Page 254 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

Welded Joint Design and Production 239

beads that were formerly in tension will have compression induced
by the subsequent passes.
For relatively flexible assemblages, these residual stresses induce
distortion. As assemblages become more rigid, the same residual
stresses can cause weld cracking problems, typically occurring near
the time of fabrication. If distortion does not occur, or when cracking
does not occur, the residual stresses do not relieve themselves but are
“locked in.” Residual stresses are considered to be at the yield point of
the material involved. Because any area that is subject to residual
tensile stress is surrounded by a region of residual compressive
stress, there is no loss in overall capacity of as-welded structures.
This does reduce the fatigue life for low-stress-range, high-cycle appli-
cations, which are different from seismic loading conditions.
Small welded assemblies can be thermally stress-relieved where
the steel is heated to 1150°F, held for a predetermined length of time
(typically 1 h/in of thickness), and allowed to return to room tempera-
ture. Residual stresses can be reduced by this method, but they are
never totally eliminated. This type of approach is not practical for
large assemblies, and care must be exercised to ensure that the com-
ponents being stress-relieved have adequate support when at the ele-
vated temperature where the yield strength and the modulus of elas-
ticity are greatly reduced as opposed to room-temperature properties.
For most structural applications, the residual stresses cause no par-
ticular problem to the performance of the system, and owing to the
complications of stress-relief activities, welded structures commonly
are used in the as-welded condition.
When loads are applied to as-welded structures, there is some
redistribution or gradual decrease in the residual stress patterns.
Typically called shake-down, the thermal expansion and contraction
experienced by a typical structure as it goes through a climatic sea-
son, as well as initial service loads applied to the building, result in a
gradual reduction in the residual stresses from welding.
These residual stresses should be considered in any structural
application. On a macro level, they will affect the erector’s overall
sequence of assembly of a building. On a micro level, they will dictate
the most appropriate weld bead sequencing in a particular groove-
welded joint. For welding applications involving repair, control of
residual stresses is particularly important since the degree of
restraint associated with weld repair conditions is inevitably very
high. Under these conditions, as well as applications involving heavy,
highly restrained, very thick steel for new construction, the experi-
ence of a competent welding engineer can be helpful in avoiding the
creation of unnecessarily high residual stresses, thus alleviating
cracking tendencies.

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