Civil Engineering Reference
In-Depth Information
y
t
c
f
pe
f
d
)
M
cr
cracking moment
(6
f
(ACI Equation 11-11)
13,824
12
(6
4000
2170
400)
2,476,193 in.
lb
206,349 ft- lb
(12)(24)
144
Beam weight
(150)
300 lb/ft
w
u
not counting beam weight
(1.2)(1.2
0.3)
(1.6)(2.1)
4.44 k/ft
M
max
(10)(4.44)(4)
(4)(4.44)(2)
142.08 ft- k
142,080 ft- lb
V
i
due to
w
u
occurring same time as
M
max
(10)(4.44)
(4)(4.44)
26.64 k
26,640 lb
V
d
dead load shear
(10)(1.2)
(4)(1.2)
7.2 k
7200 lb
d
24
3
3
18
or
(0.8)(24)
19.2
c
b
w
d
V
d
V
i
M
cr
M
max
V
ci
0.6
f
(ACI Equation 11-10)
(0.6
4000)(12)(19.2)
7200
(26,640)(206,349)
42,080
54,634 lb
but need not be less than (1.7
4000)(12)(19.2)
24,772 lb
Computing
V
cw
f
pc
calculated compressive stress in psi at the centroid of the concrete due to the
effective prestress
250,000
(12)(24)
868 psi
9
9
2
72
2
(250,000)
V
p
vertical component of effective prestress at section
9
72.5
6
(250,000)
31,009 lb
c
V
cw
(3.5
f
0.3
f
pc
)
b
w
d
V
p
(ACI Equation 11-12)
(3.5
4000
0.3
868)(12)(19.2)
31,009
142,006 lb
Using Lesser of
V
ci
or
V
cw
V
c
54,634 lb
EXAMPLE 19.6
Determine the spacing of #3
stirrups required for the beam of Example 19.5 at 4 ft from the end
support if
f
pu
is 250 ksi for the prestressing steel and
f
y
for the stirrups is 40 ksi. Use the value of
V
c
obtained by the approximate method, 50,684 lb.
