The Design of Modern Steel Bridges - TEDI

5.4.3 Effect of residual stresses
Residual stresses in rolled steel sections are mainly caused by uneven cooling
after rolling; in sections fabricated by welding together several plates, the
residual stresses are caused by the shrinkage of the material in and adjacent to
the weld. In rolled I-sections, the flange tips cool first, but the delayed cooling
of the interior parts causes compressive stresses along the flange tips; the
junction between the flange and the web stays hot the longest and is thus
subjected to tensile stresses as the adjacent colder parts tend to prevent its
shrinkage. For equilibrium, the tensile and compressive longitudinal forces in
the cross-section must balance. A typical residual stress pattern in a rolled
I-section is shown in Fig. 5.15. Compressive stress along the tip may be of the
order of 100–150 N/mm 2 .
Welding or flame-cutting is associated with very high temperatures in a
localised strip. Shrinkage due to cooling of this strip is resisted by the
remaining cold portion of the steelwork. As a result, the strip adjacent to the
weld or flame cut is subjected to high tensile strains which may be several
times the yield strain, and the rest of the steelwork is subjected to compression.
A typical pattern is shown in Fig. 5.16. The shrinkage force due to welding can
be expressed as (CAw), where Aw is the cross-sectional area of the weld
deposited and C is a constant dependent upon the welding process adopted.
C has been found experimentally to vary from 7.5 to 12.5 kN/mm 2 ; the lower
values in this range are typical of manual welding and the higher values are
associated with submerged arc welding. In multi-welds, if the steelwork is
allowed to cool down to room temperature between successive weld passes, A w
is the area of weld deposited in one pass. A simplified pattern of the residual
stresses may be derived by assuming the tensile stresses in the strip adjacent to
the weld and the compressive stresses in the remaining area to be uniform in
their respective areas, and the former to be equal to the yield stress sy of the
Figure 5.15 Residual stresses in rolled sections.
Rolled Beam and Plate Girder Design 121