Civil Engineering Reference
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4. Check negative moment sections for combined gravity plus wind load moments
(a) Exterior Negative:
Consider wind load moments resisted by column strip as defined in Fig. 4-9. Column strip width
= 0.5(20
12) = 120 in.
gravity loads only:
M
u
= 73.4 ft-kips
gravity + wind loads:
( )
+ 0.5 46.5
(
)
1.2 136
M
u
=
73.4 + 1.6
55.78 = 146.75 ft-kips (govern)
238
ACI Eq. (9-4)
Also check for possible moment reversal due to wind moments:
M
u
= 0.9(42) ± 1.6(55.78) = 127.0 ft-kips, - 51.4 ft-kips (reversal)
ACI Eq. (9-6)
where q
D
= 136 psf
M
d
= 0.29(0.136
24
18.83
2
/8) = 42 ft-kips
(b) Interior Negative:
gravity loads only:
M
u
= 134.1 ft-kips
gravity + wind loads:
( )
+ 0.5 46.5
(
)
1.2 136
M
u
=
134.1 + 1.6
55.78 = 194.3 (governs)
238
and M
u
= 0.9(76.8) ± 1.6(55.78) = 158.4 ft-kips, - 20.1 ft-kips (reversal)
where M
d
= 42(0.53/0.29) = 76.8 ft-kips
5. Check slab section for moment strength
(a) At exterior negative support section:
b = 20 M
u
/d
2
= 20
146.75/7.25
2
= 55.8 in. < 120 in. O.K.
where d = 8.5 - 1.25 = 7.25 in.
(b) At interior negative support section:
b = 20 x 194.3/9.50
2
= 43.1 in. < 120 in. O.K.
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