Civil Engineering Reference
In-Depth Information
c
=
40
(
20
)/
60
=
13.33
d
=
(
35
/
40
)
13.33
=
11.67
e
=
(
30
/
40
)
13.33
=
10.00
f
=
(
21
/
40
)
13.33
=
7.00
=
(
16
/
40
)
13.33
=
5.33
g
h
=
(
10
/
40
)
13.33
=
3.33
i
=
(
5
/
40
)
13.33
=
1.67
M
C
=
20
(
4.67
)
+
40
(
10.00
+
13.33
+
11.67
+
10.00
)
+
26
(
7.00
+
5.33
+
3.33
+
1.67
)
=
2345 ft-kips.
5.2.1.2.2 Influence Lines for Maximum Axial Forces in Statically
Determinate Truss Spans
TheinfluencelinesdevelopedinSection5.2.1.2.1forshearforceandbendingmoment
are useful in the construction of axial force influence lines for truss web and chord
members, respectively. In addition, the consideration of the moving load effect at
panel points simplifies the construction of axial force influence lines.
Influence lines for chord members may be constructed by considering free body
diagrams and equilibrium of moments. Influence lines for web members may be
constructed by considering free body diagrams and equilibrium of forces. The con-
struction of influence lines for axial forces in members of simply supported truss
spans is illustrated by examples of a Pratt truss and a Parker truss, respectively, in
Examples 5.5 and 5.6.
∗
Example 5.5
Construct influence lines for the 156.38 ft eight-panel Pratt through truss in
Figure E5.5.
The influence lines are constructed by locating unit loads at
appropriate locations and using the method of sections or the method of
joints.
Determine influence lines for the reactions and members U1-U2, U3-L3,
L1-L2, L3-L4, U1-L1, and U1-L2.
Section 1-1 may be isolated to determine the forces in members U1-U2
(Figure E5.5b), L1-L2 (
Figure E5.5c),
and U1-L2 (
Figure E5.5d).
In Figure E5.5a: with unit load at L1 and taking moments about L2, the
force in U1-U2
=[
(
1
/
8
)(
6
)(
19.55
)
]
/
27.25
=−
0.54 (compression direction to
balance reaction moment about L2).
In Figure E5.5b: with unit load at L2 and taking moments about L2, the
force in U1-U2
=[
(
2
/
8
)(
6
)(
19.55
)
]
/
27.25
=−
1.08 (compression direction to
balance reaction moment about L2).
∗
These truss forms are often used for medium span steel railway bridges.
