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If bolts are used, they should all be pretensioned high-strength bolts designed for standard or shortslotted (with direction of slot perpendicular to applied force) holes with hole deformation considered in the design (i.e., nominal bearing strength < 2.4dtFu). Although design shear strength for the bolts is permitted to be calculated as that of bearing-type joints, all faying surfaces must be prepared as required for Class A or better slip-critical joints. Bolted connections for members that form part of the seismic load resisting system shall be designed to ensure that a ductile limit state (either in the connection or in the member) controls the design. If welds are used, they shall conform to procedures and standards outlined in AWS D1.1 (AWS 2000). If both bolts and welds are used, bolts shall not be designed to share load with welds on the same faying surface.

To avoid brittle fracture of welds, a minimum Charpy V-Notch (CVN) toughness of 20 ft lb (27 J) at —20°F (—29°C) is required for all welds in members and connections that are part of the seismic lateral load resisting system. For structures with enclosed SMF or IMF maintained at a temperature of 50° F (10°C) or higher, complete joint penetration welds of beam flanges to columns, column splices, and groove welds of shear tabs and beam webs to columns, shall be made with a filler material with minimum CVN toughness of 20 ft lb (27 J) at -20°F (-29°C) and 40 ft lb (54 J) at 70°F (21°C). If the service temperature is lower than 50°F (10°C), these minimum CVN toughness values shall be reduced accordingly.

Regardless of the type of framing system used, the required strength of the connections or related components must be designed for an expected yield strength Fye of the connection members given by

where Ry is an adjustment factor to account for the increase in demand on the connections when yielding occurs in the connecting members at the true yield stress (which is often higher than the minimum specified yield stress used in the design) during a severe earthquake. Values for Ry are given in Table 4.23.

The current design philosophy for moment frames is based on the strong column-weak beam concept. This condition is enforced in the design of SMF at all beam-column joints by the following equation em;c

where E M*c is the sum of column moments acting at the joint in question and EMpb is the sum of beam moments acting at the joint in question. They can be calculated as follows:

where Zc is the column plastic section modulus, Fyc is the specified minimum yield stress of the column, Puc is the required axial strength of the column, Ag is the gross sectional area, and Ry are the values shown in Table 4.23, Mp is the plastic moment capacity of beam (for a reduced beam section, Mp can be calculated based on the minimum plastic section modulus of the reduced section), and Mv is the additional moment due to shear amplification from the location of plastic hinge to the column centerline.

While the use of the strong column-weak beam design tends to force plastic hinges to develop in the beams, there is a region in the column commonly referred to as the panel zone where high stresses are often developed. The panel zone is the region of the column to which the beams are attached. For the moment frame to be able to withstand the seismic loading, the panel zone must be strong enough to allow the frame to sustain inelastic deformation and dissipate energy before failure. For SMF, the design criterion for a panel zone (when the beam web is parallel to the column web) is fvRv > Ru (4.162)

TABLE 4.23 Value of Ry

Application

Ry

Hot rolled structural steel shapes

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