Civil Engineering Reference
In-Depth Information
Modern trends in structural engineering software are toward integrated structural
analysis, design, drafting, and fabrication. Some proprietary systems successfully
integrate many of these functions, and it is likely that, as such integrated systems
becomeevenmore“user-friendly”andreliable;theywillbeusedevenmorefrequently
in structural engineering practice.
5.2.2 L
ATERAL
L
OAD
A
NALYSIS OF
S
TEEL
R
AILWAY
S
UPERSTRUCTURES
The analysis of railway superstructures also involves the determination of the maxi-
mum deformations and stresses caused by lateral effects such as those due to moving
loads (centrifugal and nosing), wind, and earthquakes.
∗
For usual steel railway bridge superstructures, lateral load effects may be deter-
mined by simplified analyses. This enables the use of hand calculations, relatively
simple computer programs, and spreadsheets to determine the deformations and
forces. For more complex superstructures more sophisticated computerized frame
analysis software is often used.
5.2.2.1
Lateral Bracing Systems
Lateral forces on steel railway superstructures from wind, nosing, and centrifugal
forces are generally transferred to the bearings and then substructures via bracing
members in horizontal truss systems. Components of the horizontal bracing systems
may also resist the buckling propensity of compression members such as girder top
flanges or truss top chords in simply supported spans. Forces from horizontal truss
systems that are not in the plane of the bearings are transferred to the substructures
by end vertical (DPG spans and some deck truss spans) or portal (through truss and
some deck truss spans) bracing systems. Knee braces are used to provide resistance
to buckling of the compression flange and transfer wind forces from the top flanges
to the bearings in through plate girder spans.
5.2.2.1.1 Horizontal Truss Bracing
Since, for usual steel railway bridges, the determination of lateral loads is approx-
imate, it is reasonable to utilize simplifications regarding load distribution to the
horizontal bracing systems. It is generally adequate to use a horizontal Pratt or War-
ren truss and apply lateral forces at the windward side of the lateral truss panel points.
For bracing systems (horizontal trusses) with two diagonals in each panel (Pratt-type
cross-bracing) the lateral shear can be assumed transferred equally between diag-
onals and the members are designed for both maximum tension and compression
forces. When double bracing is not connected to the floor system or otherwise sup-
ported,
†
the diagonals can be assumed to act in tension only with transverse members
(struts) in compression. For bracing systems with only a single diagonal in each
panel (Warren-type bracing) the diagonals are also assumed to act as tension-only
members.
∗
Wind and earthquake forces may also have longitudinal components.
†
Therefore, relatively long and slender with a low critical buckling load and compressive force capacity.
