Civil Engineering Reference
In-Depth Information
Examples 4.18 and 4.19 illustrate that, for long open deck spans, there are conflict-
ing design requirements that the rail-to-deck-to-superstructure connection be flexible
enough to avoid excessive compressive stress in the CWR (that could precipitate
buckling), and rigid enough to reduce rail separation and relative displacements at
the rail-to-deck-to-superstructure interfaces.
Therefore,inordertoallowformovementbetweentherailandsuperstructurewhile
providing sufficient rail anchoring to preclude excessive relative displacements, the
CWR may be anchored to only a portion of the span length. The portion of the span
length to which the CWR is anchored should be adjacent to the fixed bearings to
allow the necessary movement between the rail and superstructure (anchored CWR
over the expansion bearing areas will resist the thermal movements of the span). The
effect of this is illustrated in Example 4.20.
Example 4.20
In order to reduce the relative displacement between the rail and super-
structure in Example 4.18, anchoring the CWR to only a portion of the rail is
proposed. If only 1/3 of the span length (from fixed bearings) has the CWR
anchored to the deck, determine the maximum stress in the CWR, relative
displacement between the rail and superstructure and rail separation,
Maximum stress in the CWR
:
k
d
EA
=
10
−4
in.
−1
,
λ
d
=
7.30
×
k
t
EA
3.65
10
−4
in.
−1
,
λ
t
=
×
10
−4
)
=
l
d
L
=
(
75
)(
12
)(
7.30
×
0.66.
into Equation 4.45 with
n
=
1 yields
9425
1
e
−λ
d
L
)
,
σ
cwr
=−
+
0.31
(
λ
d
L
−
1
+
σ
cwr
=−
9425 [1
+
0.31
(
0.66
−
1
+
0.52
)
]
=−
9937 psi for both rails.
Force in each rail
=
9937
(
13
)/
1000
=
129 kips compression, OK.
Relative displacement
:
Substitution into Equation 4.48 with
n
=
1 yields
0.24
1
e
−λ
L
,
Δ
x
=
+ λ
L
−
Δ
x
=
0.24
(
1
+
0.66
−
0.52
)
=
0.27 in., OK.
Rail separation
:
Substitution into Equation 4.44 yields
10
−4
1
1
Δ
x
s
=−
4.55
×
10
−4
+
=
2.65 in., OK.
10
−4
5.16
×
2.58
×
