Draft

2.3LateralLoads 35

24 Windloadsareparticularlysignicant ontallbuildings

1

.

25 Whenasteadystreamlineair
ow of velocityV is completelystoppedby a rigidbody, the

stagnationpressure(orvelocity pressure)qswas derived by Bernouilli(1700-1782)

qs=

1

2

V

2

(2.3)

wheretheairmassdensityis theairweight dividedby theaccelerationof gravityg= 32: 2

ft/sec

2

. At sealevel anda temperatureof 15

o

C (

o

F),theairweighs0.0765lb/ft

3

thiswould

yielda pressureof

qs=

1

2

(0:0765)lb/ft

3

(32:2)ft/sec

2



(5280)ft/mile

(3600)sec/hr

V

 2

(2.4)

or

qs= 0: 00256 V

2

(2.5)

whereV is themaximumwindvelocity (inmilesper hour)andqsis in psf.V canbe obtained

fromwindmaps(intheUnitedStates 70 V 110),Fig.2.5.

Figure2.5:WindMapof theUnitedStates,(UBC1995)

26 Duringstorms,windvelocitiesmay reach valuesupto or greaterthan 150 milesper hour,

which correspondsto a dynamicpressureqsof about 60 psf(ashighas theaveragevertical

occupancyloadin buildings).

27 Windpressureincreaseswithheight, Table2.5.

28 Windloadwillcausesuctionontheleewardsides,Fig.2.

29 Thismagnitudemustbe modiedto account fortheshape andsurroundingsof thebuilding.

1

Theprimarydesignconsiderationforveryhighrisebuildingsis theexcessivedriftcausedby lateralload

(windandpossiblyearthquakes).