Causes and analysis of steel structural failures - Assessment and Refurbishment of Steel Structures - page 38

hardness increase). Cracks can be found in the thermal zone of welds, e.g. when welding
thick plates (Figure 2.17), some cracks will form particularly in the very rigid parts of the
structure, i.e. in the upper part of the column and in its base. When thick plates are
connected into a rigid node, the possibility of local deformations during welding is
limited.
Figure 2.17 Structural details and
localization of cracks: (a) base (b)
upper part of the column
In Figure 2.18, the influence of rigidity of the nodes is revealed by the great number of
cracks in the upper part of the column, where the thick plates were reinforced by
additional elements.
2.3.2 Steels liable to brittle fracture
Failures caused by brittle fracture have resulted in serious collapses, particularly of steel
bridges and ships, which has led to extensive research of this problem. There are some
known cases of brittle fracture of elements without loads from external forces; these
originate in rigid structural elements where there are very high residual stresses from
rolling or welding. When fracture liable steels are cut using oxygen, brittle fracture of
elements loaded by high residual stresses may occur. Constructional parts can also be
broken by brittle fracture at low stress (considerably lower than the design strength) when
the material is not tenacious enough. Brittle cracks propagate at a high speed of 1200
m.s −1 , so they can cause immediate collapse of the structure. Brittle fracture depends
particularly on:
• Tenacity of materials (which depends on temperature)
• Size and orientation of defects, especially planar
• Level of stress, including the influence of residual voltages
• Rate of loading
• Material thickness.