Civil Engineering Reference
In-Depth Information
thelateraldeflectionassociatedwiththeAREMA-recommendeddesignforce.
29,000
100
2
((
100
/
3
(
150
))
+
(
260
/
2
(
10,000
)))
=
C
=
12.3 k
/
in.
F
cr
∼
1.80
(
45
+
(
50
/
2
/
3
))(
20
)
∼
1920 kips (using a safety factor of 1.80)
2
(
1920
)
4
(
1200
/
10
)
=
π
C
req
=
39.5 k
/
in.
Since
C < C
req
, the transverse frames are not stiff enough to preclude
excessive buckling deformations.
If, for example, the vertical member stiffness,
I
c
, was increased by the
addition of a substantial knee brace so that
I
c
=
750 in.
4
29,000
100
2
((
100
/
3
(
750
))
+
(
260
/
2
(
10,000
)))
=
C
=
50.5 k
/
in.
Using the shear force,
R
F
=
22.5 kips, from Example 5.17 (AREMA-
recommended force)
R
F
C
=
22.5
50.5
=
δ =
0.45 in.,
which appears reasonable for 100 ft long span lateral deflection (span/2666)
and 6 ft-9 in frame wall (girder web/knee brace) cantilever tip deflection
(height/160).
5.3 STRUCTURAL DESIGN OF STEEL RAILWAY
SUPERSTRUCTURES
The structural design of members and connections in the superstructure may proceed
once the bridge engineer has determined the loads (Chapter 4) on the superstructure
and the internal member forces from structural analyses (
Section 5.2)
of appropriate
load combinations.
Preliminary structural analyses use superstructure models developed through the
planning and preliminary design process that are refined, as necessary, through the
structural analysis process. The structural analysis may range from the routine analy-
sis of statically determinate superstructures (reactions and internal forces determined
from equilibrium) to continuous and more complex statically indeterminate structures
(additional equations required such as compatible displacement equations, which
require section properties and dimensions). Structural design (for strength and ser-
viceability) involves material selection and determining the dimensions or section
properties of members and connections in the superstructure. For statically indeter-
minate structures, an iterative analysis/design procedure is required. The potential
failure modes of the steel superstructure require assessment in order to examine the
strength and serviceability of the structure.
