Civil Engineering Reference
In-Depth Information
The deflections are estimated as (see Chapter 5)
0.104
M
LL
+
I
L
2
EI
2
0.104
(
7314
)(
12
)
[
90
(
12
)
]
Δ
LL+
I
=
=
=
1.64 in.,
29,000
(
223,444
)
0.104
M
DL
L
2
EI
2
0.104
(
2500
)(
12
)
[
90
(
12
)
]
Δ
DL
=
=
=
0.56 in.,
29,000
(
223,444
)
therefore consider a camber of 1/2
.
In practice, dimensions of the various main and secondary elements may
be revised further to attain greater economy of material.
7.4 STRENGTH DESIGN OF STEEL AND CONCRETE COMPOSITE
FLEXURAL MEMBERS
Railway bridges with ballasted decks are beneficial from operational, structural, and
maintenance perspectives (see Chapter 3). The ballast may be placed on timber, steel,
or concrete decks.
Timber decks are generally not effective from structural and maintenance per-
spectives. Steel plate decks have only the strength or stiffness to span small lengths
under railway live loads. Therefore, steel plate decks are generally not appropriate on
deck type bridges with girders or trusses spaced at wide distances. Floor systems are
needed to support stiffened or unstiffened steel plate decks unless the longitudinal
members are closely spaced.
∗
The use of steel plate decks
†
is often appropriate for
long span construction to reduce the superstructure dead load stresses. Steel quantity,
fabrication (welding), and shipping/erection (deck plate size) considerations need to
be carefully reviewed for steel deck plate bridges.
Reinforced and/or prestressed concrete decks have the strength and stiffness
required for use as ballasted decks in ordinary steel railway bridge construction
(Figure 7.18
).
Concrete decks may be noncomposite (not positively connected to
the steel bridge span) or made composite. Relative slip between the concrete deck
and steel span will occur with noncomposite construction and, even with the substan-
tial dead load, the deck may translate under the action of modern train braking and
locomotive traction forces (see Chapter 4). Composite steel and concrete construction
has the following benefits for steel railway bridge construction:
• Ease of site access for the materials used in railway bridge construction
(providedthatconcretetransportorsitebatchingisavailable,thereisreduced
shippinganderectionoflargesteelsectionsand/orplatedecksrequiringfield
bolting).
• Improved train ride and reduced track and deck maintenance (given that
adequate deck drainage and waterproofing are provided).
∗
The fabrication and erection difficulties associated with steel decks on closely spaced longitudinal
members may make them impractical except for use in short spans.
†
In particular, orthotropic steel plate decks.
