Page 404 - Forensic Structural Engineering Handbook

STEEL STRUCTURES 11.35

with yield strengths greater than 100 ksi (690 MPa), but it can also occur in carbon and
stainless steels at much lower strength levels. This has been a special concern in the chem-
ical process industries where a number of types of stainless steels are used in aggressive
solution service. Some large pressure vessel failures have been attributed to this cause.
Since this process is controlled by the composition of the steel, the level of stress, and the
solution contained, welded structures with high residual stresses are particularly sensitive.
In some cases, these steels cannot be thermal-stress-relieved after welding due to adverse
metallurgical effects in the weld and heat-affected zone by the heat of stress relief; thus
careful selection of appropriate alloys for various types of service is essential.
In at least one case, inappropriate use of common 18 Cr (chromium), 8 Ni (nickel),
sometimes referred to as “18-8,” stainless steel led to a significant structural failure. It was
used in a concrete ceiling slab hanger system in a swimming pool in Switzerland in which
the hangers were exposed to chloride-containing moist air as a result of the action of a ven-
tilation system. This resulted in progressive failure of the hangers over time and in the ulti-
mate collapse of the ceiling with loss of life. The use of the same material in roof hangers
for ceiling slabs in highway tunnels has also resulted in hanger cracking at tunnel ends,
where salt spray from vehicle tires during winter has been pulled across the hangers by the
ventilation system. The intended philosophy was to provide a material that would resist
general corrosion as a tunnel hanger; however, the material selected was stress-corrosion-
sensitive in chloride environments, creating dangerous potential for cracking.
Stress corrosion cracking has also been observed in high-strength steel fasteners and
other products that have exceeded specified hardness levels for atmospheric corrosion or
corrosion in other more aggressive environments. Indeed, the failure of the Silver Bridge
in West Virginia in 1969 with substantial loss of life was, in part, attributed to stress cor-
rosion cracking in a material of very low fracture toughness.
Stress corrosion has also been identified as one of the causes of cracking of wires in sus-
pension bridge main cables which have experienced long-time service. The wires were gen-
erally processed to a high strength level through mechanical deformation [greater than 200
ksi (1380 MPa) yield strength], and some, but not all, were galvanized. The general deteri-
oration of the galvanized coating through water intrusion in the cables has apparently cre-
ated electrochemical processes that lead to pitting corrosion, general corrosion, and stress
corrosion of the wires.
A system of corrosion protection widely employed in some structures is sacrificial pro-
tection. In the former, a more reactive metal, such as zinc or magnesium, is attached to, or
more commonly in civil engineering structures, coated on the steel. The more reactive metal
corrodes in preference to the steel, creating microscopic electric currents that protect the
steel surface underlying or adjacent to it. Galvanizing of steel structures or consumer prod-
ucts is the most well-known example of this protection system. Attachment of metal anodes
to ship hulls or pipelines is another system of corrosion protection. A similar protection sys-
tem using impressed current imposes a voltage differential between the structure and sur-
rounding environment to prevent corrosion. This system is sometimes used for pipelines.
In both cases these systems can fail. The sacrificial system fails when the coating or
anode material is consumed and can no longer protect the structure from rusting. The
impressed current system can fail when the imposed voltage is not maintained or is at an
insufficient level to counteract the corrosion conditions surrounding the structure.

REPAIR OF DAMAGED STRUCTURES

If the structure has not collapsed, cracked elements can almost always be repaired. The method
of repair will depend on the nature of the damage and geometric change, or misalignment that
has resulted from the crack. Depending on their magnitudes, excessive deformations can also