Page 44 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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FABRICATION AND ERECTION*

2.6 CHAPTER TWO

includes flame and plasma cutting. Laser cutting is also included but is not yet prevalent in structural
fabrication shops. Lasers offer good cut-surface profiles and little thermal distortion but are cur-
rently expensive and limited in the thicknesses they can handle. Flame cutting may be manual or
mechanically guided or automated. AISC encourages the use of a guide where practical. A guide can
be as simple as a bar clamped to the work surface. Automation in flame and plasma cutting include
torches mounted on self-propelled buggies and mounted on tracks. Tracks can be rigid for straight
cuts or flexible for curves. Cutting tables are used to strip long lengths of plate to the needed width.
Tables can be fitted with six or more torches to cut one plate into many pieces at one time. Tables
can be fitted with devices that trace templates and copy the pattern onto the plate. Much more com-
mon in recent times are tables of various sizes that are numerically controlled. These burning tables
can be coordinated with numerically controlled punches or drills.
In the flame-cutting process, the torch burns a mixture of oxygen and gas to bring the steel at
the point where the cut is to be made to a preheat temperature of about 1600°F. At that temperature, the
steel has a great affinity for oxygen. The torch then releases pure oxygen under pressure through
the cutting tip. This oxygen combines immediately with the steel. As the torch moves along the cut line,
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the oxidation, coupled with the erosive force of the oxygen stream, produces a cut about / 8 in wide. Once
cutting begins, the heat of oxidation helps to heat the material. Structural steel of certain grades and
thicknesses may require additional preheat. In those cases, flame is directed to the metal ahead of the cut.
In such operations as stripping plate-girder flange plates, it is desirable to flame-cut both edges
of the plate simultaneously. This limits distortion by imposing shrinkage stresses of approximately
equal magnitude in both edges of the plate. For this reason, plates to be supplied by a mill for mul-
tiple cutting are ordered with sufficient width to allow a flame cut adjacent to the mill edges. It is not
uncommon to strip three flange plates at one time using four torches.
Plasma-arc cutting is an alternative process for steel fabrication. A tungsten electrode may be used,
but hafnium is preferred because it eliminates the need for expensive inert shielding gases. Advantages
of this method include faster cutting, easy removal of dross, and lower operating cost. Disadvantages
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include higher equipment cost, limitation of thickness of cut to 1 / 2 in, slightly beveled edges, and a
wider kerf. Plasma is advantageous for stainless steels that cannot be cut with oxyfuel torches.
Shearing is used in the fabricating shop to cut certain classes of plain material to size. Several
types of shears are available. Guillotine-type shears are used to cut plates of moderate thickness.
Some plate shears, called rotary-plate shears, have a rotatable cutting head that allows cutting on a
bevel. Angle shears are used to cut both legs of an angle with one stroke. Rotary-angle shears can
produce beveled cuts.
Cutting to length of main material shapes can be done by the steel producer, a service center, or a
processor, but is most often done by the fabricator. Steel mills cut shapes with high-speed friction saws
that are fast enough to cut the strand into separate lengths as the strand comes off the rolls. The cut sur-
face is suitable for some applications but not for others. Structural fabrication shops cut main material
shapes to length with thermal cutting or with saws. Band saws and cold saws are most common, but
machine hacksaws and friction saws can be used. The choice of saw depends on the size being cut and
the application requirements. Band and cold saws can produce cuts suitable and accurate enough for
most applications, including bearing surfaces of columns, without further machining. The suitability of a
cut for a particular application depends on maintenance of blades and the way the saw is set up.
Bolt holes can be drilled, punched, or, in some cases, cut thermally. The three methods each have
different effects on the surrounding material. Strength limit states are established for structures sub-
ject to static loads and seismic loads such that any hole-forming method can be used. AASHTO
requires drilled or subpunched and reamed holes in main member connections. The selection of the
hole-forming method to be used depends on the thickness of the material and the hole size, the num-
ber of pieces with identical hole patterns, and the other operations that have to be performed on the
material. In the abstract general case, punching is fast but is limited in the thickness that can be
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punched. AISC used to limit punching to the diameter of the hole plus / 8 in. While that limit no
longer exists in the specification, because of advances in capabilities of available equipment, it is still
a practical rule of thumb for routine use. Practically, punching is limited by the capacity of the
machine and the punch itself relative to the ultimate strength of the material at the hole perimeter.
Punching is a form of shearing whereby the material is sheared in a ductile fashion until the punch

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