como calcular carga do vento (how to calculate wind load)

8/19/2019 Como calcular carga do vento (How to Calculate Wind Load)

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How to Calculate Wind LoadFour Methods: Wind Load Calculator Calculating Wind Load Using the Generic Formula

Calculating Wind Load Using the Electronic Industries Association Formula Calculating Wind Load Using the Uniform Building Code UBC! "#$ Formula

Wind is a mass of air that moves in a mostly horizontal direction from an area of high pressure to an area with low pressure. [1]

High winds can be very destructive because they generate pressure against the surface of a structure. The intensity of this

pressure is the wind load. The effect of the wind is dependent upon the size and shape of the structure. Calculating wind load

is necessary for the design and construction of safer more wind!resistant buildings and placement of ob"ects such as

antennas on top of buildings.

% &efine the generic formula' The generic formula for wind load is F = A x P x Cd where F is the force or wind

load A is the pro"ected area of the ob"ect P is the wind pressure and Cd is the drag coefficient.[#]

This e$uation is useful to calculate the wind load on a specific ob"ect with %nown area.[&]

( Understand the )aria*les of the e+uation' 'n order to use an e$uation properly you must first understand what

each variable stands for and what its associated units are.

A is the pro"ected area of the ob"ect. 't is the two!dimensional area of the three!dimensional ob"ect that the wind

is hitting.[(] 'ts units are feet s$uared )ft#*.

P is the wind pressure in pounds per s$uare foot or psf. [+] 't can be calculated using wind speed and a

conversion factor.

Cd is the drag coefficient for an ob"ect and is determined based on the size and shape of the ob"ect sub"ected

to the wind.[,] 't does not have units.

, Calculate -ro.ected area' The pro"ected area of your ob"ect is dependent upon its shape and size. 'f the wind is

hitting a flat wall the pro"ected area is easier to calculate than if the ob"ect is not uniform. -ro"ected area will be

an approimation of the area that the wind comes in contact with. There is no one formula for calculating pro"ected

area but you can estimate it with some basic calculations. /nits for area are ft#.

0or a flat wall use the formula rea 2 length width measuring the length and width of the wall where the wind

is hitting it.

0or a tube or column you can also approimate the area using length and width. 'n this case the width will be

the diameter of the tube or column.

/ Calculate wind -ressure' Wind pressure is given by the e$uation P = 0.00256 x V 2 where V is the speed of the

wind in miles per hour )mph*. The unit for wind pressure is pounds per s$uare foot )psf*.0or eample if the wind speed is 34 mph the wind pressure is 4.44#+, 34# 2 1#.+ psf.

n alternat ive to calculating wind pressure at a particular wind speed is to use the standard for various wind

zones. 0or eample according to the 5lectronic 'ndustries ssociation )5'* most of the /.6. is in 7one with

8,., mph wind but coastal areas might lie in 7one 9 )144 mph* or 7one C )111.8 mph*.[3]

0 &etermine the drag coefficient for the o*.ect in +uestion' :rag is the net force in the direction of flow due to

pressure on the surface of an ob"ect.[8] The drag coefficient represents the drag of an ob"ect through a fluid and is

dependent upon the shape size and roughness of an ob"ect.

The standard drag coefficient for a long cylinder tube is 1.# and for a short cylinder is .8 These apply to

antenna tubes found on many buildings.

The standard coefficient for a flat plate such as the face of a building is #.4 for a long flat plate or 1.( for ashorter flat plate.

The difference between drag coefficients for flat and cylinder items is approimately 4.,. [;]

The drag coefficient has no units.

Method

%Calculating Wind Load Using the Generic Formula

w to Calculate Wind Load (with Downloadable Load Calculator) http://www.wikihow.com/Calculate-Wind-Load

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1 Calculate the wind load' /sing the values determined above you can now calculate wind load with the e$uation

F = A x P x Cd .

0or eample let<s say you want to determine the wind load on an antenna that is & feet long with a diameter of

4.+ inches in a gust of 34mph winds.

6tart by calculating the pro"ected area. 'n this case A = l x w 2 & ft )4.+in )1 ft=1# in** 2 4.1#+ ft#.

Calculate the wind pressure> P = 0.00256 x V 2 2 4.44#+, 34# 2 1#.+ psf.

0or a short cylinder the coefficient of drag is 4.8.

-lugging into the e$uation> F = A x P x Cd 2 4.1#+ 1#.+ 4.8 2 1.#+ lbs.

1.#+ lbs is the amount of wind load on the antenna.

% &efine the formula de)elo-ed *2 the Electronic Industries Association' The formula for wind load is F = A x

P x Cd x Kz x Gh where A is the pro"ected area P is wind pressure Cd is the drag coefficient Kz is the eposure

coefficient and Gh is the gust response factor. This formula ta%es a few more parameters into account for wind load.

This formula is generally used to calculate wind load on antennas.

( Understand the )aria*les of the e+uation' 'n order to use an e$uation properly you must first understand what

each variable stands for and what its associated units are.

A P and Cd are the same variables used in the generic e$uation.Kz is the eposure coefficient and it is calculated by ta%ing into account the height from the ground to the

midpoint of the ob"ect. The units of Kz are feet.

Gh is the gust response factor and it is calculated by ta%ing into account the entire height of the ob"ect. The

units of Gh are 1=feet or ft!1.

, &etermine -ro.ected area' The pro"ected area of your ob"ect is dependent upon its shape and size. 'f the wind is

hitting a flat wall the pro"ected area is easier to calculate than if the ob"ect is rounded. -ro"ected area will be an

approimation of the area that the wind comes in contact with. There is no one formula for calculating pro"ected area

but you can estimate it with some basic calculations. /nits for area are ft#.


is hitting it.0or a tube or column you can also approimate the area using length and width. 'n this case the width will be


/ Calculate wind -ressure' Wind pressure is given by the e$uation P = 0.00256 x V 2 where V is the speed of the

wind in miles per hour )mph*. The unit for wind pressure is pounds per s$uare foot )psf*.

0or eample if the wind speed is 34 mph the wind pressure is 4.44#+, 34# 2 1#.+ psf.

n alternat ive to calculating wind pressure at a particular wind speed is to use the standard for various wind

zones. 0or eample according to the 5lectronic 'ndustries ssociation )5'* most of the /.6. is in 7one with

8,., mph wind but coastal areas might lie in 7one 9 )144 mph* or 7one C )111.8 mph*.[14]

0 &etermine the drag coefficient for the o*.ect in +uestion' :rag is the net force in the direction of flow due to

pressure on the surface of an ob"ect.[11] The drag coefficient represents the drag of an ob"ect through a fluid and

is dependent upon the shape size and roughness of an ob"ect.



The standard coefficient for a flat plate such as the face of a building is #.4 for a long flat plate or 1.( for a

shorter flat plate.

The difference between drag coefficients for flat and cylinder items is approimately 4.,. [1#]


1Calculate the e3-osure coefficient4 Kz ' Kz is calculated using the formula [z/33] (2/7) where z is the height from

the ground to the midpoint of the ob"ect.

0or eample if you have an antenna that is & ft long and (8 ft off the ground z would be e$ual to (,.+ ft.

Kz = [z/33] (2/7) 2 [(,.+=&&])#=3* 2 1.1 ft.

Method

(Calculating Wind Load Using the Electronic Industries Association Formula


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$ Calculate the gust res-onse factor4 Gh' ?ust response factor is calculated with the e$uation Gh = .65+.60/[(h

/33)(1/7) ] where h is the height of the ob"ect.

0or eample if you have an antenna that is & ft long and (8 ft off the ground Gh = .65+.60/[(h/33)(1/7) ] 2

.,+@.,4=)+1=&&*)1=3* 2 1.## ft!1

5 Calculate the wind load' /sing the values determined above you can now calculate wind load with the e$uation

F = A x P x Cd x Kz x Gh. -lug in all of your variables and do the math.


4.+ inches in a gust of 34mph winds. 't is placed on top of a (8 ft high building.


Calculate the wind pressure> P = 0.00256 x V 2 2 4.44#+, 34# 2 1#.+ psf.

0or a short cylinder the coefficient of drag is 4.8.

Calculate the eposure coefficient> Kz = [z/33] (2/7) 2 [(,.+=&&])#=3* 2 1.1 ft.

Calculate the gust response factor> Gh = .65+.60/[(h/33)(1/7) ] 2 .,+@.,4=)&=&&*)1=3* 2 1.## ft!1

-lugging into the e$uation> F = A x P x Cd x Kz x Gh 2 4.1#+ 1#.+ 4.8 1.1 1.## 2 1.,8 lbs.

1.,8 lbs is the amount of wind load on the antenna.

% &efine the UBC "#$ formula' This formula was developed in 1;;3 as part of the /niform 9uilding Code )/9C* for

calculation of wind load. The formula is F = A x P were A is the pro"ected area and P is the wind pressureA

however this formula has an alternate calculation for wind pressure. [1&]

Wind pressure )-sf* is calculated as P= Ce x Cq x ! x "w where Ce is the combined height eposure and

gust response factor Cq is a pressure coefficient )it is e$uivalent to the drag coefficient in the previous two

e$uations* ! is wind stagnation pressure and "w is importance factor. ll of these values can be calculated or

obtained from the appropriate tables.[1(]

( &etermine -ro.ected area' The pro"ected area of your ob"ect is dependent upon its shape and size. 'f the wind is

hitting a flat wall the pro"ected area is easier to calculate than if the ob"ect is rounded. -ro"ected area will be an

approimation of the area that the wind comes in contact with. There is no one formula for calculating pro"ected areabut you can estimate it with some basic calculations. /nits for area are ft#.


is hitting it.

0or a tube or column you can also approimate the area using length and width. 'n this case the width will be


, &etermine Ce4 the com*ined height4 e3-osure4 and gust res-onse factor' This value is chosen based on

table 1,!? of /9C and ta%es into account three terrain eposures with various heights and Ce values for each.[1+]

B5posure 9 is terrain with buildings trees or other surface irregularities covering at least #4 percent of the

surrounding area and etending 1., %ilometers or more from the site.[1,]

B5posure C has terrain that is flat and generally open etending 4.8 %m or more from the site.[13]

B5posure : is the most severe with basic wind speeds of 1#; %m=hr or greater and terrain that is flat and

unobstructed facing large bodies of water. [18]

/ &etermine the -ressure coefficient for the o*.ect in +uestion' The pressure coefficient Cq is the same as the

drag coefficient )Cd *. :rag is the net force in the direction of flow due to pressure on the surface of an ob"ect. [1;]

The drag coefficient represents the drag of an ob"ect through a fluid and is dependent upon the shape size and

roughness of an ob"ect.



The standard coefficient for a flat plate such as the face of a building is #.4 for a long flat plate or 1.( for a

shorter flat plate.The difference between drag coefficients for flat and cylinder items is approimately 4.,. [#4]


&etermine the wind stagnation -ressure' ! is the wind stagnation pressure and is e$uivalent to the wind pressure

Method

,Calculating Wind Load Using the Uniform Building Code UBC! "#$ Formula


5 1/!"/#!1$ !%:55



0 calculation from the previous e$uations> ! = 0.00256 x V 2 where V is the speed of the wind in miles per hour

)mph*.

0or eample if the wind speed is 34 mph the wind stagnation pressure is 4.44#+, 34 # 2 1#.+ psf.

n alternat ive to this calculation is to use the standards set for various wind zones. 0or eample according to

the 5lectronic 'ndustries ssociation )5'* most of the /.6. is in 7one with 8,., mph wind but coastal areas

might lie in 7one 9 )144 mph* or 7one C )111.8 mph*. [#1]

1&etermine im-ortance factor' "w is the importance factor and can be determined using table 1,!D of the /9C. 't

is a multiplier used in calculating loads that ta%es into account the use of the building. 'f a building contains

hazardous materials its importance factor will be higher than that of a traditional building.

Calculations for buildings with standard use have an importance factor of one.

$ Calculate the wind load' /sing the values determined above you can now calculate wind load with the e$uation

F = A x P = A x Ce x Cq x ! x "w . -lug in all of your variables and do the math.


4.+ inches in a gust of 34mph winds. 't is placed on top of a (8 ft high standard building in an area with an

eposure 9 terrain.


:etermine Ce. 9ased on table 1,!? using the height of (8 ft and eposure 9 terrain Ce is 4.8(.

0or a short cylinder the coefficient of drag or Cq is 4.8.

Calculate !> ! = 0.00256 x V 2 2 4.44#+, 34# 2 1#.+ psf.

:etermine importance factor. This is a standard building therefore "w is 1.

-lugging into the e$uation> F = A x P = A x Ce x Cq x ! x "w 2 4.1#+ 4.8( 4.8 1#.+ 12 1.4+ lbs.

1.4+ lbs is the amount of wind load on the antenna.

Dnow that wind velocity varies at various distances from the ground. Wind speed increases with structural height and is

most unpredictable closer to the ground because it is affected by interacting with things on the ground.

9e aware that this unpredictability can ma%e it difficult to ma%e accurate wind calculations.

6i-s


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http>==pec.org.p%=sCourseEfiles=C5C+!#.pdf 1+.

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