Civil Engineering Reference
In-Depth Information
(a)
From Table 16.2 of this chapter (Code 13.6.3.3) for an end span with beams between
interior supports but with no edge beams:
Interior negative factored moment
0.70
M
o
(0.70)(302.3)
212 ft- k
Positive factored moment
0.52
M
o
(0.52)(302.3)
157 ft- k
Exterior negative factored moment
0.26
M
o
(0.26)(302.3)
79 ft- k
These factored moments may be modified by 10%, according to Section 13.6.7 of the Code,
but this reduction is neglected here.
(b)
Allotting these moments to beam and column strips:
2
1
18
22
0.818
1
1
for the 22
beam
4.08
1
2
1
(4.08)(0.818)
3.34
Next an expression is given for
t
. It is the ratio of the torsional stiffness of an edge beam sec-
tion to the flexural stiffness of a width of slab equal to the span length of the beam measured center
to center of supports.
t
E
cb
C
2
E
cs
I
s
Involved in the equation is a term
C
, which is a property of the cross-sectional area of the tor-
sion arm estimating the resistance to twist.
x
3
y
3
1
0.63
x
y
C
where
x
is the length of the short side of each rectangle and
y
is the length of the long side of each
rectangle. The exterior beam considered here is described in ACI Section 13.2.4 and is shown in
Figure 16.18, together with the calculation of
C
.
t
E
cb
C
2
E
cs
I
s
(
E
c
)(12,605)
(2)(
E
c
)(3888)
1.62
Figure 16.18
Evaluation of
C
.
