Civil Engineering Reference
In-Depth Information
TABLE E7.7
Composite Steel and Concrete Section Properties—Long-Term Loads
(for Shored Construction Includes all Dead Loads and for Unshored
Construction Includes Dead Load Supported by Steel Section only and
Dead Load Supported by Composite Section (e.g., Track, Ballast,
Walkways, and Conduits))
Element
A
(in.
2
)
Ay
b
(in.
3
)
y
)
2
(in.
4
)
I
o
(in.
4
)
y
b
(in.)
y
b
−
y
(in.)
A
(
y
b
−
Steel section
153.13
45.00
6891
−
9.84
14,825
223,444
Concrete slab
(n
=
24
)
37.50
95.00
3563
40.16
60,483
313
190.63
10,454
75,308
223,757
TABLE E7.8
Composite Steel and Concrete Section Properties with
n
=
24
Location
I
(Gross or Net Depending on
nS
(Gross or Net)
Location of NA) (in.
4
)
(in.
3
)
(Figure E7.2)
n
c
(in.)
Top concrete
24
45.16
299,065
158,936
Bottom concrete
24
35.16
299,065
204,140
Top steel
1
35.16
299,065
8,506
Bottom steel
1
54.84
276,408
5,040
In this example there is not a great difference in unshored and shored flex-
ural stresses due to the relatively small dead load stress on the noncomposite
(steel only) section during unshored construction.
Shear stresses:
f
v
=
(
486
)/
[
(
85
)(
0.625
)
]=
9.15 ksi (the shear is resisted entirely by the steel
girder web)
Allowable stresses:
F
call
concrete
0.40
f
c
=
=
0.40
(
3
)
=
1.2 ksi (minimum 28 day compressive
strength of 3,000 psi)
≥
1.15 ksi OK
F
tall
steel
=
F
call
steel
=
0.55
F
y
=
0.55
(
50
)
=
27.5 ksi
≥
22.4 ksi OK
F
fat
=
16 ksi (Category B with loaded length of 90 ft)
≥
13.2 ksi OK
F
vall
=
9.15 ksi OK
Girder design for fabrication and erection loads
- see Example 7.1
Detailed design of the girder
Detailed design of the web plate
0.35
(
50
)
=
17.50 ksi
≥
Flexural buckling:
4.18
E
4.18
29,000
22.6
h
t
w
=
85
0.625
=
136
≤
f
c
≤
≤
150.
Therefore,nolongitudinalstiffenersarerequiredforwebflexuralbuckling
stability.
