Civil Engineering Reference
In-Depth Information
which may be simplified to
h
a
0.7
,
2
2.5
a
0
t
w
I
ts
=
−
(7.95)
where
a
0
is the actual stiffener spacing used in design which must be
≤
h
(see
development of Equation 7.86) and
≤
96 in.;
a
is the required stiffener spacing
(Equation 7.93).
In elastic design the stiffeners are not required to carry a force
∗
and, therefore,
there is no need to design them, nor their connections, for strength. The size of the
transversestiffenerisdeterminedfromEquation7.95,whichisbasedsolelyonrigidity
considerations, and nominal welded or bolted connection to the web is typically
used. AREMA (2008) recommends connection to the compression flange to provide
additional stability to both the stiffener itself and against torsional buckling of the
compression flange.
†
This connection may be accomplished with bolts or fillet welds
or by careful grinding to ensure a uniform and tight fit against the flange.Wrap around
fillet welds must not be used for welding transverse stiffeners to either the web or the
compression flange.
However, if lateral bracing is attached to a stiffener, the connection at the top flange
must be designed to transmit 2.5% of the compression flange force and other lateral
forces from wind, centrifugal, or nosing (see Chapters 4 and 5). From a lateral bracing
perspective, the connection at the bottom flange is less important as it resists only
forces from wind. The stiffener connection to the web must also be designed to resist
the forces from out-of-plane bending of the beams or girders they are connected to and
forces due to lateral distribution of the live load.
‡
AREMA (2008) recommends that
transverse web stiffeners be adequately attached to both the top and bottom flanges
when bracing is connected to the stiffeners (although not required for rolled beams
on single track spans without skew or curvature).
The web plate stiffeners must not be welded to the tension flange, as such a trans-
verse weld is a very poor fatigue detail. Furthermore, welds connecting transverse
stiffeners to web plates should not be made close to the tension flanges because of
stress concentration effects. Extensive testing and analytical work have established
that the stiffener weld should be a minimum of 4-6 times the web plate thickness
from the near toe of the tension flange-to-web weld (Basler and Thurlimann, 1959).
AREMA (2008) recommends this distance as 6
t
w
. Careful consideration of details is
required (such as provision of bolted angles at the bottom of the stiffeners (D'Andrea
et al., 2001) or peening pretreatments
§
) where brace frames are attached to transverse
stiffeners that may precipitate out-of-plane distortion in the web gap. Even though
fabrication cost is increased, some design engineers will provide bolted transverse
∗
Such as a compressive force if tension field action (postbuckling) is assumed in the web (analogous to
truss post with web behavior like truss diagonal).
†
Necessary,inparticular,forrelativelythintopflangeswithopendeckconstructionwherelateralflexureof
thetopflangemayprecipitateweldcrackingatthetransversestiffener-to-compressionflangeconnection.
‡
Typically present in superstructures with relatively large skew, curvature, or track eccentricity.
§
Ultrasonic impact treatment is a modern pretreatment to improve these poor fatigue details at the base
of welded transverse stiffeners (Roy and Fisher, 2006).
