Page 390 - Forensic Structural Engineering Handbook

STEEL STRUCTURES 11.21

to estimate static fracture toughness, K , directly from CVN results without using a tem-
1c
perature shift:
K ⎡ ⎤ 2 ⎛ CVN ⎞
⎢ c 1 ⎥ = 5 ⎜ ⎜ − . ⎟ ⎟ (11.9)
0 05
⎣ ⎢ σ y ⎦ ⎥ ⎝ σ y ⎠
CVN specimens should be cut so as not to weaken the existing structure and the notch
in the CVN specimens should be oriented parallel to the fracture surface at the site of fail-
ure initiation.
Moreover, once the stress (driving force for failure) and the material fracture toughness
are determined through testing or calculation, then the critical flaw size in other parts of the
structure or similar structures can also be determined. This knowledge allows for creation
of an inspection plan to seek additional flaws that may be subcritical but growing, and to
assess factors of safety. If the calculated critical flaw is too small to be readily detected, the
potential for fractures is high. The methods for the determination of critical flaw sizes in
structures are found in Refs. 3 and 8. It should be noted that many of the advances in fracture
mechanics that have found their way into design codes and standards both in the United States
and the rest of the world were facilitated by advances not only in steel making practice but in
theoretical Applied Mechanics. The advent of bigger and faster computers allowed for the
solution of many practical structural steel stress analysis problems which for reasons of time,
finances, and theoretical issues remained intractable. In particular, the analysis of the stress
fields associated with cracks or defects within structural steel members and, in particular,
the flaws and defects associated within the welds adjacent to structural steel connections.

STRUCTURAL STEEL CODES AND STANDARDS

Since most structures are intended to be in compliance with some industry- or government-
endorsed code or specification, structural failure analysis will inevitably involve a deter-
mination of whether the materials involved were or were not in compliance with the
relevant specification, and whether the structure was designed and fabricated in compliance
with the relevant code. In fact, many of these codes, standards, and specifications are inter-
locked by cross-references, creating a complex legal and engineering puzzle when a failure
or structural integrity analysis is required. For example, materials and their properties are
often referenced to ASTM Specifications. The ASTM Specifications usually include both
the chemical composition of the material and its mechanical properties. Occasionally they
also specify a heat treatment by which its properties are to be achieved. However, materi-
24
als may also be referenced to an American Iron and Steel Institute (AISI) standard or a sim-
ilar standard created by another metals industry (aluminum, copper, etc.). These standards
usually only specify composition, with properties being dependent on heat treatment (which
may or may not be part of the specification). Sometimes ASTM Specifications also refer to
AISI Standards or SAE (Society of Automotive Engineers) 25 specifications. Structural
design and fabrication codes, such as those of AISC (American Institute for Steel
26
Construction), AASHTO, AREMA (American Railway Engineering and Maintenance-of-
27
3
Way Association), and API (American Petroleum Institute), specify materials, sometimes
by reference to ASTM, but they also specify design rules and fabrication procedures, such
as bolting or welding requirements. When the latter are specified, it is normal to invoke the
AWS Structural Welding Code, D1.1, which specifies welding materials, procedures, and
procedure qualifications. On the other hand, some industries, such as the pressure vessel
industry, rely on the ASME Boiler and Pressure Vessel Code, a comprehensive code that