Page 376 - Forensic Structural Engineering Handbook

STEEL STRUCTURES 11.7

• Corrosion and stress corrosion cracking
• Fatigue
• Creep
• Metallurgical degradation

The most relevant aspects of these steel failure-damage modes and limit states are dis-
cussed in the following paragraphs.

Yielding and Ductile Failure
Of the failure modes which occur in steel structures, ductile failure or excessive distortion
(i.e., yielding or inelastic/nonrecoverable deformation/strain) occurs least often; yet
almost every design code for steel that utilizes yield strength as the critical design para-
6
meter (e.g., AISC Steel Construction Manual and ASME Boiler and Pressure Vessel
7
Code ), is devoted to the prevention of ductile failure. Yielding is a failure mode in com-
ponents wherein the elastic limit is exceeded. This can result in visible deformation as
members are stretched or bent. Ductile failure is usually characterized by excessive inelas-
tic (nonrecoverable) deformation prior to attaining the steel’s ultimate tensile strength
whereupon tearing or fracture occurs. Often this is at a connection where the section is
reduced by holes or geometric changes; large deformations and necking of the material are
often apparent. A beneficial aspect of ductile behavior is the large amount of energy
absorption that occurs prior to failure. In many instances extreme or unanticipated loads
can be redistributed within the structure without the consequence of failure. Frequently,
excessive deformation occurs over a long enough period of time that the structure can be
stabilized or abandoned before unacceptable property damage or injuries occur. In most
steel failures, excessive deformation and ductile fracture are consequential and not the
root cause of failure.
Ductile fractures are generally irregular in appearance, exhibit shear lips and localized
“necking-down” (thinning of the cross-section). An example of ductile fracture in a bridge
wind chord is shown in Fig. 11.1. On a microscopic level, ductile fractures exhibit a frac-
ture morphology known as microvoid coalescence, as shown in Fig. 11.2.
More often than not, ductile fractures can also be identified by the appearance of the
adjacent painted surface or mill scale. If the steel structure has sustained inelastic defor-
mation or applied loads in excess of the yield strength, the paint and/or mill scale—due to
its brittle nature—cracks or crazes in bands perpendicular to the direction of the local prin-
cipal stress.
The deformation capacity of a steel structure can be affected by the service temperature,
loading rate, and level of constraint. As the temperature decreases and/or the loading rate
and constraint increase, the capacity for deformation, particularly at notches (stress con-
centrations) decreases and the structure can undergo a transition from ductile-to-brittle
behavior.

Brittle Fracture
Brittle fractures occur with little or no deformation and, therefore, with little or no warn-
ing. Such fractures initiate and propagate through steel structures at very high speeds
(approaching the speed of sound in steel) often at stress levels below the yield strength
or even below design allowable stress levels. The rapidity of brittle fracture propagation