Care should be taken when setting anchor rods to ensure they are plumb. If the rods are not properly secured in the template, or if there is reinforcing steel interference, the rods may end up at an angle to the vertical that will not allow the base plate to fit over the rods.
Rods can also be damaged in the field by construction equipment and construction operations. Anchor rod locations should be clearly flagged so that they are visible to equipment operators working in the area.
Bending of high tensile class anchor rods (e.g. Property Class 8.8) is likely to result in micro-cracks that could lead to premature failures of the anchor rod [1]. Therefore repair by bending of high tensile anchor rods is not permitted. Possible field fixes for bent or out of plumb high tensile anchor include:
Cut the damaged anchor rods at base level, carefully removing the surrounding concrete and install mechanical couplers and new anchor rod extensions.
Remove sufficient concrete that the whole of the damaged anchor rods can be removed and replaced with undamaged anchors rods.
Extend the baseplate, drill and grout in new anchor rods.
Enlarge/slot the baseplate holes and use some steel shims/additional plate washers to get full contact with the nut.
Reconstruction of the whole footing.
Care is also required for bending of low tensile class (e.g. Property Class 4.6) anchor rods. Cold bending of the threaded section may result in the formation of micro-cracks [1]. Further guidance is provided by the American Institute of Steel Construction (AISC) [2]. AISC recommend that only low tensile anchor rods be bent in the field and the bend limited to 45° or less. Rods up to about 24mm in diameter can be cold bent. Rods over 24mm can be heated up to 650 °C to make bending easier. AISC recommend that bending be done using a specialist rod bending device. After bending, the rods should be visually inspected for cracks. If there is concern about the tensile strength of the anchor rod, the rod can be load tested.
Reference:
[1] Fernando, S., Specification Of Threaded Bar in Structural Applications, Australian Steel Institute, 2014
[2] Fisher, J., Kloiber, L., AISC Design Guide 1: Base Plate and Anchor Rod Design Second Edition, American Institute of Steel Construction, 2010
Assuming that continuous fillet welds are used, the welds may be sized as per the requirement of NZS3404:1997 Table 9.7.3.2 – Minimum size of a fillet weld if the member is subjected only to axial compression or tension. If the member is subjected to flexure, the shear flow between the web and flange can be calculated from the beam shear V as VQ/I and the weld sized to provide for this required strength; Q is the first moment about the neutral axis of the flange area and I is the moment of inertia of the entire cross-section. Should two sided fillet welds be used the shear flow on one fillet weld is VQ/2I. Further explanation is found on page 5.23 in [1]
[1] Clifton C, New Zealand Steelwork Limit State Design Guide Volume 1, HERA Report R4-80, New Zealand Heavy Engineering Research Association, 1994
Hole sizes in base plate may be up to 6 mm larger than the anchor bolt diameter (NZS 3404, Clause 14.3.5.2.2). 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)
(mm) Min. washer dimension
(mm) Min. washer thickness
(mm)
M16 22 50 6
M20 26 50 6
M24 30 75 10
M30 36 75 12
M36 42 90 12
Notes:
Washer diameters are sized to cover the entire hole when the anchor bolt is located at the edge of the hole.
Circular or square washers meeting the minimum dimensions may be used.
Check clearance for washer to face of column section.
Washers are usually fabricated by cutting from plate or flat bar.
Washer thickness is approximately 30% to 40% of bolt diameter.
NZS 3404 Clause 14.3.5.2.2 permits a hole diameter not more than 6 mm greater than bolt diameter.
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