Civil Engineering Reference
In-Depth Information
of paramount concern. The planning phase should yield information concerning opti-
mum crossing geometry, layout, and anticipated construction methodologies. This
information is required for the selection of span lengths, types, and materials for
preliminary design. Preliminary design concepts are often the basis of regulatory
reviews, permit applications, and budget cost estimates. Therefore, planning and pre-
liminary design can be critical to successful project implementation and, particularly
for large or complex bridges, warrants due deliberation. Detailed design of the bridge
for fabrication and construction can proceed following preliminary design.
3.2 PLANNING OF RAILWAY BRIDGES
Planning of railway bridges involves the careful consideration and balancing of multi-
faceted, and often competing, construction economics, business, public, and technical
requirements.
3.2.1 B
RIDGE
C
ROSSING
E
CONOMICS
In general, other issues not superseding, the bridge crossing should be close to per-
pendicular to the narrowest point of the river or flood plain.The economics of a bridge
crossing depends on the relative costs of foundations, substructures, and superstruc-
tures.
∗
Estimates related to the cost of foundation and substructure construction are
often less reliable than those for superstructures. Superstructure fabrication cost esti-
mates are often more dependable than erection estimates due to the inherently greater
uncertainty and risk associated with field construction. Excluding, in particular, pub-
lic and technical (hydraulic and geotechnical) constraints from the cost-estimating
procedure enables the economical span length,
l
, to be established based on very
simple principles. Considering a fairly uniform bridge with similar substructures and
multiple equal length spans, the total estimated cost, CE, of a bridge crossing may be
expressed as
CE
=
n
s
C
sup
w
s
l
+
(n
s
−
1
)C
pier
+
2
C
abt
,
(3.1)
where
n
s
is the number of spans and is equal to
L
/
l
, where
L
is the length of the bridge;
C
sup
is the estimated cost of steel per unit weight (purchase, fabricate, and erect);
w
s
is the weight of span elements that depend on span length,
l
(e.g., the bridge deck
of a span or the floor system of a through span is independent of span length and
excluded from
w
s
);
C
pier
is the average cost of one pier (materials, foundation, and
construction); and
C
abt
is the average cost of one abutment (materials, foundation,
and construction).
If
w
s
= α
are constants independent of span length and
dependent only on span type and design live load, Equation 3.1 may be expressed as
l
+ β
, where
α
and
β
C
pier
L
L
−
l
CE
=
C
sup
L(
α
l
+ β
)
+
+
2
C
abt
,
(3.1a)
lL
∗
A rule of thumb for economical relatively uniform multispan bridges is that the cost of superstructure
(fabrication and erection) equals the cost of foundation and substructure construction (Byers, 2009).
