Mixing Weld and Bolt Part2
Mixing Weld and Bolt Part2
Mixing Weld and Bolt Part2
In a previous edition of Welding Innovation (Volume XVIII, AISC LRFD Steel Specification require that the welds be
Number 2, 2001), Part 1 of “Mixing Welds and Bolts” was designed to carry the entire load under these conditions. The
published. That column dealt with snug-tightened and pre- Canadian standard CAN/CSA-S16.1-01 provides a more
tensioned mechanical fasteners, including rivets, combined rational criterion by permitting load sharing between welds
with welds, as well as existing specification requirements and bolts for service loads, providing the higher of the two
for such combinations. Part 1 can be obtained by down- capacities can carry all factored loads alone.
loading a PDF file from the Welding Innovation web site at
www.weldinginnovation.com. Part 2 will address combining Part 2 focuses on slip-critical joints, combined with welds.
welds with slip-critical, high-strength bolted connections, As mentioned in Part 1, this topic is the subject of ongoing
and will also examine existing specification provisions for research and consideration by the various technical com-
various combinations of welds and bolts in light of recent mittees. Much of this work has been done by Drs. G. Kulak
research. and G. Grondin and their co-workers of the University of
Alberta, Canada, and definitive conclusions have not yet
Review of Part 1 been reached as to how these findings should be incorpo-
rated into US standards, such as AWS D1.1 and AISC
In Part 1, general information was provided on bolted con- LRFD. However, at least some parts of current standards
nections. Snug-tightened, pretensioned, and slip-critical are likely to be determined to be unconservative, and prac-
bolted connections were defined. ASTM A325 and A490 ticing engineers should review these data and determine
bolts were identified, and the capacity of rivets identified as how specific projects should be addressed in light of these
typically about half of the strength of A325 bolts. Slip-criti- findings. The same research has drawn into question some
cal joints have bolts that have been installed in a manner of the current specification requirements for snug-tightened
so that the bolts are under significant tensile load with the connections when welds are added, and these findings will
plates under compressive load. They have faying surfaces be reviewed.
that have been prepared to provide a calculable resistance
against slippage. Slip-critical joints work by friction: the
pretension forces create clamping forces and the friction
Code Provisions for Slip-Critical Connections
between the faying surfaces work together to resist slip- with Welds
page of the joint. The basic design philosophy relies on The issue of mixing mechanical fasteners and welds is
friction to resist nominal service loads. The provisions for addressed in AWS D1.1: 2002 Structural Welding
design of slip-critical connections are intended to provide Code–Steel. Provision 2.6.7 states:
90–95% reliability against slip at service load levels. In its
strength limit state, slip can occur and the bolts will go into “Connections that are welded to one member and bolted
bearing. This should not be the case for service loads. or riveted to the other shall be allowed. However, rivets
and bolts used in bearing connections shall not be con-
The focus of this Design File series is not upon bolted sidered as sharing the load in combination with welds in
connections, but rather upon connections that are composed a common faying surface. Welds in such connections
of both welds and bolts. For the snug-tightened and preten- shall be adequate to carry the entire load in the connec-
sioned bolted connections, it was shown that welds cannot tion. High-strength bolts installed to the requirements for
be assumed to be capable of sharing loads with the mechan- slip-critical connections prior to welding may be consid-
ical fasteners. AWS D1.1 Structural Welding Code-Steel and ered as sharing the stress in the welds. (See:
The fourth sentence deals with slip-critical connections. The concept presented in codes with respect to slip-critical
Notice that, in order for sharing to be considered, this pro- connections was presumably based upon the lack of slip
vision requires that the high-strength bolts be installed in the connection (that is, their “stiffness”), justifying the
“prior to welding.” More will be said on this issue later. assumption that the capacities of the two types of joining
AISC LRFD – 1999, Provision J 1.9, expresses the same systems (welds and bolts) can be joined. Ultimately, a slip-
general philosophy when it states: critical bolted connection will slip, but if a weld is added,
such a connection cannot slip. Thus, the capacities of the
“In slip-critical connections, high-strength bolts are two elements cannot be combined in terms of the ultimate
permitted to be considered as sharing the load with strength capacity.
the welds.”
Figure 1 contains a conceptual plot of the load/displace-
The commentary to this provision provides some additional ment relationships for welds and bolts. Note that the
understanding of both the AISC and AWS provisions: load/deformation relationships are different for each of the
three elements. It should be noted that the two types of
“For high-strength bolts in slip-critical connections to welds shown are not equally “stiff.” The actual curve for the
share the load with welds it is advisable to fully tension bolted connection is illustrative only; in fact, there would be
the bolts before the weld is made. If the weld is placed various curves for the different types of bolted connections.
Recall from Part 1 that in the general case, AWS and AISC Manuel, Thomas J. and Kulak, Geoffrey L., (2000). “Strength of Joints That
require that combinations of welds and bolts of the bearing Combine Bolts and Welds,” J. of Struct. Engineering, ASCE, 126(3),
279-287.
type be designed such that the entire load is transferred