Frequently Asked Questions

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

A: Hole sizes in base plate may be up to 6 mm larger than the anchor bolt diameter (NZS 3404, Clause  Holes would normally be drilled. Holes require a special plate washer of 4 mm minimum thickness under the nut if the bolt hole is more than 3 mm larger than the anchor bolt diameter. The oversize holes will accommodate reasonable misalignment in the location of the anchor bolts and allow adjustment of column bases to meet the tolerance requirements between column centres permitted in NZS 3404. Recommended diameter of bolt holes, minimum washer dimensions and thickness are given in the table below. 

For further guidance refer to:

[1] Hogan, T.J., Design Guide 7 Pinned base plate connections for columns, Australian Steel Institute, 2011

[2] Fisher, J.M. and Kloiber, L.W., Base plate and anchor rod design 2nd Edition, AISC Design Guide 1, American Institute of Steel Construction, 2010

Table 1: Recommended Sizes of Anchor Bolt Holes and Washers in Base Plates (after Ref 2 and reproduced in Ref 1]
Anchor bolt diameter Hole diameter
(Note 6)
Min. washer dimension
Min. washer thickness
M16 22 50 6
M20 26 50 6
M24 30 75 10
M30 36 75 12
M36 42 90 12

  1. Washer diameters are sized to cover the entire hole when the anchor bolt is located at the edge of the hole.
  2. Circular or square washers meeting the minimum dimensions may be used.
  3. Check clearance for washer to face of column section.
  4. Washers are usually fabricated by cutting from plate or flat bar.
  5. Washer thickness is approximately 30% to 40% of bolt diameter.
  6. NZS 3404 Clause permits a hole diameter not more than 6 mm greater than bolt diameter.

Connections 11/02/2016