Design of Connections for Steel Members - Design of Modern Steel Railway Bridges - page 381

TABLE 9.5
Mean Slip Coefficients for Steel Surfaces
Class Surface Description Mean Slip Coefficient, k sm

A Clean mill scale and blast cleaned surface before coating 0.33

B Blast cleaned surface with or without coating 0.50

C Galvanized and roughened surfaces 0.40

AREMA (2008) outlines three slip critical connection faying surface conditions for
design(Table9.5).Testsdonewithturn-of-nutandcalibratedwrenchboltinstallations
willyielddifferentresultsfortheslipfactor, D .The“SpecificationforStructuralJoints
Using ASTM A325 or A490 Bolts” (RCSC, 2000) provides values of slip factor, D ,
based on a 5% slip probability ∗ and method of installation as shown in Table 9.6. †
Substitution of the mean slip coefficient, k sm (Table 9.5), and slip factor, D
(Table 9.6), into Equation 9.22 provides the effective allowable shear stress for a
5% slip probability as shown in Table 9.7. It is usual practice to specify turn-of-nut
bolt installation and use f bv = 17.0 ksi for the design of slip-resistant connections.

This provides an allowable shear force per bolt of 10.2 kips for a 7/8 in diameter bolt
with a nominal cross-sectional area of 0.60 in. 2 .

TABLE 9.6
Slip Coefficient for A325 Bolts with 5% Slip Probability

Slip Coefficient, D

Mean Slip Coefficient, k sm Turn-of-Nut Installation Calibrated Wrench Installation

0.33 0.82 0.72

0.50 0.90 0.79

0.40 0.90 0.78

TABLE 9.7
Effective Allowable Shear Stress for A325 Bolts Based on 5%
Slip Probability

Effective Allowable Shear Stress, f bv

(ksi)

Mean Slip Coefficient, k sm Turn-of-Nut Installation Calibrated Wrench Installation

0.33 17.0 15.0

0.50 28.4 (AREMA uses 28.0) 24.9

0.40 22.7 (AREMA uses 22.0) 19.7

∗ A slip probability of 5% (corresponds to a 95% confidence level for the test data) is appropriate for usual
steel railway superstructure design.
† RCSC (2000) also provides slip factors for 1% and 10% slip probability.