Page 56 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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FABRICATION AND ERECTION*

2.18 CHAPTER TWO

steel from corrosion, but only as long as this scale is intact and adheres firmly to the steel. Intact mill
scale, however, is seldom encountered on fabricated steel because of weathering during storage and
shipment and because of loosening caused by fabricating operations. Undercutting of mill scale, which
can lead to paint failure, is attributable to the broken or cracked condition of mill scale at the time of
painting. When structural steel is exposed to view, even small amounts of mill scale lifting and result-
ing rust staining will likely detract from the appearance of a building. On industrial buildings, a little
rust staining might not be objectionable. But where appearance is of paramount importance, descaling by
blast cleaning is the preferred way of preparing the surface of architecturally exposed steel for painting.
Steels are available which can be exposed to the weather and can be left unpainted, such as A588
steel. This weathering steel forms a tight oxide coating that will retard further atmospheric corrosion
under common outdoor exposures. Many bridge applications are suited to this type of steel. Where the
steel would be subjected to salts around expansion devices, owners often choose to paint that area. The
steel that is to be left unpainted is generally treated in one of two ways, depending on the application.
For structures where appearance is not important and minimal maintenance is the prime consid-
eration, the steel may be erected with no surface preparation at all. While it retains mill scale, the
steel will not have a uniform color. But when the scale loses its adherence and flakes off, the exposed
metal will form the tightly adherent oxide coating characteristic of this type of steel, and eventually,
a uniform color will result.
Where uniform color of bare, unpainted steel is important, the steel must be freed of scale by blast
cleaning. In such applications, extra precautions must be exercised to protect the blasted surfaces
from scratches and staining.
Steel may also be prepared by grinding or blasting to avoid problems with welding through heavy
scale or to achieve greater nominal loads or allowable loads in slip-critical bolted joints.
(Steel Structures Painting Manual, vol. I, Good Painting Practice, vol. II, Systems and
Specifications, Society for Protective Coatings, 40 24th St., Pittsburgh, PA 15222.)

2.9 FABRICATION TOLERANCES

Variations from theoretical dimensions occur in hot-rolled structural steel because of the routine pro-
duction process variations and the speed with which they must be rolled, wear and deflection of the
rolls, human differences between mill operators, and differential cooling rates of the elements of a
section. Also, mills cut rolled sections to length while they are still hot. Tolerances that must be met
before structural steel can be shipped from mill to fabricator are listed in ASTM A6, “General
Requirements for Delivery of Rolled Steel Plates, Shapes, Sheet Piling and Bars for Structural Use.”
Tolerances are specified for the dimensions and straightness of plates, hot-rolled shapes, and bars.
For example, flanges of rolled beams may not be perfectly square with the web and may not be per-
fectly centered on the web. There are also tolerances on surface quality of structural steel.
Specifications covering fabrication of structural steel do not, in general, require closer tolerances
than those in A6, but rather extend the definition of tolerances to fabricated members. Tolerances for
the fabrication of structural steel, both hot-rolled and built-up members, can be found in standard
codes, such as the AISC “Specification for Structural Steel Buildings”; the AISC “Code of Standard
Practice for Steel Buildings and Bridges”; AWS D1.1, “Structural Welding Code-Steel”; AWS D1.5,
“Bridge Welding Code”; and AASHTO specifications.
The tolerance on length of material as delivered to the fabricator is one case where the tolerance
as defined in A6 may not be suitable for the final member. For example, A6 allows wide flange beams
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24 in or less deep to vary (plus or minus) from ordered length by / 8 in plus an additional / 16 in for each
additional 5-ft increment over 30 ft. The AISC specification for length of fabricated steel, however,
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allows beams to vary from detailed length only / 16 in for members 30 ft or less long and / 8 in for
members longer than 30 ft. For beams with framed or seated end connections, the fabricator can toler-
ate allowable variations in length by setting the end connections on the beam so as to not exceed the
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overall fabrication tolerance of ± / 16 or ± / 8 in. Members that must connect directly to other members,
without framed or seated end connections, must be ordered from the mill with a little additional length
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to permit the fabricator to trim them to within ± / 16 or ± / 8 in of the desired length.
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