1

Determining Wind Loads

(Part 1)

Determining Determining Wind LoadsWind Loads

(Part 1)(Part 1)

BackgroundBackground• Wind perpendicular to ridge:• Wind perpendicular to ridge:

WINDWIND

2

WINDWIND

TOP VIEW OF GABLE ROOFTOP VIEW OF GABLE ROOF

• Wind parallel to ridge:• Wind parallel to ridge:

3

Wind damage from tornadoWind damage from tornado

Wind Load ProceduresWind Load Procedures• Basic procedures for calculating wind loads

(pressures) are in ASCE 7

• Building codes have variations on the methods given in the building codes -particularly for buildings < 60 ft tall• we will use Standard Building Code

• in practice, be prepared to use International Building Code

• different parts of the US may use different codes

• Basic procedures for calculating wind loads (pressures) are in ASCE 7

• Building codes have variations on the methods given in the building codes -particularly for buildings < 60 ft tall•• we will use we will use Standard Building CodeStandard Building Code

•• in practice, be prepared to use in practice, be prepared to use International International Building CodeBuilding Code

•• different parts of the US may use different different parts of the US may use different codescodes

4

Wind Load TermsWind Load Terms• Main Wind Force Resisting System (MWFRS)

• an assemblage of major structural elements assigned to provide support for secondary members and cladding, it primarily receives wind loading from more than one surface

• Rigid and braced frames

• trusses

• roof and floor diaphragms

• shear walls

• Main Wind Force Resisting System (MWFRS)• an assemblage of major structural elements

assigned to provide support for secondary members and cladding, it primarily receives wind loading from more than one surface

• Rigid and braced frames

• trusses

• roof and floor diaphragms

• shear walls

Rigid FramesRigid Rigid FramesFrames

5

Main columns, beams, trusses, roof diaphragm

Main Main columns, columns, beams, beams, trusses, roof trusses, roof diaphragmdiaphragm

Columns, trussesColumns, Columns, trussestrusses

6

Shear wallsShear wallsShear walls

7

Wind Load TermsWind Load Terms• Components and Cladding

• structural elements that are either directly loaded by wind or receive wind loads originating at relatively close locations and that transfer these loads to the main wind force resisting system

• curtain walls, studs

• roof and wall sheathing• purlins, girts

• Components and Cladding• structural elements that are either directly

loaded by wind or receive wind loads originating at relatively close locations and that transfer these loads to the main wind force resisting system

• curtain walls, studs

• roof and wall sheathing• purlins, girts

Roof sheathingRoof Roof sheathingsheathing

8

PurlinsPurlinsPurlins

GirtsGirtsGirts

9

10

Wall sheathingWall Wall sheathingsheathing

• Design Wind Pressure calculated by (1999 Standard Building Code):

• Design Wind Pressure calculated by (1999 Standard Building Code):

UGCqp p= UGCqp p=• p = design wind pressure, psf

• q = basic velocity pressure

• GCp = combined gust factor and pressure coefficient

• U = use factor (like importance factor)

• p = design wind pressure, psf

• q = basic velocity pressure

• GCp = combined gust factor and pressure coefficient

• U = use factor (like importance factor)

11

• Basic Velocity Pressure• Basic Velocity Pressure

• q, basic velocity pressure based on:

• wind speed - Figure 1606

• mean roof height

• q, basic velocity pressure based on:

• wind speed - Figure 1606

• mean roof height

12

• Basic Velocity Pressure• Basic Velocity Pressure

• q can be determined from Table 1606.2A or by:

• V = fastest mile wind speed from Figure 1606 (mph)

• H = mean roof height (ft)

•• q can be determined from Table q can be determined from Table 1606.2A or by:1606.2A or by:

•• V = fastest mile wind speed from Figure V = fastest mile wind speed from Figure 1606 (mph)1606 (mph)

•• H = mean roof height (ft)H = mean roof height (ft)

72

2

33HV00256.0q

=

72

2

33HV00256.0q

=

13

Wind pressure increases with

increasing elevation above the

ground

Wind pressure increases with

increasing elevation above the

ground

WINDWIND

Mean Roof HeightMean Roof Height

• H = Mean roof height

• H > 15 ft

• for roofs with slope less than 10o, H can be the eave height

•• H = Mean roof heightH = Mean roof height

•• H H >> 15 ft15 ft

•• for roofs with slope less than 10for roofs with slope less than 10oo, H can , H can be the eave heightbe the eave height

14

• Example q• Example q

• Given:

• commercial building in Mobile, AL

• gable roof, 4:12 roof slope (18o angle)

• eave height of 20 ft, half width = 40 ft

• Find:

• q

• Given:

• commercial building in Mobile, AL

• gable roof, 4:12 roof slope (18o angle)

• eave height of 20 ft, half width = 40 ft

• Find:

• q20 ft20 ft

40 ft40 ft

15

• Example q• Example q

• Solution:• Solution:

26.6 ft26.6 ft

( ) ft6.26ft20ft40124

21H =+

= ( ) ft6.26ft20ft40124

21H =+

=

• For Mobile, use wind speed = 100 mph

•• For Mobile, For Mobile, use wind use wind speed = speed = 100 mph100 mph

16

• q can be determined from Table 1606.2A or by:

•• q can be determined from Table q can be determined from Table 1606.2A or by:1606.2A or by:

( ) 72

2

332710000256.0q

= ( ) 7

2

2

332710000256.0q

=

psf1.24q = psf1.24q = ANSWERANSWERANSWER

• Gust Factor - Pressure Coefficient• Gust Factor - Pressure Coefficient

• GCp is a combined term that SBC uses• the gust factor (G) accounts for

increases in pressure due to higher speed wind gusts

• the pressure coefficient (Cp) indicates the magnitude and direction of the actual wind pressure on a given surface of the building

• GCp is a combined term that SBC uses• the gust factor (G) accounts for

increases in pressure due to higher speed wind gusts

• the pressure coefficient (Cp) indicates the magnitude and direction of the actual wind pressure on a given surface of the building

17

• Gust Factor - Pressure Coefficient• Gust Factor - Pressure Coefficient

• GCp terms are different for:• each surface of the building

• MWFRS or C&C

• i.e. to design different components, you will need to use different GCpvalues

• GCp terms are different for:• each surface of the building

• MWFRS or C&C

• i.e. to design different components, you will need to use different GCpvalues

• GCp for MWFRS• GCp for MWFRS

• for MWFRS - use:

• Tables 1606.2.B and 1606.2.C

• Figure 1606.2B1 and 1606.2B2 and its notes

• Note how the building is broken down into different zones

• Note interior and exterior zones

• for MWFRS - use:

• Tables 1606.2.B and 1606.2.C

• Figure 1606.2B1 and 1606.2B2 and its notes

• Note how the building is broken down into different zones

• Note interior and exterior zones

18

• Note different wind directions

• Note different load cases

• Note different wind directions

• Note different load cases

19

• Note the different types of buildings:

• Enclosed

• Partially Enclosed

• Completely Open

• Note the different roof angles

• See NOTES on page 217• internal pressure - for buildings where door

or window may have been punctured by missile

• internal suction - for completely enclosed buildings (all doors and windows closed)

• Note the different types of buildings:

• Enclosed

• Partially Enclosed

• Completely Open

• Note the different roof angles

• See NOTES on page 217• internal pressure - for buildings where door

or window may have been punctured by missile

• internal suction - for completely enclosed buildings (all doors and windows closed)

20

• These notes explain different coefficients in Table 1606.2B and 2C

• These notes explain different coefficients in Table 1606.2B and 2C

• Example GCp for MWFRS• Example GCp for MWFRS• Given:

• commercial building in Mobile• gable roof, 4:12 roof slope (18o angle)• totally enclosed building• internal suction

• Find:• MWFRS GCp for winds perpendicular to

ridge• windward wall, windward roof,• leeward wall, leeward roof

• Given:• commercial building in Mobile• gable roof, 4:12 roof slope (18o angle)• totally enclosed building• internal suction

• Find:• MWFRS GCp for winds perpendicular to

ridge• windward wall, windward roof,• leeward wall, leeward roof

21

• From Figure 1606.2B1• From Figure 1606.2B1

• GCp values• GCp values

22

• GCp values for end zones• GCp values for end zones

23

+1.10+1.10+1.10

-1.00--1.001.00

-0.60--0.600.60-0.55--0.550.55

• GCp for end zones of building• GCp for end zones of building

• GCp values for interior zones• GCp values for interior zones

24

+0.80+0.80+0.80

-0.60--0.600.60

-0.35--0.350.35-0.30--0.300.30

• GCp for interior zones of building• GCp for interior zones of building

• Use factors• Use factors

25

• Design Wind Pressure :• Design Wind Pressure :

UGCqp p= UGCqp p=• p = design wind pressure, psf

• q = basic velocity pressure

• GCp = combined gust factor and pressure coefficient

• U = use factor

• p = design wind pressure, psf

• q = basic velocity pressure

• GCp = combined gust factor and pressure coefficient

• U = use factor

• Example• Example• Given:

• commercial building in Mobile, AL• gable roof, 4:12 roof slope (18o angle)• eave height of 20 ft, half width = 40 ft

• Find:• design wind pressures for MWFRS:

• wind perpendicular to ridge• resistance in transverse direction

• Given:• commercial building in Mobile, AL• gable roof, 4:12 roof slope (18o angle)• eave height of 20 ft, half width = 40 ft

• Find:• design wind pressures for MWFRS:

• wind perpendicular to ridge• resistance in transverse direction

26

• Solution:

• q = 25.6 psf

• U = 1.0 for commercial building with low human occupancy

• GCp is from previous examples

• Solution:

• q = 25.6 psf

• U = 1.0 for commercial building with low human occupancy

• GCp is from previous examples

UGCqp p= UGCqp p=

( )( )0.1psf1.24GCp p= ( )( )0.1psf1.24GCp p=

+0.80+0.80+0.80

-0.60--0.600.60

-0.35--0.350.35-0.30--0.300.30

• GCp for interior zones of building• GCp for interior zones of building

27

+1.10+1.10+1.10

-1.00--1.001.00

-0.60--0.600.60-0.55--0.550.55

• GCp for end zones of building• GCp for end zones of building

+19.3 psf+19.3 +19.3 psfpsf

-14.5 psf--14.5 14.5 psfpsf

-8.4 psf--8.4 8.4 psfpsf

-7.2 psf--7.2 7.2 psfpsf

• Final design pressure, p, for interior zones of building

• Final design pressure, p, for interior zones of building

28

+26.5 psf+26.5 +26.5 psfpsf

-24.1 psf--24.1 24.1 psfpsf

-14.5 psf--14.5 14.5 psfpsf

-13.3 psf--13.3 13.3 psfpsf

• Final design pressure, p, for end zones of building

• Final design pressure, p, for end zones of building

• Important note• Important note

• Calculated wind pressures are normal to the roof or wall surface

• (not like snow pressures)

• Calculated wind pressures are normal to the roof or wall surface

• (not like snow pressures)

• DON’T FORGET• DON’T FORGET

• Check wind pressures from different directions and for different load cases

• internal suction vs. internal pressure

• longitudinal vs. transverse wind

• Check wind pressures from different directions and for different load cases

• internal suction vs. internal pressure

• longitudinal vs. transverse wind

29

• NEXT STEP• NEXT STEP

• Calculate wind pressures for Components and Cladding

• Calculate wind pressures for Components and Cladding