Page 383 - Forensic Structural Engineering Handbook

11.14 MATERIAL-SPECIFIC FORENSIC ANALYSES

ENGINEERING PROPERTIES OF STRUCTURAL
STEELS

General

A review of the compositions and properties of structural steels indicates that, in spite of
the variety of specifications, most structural steels share the same basic composition: car-
bon contents of 0.10 to 0.30 percent, manganese contents of 0.5 to 1.3 percent, silicon con-
tents up to 0.4 percent, and phosphorus and sulfur contents up to 0.05 percent (as residuals
from the steelmaking process). For grades which require greater corrosion resistance, or for
which greater strength or toughness is desired, alloy additions of copper, nickel, chromium,
and molybdenum up to 0.7 percent may be employed. The vast majority of these materials
are not heat-treated, but rather their microstructures and properties reflect the hot rolling
and air cooling cycles that result from the production of plates and shapes (it is for this rea-
son that most structural steel do not sustain significant deterioration in mechanical proper-
ties during a fire). Since the microstructure is coarser in heavier sections, the resulting
strength is lower, which is often compensated for by increasing the carbon and/or alloy con-
tent as the section thickness increases. This is reflected in some ASTM specifications. From
the late 1960s until present, structural steels with higher strength, toughness, or corrosion
resistance are increasingly being used. Some of these steels require heat treatment to
achieve their properties, typically quenching and tempering, and they usually have higher
alloy content. The typical strength of structural steels, as measured by the normal tension
testing, has also gradually increased over the last 50 years. The typical, as-measured yield
strength of earlier steels was 35 to 40 ksi (240 to 275 MPa), while tensile strengths were 60
to 70 ksi (415 to 485 MPa). Older structural steels exhibited yield strengths closer to 30 ksi
(210 MPa). Currently, many structures utilize steels with yield strengths of 50 ksi (345 MPa),
and tensile strengths over 65 ksi (450 MPa), with some having yield strengths as high as
100 ksi (690 MPa) and tensile strengths greater than 115 ksi (795 MPa).
Toughness requirements for structures, typified by the use of Charpy V-notch (CVN)
impact test specifications, have also grown steadily in the last 50 years. Most structures
built prior to 1950 were designed without any toughness requirements. Analysis of the brit-
tle fracture of ships in World War II led to the development of the Charpy impact test and
to toughness requirements for some ship hull steels. In the 1950s and 1960s, toughness
requirements were extended to pressure vessels and in the 1970s, to bridges. The 1960s and
1970s were also the period during which new and more sophisticated methods of analysis
of brittle fractures, primarily using the concepts of fracture toughness and fracture mechan-
ics, were developed for aerospace structures, and are now widely applied for many other
structures. Results of these efforts have been an increasing expectation that structural steels
will be both strong and tough. It should be remembered during forensic or root cause analy-
sis of older structures that toughness requirements were typically not a part of the design,
materials selection, or construction process, and are still not a part of the design or materi-
als selection process for most buildings. In root cause analysis, however, an understanding
of the toughness properties of the steels involved in a failure may be the key to explaining
the sequence of events, whether or not the properties were part of design consideration.

Strength and Strength-Related Properties
The most common measure of the resistance of structural steel to failure is the strength of
the members. The normal strength attributes for structural steel are yield and tensile
strength, but ductility characteristics (i.e., total elongation and reduction of area) also play