Page 54 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch02.qxd 9/29/05 5:01 PM Page 2.16

FABRICATION AND ERECTION*

2.16 CHAPTER TWO

Cover-plate material is ordered to multiple widths for flame cutting or stripping to the required
width in the shop. For this reason, when several different design conditions exist in a project, it is good
practice, as well as good economy, for the designer to specify as few different cover-plate thicknesses
as possible and to vary the width of plate for the different members.
Plate girders are specified when the moment capacity, stiffness, or on occasion, web shear capacity
cannot be obtained in a rolled beam. They usually are fabricated by welding.
Welded plate girders consist of a web plate, a top flange plate, a bottom flange plate, and stiffener
plates. Web material is ordered from the mill to the width between flange plates plus an allowance
for trim and camber, if required. Flange material is ordered to multiple widths for stripping to the
desired widths in the shop.
When an order consists of several identical girders having shop flange splices, fabricators usually
first lay the flange material end to end in the ordered widths and splice the abutting ends with the
required groove welds. The long, wide plates thus produced are then stripped to the required widths.
For this procedure, the flanges should be designed to a constant width over the length of the girder.
This method is advantageous for several reasons: Flange widths permit groove welds sufficiently
long to justify use of automatic welding equipment. Run-out tabs for starting and stopping the welds
are required only at the edges of the wide, unstripped plate. All plates can be stripped from one setup.
And much less finishing is required on the welds.
After web and flange plates are cut to proper widths, they are brought together for fit-up and final
welding. The web-to-flange welds, usually fillet welds, are positioned for welding with maximum
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efficiency. For relatively small welds, such as / 4- or / 16-in fillets, a girder may be positioned with
web horizontal to allow welding of both flanges simultaneously. The girder is then turned over, and
the corresponding welds are made on the other side. When relatively large fillet welds are required,
the girder is held in a fixture with the web at an angle of about 45° to allow one weld at a time to be
deposited in the flat position. In either method, the web-to-flange welds are made with automatic
welding machines that produce welds of good quality at a high rate of deposition. For this reason,
fabricators would prefer to use continuous fillet welds rather than intermittent welds, though an
intermittent weld may otherwise satisfy design requirements.
After web-to-flange welds are made, the girder is trimmed to its detailed length. This is not done
earlier because of the difficulty of predicting the exact amount of girder shortening due to shrinkage
caused by the web-to-flange welds.
If holes are required in web or flange, the girder is drilled next. This step requires moving the
whole girder to the drills. Hence, for economy, holes in main material should be avoided because of
the additional amount of heavy-load handling required. Instead, holes should be located in detail
material, such as stiffeners, which can be punched or drilled before they are welded to the girder.
The next operation applies the stiffeners to the web. Stiffener-to-web welds often are fillet welds.
They are made with the web horizontal. The welds on each side of a stiffener may be deposited
simultaneously with automatic welding equipment. For this equipment, many fabricators prefer con-
tinuous welds to intermittent welds. When welds are large, however, the girder may be positioned
for flat, or downhand, welding of the stiffeners.
Variation in stress along the length of a girder permits reductions in flange material. For minimum
weight, flange width and thickness might be decreased in numerous steps. But a design that optimizes
material seldom produces an economical girder. Each change in width or thickness requires a splice.
The cost of preparing a splice and making a weld may be greater than the cost of material saved to
avoid the splice. Therefore, designers should hold to a minimum flange splices made solely to save
material. Sometimes, however, the length of piece that can be handled may make splices necessary.
Welded crane girders differ from ordinary welded plate girders principally in that the upper surface
of the top flange must be held at constant elevation over the span. A step at flange splices is undesirable.
Since lengths of crane girders usually are such that flange splices are not made necessary by available
lengths of material, the top flange should be continuous. In unusual cases where crane girders are long
and splices are required, the flange should be held to a constant thickness. (It is not desirable to com-
pensate for a thinner flange by deepening the web at the splice.) Depending on other elements that con-
nect to the top flange of a crane girder, such as a lateral-support system or horizontal girder, holding the
flange to a constant width also may be desirable.

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