Civil Engineering Reference
In-Depth Information
Each brace is assumed to resist 1/2 of the horizontal shear. The force in
each brace is estimated as
12 2
10 2
( 35.5 )
2
+
F A =−
F B =−
=−
27.7 kips.
10
5.2.2.1.3 Intermediate Vertical and Sway Bracing
Intermediate vertical bracing in deck spans and sway bracing in through truss spans
are required to provide compression chord or flange stability and adequate stiffness
for serviceability conditions.
In through truss spans intermediate vertical sway bracing carries only small forces
because of the negligible difference in relative lateral deformation of top and bottom
lateral systems. It is often estimated that 50% of panel load due to wind in addition
to 2.5% of the total compressive axial force in the chord at the panel point is carried
by the sway bracing. The analysis of forces may then proceed in a similar manner
to that for the end portal frames of through truss spans (see Examples 5.11 through
5.14). Intermediate sway bracing is often designed as the knee-braced frame type
(see Figure E5.13). Where estimated lateral forces are small it may be sufficient to
proportion members based on maximum slenderness criteria for buckling. AREMA
(2008) provides recommendations regarding types and geometry of through truss
span sway bracing.
In deck spans, the intermediate cross frames or diaphragms provide for proper
load distribution between main girders or trusses and, therefore, in addition to the
lateral forces from equipment, wind and stability-related forces, must be designed
to resist the forces induced by differential vertical deflections of trusses or girders.
AREMA (2008) indicates that for deck spans, diaphragms may be used in lieu of
cross frames for closely spaced shallow girders. AREMA (2008) also provides the
guidelinesshown in Table5.2 f orrecommendedspacingofintermediateverticalbrace
frames.
5.2.2.1.4 Knee Bracing in Through Spans
The members (knee braces) that provide intermittent lateral bracing to pony truss
compression chords and through plate girder compression flanges must have adequate
transverse elastic frame stiffness to ensure that the overall chord or flange has panel
lengths with appropriate stiffness to attain the buckling load, P c ( Figure 5.29) . Nodal
points are created at each knee brace/frame (panel point) location if the transverse
frame stiffness is very large. Conversely, if the transverse frame is too flexible, the
entire compression chord or flange may buckle in a single half-wave. In structures
Provided that there are no substantial live load eccentricities. Track eccentricity can create additional
forces in the bracing members that may be determined by the simple tension member only assumption
or by a more rigorous analysis.
These forces can be particularly large in skewed spans (see Chapter 3) or spans with a substantial track
eccentricity.
Bracing of the compression chord or flange in the vertical direction is provided by truss and girder web
members, respectively.
 
 
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