Page 55 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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FABRICATION AND ERECTION*

FABRICATION AND ERECTION 2.17

The performance of crane girders is quite sensitive to the connection details used. Care must be
taken in design to consider the effects of wheel loads, out-of-plane bending of the web, and permit-
ting the ends of the girders to rotate as the crane travels along the length of the girder. The American
Iron and Steel Engineers and the AISC both provide information concerning appropriate details.
Horizontally curved plate girders for bridges constitute a special case. Two general methods
are used in fabricating them. In one method, the flanges are cut from a wide plate to the prescribed
curve. Then the web is bent to this curve and welded to the flanges. In the second method, the girder
is fabricated straight and then curved by application of heat to the flanges. This method, which is
recognized by the AASHTO specifications, is preferred by many fabricators because less scrap is
generated in cutting flange plates, savings may accrue from multiple welding and stripping of flange
plates, and the need for special jigs and fittings for assembling a girder to a curve is avoided.
(“Fabrication Aids for Continuously Heat-Curved Girders” and “Fabrication Aids for Girders
Curved with V-Heats,” American Institute of Steel Construction, Chicago, Ill.)
Procedures used in fabricating other built-up sections, such as box girders and box columns, are
similar to those for welded girders.
Columns generally require the additional operation of end finishing for bearing. For welded
columns, all the welds connecting main material are made first, to eliminate uncertainties in length
due to shrinkage caused by welding. After the ends are finished, detail material, such as connection
plates for beams, is added.
The selection of connection details on built-up sections has an important effect on fabrication
economy. If the pieces making up the section are relatively thick, welded details can provide bolt
holes for connections and thereby eliminate punching the thick material. On the other hand, fabrica-
tors that trim sections at the saw after assembly may choose to drill holes using a combination drill-
saw line, thus avoiding manual layout for welded detail material.

2.8 CLEANING AND PAINTING

The AISC “Specification for Structural Steel Buildings” provides that, in general, steelwork to be
concealed within the building need not be painted and that steel encased in concrete should not be
painted. Inspection of old buildings has revealed that the steel withstands corrosion virtually the
same whether painted or not.
Paint is expensive to apply, creates environmental concerns in the shop, and can create a slip haz-
ard for erectors. Environmental requirements vary by region. Permitting flexibility in coating selec-
tion may lead to savings. When paint is required, a shop coat is often applied as a primer for
subsequent field coats. It is intended to protect the steel for only a short period of exposure.
Many fabricators have invested in the equipment and skills necessary to apply sophisticated coat-
ings when required. Compared with single-coat, surface-tolerant primers used in normal applica-
tions, these multiple-coat or special systems are sensitive to cleaning and applicator skill. While
these sophisticated coating systems are expensive, they can be useful when life cycle costs are con-
sidered in very-long-term exposures or aggressive environments.
Steel which is to be painted must be thoroughly cleaned of all loose mill scale, loose rust, dirt,
and other foreign matter. Cleaning can be done by hand tool, power tool, and a variety of levels of
abrasive blasting. Abrasive blasting in most fabrication shops is done with centrifugal wheel blast
units. The various surface preparations are described in specifications by the Society for Protective
Coatings. Unless the fabricator is otherwise directed, cleaning of structural steel is ordinarily done
with a wire brush. Sophisticated paint systems require superior cleaning, usually abrasive blast
cleaning and appropriate quality systems. Knowledge of the coating systems, equipment mainte-
nance, surface preparation, and quality control are all essential.
Treatment of structural steel that will be exposed to close public view varies somewhat from that for
steel in unexposed situations. Since surface preparation is the most important factor affecting performance
of paint on structural steel surfaces, it is common for blast cleaning to be specified for removing all mill
scale on steel that is to be exposed. Mill scale that forms on structural steel after hot rolling protects the

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