APPENDIX A SPECIAL CONSIDERATIONS FOR DOUBLE-NUT JOINTS, PRETENSIONED JOINTS, AND SPECIAL STRUCTURES A1. Design Requirements Anchor rods are sometimes used in special applications that require special design details, such as anchor rods designed without a grout base (double-nut anchor rods), anchor rods in sleeves, pretensioned applications, and special moment bases or stools. Double-nut anchor rods, are different from building column anchor rods that may use a setting nut but are not designed for compression in the completed structure. Doublenut joints are very stiff and reliable for transmitting moment to the foundation. Because tall pole-type structures are nonredundant and are subject to fatigue due to wind flutter special inspection and tightening procedures should be used. Studies have shown that pretension in the rod between the two nuts improves fatigue strength and ensures good load distribution among the anchor rods (Frank, 1980; Kaczinski et al., 1996). The base plates of light and sign standards are not grouted after erection, and the rod carries the all of the structural load. The anchor rods must be designed for tension, compression, and shear, and the foundation must be designed to receive these loads from the anchor rods. Machinery bases and certain columns may require very close alignment of the anchor rods. Oversized sleeves can be used when setting the rods to provide substantial flexibility in the rod so that it can be adjusted to fit the machinery base. The anchorage at the bottom of the rod must be designed to span the sleeve and develop the required bearing on the concrete.
Figure A1.1. Anchor rods with sleeves.
Often machinery, process equipment, and certain building columns may be subject to vibration or cyclical loads, which may in turn subject the anchor rod to fatigue. Pretensioning the rod can improve its fatigue life, but anchor rods can effectively be pretensioned only against steel. Even when tensioning a Grade 55 rod 24 in. long, it only takes concrete creep/shrinkage of 0.05 in. to relieve all of the pretension. Thus, it is recommended, when it is necessary to pretension anchor rods, that a steel sleeve be used that is adequate to transfer the anchor rod pretension from the anchor plate to the base plate. See Figure A1.1. Large mill building columns that have to be set accurately and have large moments at the base can be designed using a stool-type detail as shown in Figure A1.2. The advantage of this type of detail is that the base plate can be set in advance using large oversized holes. The use of the fillet welded stool avoids having to complete joint penetration groove weld the column base to the heavy base plate. If the column and base plate are over 2 in. thick, using a complete joint penetration weld detail would require special material toughness. The use of the stool has the added advantage that the extended anchor rod length will allow easier adjustment to meet the holes in the stool cap plate. A1.1 Compression Limit State for Anchor Rods With the usual short length involved, the nominal steel compressive strength for anchor rods in double-nut moment joints is the product of its yield stress and the gross area. Yielding could initiate at lower load levels on the reduced area of the threads, but it is assumed that the consequences of this yielding would be relatively minor. The available strength, φRc or Rc/Ω, is determined with Rc = FyAg φ = 0.90 Ω = 1.67
Figure A1.2. Column moment base using stool.
DESIGN GUIDE 1, 2ND EDITION / BASE PLATE AND ANCHOR ROD DESIGN / 47