Page 21 - Handbook of Structural Steel Connection Design and Details

Fasteners and Welds for Structural Connections

6 Chapter One

moving primarily by the bolt shank). In bearing-type connections, bolt
threads may be included in or excluded from the shear plane. Different
design strengths are used for each condition. Also, bearing-type connec-
tions may be either pretensioned or snug-tight, subject to the limita-
tions already discussed. Snug-tight bolts are much more economical to
install and should be used where permitted. The slip-critical connection
is the most expensive, because it requires that the faying surfaces be free
of paint, grease, and oil, or that a special paint be used. Hence this type
of connection should be used only where required by the governing design
specification, for example, where it is undesirable to have the bolts slip
into bearing or where stress reversal could cause slippage. The 2005
AISC specification requires the use of slip-critical connections when
(a) Bolts are installed in oversized holes
(b) Bolts are installed in slotted holes with the direction of the load par-
allel to the slot
The RCSC specification further requires slip-critical connections for

(c) Joints that are subject to fatigue load with reversal of the loading
direction
(d) Joints in which slip at the faying surfaces would be detrimental to
the performance of the structure.

The 2005 AISC specification includes provisions for designing slip-
critical connections at either the strength level or the serviceability
level. As the name implies the serviceability limit state assumes that
slip in the joint would affect only the serviceability of the structure and
not lead to collapse. The minimal slip that could occur in a joint with
standard holes is generally thought to be negligible. Therefore the
specification recommends that joints utilizing standard holes or slots
perpendicular to the load should be designed at the serviceability level.
In contrast, connections where slip at the joint could lead to a collapse,
should be designed considering slip as a strength level limit state. The
specification conservatively recommends designing joints utilizing over-
sized holes or slots parallel to the direction of the load at the strength
level. However, the choice of strength versus serviceability is ultimately
left to the discretion of engineer. If for example during the design of the
main members the P-∆ effects resulting from joint slip are considered,
the connection could safely be designed with slip as a serviceability
limit state.
Threads included in shear planes. The bearing-type connection with
threads in shear planes is most frequently used. Since location of threads
is not restricted, bolts can be inserted from either side of a connection.

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