Civil Engineering Reference
In-Depth Information
which may be differentiated in terms of span length,
l
, to determine an expression for
the minimum total estimated cost, CE, as
dCE
d
l
=
C
pier
L
l
2
C
sup
α
L
−
=
0.
(3.2)
Rearrangement of Equation 3.2 provides the economical span length,
l
,as
C
pier
C
sup
α
l
=
.
(3.3)
However,whileEquation3.3providesasimpleestimateofeconomicalspanlength,
thefinalgeneralarrangementintermsofspanlengths,
l
,maydependonotherbusiness,
public, and technical requirements.
3.2.2 R
AILROAD
O
PERATING
R
EQUIREMENTS
Most new freight railway bridges are constructed on existing routes on the same
alignment. Construction methods that minimize the interference to normal rail, road,
and marine traffic enable simple erection and are cost-effective must be carefully
considered during the planning process. Often, in order to minimize interruption
to railroad traffic, techniques such as sliding spans into position from falsework,
floated erection of spans from river barges, span installation with movable derricks
and gantries, construction on adjacent alignment,
∗
and the use of large cranes must be
developed (Unsworth and Brown, 2006). These methodologies may add cost to the
reconstruction project that are acceptable in lieu of the costs associated with extended
interruption to railway or marine traffic.
New rail lines are generally constructed in accordance with the requirements estab-
lished by public agencies
†
and railroad business access. It is not often that bridge
crossings are selected solely on the basis of localized bridge economics planning
principles
‡
as outlined in Section 3.2.1. Therefore, site reconnaissance (surveying
and mapping) and route selection are performed on the basis of business, technical,
and public considerations.
The railroad operating environment presents specific challenges for bridge design,
maintenance, rehabilitation, and construction. The design of steel railway bridges
involves the following issues related to railroad operations:
• The magnitude, frequency, and dynamics of railroad live loads
• Other loads specific to railroad operations
∗
Either the new bridge is constructed on an adjacent alignment or a temporary bridge is built on an adjacent
alignment (shoo-fly) in order to not interrupt the flow of rail traffic. This may not always be feasible due
to cost, site conditions, and/or route alignment constraints.
†
Generally, the requirements relate to environmental, fish and wildlife, land ownership, and cultural
considerations and/or regulations.
‡
The exception might be very long bridges.
