7.2. Base Plates

7.2.1. How can one account for base-plate distortion due to welding?

When grouted, such curling or distortion is of no consequence. Otherwise (e.g., if leveling plates are used or if the base mates to a steel element), the gap allowance of 2005 AISC Specification Section M4.4 should not be exceeded. Additionally, see Specification Section M2.8, Finish of Column Bases.

last modified on 1 January 2006


7.2.2. When a steel base plate bears on less than the full area of concrete, the nominal bearing strength 0.85f'cA is multiplied by the lesser of 2 or the square root of the ratio of geometrically similar concrete area to base-plate area. Why?

The 2005 AISC Specification Section J8 is consistent with ACI 318 provisions, which recognize the increase in bearing strength that results from the confinement that is provided by the concrete surrounding that providing direct bearing resistance.

last modified 1 January 2006

7.2.3. What are the preferred hole diameters in base plates?

The recommended maximum hole sizes for anchor rods in base plates are given in the 13th Edition Manual Table 14-2. It is noted that these hole sizes permit a reasonable tolerance for misalignment in setting the bolts and more precision in the adjustment of the base plate or column to the correct centerlines. Note that these hole sizes are such that flame-cutting will often be required. An adequate washer (see 7.2.4) should be provided for each anchor rod. Because these hole sizes are recommended as maximum sizes, the use of smaller hole sizes is often justified if anchor-rod groups are set accurately.

last modified 1 January 2006

7.2.4. What thickness and size of washer is required for the preferred hole diameters in base plates?

A general rule of thumb is given in AISC Design Guide #10 Erection Bracing of Low-Rise Structural Steel Frames1: the minimum thickness should be one-third the diameter of the anchor rod and that the minimum diameter (or length and width for a non-circular washer) should be 1-in. larger than the hole diameter. When the anchor rod transmits tension, the washer size must be sufficient to transmit the force to the base plate. Washers of the appropriate size can generally be fabricated from plate.

[1] Fisher, J.M. and M.A. West, 1997, Erection Bracing of Low-Rise Structural Steel Frames, AISC, Chicago, IL.

last updated 30 August 2002

7.2.5. When should grout holes be provided for base plates?

Grouting of base plates can be accomplished for common base-plate sizes without the need for grout holes. However, the use of a grout hole should be considered when the smaller side dimension of the base plate exceeds approximately 24 in. Two grout holes may be required for very large plates or long rectangular plates. See the discussion on grouting in Appendix B of AISC Design Guide #11.

[1] DeWolf, John T. and David T. Ricker, 1990, Column Base Plates, AISC, Chicago, IL. Second Edition


last updated 1 January 2006