Civil Engineering Reference
In-Depth Information
Lateral bracing
(a)
(b)
(c)
Figure 8.1
Steel box girders. (a) Unstiffened closed box girder. (b) Unstiffened tub girder
with lateral bracing. (c) Stiffened closed box girder.
Top lateral-bracing system
Internal vertical cross frame
External cross frame
Figure 8.2
Twin-box girder bridge. (Courtesy of TXDOT.)
the torsional stiffness. Bracing systems commonly consist of a horizontal
truss attached to the girder near its top flange to increase its torsional
stiffness. Consideration of distortional effects may be limited to local
regions between internal intermediate diaphragms. The distortion of
the cross section can be reduced by using closer internal cross frames
and diaphragms. External bracing between girders may be necessary in
curved bridges to control the deflections and rotations of the girders,
thereby facilitating the placement of the concrete roadway deck. The box
girder cross section possesses a high torsional stiffness after the concrete
deck gains its full strength because the cross section is considered as a
fully closed section. However, internal intermediate diaphragms and top-
flange lateral bracing may still be needed as a box girder is an unstable
open section with very little torsional stability before the concrete is
hardened.
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