Design of Connections for Steel Members - Design of Modern Steel Railway Bridges - page 376

the bolt in statically loaded structures. These are bearing-type connections. Bearing-
type connections are generally not used in steel railway superstructures because of
live load stress reversals and cyclical stresses in main members and vibration in both
main and secondary members.

9.3.1.2 Pretensioned Bolt Installation

Connections made with pretensioned bolts rely on friction between plates or element
surfaces (faying surfaces) for strength. These are slip-resistant connections and are
used extensively in modern steel railway superstructures. Pretensioned bolted joints
are made by snug-tight bolt installation followed by increasing the torque applied to
the bolt. The applied torque creates tension in the bolt (and corresponding compres-
sion of, and friction between, the connection elements). The minimum required bolt
pretension, T bP ,is

T bP ≥ 0.70 P bU ≥ 0.70 F bU A st , (9.17)

where P bU is the minimum specified tensile strength of the bolt; F bU is the minimum
specified tensile stress of the bolt material; A st is the tensile stress area of the bolt and
is equal to the cross-sectional area through the threaded portion of the bolt.
To attain this minimum bolt tension, AREMA (2008) recommends that nuts be
rotated between 1/3 and 1 turn from the snug-tight condition, depending on bolt
lengthandangleofconnectionplateswithrespecttotheboltaxis. ∗ Thiswillestablisha
pretensioninthebolt, P bP ,whichisgreaterthantheminimumrequiredboltpretension,
T bP , for the bolt, as shown in Figure 9.9. Alternatively, slip-resistant bolted joints may
be made using specialized twist-off-type bolts or direct tension indicators.

P bU

P bP

T bP

Bolt elongation

FIGURE 9.9 Bolt tension forces and elongation during application of bolt torque.

∗ Bolts will generally not fail until nut rotation exceeds about 1.75 times from the snug-tight condition
(Kulak, 2002).