How to Proof-Load Bolts in Cover Plate Splices

Bolts in a cover plate splice are typically installed as slip-critical connections — meaning they rely on friction between the clamped surfaces rather than bolt shank bearing against the hole edges. For friction to work, the bolts must be tightened to a specific minimum preload (proof load). Under-tightened bolts allow the plates to slip, which changes the load path and can lead to fatigue or progressive failure.

Accepted Methods for Proof Loading

The following methods are listed in order of preference (most reliable first):

1. Direct Tension Indicator (DTI) Washers — Preferred Method

DTI washers are hardened washers with small protrusions on one face. As the bolt is tightened, the protrusions flatten. Proof load is achieved when a feeler gauge of specified thickness can no longer pass between the protrusions and the bearing face — confirming the required preload has been reached. This is the most reliable because the verification does not depend on the coefficient of friction between bolt threads, which varies.

2. Skidmore-Wilhelm Calibrator

A hydraulic device that measures actual bolt tension directly. A sample of bolts from the batch is torqued in the calibrator to confirm that the torque required in the field achieves the target preload. Site torquing then proceeds to that confirmed value.

3. Calibrated Torque Wrench

A torque wrench pre-set to the manufacturer’s torque value for the bolt grade, diameter, and thread condition. Reliable provided the wrench is in current calibration and the thread lubrication state matches the calibration condition. Accuracy is lower than methods 1 and 2.

4. Hardened Washer + Torque (Turn-of-Nut)

The nut is first snug-tightened, then turned a specified number of degrees beyond snug (defined by the bolt standard and diameter). Less accurate than DTI washers because it relies on correct identification of the snug-tightened position.

5. Fully Welded Splice (Alternative to Bolted)

If proof loading cannot be satisfactorily achieved on site, a fully welded splice eliminates the need for bolt preloading entirely. This requires a coded site welder and appropriate inspection but removes uncertainties around bolt installation.

Why This Matters

At proof load, the friction capacity of the joint is fully mobilised. Below proof load, the joint may slip under service loads, causing visible movement, noise from the structure, and potential fatigue damage to the bolts and plates. Getting bolt tightening right is not a site “best effort” — it is an engineered requirement.

Contact us if you have questions about the appropriate method for your specific project.