Civil Engineering Reference
In-Depth Information
Shear resistance
Overstress
Shear flow
s
i+1
2
s
i
s
1
s
2
FIGURE 7.24
Distribution of shear resistance (studs or channels) along the span length.
7.5 SERVICEABILITY DESIGN OF COMPOSITE
FLEXURAL MEMBERS
AREMA (2008) recommends that mid-span deflection of simply supported spans
due to live load plus impact,
I, should not exceed
L/
640, where
L
is the span
length. Some engineers or bridge owners recommend even more severe live load plus
impact deflection criteria to attain stiffer spans, which offer improved performance
from a structural
∗
and track-train dynamics perspective.
It is also recommended that camber be provided for dead load deflections exceed-
ing 1 in. For composite spans, the dead load deflections depend on the construction
method employed (shored or unshored).
The serviceability criteria for steel railway spans are also discussed in Chapter 5.
Example 7.2 outlines the design of a composite steel-concrete span for Cooper's
E80 live load considering both shored and unshored construction for the flexural
design.
Δ
LL
+
Example 7.2
A 90 ft simple span steel (
F
y
=
50 ksi) ballasted deck plate girder (BDPG) is to
be designed for the forces shown in
Table E7.4a
.
Section properties of the span
The steel girder section properties are shown in
Figure E7.1
(see Exam-
ple E7.1) and
Table E7.4b.
The composite steel and concrete girder section
properties are shown in Figure E7.3 and
Tables E7.5
and
E7.6
for short-term
loads and in
Tables E7.7
and
E7.8
for long-term loads.
Steel section properties: see Example 7.1
∗
Concrete bridge decks generally exhibit better behavior on stiffer spans.
