Page 80 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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CONNECTIONS

3.12 CHAPTER THREE

1. A hardened beveled washer should be used to compensate for the lack of parallelism where the outer
face of the bolted parts has a greater slope than 1:20 with respect to a plane normal to the bolt axis.
2. For A325 and A490 bolts for slip-critical connections and connections subject to direct tension,
hardened washers are required as specified in items 3 through 7 below. For bolts permitted to be
tightened only snug-type, if a slotted hole occurs in an outer ply, a flat hardened washer or com-
mon plate washer shall be installed over the slot. For other connections with A325 and A490
bolts, hardened washers are not generally required.
3. When the calibrated-wrench method is used for tightening the bolts, hardened washers shall be
used under the element turned by the wrench.
4. For A490 bolts tensioned to the specified tension, hardened washers shall be used under the head
and nut in steel with a specified yield point less than 40 ksi.
5. A hardened washer conforming to ASTM F436 shall be used for A325 or A490 bolts 1 in or less
in diameter tightened in an oversized or short slotted hole in an outer ply.
5
6. Hardened washers conforming to F436 but at least / 16 in thick shall be used, instead of washers
of standard thickness, under both the head and nut of A490 bolts more than 1 in in diameter tight-
ened in oversized or short slotted holes in an outer ply. This requirement is not met by multiple
5
washers even though the combined thickness equals or exceeds / 16 in.
7. A plate washer or continuous bar of structural-grade steel, but not necessarily hardened, at least
5 / 16 in thick and with standard holes, shall be used for an A325 or A490 bolt 1 in or less in diam-
eter when it is tightened in a long slotted hole in an outer ply. The washer or bar shall be large
enough to cover the slot completely after installation of the tightened bolt. For an A490 bolt more
than 1 in in diameter in a long slotted hole in an outer ply, a single hardened washer (not multi-
5
ple washers) conforming to F436, but at least / 16 in thick, shall be used instead of a washer or bar
of structural-grade steel.
The requirements for washers specified in items 4 and 5 above are satisfied by other types of fasten-
ers meeting the requirements of A325 or A490 and having a geometry that provides a bearing circle
on the head or nut with a diameter at least equal to that of hardened F436 washers. Such fasteners
include “twist-off” bolts with a splined end that extends beyond the threaded portion of the bolt.
During installation, this end is gripped by a special wrench chuck and is sheared off when the spec-
ified bolt tension is achieved.
The RCSC Specification also permits direct tension-indicating devices, such as washers incorpo-
rating small, formed arches designed to deform in a controlled manner when subjected to the tight-
ening force. The specification provides guidance on use of such devices to assure proper installation.
Carbon-steel bolts (also referred to as machine, common, or ordinary bolts) can prove econom-
ical in a number of applications. “Secondary connections may be made with unfinished bolts con-
forming to the Specification for Carbon Steel Bolts and Studs, ASTM A307” is an often-used job
specification. When this specification is used, secondary connections should be carefully defined to
preclude selection by ironworkers of the wrong type of bolt for a connection. A307 bolts generally
have no identification marks on their square, hexagonal, or countersunk heads, as high-strength bolts
do. Use of high-strength bolts where A307 bolts can provide the required strength merely adds to the
cost of a structure. High-strength bolts cost at least 10% more than A307 bolts. A disadvantage of
A307 bolts is the possibility that the nuts may loosen, but this may be eliminated by use of lock
washers. Alternatively, lock nuts can be used or threads can be jammed, but either is more expensive
than lock washers. Also, if A307 bolts in a connection carry calculated stress and have grips exceed-
ing five diameters, the number of these fasteners used in the connection must be increased 1% for
1
each additional / 16 in in the grip.
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1
3
Fastener diameters for building construction should be / 2 in or more, and diameters of / 4, / 8,
and 1 in are preferred. In general, a connection with a few large-diameter fasteners costs less than
one of the same capacity with many small-diameter fasteners. The fewer the fasteners, the fewer the
number of holes that must be formed and the less is the installation work required. Larger-diameter
fasteners are generally favored in connections, because the available strength (load capacity) of a fas-
tener varies with the square of the fastener diameter.

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