Civil Engineering Reference
In-Depth Information
train loads are transmitted from the rails to the sleepers, from the sleepers to
the stringers, from the stringers to the cross girders, from the cross girders
to main plate girders, from the main plate girders to the bearings, from the
bearings to the foundations or piers, and finally from foundations or piers
to the ground. Wind and lateral loads acting on the bridge can be transmit-
ted by systems of horizontal (
upper and lower wind bracings
)andvertical(
cross
wind bracings
) bracing systems, which carry out wind loads safely to the
bearings. Also, the stringers can be attached to horizontal systems of brac-
ings called
stringer bracing
or
lateral shock
(
nosing force
)
bracing
,whichtransmit
lateral shock (nosing) forces resulting from the moving train safely to cross
girders where it causes additional small axial force on the cross girders. The
web of the main I-shaped plate girder bridge is very sensitive to buckling
since it has a thin thickness compared to its depth. Therefore, the web of
the plate girder is strengthened by vertical and horizontal
stiffeners
.The
spacing between the vertical stiffeners should be reasonably assumed
(1.5-2 m) not to increase the thickness of the web. Hence, the spacing
between cross girders (
a
)isdependentonthenumberofverticalstiffeners
used between two adjacent cross girders. Finally, the length of the bridge
(
L
) is equal to the number of (
a
).
The structural components of a traditional highway bridge can be
reviewed as shown in
Figure 1.20
. The figure shows the general layout of
a through truss highway steel bridge. The bridge has a RC floor supported
by a number of stringers. The stringers are spaced at designed distances (
a
3
)
reasonably assumed between 2 and 3 m. Similar to railway bridges, the
stringers of this type of bridges are supported by cross girders. The cross
girders are supported by two longitudinal trusses, which are the main struc-
tural system for this type of bridges. The main trusses are supported on
hinged and roller bearings, which rest on foundations or piers. The truck
and car loads are transmitted from the RC floor to the stringers, from the
stringers to the cross girders, from the cross girders to the main trusses, from
the main trusses to the bearings, from the bearings to the foundations or
piers, and finally from the foundations or piers to the ground. Wind loads
acting on the bridge can be transmitted by systems of horizontal upper, since
this bridge is a through bridge with enough height to contain traffic in addi-
tion to overhead clearance, and end portal frames, since cross bracing
will close the bridges, which carry out wind loads safely to the bearings.
The bracing systems are also important to define the buckling lengths of
compression members of the main trusses. However, the stringers do not
need a bracing since the RC concrete floor takes care of any lateral and
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