4.1 Pre-Installation Verification Testing

4.1 Pre-Installation Verification Testing

Pre-installation verification testing is a critical quality assurance step in structural steel bolting. Before ironworkers begin installing high-strength bolts in the structural frame, the bolting assemblies must prove they are capable of achieving the required tension under job-site conditions. This testing is not merely a formality; it is a mandatory requirement under the Research Council on Structural Connections (RCSC) Specification for pretensioned and slip-critical joints; snug-tightened joints do not require it.

The Purpose of Pre-Installation Verification

The primary goal of pre-installation testing is to verify that the combination of the fastener components (bolt, nut, and washer), when assembled and tightened using the intended installation method, will develop the required pretension. High-strength bolting relies heavily on the interaction between the nut threads and bolt threads, the friction under the turned element, and the lubrication provided by the manufacturer.

When a bolt assembly arrives on site, it has been exposed to transportation, handling, and potentially varying environmental conditions. Pre-installation testing serves several vital functions:

• Validating the Assembly: It confirms that the specific lot of bolts, nuts, and washers functions together as a cohesive unit.
• Verifying the Installation Method: It ensures that the bolting crew's tools (such as impact wrenches or electric tone wrenches) and techniques (like the turn-of-nut method or use of direct tension indicators) are appropriate and correctly calibrated.
• Assessing Lubrication: It checks that the factory lubrication has not degraded due to weather or time, which would otherwise dramatically increase the torque required to achieve tension, potentially causing bolt torsion failure before reaching the target pretension.

Hydraulic Tension Calibrators (Skidmore-Wilhelm)

The standard device used for pre-installation verification is a hydraulic tension calibrator, most commonly known by the brand name Skidmore-Wilhelm. This heavy, rugged instrument is typically mounted to a steel column or beam on the job site.

How It Works

The bolt assembly is installed into the calibrator precisely as it will be installed in the actual steel connection. When the bolt is tightened, it compresses a hydraulic fluid chamber within the device. A pressure gauge attached to the chamber translates this compressive force directly into pounds or kips (thousands of pounds) of tension. This provides a direct reading of the actual tension achieved in the bolt, which is fundamentally different from measuring the torque applied by a wrench.

Calibration Requirements

Because the accuracy of the tension calibrator is paramount, RCSC Section 7.1 requires the accuracy of a hydraulic tension calibrator to be confirmed through calibration at least annually; good practice is to recalibrate sooner if it is dropped, mishandled, or gives suspect readings. Special inspectors must verify the calibration sticker on the hydraulic calibrator before allowing any pre-installation testing to proceed.

Inspector Callout: Always check the calibration sticker on the Skidmore-Wilhelm before testing begins. A device with an expired calibration invalidates all pre-installation verification tests performed with it.

Bolts Too Short for the Calibrator

The RCSC commentary allows a direct tension indicator, first calibrated to find its gap at the Table 7.1 pretension, to serve as the tension calibrator for short bolts—except with the turn-of-nut method, because DTI deformation would consume part of the required rotation. For turn-of-nut verification of bolts too short for a hydraulic calibrator, the assembly is installed in a solid steel plate with the proper hole size and given the required turns.

Wrench Capacity

RCSC Section 7.2 also requires impact wrenches, if used, to be of adequate capacity and supplied with enough air to pretension each bolt in approximately 10 seconds for bolts up to 1-1/4 in. diameter and approximately 15 seconds for larger bolts.

The 1.05 Minimum Tension Multiplier

When conducting pre-installation verification, simply reaching the minimum design pretension is not sufficient. The RCSC Specification mandates that the bolt assembly must be tightened to a tension that is at least 1.05 times the required minimum installation pretension.

For example, if RCSC Table 8.1 requires a minimum pretension of 39 kips for a 7/8-inch diameter ASTM F3125 Grade A325 bolt, the pre-installation verification test must achieve at least: 39 kips × 1.05 = 40.95 kips, which RCSC Table 7.1 rounds to the nearest kip: 41 kips.

These values are from RCSC 2014 (15th Edition Manual). AISC 360-16 Table J3.1 raised the Group A minimum pretension for bolts over 1 in. to 64, 81, 97 and 118 kips (1-1/8 through 1-1/2 in.) because F3125 Grade A325 has a 120 ksi tensile strength in every diameter, so read which document a question cites.

This 5 percent over-tension provides a crucial safety margin. It accounts for the inherent variability in the installation tools, the slight differences in technique among ironworkers, and the phenomenon of elastic relaxation (where previously tightened bolts lose a small amount of tension as adjacent bolts in the joint are tightened).

Testing Frequency and Requirements

The frequency and scope of pre-installation testing are strictly defined. Testing must be performed:

• At the start of the work, for each lot: RCSC Section 8.2 requires pre-installation testing for each fastener assembly lot before that lot is used in the work. Only the calibrated wrench method requires the testing daily, because the test also calibrates the installation wrench (Section 8.2.2).
• When conditions change: Components that accumulate rust or dirt must be requalified by this test before use (Section 2.2), and the test assemblies must represent the condition of those being installed.
• For Each Combination: The test must be conducted for every unique combination of bolt diameter, bolt length, bolt grade, and production lot that will be used in the work.
• For Each Installation Method: If the crew is using both the turn-of-nut method and twist-off-type tension-control (TC) bolts, both methods must be verified independently.

The Representative Sample

For each unique combination, RCSC Section 7.2 requires a representative sample of not fewer than three complete fastener assemblies, checked at the site of installation in a tension calibrator. AISC 360 Table N5.6-1 makes observing and documenting this testing a Perform task for quality control and an Observe task for quality assurance, and RCSC Section 9.2 requires the inspector to observe it.

• A "representative" assembly means the exact bolt, nut, and washer from the lots to be installed.
• All three assemblies must successfully achieve the minimum 1.05 times the required tension.
• If the pretension developed in any assembly is less than the Table 7.1 value, RCSC Section 7.2 requires the cause to be determined and resolved before the assemblies are used. Cleaning, lubrication and retesting are permitted, except for F1852 and F2280 twist-off assemblies, provided all assemblies are treated in the same manner.
• Washers are used in the test assemblies exactly as they will be used in the work (Section 6.2).

Effects of Temperature and Lubrication

High-strength bolts are sensitive to environmental conditions, particularly when using installation methods that rely on torque control, such as calibrated wrench or TC bolts.

Loss of Lubrication

The relationship between the torque applied and the tension achieved is governed by friction. If a bolt assembly loses its lubrication—due to rain, dust, or prolonged exposure to the sun—the friction between the threads increases dramatically. An impact wrench will expend all its energy overcoming friction rather than stretching the bolt, resulting in a joint that is severely under-tensioned despite feeling "tight."

Temperature Variations

Extreme cold or heat can also alter the performance of the lubricant and the behavior of the installation tools. Pre-installation testing in the morning might not accurately reflect conditions in the mid-afternoon if the temperature shifts by 40 degrees. The RCSC commentary therefore presumes that verified assemblies will be pretensioned before their condition, the equipment or the steelwork change significantly; when exposure time is a concern, verification can be repeated on assemblies taken from the work or set aside with them.

Documentation and Inspection Logs

The special inspector's role is to witness the testing and thoroughly document the results. Inspection logs must record:

• The date and time of the test.
• The specific hydraulic calibrator used (and its calibration date).
• The bolt grade, diameter, length, and lot numbers for the bolt, nut, and washer.
• The installation method being verified.
• The target tension (1.05 times the minimum).
• The actual tension achieved for each of the three sample assemblies.

Properly executed and documented pre-installation verification testing is the foundation of a reliable structural steel frame, ensuring that the theoretical design strength is safely realized in the field.