Page 86 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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CONNECTIONS

3.18 CHAPTER THREE

step of installation, when the plies are being brought into contact, the fastener must be removed
and replaced.
Direct-tension indicators are hardened washer-shaped discs with arched protrusions that flatten
when the specified tension is achieved. The protrusions must bear against the bolt head or nut or
against a hardened flat washer. If the protrusions flatten to the job-inspection gap while the connec-
tion is being brought into the snug-tight condition, the direct-tension indicator must be removed and
replaced.

3.2.6 Connection Resistance: Bearing versus Slip-Critical Connections

It is common for bolts in structural steel connections to be referred to as being either bearing bolts
or slip-critical bolts. This is a misnomer, since the same high-strength bolts can be used for both
bearing and slip-critical connections, though common bolts (A307) are restricted to use in bearing
connections. In bearing connections, movement within the joint is prevented through contact
between the shank of the bolt and the material. In slip-critical connections, movement of the joint is
resisted through the friction between the faying surfaces caused by the tension in the bolt. Therefore,
though either snug-tight or fully tensioned bolts may be installed in bearing-type connections, only
fully tensioned bolts may be installed in slip-critical connections.
By definition, slip-critical connections are required where slip cannot be tolerated, which would
seem to be a definitive statement, but in reality there is a range of intolerance to slip. This range can
be divided into two distinct levels, strength and serviceability.
Designing a slip-critical connection for serviceability can be viewed in two ways. It can be
viewed as designing for a lower safety factor against slip than when designing as a strength limit
state, or, more accurately, can be viewed as designing for an equal safety factor with a lower period
of nonexceedance. This is analogous to what is done with sway due to wind loads in tall buildings.
In such buildings, the limit states that govern strength and stability of the structure are designed to
perform satisfactorily for a 50- or 100-year storm, while the sway is limited based on an 8- to
10-year storm. Because of this approach, all limit states for bearing-type connections must also be
checked when designing against slip as a serviceability limit state. This includes bearing at the bolt,
bolt tear-out, and development of fills. Though currently required by the AISC Specification, it is the
opinion of the authors that these limit states, associated with bearing-type connections, need not be
checked when designing against slip as a strength limit state.
Slip-criticality should be considered as a strength limit state where slip in the connection could
be large enough to alter the usual analysis assumption that the undeformed structure can be used to
calculate the internal forces. Examples might include braced frames where oversized holes are used,
which could potentially result in large P-delta effects, or long-span roof trusses with oversized holes,
where slip could result in excessively large loads due to ponding.
Slip-criticality should be considered as a serviceability limit state where slip in the connection
would not violate the analysis assumptions of the structure. Since only a negligible amount of slip
can occur at bolts installed in standard holes, these connections should be designed for serviceabil-
ity, when slip must be resisted. Examples might include structures that contain sensitive communi-
cation or testing equipment, where slip is undesirable but would not result in structural failure. Slip
should also be viewed as a serviceability limit state when slip-critical connections are used for joints
subjected to fatigue load with reversal of the loading direction.
Slip-critical connections are required for very few situations in building design. The RCSC
Specification requires the use of slip-critical connections for the following conditions:

1. Joints that are subject to fatigue load with reversal of the loading direction
2. Joints that utilize oversized holes
3. Joints that utilize slotted holes, except those with applied load approximately normal (within 80°
to 100°) to the direction of the long dimension of the slot
4. Joints in which slip at the faying surface would be detrimental to the performance of the structure

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