Building Design and Construction Handbook - Merritt - Ventech!

STRUCTURAL STEEL CONSTRUCTION 7.9
quiring bolt diameters greater than 1 1 ⁄2 in. Furthermore, when they are required to
be tightened to more than 50% of their specified minimum tensile strength, hardened
steel washers should be installed under the heads.
When high-strength bolts are used in a connection, they are highly tensioned by
tightening of the nuts and thus tightly clamp together the parts of the connection.
For convenient computation of load capacity, the clamping force and resulting
friction are resolved as shear. Bearing between the bolt body and connected material
is not a factor until loads become large enough to cause slippage between the parts
FIGURE 7.2 Identification markings on heads
and nuts of high-strength bolts.
7.3.2 High-Strength Bolt Installation
of the connection. The bolts are assumed
to function in shear following
joint slippage into full bearing.
The clamping and bearing actions
lead to the dual concept: slip-critical
connections and bearing-type connections.
For the latter, the allowable shear
depends on the cross-sectional bolt area
at the shear plane. Hence, two shear values
are assigned, one for the full body
area and one for the reduced area at the
threads.
Identification. There is no difference
in appearance of high-strength bolts intended
for either slip-critical or bearingtype
connections. To aid installers and
inspectors in identifying the several
available grades of steel, bolts and nuts
are manufactured with permanent markings
(Fig. 7.2).
Washer requirements for high-strength bolted assemblies depend on the method of
installation and type of bolt holes in the connected elements. These requirements
are summarized in Table 7.5.
Bolt Tightening. Specifications require that all high-strength bolts be tightened to
70% of their specified minimum tensile strength, which is nearly equal to the proof
load (specified lower bound to the proportional limit) for A325 bolts, and within
10% of the proof load for A490 bolts. Tightening above these minimum tensile
values does not damage the bolts, but it is prudent to avoid excessive uncontrolled
tightening. The required minimum tension, kips, for A325 and A490 bolts is given
in Table 7.6.
There are three methods for tightening bolts to assure the prescribed tensioning:
Turn-of-Nut. By means of a manual or powered wrench, the head or nut is turned
from an initial snug-tight position. The amount of rotation, varying from one-third
to a full turn, depends on the ratio of bolt length (underside of heat to end of point)
to bolt diameter and on the disposition of the outer surfaces of bolted parts (normal
or sloped not more than 1:20 with respect to the bolt axis). Required rotations are