Page 255 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

240 Chapter Three

Triaxial stresses and ductility. The commonly reported values for duc-
tility of steel generally are obtained from uniaxial tensile coupons. The
same degree of ductility cannot be achieved under biaxial or triaxial
loading conditions. This is particularly significant since residual
stresses are always present in any as-welded structure.
Flat position welding. Whenever possible, it is desirable to orient weld
details so that the welding can be performed in the flat position, tak-
ing advantage of gravity which helps hold the molten weld metal in
place. These welds are made with a lower requirement for operator
skill, and at the higher deposition rates that correspond to more eco-
nomical fabrication. This is not to say, however, that overhead weld-
ing should be avoided at all costs. An overhead weld may be advanta-
geous if it allows for double-sided welding and a corresponding
reduction in the weld volume. High-quality welds can be made in the
vertical plane and with the welding consumables available today, can
be made at an economical rate.

3.9.3 Unique aspects of seismically loaded
welded structures

Demands on structural systems. Structures designed for seismic
resistance are subject to extreme demands during earthquakes. By
definition, any structure designed with an R greater than unity will

be loaded beyond the yield stress of the material. This is far more
demanding than other anticipated types of loading. Because of the
inherent ductility of the material, stress concentrations within a
steel structure are gradually distributed by plastic deformation. If
the materials have a moderate degree of notch toughness, this redis-
tribution eliminates localized areas of high stress, whether due to
design, material, or fabrication irregularities. For statically loaded
structures, the redistribution of stresses is of little consequence. For
cyclically loaded structures, repetition of this redistribution can lead
to fatigue failure. In seismic loading, however, it is expected that por-
tions of the structure will be loaded well beyond the elastic limit,
resulting in plastic deformation. Localized areas of high stress will
not simply be spread out over a larger region by plastic deformation.
The resultant design, details, materials, fabrication, and erection
must all be carefully controlled in order to resist these extremely
demanding loading conditions.
Demand for ductility. Seismic designs have relied on “ductility” to pro-
tect the structural system during earthquakes. Unfortunately, much
confusion exists regarding the measured property of ductility and steel,
and ductility can be experienced in steel configured in various ways.

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