Direct Answer
Erection bolts (or drift pins and erection bolts) are used to temporarily align and hold steel members in position while permanent bolts are installed. A minimum of two erection bolts per connection (or one drift pin plus one bolt) are required by AISC CoSP before the ironwork crew can release the crane from a pick. Erection bolts are then replaced by the permanent bolt complement after alignment.
Full Explanation
Erection bolt practice: (1) The ironwork crew places a drift pin in one hole to align the connection, then installs erection bolts (2 minimum, or 1/3 of hole count minimum per AISC) hand-tight. (2) The crane is released once the minimum erection bolt count is reached and the connection is stable. (3) After adjacent members are set and the bay geometry is checked, the full permanent bolt complement is installed. (4) Erection bolts are the same grade as permanent bolts (A325 or A490) — do not use temporary bolts of lower grade. (5) In final bolting, erection bolts are replaced one at a time, not all at once, so the connection always has a minimum number of bolts.
What This Means for Your Shop
Ensure your piece mark drawings show all bolt holes in erection connections — erection bolts go in the same holes as permanent bolts. There are no separate 'erection bolt holes.' If the ironwork crew needs extra holes for drift pins, this must be requested as an RFI and the holes added to the drawing.
Common Mistakes
Allowing the ironwork crew to use unmatched or lower-grade temporary bolts as erection bolts. If a temporary bolt is left in a connection (accidentally passed over during final bolting), a lower-grade bolt in a permanent location is a structural deficiency.
Recommended Process
Brief the bolting crew before erection begins on the minimum erection bolt requirement per AISC and the requirement that all bolts (including temporary) must be A325 or A490 as specified. Include erection bolt requirements in the pre-erection meeting agenda.