A Beginner's Guide to the Steel Construction Manual, 15th ed.

Chapter 4 - Bolted Connections

© 2006, 2007, 2008, 2011, 2017 T. Bartlett Quimby

Overview

Mechanics of Load Transfer

Finding Forces on Bolts

Hole Size and Bolt Spacing

Tensile Rupture

Shear Rupture

Slip Capacity

Chapter Summary

Example Problems

Homework Problems

References


Report Errors or Make Suggestions

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Section 4.7

Slip Capacity

The limit state of slip is introduced in SCM J3.8 (SCM pg 16.1-134). 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 under normal loading, 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.

The commentary on SCM J3.8 (SCM pgs 16.1-439 to 16.1-443) 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:

LRFD ASD
Pu < ftRn Pa < Rn/Wt
Req'd Rn = Pu / ft < Rn Req'd Rn = Pa Wt < Rn
Pu / (ftRn< 1.00 Pa / (Rn/Wt) < 1.00
ft = 1.0 for standard and short slotted holes perpendicular to direction of load,
ft = 0.85 for oversized holes and short-slotted holes parallel to direction of load,
f
t = 0.70 for long-slotted holes
Wt = 1.50 for standard and short slotted holes perpendicular to direction of load,
Wt = 1.76 for oversized holes and short-slotted holes parallel to direction of load,
W
t = 2.14 for long-slotted holes

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

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

Rn = mDuhfTbns

Where:

  • m is the coefficient of friction for the connected surfaces. It is taken as 0.30 for Class A surfaces and 0.50 for Class B surfaces. See SCM pg 16.1-135 for the required surface preparation for each case.
  • Du 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.
  • hf is a factor for fillers. Fillers are used to match surfaces--they are sometimes called shims. See SCM Figure C-J3.3 (SCM page 16.1-441) for an illustration.
  • Tb is the minimum specified bolt pretension. This is obtained from SCM Table J3.1 (SCM pg 16.1-127)
  • ns is the 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 ks is the reduction factor applied to the nominal slip resistance Rn.

Rn = mDuhfTbNsks

The equations (SCM equations J3-5, pg 16.1-127) for ks 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 ks in the Rn equation since ks is 1.0, and hence transparent to the equation, when there is no applied tension (i.e., Ta or Tu = 0) on the connection. Applying ks to SCM equation J3-4 results in:

LRFD ASD
fRn = fmDuhfTbns[1 - Tu/(DuTbnb)]

fRn = fmhfns[DuTb - Tu/nb]

Rn/W= mDuhfTbns[1 - 1.5Ta/(DuTbnb)]/W

Rn/W= mhfns[DuTb - 1.5Ta/nb]/W

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. Note that, in this case, if there were no externally applied tension on the connection, that ks (for both LRFD and ASD) would be 1.0, and the connection would be adequate in shear for both LRFD and ASD.

Bolt Slip Capacity

SCM J3.8&9

 

 

 

 

 

 

 

 

 

 

 

Bolt:

A325-N

 

 

 

 

 

 

Ab

0.4418

 in

 

 

 

 

 

Fnt

90

 ksi

 

 

 

 

 

m

0.30

 

 

 

 

 

 

Du

1.13

 

 

 

 

 

 

hf

1

 

 

 

 

 

 

Tb

28

 k

 

 

 

 

 

Ns

1

2

per bolt

 

 

 

 

Nb

8

8

bolts

 

 

 

 

 

 

 

 

 

 

 

 

Total bolts

16

 

 

 

 

 

Total Shear Planes

24

 

 

 

 

 

Rnv

9.5

k/shear plane

Rnt

39.8

k/bolt

 

Rnv

227.8

k/connection

Rnt

636.2

k/connection

 

 

 

 

 

 

 

 

LRFD

 

 

 

ASD

 

 

 

Tension, Tu

250

k/connection

Tension, Ta

200

k/connection

Shear, Vu

200

k/connection

Shear, Va

150

k/connection

fv

1

 

 

Wv

1.5

 

 

ks

0.506

 

 

ks

0.407

 

 

f Rnv =

115.3

k/connection

Rnv / W =

61.9

k/connection

Vu/f Rnv =

173.4%

…NG

 

Va / (Rn / W ) =

242.4%

…NG

 

 

 

 

 

 

 

 

 

Check tension: (SCM J3.6)

 

 

 

 

 

ft

0.75

 

 

Wt

2

 

 

ft Rnt =

477.1

k/connection

Rnt / W =

424.1

k/connection

Tu/f Rnt =

52.4%

… OK

 

Ta / (Rn / W ) =

47.2%

… OK

 

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