Page 26 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch01.qxd 9/29/05 4:59 PM Page 1.24

PROPERTIES OF STRUCTURAL STEELS AND EFFECTS OF STEELMAKING AND FABRICATION

1.24 CHAPTER ONE

lamellar tearing. This is a cracking phenomenon that starts underneath the surface of steel plates as
a result of excessive through-thickness strain, usually associated with shrinkage of weld metal in
highly restrained joints. The tear has a steplike appearance consisting of a series of terraces parallel
to the surface. The cracking may remain completely below the surface or may emerge at the edges
of plates or shapes or at weld toes.
Careful selection of weld details, filler metal, and welding procedure can restrict lamellar tearing
in heavy welded constructions, particularly in joints with thick plates and heavy structural shapes.
Also, when required, structural steels can be produced by special processes, generally with low sulfur
content and inclusion control, to enhance through-thickness ductility.
The most widely accepted method of measuring the susceptibility of a material to lamellar tear-
ing is the tension test on a round specimen, in which is observed the reduction in area of a section
oriented perpendicular to the rolled surface. The reduction required for a given application depends
on the specific details involved. The specifications to which a particular steel can be produced are
subject to negotiations with steel producers.
(R. L. Brockenbrough, Chap. 1.2 in Constructional Steel Design—An International Guide,
R. Bjorhovde et al., eds., Elsevier Science Publishers, New York.)

1.16 WELDED SPLICES IN HEAVY SECTIONS

Shrinkage during solidification of large welds in structural steel members causes, in adjacent restrained
metal, strains that can exceed the yield-point strain. In thick material, triaxial stresses may develop
because there is restraint in the thickness direction as well as in planar directions. Such conditions
inhibit the ability of a steel to act in a ductile manner and increase the possibility of brittle fracture.
Therefore, for members subject to primary tensile stresses due to axial tension or flexure in buildings,
the American Institute of Steel Construction (AISC) Specification for Structural Steel Buildings
imposes special requirements for welded splicing of either hot-rolled shapes with a flange thickness
more than 2 in thick or of shapes built up by welding plates more than 2 in thick. The specifications
include requirements for notch toughness, generous-sized weld-access holes, preheating for thermal
cutting, and grinding and inspecting cut edges. Even for primary compression members, the same
precautions should be taken for sizing weld access holes, preheating, grinding, and inspection.
Most heavy wide-flange shapes and tees cut from these shapes have regions where the steel has
low toughness, particularly at flange-web intersections. These low-toughness regions occur because
of the slower cooling there and, because of the geometry, the lower rolling pressure applied there
during production. Hence, to ensure ductility and avoid brittle failure, bolted splices should be considered
as an alternative to welding.
“Specification for Structural Steel Buildings,” American Institute of Steel Construction; R. L.
Brockenbrough, Sec. 9 in Standard Handbook for Civil Engineers, 4th ed., McGraw-Hill, New York.)

1.17 k-AREA CRACKING

Wide flange sections are typically straightened as part of the mill production process. Often a rotary
straightening process is used, although some heavier members may be straightened in a gag press.
Some reports have indicated a potential for crack initiation at or near connections in the “k” area of
wide flange sections that have been rotary straightened. The k area is the region extending from approx-
1
imately the mid-point of the web-to-flange fillet, into the web for a distance approximately 1 to 1 / 2 in
beyond the point of tangency. In some cases, this limited region had a reduced notch toughness due to
cold working and strain hardening. Most of the incidents reported occurred at highly restrained joints
with welds in the k area. However, the number of examples reported was limited and these occurred
during construction or laboratory tests, with no evidence of difficulties with steel members in service.
Most of the concern was related to welding of continuity plates and doubler plates in beam-to-
column connections. Recent research has shown that such cracking can be avoided if the continuity plates

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