mentary to the AISC Seismic Provisions notes some significant differences: 1. Long anchor rods embedded in concrete will strain much more than high-strength bolts or welds in beam-to-column connections. 2. Column base plates are bearing on grout and concrete, which is more compressible than the column flanges of the beam-to-column connections. 3. Column base connections have significantly more longitudinal load in the plane of the flanges and less transverse load when compared to beam-to-column connections. 4. The shear mechanism between the column base and the grout or concrete is different from the shear mechanism between the beam end plate and the column flange. 5. AISC standard hole diameters for column base anchor rods are different than AISC standard holes for highstrength bolts. 6. Foundation rocking and rotation may be an issue, especially on isolated column footings. As the Commentary to the AISC Seismic Provisions suggests, research is lacking regarding the performance and design of base details for high seismic loading. However, the Commentary also acknowledges that these details are very important to the overall performance of the SLRS. Therefore, careful consideration must be given to the design of these details.
Figure 2.6. Typical moment base detail.
3.0 DESIGN OF COLUMN BASE PLATE CONNECTIONS This section of the Design Guide provides the design requirements for typical column base plate connections in buildings, such as the one shown in Figure 1.1. Five different design load cases in column base plate connections are discussed: • Section 3.1 Concentric Compressive Axial Loads • Section 3.2 Tensile Axial Loads • Section 3.3 Base Plates with Small Moments • Section 3.4 Base Plates Large Moments • Section 3.5 Design for Shear In column base connections, the design for shear and the design for moment are often performed independently. This assumes there is no significant interaction between them. Several design examples are provided in the following sections for each loading case. The general behavior and distribution of forces for a column base plate connection with anchor rods will be elastic until either a plastic hinge forms in the column, a plastic mechanism forms in the base plate, the concrete in bearing crushes, the anchor rods yield in tension, or the concrete pullout strength of the anchor rod group is reached. If the concrete pullout strength of the anchor rod group is larger than the lowest of the other aforementioned limit states, the behavior generally will be ductile. However, it is not always necessary or even possible to design a foundation that prevents concrete failure.
Figure 2.7. Embedded moment base detail.
DESIGN GUIDE 1, 2ND EDITION / BASE PLATE AND ANCHOR ROD DESIGN / 13