A Beginner's Guide to the Steel Construction Manual, 13^th ed. (old)

Chapter 4 - Bolted Connections

© 2006, 2007, 2008 T. Bartlett Quimby

Section 4.7

Slip Capacity

The limit state of slip is introduced in the specification in section J3.8 (SCM pg 16.1-109).  You will also want to read the discussion about slip-critical joints on SCM pg 7-5.  Note the statement in the second paragraph that states that slip-critical joints are rare in building design.  Part of the reason is the cost of surface preparation.  Another reason is that there are very few connections in a building that are subject to load reversal, fatigue, or where slip would cause adverse effects to serviceability.  Slip resistance should be considered when connections are subject to fatigue or the connections include oversized holes or slots parallel to the direction of load.

Slip-critical connections can be designed either for a serviceability limit state or at the required strength level.  The second paragraph of SCM section J3.8 clarifies when each of the two conditions are typically used.  For most cases, when using standard holes, the serviceability limit state is used.  The commentary on J3.8 (SCM pgs 16.1-346 to 16.1-349) should be read to understand the code requirements for slip resistance.

The Limit State:

The basic limit state follows the standard form.  The statement of the limit states and the associated reduction factor and factor of safety are given here:

The values of P_u and P_a are the LRFD and ASD factored loads, respectively, applied to the bolt. These forces are computed using the mechanics principles discussed in Section 4.3.

In this case R_n is the nominal shear strength of the bolt is computed using SCM equation J3-4:

R_n = mD_uh_scT_bN_s

[2010 Spec note:  In the 2010 Specification 'h_sc' has been replaced by a 'h_f' term which accounts for the presence of fillers.  'N_s' is also now 'n_s'. The hole size variable is now included in the determination of f_t  and W_t. The distinction used in the 2005 Specification to determine f_t  and W_t has been dropped.]

Where:

• m  is the coefficient of friction for the connected surfaces.  It is taken as 0.35 for Class A surfaces and 0.50 for Class B surfaces.  See SCM pg 16.1-110 for the required surface preparation for each case.
• D_u is a multiplier that accounts for average bolt pretension vs minimum required pretension and is taken as 1.13 unless otherwise determined by the engineer of record.
• h_sc is a hole factor that accounts for the effects of oversized or slotted holes.  See the SCM pg 16.1-110 for the values.
• T_b is the minimum specified bolt pretension.  This is obtained from SCM Table J3.1 (SCM pg 16.1-103)
• N_s number of slip planes.

Combined Tension and Shear in Slip Critical Connections

When applied tension is present in a connection, the tension reduces the clamping (normal) force between the connected parts, which reduces the capacity of the connection to resist slip due to shearing forces.

The approach taken by the specification is to linearly reduce the slip capacity as applied tension increases from zero to the pretension in the bolts.  The factor k_s is the reduction factor applied to the nominal slip resistance R_n.

R_n = mD_uh_scT_bN_sk_s

The equations (SCM equations J3-5, pg 16.1-110) for k_s vary a little for LRFD and ASD.  This is because one uses factored loads while the other does not.

Note that you can always include k_s in the R_n equation since k_s is 1.0, and hence transparent to the equation, when there is no applied tension (i.e. T_a or T_u = 0) on the connection. Applying k_s to SCM equation J3-4 results in:

Comments on Surface Preparation

When designating a connection as being slip critical, you are assuming a particular surface condition is present on the faying surfaces.  This assumption is used to determine the coefficient of friction, m, to be used in capacity equations.

As the computed capacity of the connection is critically dependent on the attainment of this surface condition, it is imperative that you specify in the construction documents means to insure that you get the surface that you want.  This will typically involve additional labor by the fabricator and the inspection team.

The added expense of preparing and monitoring surface condition is not necessary in a bearing type connection since the slip capacity of the connection is not critical to the design strength of the connection.  Consequently, slip critical connections are much more expensive than are bearing type connections.

Reference 13 gives an excellent treatment on the relative costs and the considerations associated with the choice of joint types.

Sample Spreadsheet Computation

This spreadsheet considers both straight slip resistance and combined tension and shear since the modifier due the presence of tension is a simple modifier to the computation for shear capacity.

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