Page 253 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

238 Chapter Three

amount of ductility, lowest strength, and lowest toughness properties.
When possible, it is always desirable to allow the residual stresses of
welding to elongate the steel in the X direction. Of particular concern
are large welds placed on either side of the thickness of the steel where
the weld shrinkage stress will act in the Z axis. This can result in
lamellar tearing during the time of fabrication, or it can result in sub-
surface fracture during seismic loading.
Provide ample access for welding. It is essential that the weld joint
design as well as the surrounding configuration of material offer ade-
quate access and visibility for the welder and the welding equipment.
If the operator cannot adequately observe the joint, weld quality will
suffer. As a general rule, if the welder cannot see the joint, neither can
the inspector. Weld quality will naturally suffer. It is important that
adequate access is provided for the proper placement of the welding
electrode with respect to the joint. This is a function of the welding
process. Gas-shielded processes, for example, must have ample access
for insertion of the shielding gas nozzle into the weld joint.
Consideration of these issues has been incorporated into the prequali-
fied groove weld details as listed in AWS D1.1. Overall access to the
joint is a function of the configuration of the surrounding material.
The designer and detailer should be aware of these general constraints
in order to provide adequate access for high-quality fabrication.

No secondary members in welded design. A fundamental premise of
welding design is that there are no secondary members. Anything
that is joined by welding can, and will, transfer stress between joined
materials. Segmented pieces of steel used for weld backing, for example,
can result in a stress-concentration factor at the interface of the back-
ing. Attachments that are merely tack-welded in place may become
major load-carrying members, resulting in the initiation of fracture
and propagation throughout the structure. These details must be con-
sidered in the design stage, and also controlled during fabrication and
erection.
Residual stresses in welding. As hot expanded weld metal and the
surrounding base metal cool to room temperature, they must shrink
volumetrically. Under most welding conditions, this contraction is
restrained or restricted by the surrounding material, which is rela-
tively rigid and resists the shrinkage. This causes the weld to
induce a residual stress pattern where the weld metal is in residual
tension, and the surrounding base metal is in residual compression.
The residual stress pattern is three-dimensional since the metal
shrinks volumetrically. The residual stress distribution becomes
more complicated when multiple-pass welding is performed. The
final weld pass is always in residual tension, but previous weld

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