Page 90 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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CONNECTIONS

3.22 CHAPTER THREE

end of the stay plate. Other reinforcing plates should extend at least 6 in beyond the near edge. All
plates should be connected with fasteners or welds arranged to transmit the bearing pressure uni-
formly over the full section.
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In buildings, pin hole diameters should not exceed pin diameters by more than / 32 in. The length
of pin should be sufficient to secure full bearing on the turned body of the pin of all connected parts.
Pins should be secured in position and connected material restrained against lateral movement on the
pins. In building work, a pin may be secured with cotter pins (Fig. 3.13c and d). The most econom-
ical method is to drill a hole in each end for cotter pins. However, this method can be used only for
horizontal pins. When a round pin must be turned down to obtain the required fit, a head can be
formed to hold the pin at one end. The other end can be held by a cotter pin or threaded for a nut.
This headed pin can be used in vertical installations with the head at the upper end.

3.2.12 General Considerations for Welds
Welded connections are used because of simplicity of design, fewer parts, less material, and decrease
in shop handling and fabrication operations. Frequently, a combination of shop welding and field
bolting is advantageous. With connection angles shop welded to a beam, field connections can be
made with high-strength bolts without the clearance problems that may arise in an all-bolted
connection.
Weldable structural steels permissible in buildings are listed in AISC Specification A3. Matching
electrodes are given in American Welding Society AWS D1.1 (Table 3.1).
Welded connections have a rigidity that can be advantageous if properly accounted for in design.
Welded trusses, for example, deflect less than bolted trusses, because the end of a welded member
at a joint cannot rotate relative to the other members there. If the end of a beam is welded to a col-
umn, the rotation there is practically the same for column and beam.
A disadvantage of welding, however, is that shrinkage of large welds must be considered. This is
particularly important in large structures, where there will be an accumulative effect.
Properly made, a weld is stronger than the base metal. Improperly made, even a good-
looking weld may be worthless. Properly made, a weld has the required penetration and is not
brittle.
Prequalified joints, welding procedures, and procedures for qualifying welders are covered by
AWS D1.1, “Structural Welding Code—Steel.” Common types of welds with structural steels,
intended for welding when made in accordance with AWS specifications, can be specified by note or
by symbol with assurance that a good connection will be obtained.
In making a welded design, designers should specify only the amount and size of weld actually
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required. Generally, a / 16-in weld is considered the maximum size for a single pass. A / 8-in weld,
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while only / 16 -in larger, requires three passes and engenders a great increase in cost.
The cost of fit-up for welding can range from about one-third to several times the cost of weld-
ing. In designing welded connections, therefore, designers should consider the work necessary for
the fabricator and the erector in fitting members together so they can be welded.
3.2.13 Types of Welds
The main types of welds used for structural steel are fillet, groove, plug, and slot. The most com-
monly used weld is the fillet. For light loads, it is the most economical, because little preparation of
material is required. For heavy loads, groove welds are the most efficient, because the full strength
of the base metal can be obtained easily. Use of plug and slot welds generally is limited to special
conditions where fillet or groove welds are not practical.
More than one type of weld may be used in a connection. If so, the available strength of the con-
nection is the sum of the available strengths of each type of weld used, separately computed with
respect to the axis of the group.
Tack welds may be used for assembly or shipping. They are not assigned any stress-carrying
capacity in the final structure. In some cases, these welds must be removed after final assembly or
erection.

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