Relatively few structural members in a building are ever subjected to large, repeated variations of stress or stress reversals (tension to compression, and vice versa) that could cause fatigue damage to the steel. Members need not be investigated for this possibility unless the number of cycles of such stresses exceeds 20,000, which is nearly equivalent to two applications every day for 25 years.
DESIGN OF CONNECTIONS
Design of connections and splices is a critical aspect of the design process. Because each fabricator has unique equipment and methods, the detailed configuration of 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.