The Three-Way Verification
Every hole group on a shop drawing should be verified against three references: the structural connection design (are the bolt locations geometrically consistent with the required connection?), the mating member's drawing (do the holes align when the connection is assembled?), and the CNC file (does the NC file produce holes at the same locations as the drawing?). All three must match before the drawing is released.
Edge Distance and Spacing Checks
AISC Table J3.4 specifies minimum edge distances from center of hole to edge of member. Edge distance violations are not caught by Tekla automatically — they are a detailer/checker responsibility. Check that every bolt in every connection meets the minimum edge distance for its bolt diameter and plate thickness. For seismic connections, additional maximum edge distance requirements apply. Spacing between bolts must also meet the minimum 2-2/3 × bolt diameter requirement.
The CNC File Cross-Check
Before CNC files are released, compare the NC file's hole locations against the drawing. The Tekla NC preview tool displays hole positions graphically — use it to verify that all holes shown on the drawing appear in the NC file, that no holes are missing at the ends of members (a common trimming error), and that hole diameters in the NC file match the drawing callout (bolt diameter plus required clearance, not equal to bolt diameter).
Checking Mating Member Compatibility
One of the most error-prone aspects of hole checking is verifying that two mating members' hole patterns actually align when assembled. A shear tab on a column with holes at 3" spacing and a beam web with holes at 3" spacing appear compatible — but if the first hole row is at different distances from the top of the respective members, the pattern is offset and the bolts will not install. The Tekla assembly model resolves this visually; when 3D models are not used, the checker must manually verify that the first-hole reference dimension on both the column connection plate and the beam web are consistent. This check is among the most frequently skipped — and among the most costly to miss.
Standard vs. Short-Slotted vs. Oversized Holes
AISC Chapter J and project specifications govern which hole types are permitted for each connection type. Standard holes (bolt diameter + 1/16") are the default. Short-slotted holes provide ±3/16" adjustment perpendicular to the slot and are used where minor field adjustment is needed. Oversized holes (bolt diameter + 5/16") are permitted only in slip-critical connections per AISC J3.2. When drawings show short-slotted or oversized holes, the checker must confirm the slot orientation (perpendicular or parallel to load), verify that the connection type is compatible with non-standard holes, and ensure that hardened washers are called out where AISC requires them (short-slotted holes under bolt head or nut; oversized holes always).
Hole Diameter Tolerance in CNC Files
Hole diameters in CNC files must account for the difference between bolt diameter and hole diameter — they are not the same. A 7/8" A325 bolt requires a standard 15/16" hole diameter. A CNC file that programs a 7/8" hole diameter will produce a hole the bolt cannot enter. Tekla connection components assign hole diameters based on bolt diameter plus the required clearance from AISC Table J3.3, but custom connections built with generic geometry must have hole diameters set manually — and this is where errors occur. Verify that every hole in every CNC file is the specified diameter plus clearance, not the raw bolt diameter.
- Standard hole: bolt diameter + 1/16" (e.g., 15/16" for 7/8" bolt)
- Oversized hole: bolt diameter + 5/16" (e.g., 1-3/16" for 7/8" bolt)
- Short-slotted: width = bolt diameter + 1/16"; length = bolt diameter + 5/16"
- Long-slotted: width = bolt diameter + 1/16"; length = bolt diameter + 2-1/2 times diameter
- CNC diameter programmed must match drawing callout — verify before every production run
Common Hole Checking Errors and How to Catch Them
Experience in steel detailing QA reveals consistent patterns in the errors that reach the shop floor when checking is rushed. The most common: a connection plate with the correct hole count but wrong spacing (the group is right, the geometry is wrong); a beam web with the correct edge distance from the flange but a wrong edge distance from the cut end of the member; and a column connection plate where holes are mirrored from the correct orientation because a 'mirror' function was applied in the model without verifying that hole patterns are symmetric. Each of these is caught by a physical dimension check of the actual hole coordinates — not by a visual scan of the drawing. Building a formal hole-check step into the QA workflow, separate from the general drawing review, is the most reliable prevention.