nearby cities were reduced by the seventh-root

rule to the same height as the airport anemometers.

Apparently the wind speeds recorded by anemometers over these cities are influenced by friction and

obstacles. For example, a building may produce

a vertical component of air motion which serves to raise the effective friction layer and which at the same time may cause errors in the wind-speed 1 measurements.

Some authorities [10, 11] doubt the validity of the seventh-root rule for use with very strong

winds. However, this rule has considerable backing among other authorities and is believed suitable

for use until additional data indicate need for change.

Grateful acknowledgment for scientific and technical assistance is made to Louis P. Harrison of the

Observations and Station Facilities Division of the

Ik S. Weather Bureau.

9. References

I

[1] H. A. Giblett et al., The structure of wind over level country (H. M. Stationery Office, London, England,

1932).

[2] Great Britain, Meteorological Office, Geophysical

Memoirs No. 54, 6, 4.

[3] National fire code for the prevention of dust explosions

(National Fire Protection Association, Boston 10, Mass.).

[4] J. Hartman and J. Nagy, Effect of relief vents on

reduction of pressures developed by dust explosions

(U.S. Bureau of Mines, Washington 25, D.C.).

[5] Hylton R. Brown and Richard L. Hanson, Venting

j dual explosions, National Fire Protection Associ- | ation Quart. 26 , 328-41 (1933).

[6] Weather Bureau Forecasters Forum 4 , 9 (1952).

[7] W. T. Van Orman, Preliminary meteorological survey

for airship bases on the middle Atlantic seaboard,

pp. 7 and 11 (for the International Zepplin Transport Corn., 52 Wall St., New York, N.Y., Feb. 21 , ! 1931).

[8] E. J. Gumbel, Return period—reduced variate chart, School of Engineering, Manhattan College,

Riverdale, N. Y.

[9] R. A. Fisher and L. H. C. Tippett, Limiting forms of the frequency distribution of the largest or smallest member of a sample, Proc. Cambridge Phil. Soc. | 24, pt. 2, 180-190 (1928).

[10] W. Kaufman, Hydromechanik, pt. 2 (Julius Springer,

Berlin, 1934) ; O. Flaschbart, Die belastung von Bauwerken durch Winkkrafte, ch. 9. [11] E. L. Deacon, Gust variation with heights up to 150 m, Quart. J. Rov. Meteorol. Soc. 81 , 562-573

(1955). Washington, October 17, 1958

8 U. S. GOVERNMENT PRINTING OFFICE: 1959
BUILDING MATERIALS AND STRUCTURES REPORTS

[Continued from cover page n]

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[Continued from cover page m]

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0

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