Page 375 - Forensic Structural Engineering Handbook

11.6 MATERIAL-SPECIFIC FORENSIC ANALYSES

The discussion of engineering properties of structural materials in this chapter
assumes some familiarity with both the properties of standard structural and alloy steels
and the basic behavior of steels as found in a variety of handbooks and textbooks. For
example, the mechanical properties of structural and other steels are found in current
editions of the Metals Handbook, (ASM International, Materials Park, OH 44073) or
within the current American Society for Testing Materials (ASTM) Specifications, the
most relevant being in volume 01.04, Steel-Structural Reinforcing, Pressure Vessel, and
Railway (American Society for Testing and Materials, 100 Bar Harbor Drive, West
Conshohocken, PA 19428).
In the presentation of failures related to design problems in this chapter, a basic knowl-
edge of allowable stress design (ASD) is also assumed. Until recent time, in one form or
another, this design philosophy was the principal one used in structures. The use of ASD is
widely discussed in basic textbooks such as Structural Steel Design, ASD Method by Jack
C. McCormick, 4th ed., HarperCollins, 1992. The pre-1986 versions of the American
Institute of Steel Construction Codes employed ASD.
In this chapter, however, the analysis of structures by load and resistance factor design
(LRFD) is recommended when a forensic investigation is undertaken. It is recognized that
many existing structures were designed on the basis of ASD concepts, and it may be, and
probably will be, necessary to use ASD to determine whether or not a failed structure was
originally designed in an acceptable manner. In a post-failure analysis, however, the ques-
tion to be answered is a broader one—not how it was designed, but why it failed. There is
substantial support for the use of LRFD analysis to determine the true capacity of struc-
tures, and for this reason, analysis on the basis of LRFD is emphasized in this chapter. A
basic text on LRFD concepts is LRFD Steel Design by William T. Sequi (PWS Publishing
Company, 1994). A text comparing ASD and LRFD is Design of Steel Structures by E. H.
Gaylord, Charles N. Gaylord, and James E. Stallmeyer, 3d ed., McGraw-Hill, 1992. This
latter text will also be useful to those who do not have a working familiarity with tension,
fatigue, and impact testing. The basics of these tests, which are discussed in the next sec-
tion, are found in Design of Steel Structures.

FAILURE-DAMAGE MODES AND LIMIT STATES
IN STEEL STRUCTURES

To properly assess the root cause of a steel structural failure, it is important to recognize the
potential failure modes, as well as the types of flaws and damage mechanisms that are most
likely to occur. Generally, failure modes are either instantaneous or progressive in nature;
i.e., they occur with little or no warning or they may be preceded by considerable crack
growth, deformation, or corrosion deterioration. Flaws and damage mechanisms can
develop during service or may be preexistent. For example, a fatigue crack at the toe of a
fillet weld arises due to in-service cyclic loading, whereas a weld fabrication discontinuity,
such as incomplete fusion, would be considered a pre-existent flaw. It is essential, there-
fore, that steel damage mechanisms be accurately assessed along with potential failure
modes so that proper root cause analysis can be performed. Thus, the basic failure-damage
modes or limit states that govern steel structures provide the basis for understanding why a
steel member, an assemblage of members, or the entire structure experiences distress or
failure. 4,5 These failure modes or limit states are as follows:
• Yielding and ductile failure
• Brittle fracture
• Buckling/instability