Anchor bolt setting-out, and why tolerances matter
A few millimetres at the base becomes a much larger problem at the top of the column. Here is how that happens.
Holding-down bolts are set by one trade and used by another, often weeks apart. That gap is why setting-out errors are usually discovered at the worst possible moment: with a column hanging from a crane.
What can be wrong
- Position — bolt group out of place relative to the grid
- Spacing — bolts correct as a group but wrong relative to each other, so the base plate holes do not line up
- Level — bolts at differing heights, or insufficient projection for nut and washer
- Plumb — bolts leaning, so the nut will not seat square
- Condition — threads damaged by concrete, impact or corrosion
Why small errors grow
A column sitting a few millimetres off position leans slightly to meet the beam above. That lean carries through the frame: connections that will not draw together, bolts that will not enter, cladding rails that do not line up, and a roof that is out of square. Correcting it at height is slow and often needs design approval.
Practical measures
Use a template to set the group rather than positioning bolts individually. Survey after the pour and before the steel is released, and issue the record. Protect threads from the moment the bolts are cast in — a plastic cap costs almost nothing against a site remedial.
When it is wrong anyway
Stop and get it assessed. The remedies — oversized or slotted holes, welded base plates, chemical anchors — are all engineering decisions with consequences for how the base transfers load. None of them belongs to the person holding the spanner, and none should be decided on the spot to keep a crane moving.