Civil Engineering Reference
In-Depth Information
8 @ 50 ft = 400 ft
1
Sliding mechanism
U1
U2
y
D
25 ft
L4
L2
150 ft
4
5
ft
135 ft
55 ft
1
Pin, typ.
R
A
1.00
h
D
= 55 + 45 = 100 ft
0.89
a
D
= 100 ft
FIGURE E5.8
75 ft, the influence line for axial force in chord
U1-U2 (shown by the shaded area in Figure E5.8) can be determined by the
superposition of the influence lines for
R
A
and
H
A
.
With
y
D
=
175
−
45
−
55
=
5.2.1.2.4 Influence Lines for Maximum Effects in Statically
Determinate Cantilever Bridge Spans
Long-span steel railway bridges may also be economically constructed as cantilever
bridges (see Chapter 1). The economical relative lengths of the cantilever arm,
L
c
,
anchor,
L
a
, and suspended,
L
s
, spans will vary with live to dead load bending moment
ratio. For the relatively high live to dead load bending moment ratios of steel railway
superstructures, typical
L
a
/L
c
values of between 1 and 2 are used depending on the
suspended span length,
L
s
. In steel railway superstructures,
L
c
/L
s
values typically
range from 0.4 to 2. The relative lengths of the cantilever arm, anchor, and suspended
spans may also vary based on site conditions that dictate the location of piers at a
crossing (see Chapter 3). Influence lines for cantilever superstructures may also be
constructed by consideration of unit loads traversing the bridge. The ordinates of the
influencelinesarereadilydeterminedbycalculationofthereaction,bendingmoment,
and shear due to unit loads at locations where the influence lines change direction.
5.2.1.2.4.1 Cantilever Bridge Span Influence Lines [with Loads Applied
Directly to the Superstructure]
Influence lines for reactions at locations A and
B, bending moment in the anchor span at location E and at location F in the cantilever
