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8/26/2016 Wind?Example?#1
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A?Beginner's?Guide?to?ASCE?7?05
Chapter?7???W:?Wind?Loads
??2007,?T.?Bartlett?Quimby
Overview
Nature?of?Wind
Method?2
Provisions
?
Example
Problems
Homework
Problems
References
Report?Errors?or
Make
Suggestions
?
Section?7.4.1
Example?Problem?7.1
Two?Story?Building
Method?2
Last?Revised:?11/04/2014
?
Given:??The??enclosed?office?building?shown?in?Figure?7.4.1.1.??The?building?is?located?in?a?region?with?a?wind?speed?(3?sec?gust)
of?120?mph.??The?exposure?is?Exposure?C.??The?building?is?on?flat?terrain.
Figure?7.4.1.1
Building?Definition
?
Wanted:??The?wind?pressures?applied?to?the?surfaces?and?the?net?forces?applied?to?the?building.
Solution:??To?solve?this?problem,?we?need?to?independently?look?at?two?different?wind?directions,?but?first?we?will?define?a?few
parameters.
Determine?critical?elevations:
Mean?Roof?Height:??h?=?2*11'?+?(3/12)*25'/2?=?25.1?ft
Mean?2nd?Floor?Height:??h?=?11'?+?11'/2?=?16.5?ft
Mean?1st?Floor?Height:??h?=?11'/2?=?5.5?ft
Compute?the?Velocity?Pressures,?qz?=?.00256?Kz?Kzt?Kd?V2?I??(ASCE?7?05?Equation?6?15)
Kzt?=?1?(Flat?Terrain)
Kd?=?.85?(ASCE?7?05?Table?6?4,?Buildings)
I?=?1.0?(ASCE?7?05?Table?6?1,?Category?II?building)
Kz?=?varies?with?elevation?=?2.01(max(h,15)/900)(2/9.5),?(ASCE?7?05?Tables?6?2?and?6?3)
? h Kz qz
? (ft) ? (psf)
Roof 25.125 0.946 29.7
2nd?flr 16.5 0.866 27.1
1st?flr 5.5 0.849 26.6
Compute?the?Internal?Pressures,?qhGCpi:
GCpi?=?+?0.18??(ASCE?7?05?Figure?6?5)
qh?=?29.7?psf
qhGCpi?=?+?5.34?psf
Determine?the?Gust?Factor,?G

G?=?0.85??(ASCE?7?05,?6.5.8.1)
Wind?in?the?N/S?Direction:
For?this?part?of?the?problem?we?need?to?determine?pressure?coefficients?for?the?locations?shown?in?Figure?7.4.1.2?as?well?as?for?the
side?walls.??These?coefficients?are?then?combined?with?the?gust?factor?and?velocity?pressures?to?obtain?the?external?pressures?in
each?region.
Figure?7.4.1.2
N/S?Building?Section
The?pressure?coefficients?for?the?walls?are?found?in?ASCE?7?05?Figure?6?6?(pg?49)
For?the?Windward?wall?(P1?&?P2),?Cp?is?0.8?for?all?elevations.
For?the?Leeward?wall?(P5?&?P6),?Cp?is?dependent?on?the?ration?of?L/B.??In?this?case?L/B?=?50'/90'?=?0.556,?so?Cp?=??0.50?for?all
elevations.
For?the?sidewalls?(not?shown?in?Figure?7.4.1.2),?the?value?of?Cp?is??0.7?in?all?cases.
For?the?roof,?the?slope?angle?is?14.0?degrees.??This?is?close?to?15?degrees?and?probably?not?worth?interpolating?between?the?values
given?in?ASCE?7?05?Figure?6?6.??We?also?need?to?know?that?h/L?=?25.1'/50'?=?0.50.??From?the?Figure?we?get?that?the?values?of?Cp
for?the?Windward?side?of?the?roof?is??0.70?and??0.18.??These?values?represent?two?different?load?cases.??For?the?Leeward?side,?Cp?is
?0.50.?
We?can?now?compute?the?external?pressures,?qGCp,?for?each?surface.??The?following?table?shows?the?computation?results:
Pressure Cp q qGCp
? ? ? (psf) (psf)
Windward?Wall P1 0.8 26.6 18.1
P2 0.8 27.1 18.5
Windward?Roof P3 ?0.7 29.7 ?17.6
P3 ?0.18 29.7 ?4.5
Leeward?Roof P4 ?0.5 29.7 ?12.6
Leeward?Wall P5 ?0.5 29.7 ?12.6
? P6 ?0.5 29.7 ?12.6
Side?Walls P7 ?0.7 29.7 ?17.6
Combining?with?the?internal?pressures?you?get?the?following?four?load?cases?where:
Case?I?includes?the?maximum?windward?pressure?(?17.6?psf)?and?positive?internal?pressure
Case?II?includes?the?minimum?windward?pressure?(?4.5?psf)?and?positive?internal?pressure
Case?III?includes?the?maximum?windward?pressure?(?17.6?psf)?and?negative?internal?pressure
Case?IV?includes?the?minimum?windward?pressure?(?4.5?psf)?and?negative?internal?pressure
The?net?forces?are?found?by?multiplying?the?appropriate?pressures?by?the?areas?over?which?they?act.??In?this?building?all?but?the
gable?ends?are?rectangles,?making?the?area?calculation?easier.??Note?that?we?are?computing?actual?surface?areas?(as?opposed?to
projected?areas)?in?each?of?the?cases?below.??Also,?the?sign?is?important.??Negative?signs?indicate?a?force?that?is?outward?from?the
surface?and?a?positive?sign?is?inward.??All?forces?are?normal?to?their?respective?surfaces.
? Net?Pressures: ? ? ? Net?Force ? ? ?
Pressure Case?I Case?II Case?III Case??IV Areas Case?I Case?II Case?III Case??IV
? ? (psf) (psf) (psf) (psf) (ft2) (k) (k) (k) (k)
Windward?Wall P1 12.8 12.8 23.4 23.4 990 12.62 12.62 23.19 23.19
P2 13.1 13.1 23.8 23.8 990 12.99 12.99 23.55 23.55
Windward?Roof P3 ?23.0 ?9.9 ?12.3 0.8 2319 ?53.29 ?22.90 ?28.54 1.86
Leeward?Roof P4 ?17.9 ?17.9 ?7.3 ?7.3 2319 ?41.60 ?41.60 ?16.85 ?16.85
Leeward?Wall P5 ?17.9 ?17.9 ?7.3 ?7.3 990 ?17.76 ?17.76 ?7.19 ?7.19
? P6 ?17.9 ?17.9 ?7.3 ?7.3 990 ?17.76 ?17.76 ?7.19 ?7.19
Side?Walls P7 ?23.0 ?23.0 ?12.3 ?12.3 1413 ?32.46 ?32.46 ?17.38 ?17.38
It?is?often?useful?to?resolve?each?force?into?it's?global?components?so?that?they?can?be?easily?added?vectorially.??Figure?7.4.1.3
shows?the?location?of?each?of?the?resulting?forces.
Figure?7.4.1.3
Building?Forces?for?N/S?Wind

? Case?I ? ? Case?II ? ? Case?III ? ? Case??IV ? ?
Force E/W N/S vert. E/W N/S vert. E/W N/S vert. E/W N/S vert.
? ? (k) (k) (k) (k) (k) (k) (k) (k) (k) (k) (k) (k)
Windward?Wall F1 0.00 12.62 0.00 0.00 12.62 0.00 0.00 23.19 0.00 0.00 23.19 0.00
F2 0.00 12.99 0.00 0.00 12.99 0.00 0.00 23.55 0.00 0.00 23.55 0.00
Windward?Roof F3 0.00 ?12.93 51.70 0.00 ?5.55 22.22 0.00 ?6.92 27.69 0.00 0.45 ?1.80
Leeward?Roof F4 0.00 10.09 40.36 0.00 10.09 40.36 0.00 4.09 16.34 0.00 4.09 16.34
Leeward?Wall F5 0.00 17.76 0.00 0.00 17.76 0.00 0.00 7.19 0.00 0.00 7.19 0.00
F6 0.00 17.76 0.00 0.00 17.76 0.00 0.00 7.19 0.00 0.00 7.19 0.00
Side?Walls F7a ?32.46 0.00 0.00 ?32.46 0.00 0.00 ?17.38 0.00 0.00 ?17.38 0.00 0.00
? F7b 32.46 0.00 0.00 32.46 0.00 0.00 17.38 0.00 0.00 17.38 0.00 0.00
Sum ? 0.00 58.29 92.06 0.00 65.66 62.58 0.00 58.29 44.03 0.00 65.66 14.54
Note?that?the?maximum?uplift?and?maximum?horizontal?force?do?not?occur?in?the?same?load?cases!??Do?not?combined?the?two
cases,?design?for?each?individually.?
You?will?also?notice?that?the?internal?pressure?has?no?effect?on?the?net?horizontal?force.
The?net?force?in?the?lateral?direction?is?zero?since?the?forces?on?the?side?walls?will?cancel?each?other.
Wind?in?the?E/W?Direction
Figure?7.4.1.4?defines?the?pressures?(with?the?exception?of?the?lateral/side?wall?pressures)?that?need?to?be?computed?for?wind
loading?from?the?E/W?direction.??In?this?case?we?combined?all?the?leeward?wall?segments?into?one?because?they?all?have?the?same
pressures.
Figure?7.4.1.4
E/W?Building?Section
The?pressure?coefficients?are?taken?from?ASCE?7?05?Figure?6?6.??Note?that?the?coefficient?for?the?leeward?wall?is?obtained?by
interpolation?with?an?L/B?ratio?of?1.8.
? Pressure Cp q qGCp
? ? ? (psf) (psf)
Windward?Wall P8 0.8 26.6 18.1
P9 0.8 27.1 18.5
P10 0.8 29.7 20.2
Roof? P11 ?0.9 29.7 ?22.7
P11 ?0.18 29.7 ?4.5
P12 ?0.5 29.7 ?12.6
P12 ?0.18 29.7 ?4.5
P13 ?0.3 29.7 ?7.6
? P13 ?0.18 29.7 ?4.5
Leeward?Wall P14 ?0.34 29.7 ?8.6
Side?Walls P15 ?0.7 29.7 ?17.6

Note?that?some?of?the?pressures?are?applied?to?differently?oriented?surfaces.??When?the?same?pressure?is?applied?to?a?different
surface,?we?have?chosen?to?label?on?as?"a"?and?the?other?as?"b".??See?Figure?7.4.1.5?for?force?applications.?Four?cases?are
computed,?based?on?combinations?of?maximum/minimum?roof?pressures?and?+?internal?pressures.
Figure?7.4.1.5
Building?Forces?for?E/W?Wind
The?net?forces?on?each?surface,?in?terms?of?direction?relative?the?surface,?are?as?follows:
? Pressure Case?I Case?II Case?III Case??IV Area Case?I Case?II Case?III Case??IV
? ? (psf) (psf) (psf) (psf) (ft2) (k) (k) (k) (k)
Windward?Wall P8 12.8 12.8 23.4 23.4 550 7.01 7.01 12.88 12.88
P9 13.1 13.1 23.8 23.8 550 7.21 7.21 13.09 13.09
P10 14.8 14.8 25.5 25.5 156 2.32 2.32 3.98 3.98
Roof? P11a ?28.0 ?9.9 ?17.3 0.8 647 ?18.12 ?6.39 ?11.22 0.52
P11b ?28.0 ?9.9 ?17.3 0.8 647 ?18.12 ?6.39 ?11.22 0.52
P12a ?17.9 ?9.9 ?7.3 0.8 647 ?11.60 ?6.39 ?4.70 0.52
P12b ?17.9 ?9.9 ?7.3 0.8 647 ?11.60 ?6.39 ?4.70 0.52
P13a ?12.9 ?9.9 ?2.2 0.8 1026 ?13.23 ?10.13 ?2.28 0.82
? P13b ?12.9 ?9.9 ?2.2 0.8 1026 ?13.23 ?10.13 ?2.28 0.82
Leeward?Wall P14 ?13.9 ?13.9 ?3.2 ?3.2 1256 ?17.47 ?17.47 ?4.06 ?4.06
Side?Walls P15a ?23.0 ?23.0 ?12.3 ?12.3 1980 ?45.50 ?45.50 ?24.36 ?24.36
? P15b ?23.0 ?23.0 ?12.3 ?12.3 1980 ?45.50 ?45.50 ?24.36 ?24.36
Restating?the?forces?in?terms?of?the?global?coordinate?system?we?get:
? Case?I ? ? Case?II ? ? Case?III ? ? Case??IV ? ?
Pressure E/W N/S vert. E/W N/S vert. E/W N/S vert. E/W N/S vert.
? ? (k) (k) (k) (k) (k) (k) (k) (k) (k) (k) (k) (k)
Windward?Wall F8 7.01 0.00 0.00 7.01 0.00 0.00 12.88 0.00 0.00 12.88 0.00 0.00
F9 7.21 0.00 0.00 7.21 0.00 0.00 13.09 0.00 0.00 13.09 0.00 0.00
F10 2.32 0.00 0.00 2.32 0.00 0.00 3.98 0.00 0.00 3.98 0.00 0.00
Roof? F11a 0.00 4.40 17.58 0.00 1.55 6.20 0.00 2.72 10.88 0.00 ?0.13 ?0.50
F11b 0.00 ?4.40 17.58 0.00 ?1.55 6.20 0.00 ?2.72 10.88 0.00 0.13 ?0.50
F12a 0.00 2.81 11.26 0.00 1.55 6.20 0.00 1.14 4.56 0.00 ?0.13 ?0.50
F12b 0.00 ?2.81 11.26 0.00 ?1.55 6.20 0.00 ?1.14 4.56 0.00 0.13 ?0.50
F13a 0.00 3.21 12.83 0.00 2.46 9.82 0.00 0.55 2.21 0.00 ?0.20 ?0.80
? F13b 0.00 ?3.21 12.83 0.00 ?2.46 9.82 0.00 ?0.55 2.21 0.00 0.20 ?0.80
Leeward?Wall F14 17.47 0.00 0.00 17.47 0.00 0.00 4.06 0.00 0.00 4.06 0.00 0.00
Side?Walls F15a 0.00 45.50 0.00 0.00 45.50 0.00 0.00 24.36 0.00 0.00 24.36 0.00
? F15b 0.00 ?45.50 0.00 0.00 ?45.50 0.00 0.00 ?24.36 0.00 0.00 ?24.36 0.00
Sum ? 34.01 0.00 83.34 34.01 0.00 44.43 34.01 0.00 35.31 34.01 0.00 ?3.60

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