Page 78 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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CONNECTIONS

3.10 CHAPTER THREE

These forces, however, may be transferred through the diaphragm and directly to the horizontal
members, so that no transfer forces must be designed for. The 55.4-kip force at the second level is a
member force and is not transferred through the beam-to-column connection. Designing the beam-
to-column connection for an axial force of 55.4 kips would be extremely conservative, unnecessary,
and costly.
For the bracing along line 3 in Fig. 3.3c, significant transfer forces occur at line B at both the roof
and the second floor. In both cases the transfer force is equal to the horizontal component of the
brace. At the roof this horizontal component, and therefore the transfer force, is equal to 18.4 kips,
which is also the member force since the floor beam between lines A and B is a zero-force member.
However, at the second floor, the transfer force through the column is 26.4 kips. If the connection to
the column at line B were designed for the member force of the beams between A and B and B and
C instead of the transfer force, the axial load that would be designed for would be zero or at most
5.19 kips, resulting in an unconservative design.

3.2 DESIGN OF FASTENERS AND WELDS

3.2.1 Limitations on Use of Fasteners and Welds
Structural steel fabricators prefer job specifications to state that “shop connections shall be made
with bolts or welds” rather than restricting the type of connection that can be used. This allows the
fabricator to make the best use of available equipment and to offer a more competitive price.
High-strength bolts may be used in either slip-critical or bearing-type connections. Bearing-type
connections have higher allowable loads and should be used where permitted. Also, bearing-type
connections may be either fully tensioned or, in most cases, snug-tight. Snug-tight bolts are gener-
ally more economical to install and should be allowed, except where loosening or fatigue due to
vibration or load fluctuations are design considerations.
Carbon-steel (common) bolts should not be used in connections subject to fatigue.
The AISC Specification imposes special requirements on use of welded splices and similar con-
nections in heavy sections. This includes ASTM A6 hot-rolled shapes with a flange thickness exceed-
ing 2 in and built-up cross sections with plates over 2 in thick, subject to tensile stresses due to
tension or flexure, and spliced using complete-joint-penetration groove welds that fuse through the
thickness. Charpy V-notch tests are required, as well as special fabrication and inspection proce-
dures. Where feasible, bolted connections are preferred to welded connections for such sections.

3.2.2 Bolts in Combination with Welds
Because of the significant differences in the load-deformation behavior of bolts and welds, it is dif-
ficult to properly design connections that employ both to share the load. For this reason the AISC
Specification puts severe limitations on the design of connections employing both welds and bolts to
resist loads on a common faying surface.
In new work, only longitudinally loaded welds can be considered to share loads with bolts in stan-
dard or short slotted holes loaded perpendicular to the axis of the slot. Transversely loaded welds do
not have sufficient ductility to allow the bolts to “take up” before the weld fractures. In cases where
bolts and welds act together to resist a common load, the capacity of the bolts is reduced by 50%.
These restrictions are sometimes interpreted, incorrectly, to mean that connections in general
should not employ both welds and bolts. This is not the intent. For instance, direct flange-welded
moment-connected beams can utilize bolted web connections without penalty. The reason is that the
flanges are assumed to resist only the moment, while the web is assumed to resist only the shear. The weld
access hole separates the web and the flanges sufficiently to allow this assumed behavior in practice.
In welded alterations to structures, existing rivets and high-strength bolts tightened to the
requirements for slip-critical connections are assumed to carry the loads present at the time of alter-
ation. The welding only needs to be adequate to carry the additional load. Of course this assumes

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