Civil Engineering Reference
In-Depth Information
The approximate determination of forces in lateral bracing systems is shown in
Example 5.10.
5.2.2.1.1.1 Members in Top Lateral Systems
In addition to lateral forces from
wind, the top lateral system in through spans
∗
requires bracing members that resist
a transverse shear force of 2.5% of the total compressive axial force in the chord or
flange at that panel point. The top lateral system in deck spans requires bracing to
resist a transverse shear force of 2.5% of total compressive axial force in the chord
or flange at that panel point in addition to other lateral forces from wind, nosing,
and centrifugal forces. Deck span top lateral systems are often the heaviest bracing
system required in steel railway superstructures. Concrete and steel plate decks may
be used, which act as diaphragm, for resisting these lateral forces.
5.2.2.1.1.2 MembersinBottomLateralSystems
Lower lateral bracing is gen-
erally required when the span supports are at the bottom chord of a truss or bottom
flange of a girder span. When the span supports are at the top chord of a truss
†
only
struts at bottom panel points are strictly required. However, a nominal lateral bracing
system is often employed for adequate overall lateral rigidity of the span.
The bottom lateral system in through spans may use the floorbeams as struts of the
bracing system. The bracing is designed to resist lateral wind, nosing, and centrifugal
forces. Concrete and steel decks may also be used to act as a diaphragm.
The bottom lateral bracing system in deck spans is lightly loaded by wind and, for
short spans in particular, may not be required. However, in order to ensure overall
rigidity of longer spans, a light bracing system (based on the maximum slenderness
ratioforcompressionmembers)isoftenused.Ataminimum,strutsshouldbeinstalled
at each panel point in the chord or flange.AREMA (2008) recommends bottom lateral
bracing for all deck spans greater than 50 ft long.
Example 5.10
The forces in the top and bottom lateral bracing system members of the
through truss span in
Figure E5.10
are required. The lateral wind and nosing
forces, and compression forces required to be resisted for bracing of main
compression members are as follows:
Wind load at the top chord
350 lb/ft.
Wind load at top lateral bracing panels
=
=
19.55
(
0.35
)
=
6.8 kips per panel.
Wind load at the bottom chord
=
200 lb/ft.
Wind load on train
300 lb/ft.
Wind load at the bottom lateral bracing panels
=
=
19.55
(
0.5
)
=
9.8 kips per
panel.
∗
Through spans such as plate girder and pony truss spans without room for top lateral bracing generally
utilize knee brace frames to resist the transverse shear force of 2.5% of total compressive axial force in
the chord or flange and the lateral forces from wind.An analysis of knee-braced through span transverse
frames is outlined in
Section 5.2.2.1.4.
†
Such as a “fish-bellied” deck truss span.
