Building Design and Construction Handbook - Merritt - Ventech!

STRUCTURAL STEEL CONSTRUCTION 7.91
connections plays an important part in determining the cost of the fabricated product.
Consequently, the detailed design of these elements is a part of the work
performed by the fabricator. In the industry, this work is known as detailing.
Usually, the structural engineer indicates the type of connections and type and
size of fasteners required; for example, ‘‘framed connections with 7 ⁄8-in A325 bolts
in bearing-type joints,’’ or the type of connection with reference to AWS D1.1
requirements. For beams, the design drawings should specify the reactions. If, however,
the reactions are not noted, the detailer will determine the reactions from the
uniform-load capacity (tabulated in the AISC Manual), giving due consideration to
the effect of large concentrated loads near the connection. For connections resisting
lateral loads, live, wind, or seismic, the design drawing should stipulate the forces
and moments to be carried. Generally, the design should also include a sketch
showing the type of moment connection desired.
Design Criteria for Connections. Either ASD or LRFD may be used to design
the connections of a structure. Selection of the design procedure, however, must be
consistent with the method used to proportion the members. When LRFD procedures
are used, the loads and load factors discussed in Arts. 7.15 to 7.28 should
be incorporated. The AISC Manual, Vol. II, Connections, provides many design
aids for both design procedures.
7.29 COMBINATIONS OF FASTENERS
The AISC ASD and LRFD ‘‘Specification for Structural Steel for Buildings’’ distinguish
between existing and new framing in setting conditions for use of fasteners
in connection design.
In new work, A307 bolts or high-strength bolts in bearing-type connections
should not be considered as sharing the load with welds. If welds are used, they
should be designed to carry the load in the connection. However, when one leg of
a connection angle is connected with one type of fastener and the other leg with a
different type, this rule does not apply. The load is transferred across each joint by
one type of fastener. Such connections are commonly used, since one type of fastener
may be selected for shop work and a different type for field work.
High-strength bolts in slip-critical joints may share the load with welds on the
same connection interface if the bolts are fully tightened before the welds are made.
For connections in existing frames, existing rivets and high-strength bolts may
be used for carrying stresses from existing dead loads, and welds may be provided
for additional dead loads and design live loads. This provision assumes that whatever
slip that could occur in the existing joint has already occurred.
7.30 LOAD CAPACITY OF BOLTS
Under service conditions, bolts may be loaded in tension, shear, or a combination
of tension and shear. The load capacities specified in AISC ASD and LRFD specifications
are closely related and are based on the ‘‘Specification for Structural Joints
Using ASTM A325 or A490 Bolts,’’ Research Council on Structural Connections
of the Engineering Foundation. Both bearing-type and slip-critical bolted connections
are proportional for the shear forces on the gross area of bolts.