Draft
14.4LateralDe
ections 261
E = 3 10
6
psi= 432; 000 ksf (14.49-d)=
(4:8)k.ft(156)
4
ft4
8(432;000)ksf(9;400)ft
4
= 0: 087 ft (14.49-e)
h
=
0 : 087
156
=
1
1 ; 800
p
(14.49-f)
The
h
ratiois much lessthan1/500as permittedin mostbuildingcodes,ands within
theusualindexforconcretebuildings,which rangesbetween1/1,000and1/2,500.
If thewallthicknessis reduced,andif door openingsareconsidered,thede
ectionwill
be correspondinglysmaller.
Thede
ectiondueto moment increasesrapidlyat thetop,thevalueof 1/1,800indicates
onlytheaveragedriftindexfortheentirebuilding,whereasthestorydriftindexmay be
higher,especiallyforthetop
oor.
- We nextconsiderthede
ectionof thetopof theframe.Assumingthateach frametakes
1/9of thetotalwindloadandshear,andneglectingcolumnshortening,then:
=
Vcol
Eh
2
12 E
"
h
Icol
E2 L
Igdr
(14.50-a)
Icol =
bh
3
12
=
(20=12)(20=12)
3
12
= 0: 64 ft
4
(14.50-b)Igdr = 3 : 64 ft
4
(14.50-c)V
ground
colI
=
(4:8)k.ft(156)ft
(2)(9)
= 41: 7 k=col (14.50-d)
=
(41:7)k(12)
2
12(432;000)ksf
(12)ft(0:64)ft
4
2(60)ft
(3:46)ft
4
(14.50-e)
= 0 :00116(18:8 + 34:7) = 0: 062 ft (14.50-f)
- Sincethestorydriftvarieswiththeshearin thestory, which decreaseslinearlyto the
top,theaveragedriftwillbe 0: 062 =2 = 0: 31 ftper storyandthede
ectionat topof the
buildingis approximately
= (13)(0:031)= 0 : 40 ft (14.51)
which indicatesa driftratioof
DriftRatioforBuilding =
(0:4)ft
(156)ft
=
1
400
(14.52-a)
DriftRatioforGroundFloor =
(0:062)ft
(12)ft
=
1
194
(14.52-b)
- Comparingtheframede
ectionof 0: 40 ftwiththeshaftde
ectionof 0: 087 ft, it is seen
thattheframeis about ve timesmore
exiblethantheshaft. Furthermore,theframe
wouldnotbe sti enoughto carryallthelateralloadby itlself.Proportioningthelateral
loadtotherelative sti nesses, theframewouldcarryabout1/6of theload,andthe
remaining5/6willbe carriedby theshaft.
Increasingthecolumnsizewillsti entheframe,butin orderto be reallye ective, the
girdersti nesswillalsoneedto be increased,sincethegirderscontributeabout2/3of the