Field bolting

Field-bolted joints follow the erection drawings, the RCSC Specification for Structural Joints Using High-Strength Bolts, and this guide. Joints are snug-tight or pretensioned. Unless the drawings say otherwise:

Connections of purlins, girts, truss purlins, bar joists, flange braces, bracing struts, and primary framing may be snug-tight except as listed below.

Pretension these

• Any connection using A490 bolts.
• Connections in frames supporting cranes of 5 tons capacity or more.
• Framing supporting machinery that creates vibration, impact, or stress reversal.
• Frames and bracing in high seismic areas — see the drawings for seismic category.
• Any connection designated slip-critical (A325-SC). Faying surfaces must be clean: mill scale, dirt, and heavy rust off the contact area. Galvanized or lightly rusted surfaces are permitted. Primer off the faying surface on SC joints.

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Snug-tight

Snug-tight is the tightness from a few impacts of an impact wrench, or the full effort of an ironworker with an ordinary spud wrench, bringing the plies into firm contact.

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Systematic tightening

Install every bolt in a connection loosely before you tighten. Snug from the most rigid part of the joint toward the free edge until the plies contact. If pretensioning is required, go around again in the same order. After tightening, the bolt end must be at least flush with the face of the nut — if it is below, put in a longer bolt. There is no maximum stick-out.

Snug-tightened bolts may be removed and reused. Pretensioned bolts are NOT reused unless engineering approves.

Gaps in joints

Welding distortion on primary end plates can leave a gap that will not close even with a fully tightened bolt. Small gaps are permitted as follows. Larger gaps require shims. Gaps greater than ¼″ need engineering review.

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Slip-critical connections: no gaps at bolt locations. Uncoated surfaces in firm contact around the hole. Galvanized faying surfaces are permitted as noted above. About 1″ of contact around the hole is the visual check.

Shims

The erector furnishes shims. Typical flat shim: length = flange width, 1½″ deep, 0.060″ thick, stacked as required. Finger shims between bolts. Align with plates and gussets; insert until tight; tack-weld to the edge of the plate if needed to hold. Shim dimensions may vary for fit-up.

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Pretension methods

Joints that require pretension use one of: turn-of-nut, calibrated wrench, twist-off tension-control bolts, or direct-tension-indicator washers. ALL methods must be calibrated.

Tension calibration

A hydraulic tension calibrator (Skidmore-Wilhelm or equal) validates that the method produces the required tension. Take three fastener assemblies of each diameter, length, and grade. Lubrication of the test assembly must match the work. Log the results.

Turn-of-nut

1. Complete pre-installation tension calibration.
2. Bring the joint to snug-tight. Turn-of-nut does not require washers unless the drawings call for them.
3. Match-mark each nut, bolt, and steel surface at a corner of the nut.
4. Using the same pattern as snugging, rotate each nut or bolt head the required turn.
5. Impact wrenches or spud wrenches are both acceptable for the additional turn.

Tolerances: for ⅓ and ½ turn, ±30°. For ⅔ turn, ±45°. Table is for flat surfaces only.

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Calibrated wrench

1. At the start of each day, calibrate each assembly you will install.
2. A hardened washer is required under the turned element.
3. Snug all bolts, systematic pattern.
4. Match-mark the nut or chuck.
5. Hold the opposite end and tighten until the wrench shuts off.
6. Verify rotation is within the turn-of-nut tolerances.

Twist-off (TC) bolts

Follow the manufacturer. Calibrate at the start of each day. Snug first, systematic. Then run each assembly until the spline shears. Clearance: 1″ minimum around a ¾″ TC bolt, 1⅛″ around a ⅞″, 1¼″ around a 1″. Where pitch, stiffeners, or beam depth will not allow a shear wrench, substitute standard nuts and bolts and tighten with wrenches — see the drawings.

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DTI washers

Calibrate each day. Snug without fully compressing the DTI. Then tighten until the inspection gap is reached. Uncoated DTI under the bolt head without a washer: 0.015″ feeler. Uncoated DTI with protrusions against a washer: 0.005″ feeler. At inspection, one half or more of the gaps must be compressed — all gaps need not be fully compressed.

Inspection (minimum)

Special inspections as required by the AHJ. Field inspection cost is not the manufacturer’s. QC is the erector’s inspector. QA is a third-party when the AHJ or owner requires it — that firm must be pre-approved by the AHJ.

Snug-tight joints: QC and QA need not be present during installation. After snugging, confirm firm contact (small gaps as allowed) and that nuts cannot be turned by hand on a random sample.

Turn-of-nut: if match marks were used, confirm rotation after pretensioning — QC/QA need not watch the work. If no match marks, they observe the installations.