The RCSC/AISC specification mandates onsite Pre-Installation Verification testing using the hardware delivered to the site. This applies to all methods. Test results from the distributor or manufacturer do not meet the specification's requirements. However, supplier Material Test Reports (MTRs) must include all the details of the bolts being tested. It is critical that the MTRs accurately reflect the coating when bolts are coated.

TC bolts, also known as tension-control or twist-off bolts, are commonly used in this method. These bolts have a splined extension built into the shank. The torque required to twist off this extension is calibrated during assembly. When done correctly, the twist-off happens at a bolt pretension that exceeds the minimum requirement. Bolt installers have recently discovered that both regular hex bolts and DTIs can be installed by one person from only one side. This is the same advantage that was once thought to be unique to twist-off bolts.

Disadvantages of the "Twist-Off" System

1. Requires specialized wrenches.

2. Limited availability in lengths and diameters.

3. Often fails ASTM F3125/F1852/F2280 Rotational Capacity testing if required.

4. Special connection clearances must be designed for wrench access.

5. Generally more expensive than hex bolts and DTIs.

6. Galvanized assemblies are in extremely limited supply.

7. Field re-lubrication is prohibited by code.

8. Plies must be compacted before twist-off.

9. Thread condition degradation can alter the torque-tension relationship, with studies showing that improper storage for just three days caused one-third of bolts to fail the required pretension at break-off.

10. Research from the University of Toronto indicates that TC bolt performance is significantly affected by temperature and humidity variations.

Pre-Installation Verification

According to AISC/RCSC Section 7 on Pre-Installation Verification, fastener assembly testing must be performed onsite by the installation crew using the actual tools intended for installation. Contrary to common belief, Pre-Installation Verification testing does not determine whether fastener assemblies pass or fail. Instead, its purpose is to identify potential issues between fasteners, tools, and installers before field bolting operations begin.

The following sections outline potential issues that Pre-Installation testing may reveal.

Fasteners

Are the fastener assemblies capable of achieving at least 105% of the minimum required bolt pretension?

The installation tool must be applied to the fastener assembly until either the tension calibrator confirms adequate pretension or the bolt fails. Failure is identified by one of two scenarios: the splined end breaks off before reaching minimum pretension, or the tensile load increases and then decreases, as shown by the tension calibrator, without meeting the minimum pretension. For TC bolts, there is no acceptable method to rework and retest the fastener. Production lots that do not comply must be either rejected and replaced or reworked by the TC bolt manufacturer.

Tools

Are the installation tools capable of providing sufficient output torque to tighten properly lubricated fastener assemblies to at least 105% of the minimum required pretension?

If an installation tool applied to a fastener assembly fails to achieve at least 105% of the required minimum pretension, the cause must be identified and resolved before final pretensioning of any bolts in the steelwork. Additionally, manual, non-powered TC bolt tools will not achieve the required minimum pretension.

Installation Crews

Are the installers familiar with the proper method of installation?

Installers must ensure that a snug-tightened condition is achieved and that the splined end of the fastener remains intact before using a shear wrench on any TC bolt. If the splined end shears off during the snugging process, the affected TC bolts must be removed and replaced.

Snug-tightened Joints

Before any pretensioning method can be applied, every bolted joint must first be snug-tightened. Snug-tightening involves applying sufficient effort to bring the steel plies into firm contact. According to AISC/RCSC, this effort is defined as either the full effort of an ironworker or a few impacts from an impact wrench (Section 8.1), stopping once the nut can no longer be removed by hand. Inspection only requires an attempt at manual nut removal (Section 9.1). It’s important to note that a “snugged” joint does not require pretension.

“There are no specific minimum or maximum tension requirements for snug-tight bolts.”

Since hand tightness is the sole inspection criterion, snug-tightening can begin with negligible or zero pretension. However, overly aggressive snug-tightening may result in pretension levels that approach or exceed the minimum values specified in AISC/RCSC Table 7.1, potentially leading to bolt failure. This risk is particularly high for bolt diameters of ¾” and smaller.

Verification Basics

This content pertains specifically to AISC/RCSC Pre-installation testing. Please note that individual projects, State DOTs, and Federal Highway specifications may vary significantly and are not addressed here. The following reflects Applied Bolting Technology’s interpretation of AISC/RCSC Pre-installation testing, based on the Specification for Structural Joints Using High-Strength Bolts (AISC 348-20) by The Research Council on Structural Connections and the Specification for Structural Steel Buildings (AISC 360-16) by the American Institute of Steel Construction. Readers seeking alternative interpretations are encouraged to review these documents directly.

Verification testing involves snug-tightening at least three sample fastener assemblies using a tension calibrator, applying the chosen pretensioning method, and ensuring that the achieved pretension meets or exceeds 105% of the minimum required level. While the specific tools, components, and techniques vary across the four pretensioning methods, the core steps remain consistent: snug the fastener, apply the method, and verify compliance with the specifications.

Finally, all acceptable methods are designed to permanently deform the fastener into its inelastic range.

Pre-Installation Verification Testing: Key Points from AISC/RCSC Section 7.2

Pre-installation verification begins with sampling. According to AISC/RCSC Section 7.2:

“A representative sample of not fewer than three complete fastener assemblies of each combination of diameter, length, grade, and lot to be used in the work shall be checked at the site of installation in a tension calibrator to verify that the pretensioning method develops a pretension that is equal to or greater than that specified in Table 7.1.”

Regardless of the method used, it is essential that the sampled assemblies are TRULY REPRESENTATIVE of the fasteners being pretensioned. This means the condition of the samples must closely match the condition of the fasteners in active use. Testing "new" fasteners taken directly from sealed shipping containers does not provide a representative sample unless all fasteners in use are in the same new condition.

If fasteners have been snug-tightened and exposed to weather, they must be verified in that state, provided it reflects the actual condition of the fasteners being used in the steelwork. Ensuring truly representative samples is particularly critical for Tension Control (TC) bolts and Calibrated Wrench methods, as these are significantly impacted by weathering and lubrication degradation.

Tension-Calibrator Steps

1. Conduct this procedure with every installation tool and repeat it whenever there are changes to the TC bolts' original lubricated condition.

2. Place each unique configuration of the sample fastener assembly into a tension calibrator, ensuring washer placement aligns with Table 6.1 and, if applicable, Section 6.2.

3. Tighten the fastener to a snug-tight condition in accordance with AISC/RCSC Section 8.1, then inspect as outlined in Section 9.1.

4. Use the TC shear tool on the fastener and operate it until the splined end breaks off.

5. Confirm that the resulting pretension meets or exceeds the required bolt pretension specified in AISC/RCSC Table 7.1.

6. If the resulting pretension fails to meet the values in AISC/RCSC Table 7.1, return the fasteners to the manufacturer for rework. Non-compliant TC fasteners must not be used in steelwork. Only the manufacturer is authorized to alter or rework TC bolts, and noncompliant assemblies may not be remedied by any other party.

Bolting Methodology

1. Only retrieve the number of bolts required for snugging and tightening during a single shift from protected storage to avoid lubrication degradation.

2. Limit the exposure of fasteners to environmental elements and keep them assembled in their original condition until installation. Do not disassemble fasteners into individual components.

3. Snug the steel plies in multiple steps, if necessary, to bring the steel into firm contact as outlined in AISC/RCSC Section 8.1. Avoid twisting off the splined end of the fastener during snugging. If the splined end shears off, the bolt must be discarded and replaced.

4. Immediately after snugging, or as soon as possible, use a shear wrench to tighten each bolt until the tool twists off the splined end and releases the wrench.

5. If the joint remains snugged for more than a day or a single shift, additional testing with a tension calibrator may be required to ensure proper tension. If significant time has passed or environmental exposure has occurred before final pretensioning, remove representative samples for re-testing in a tension calibrator (refer to Pre-Installation Verification).

Inspection

The inspector MUST:

1. Ensure Pre-Installation Verification has been performed.

2. Ensure the joint has been snug-tightened.

3. Ensure the splined ends have not twisted off during snugging operations.

4. Ensure assemblies in the steelwork match the condition of bolts tested during Pre-Installation verification, or remove non-similar bolts for additional tension calibrator testing.

5. Ensure the splined ends have properly severed during installation by the installation tool.

The inspector MUST NOT:

1. Authorize Pre-Installation test results on new fasteners to represent assemblies that are not in similar condition due to exposure to the elements.

2. Authorize TC bolts unassembled or allow individual component replacement.

3. Authorize relubrication by anyone except the manufacturer.

4. Authorize TC manufacturer MTR test results as a substitute for actual on-site Pre-Installation verification testing.

A pretension exceeding the [required] value shall not be grounds for rejection.

Minimum Bolt Pretension Per AISC/RCSC table 5.2

Equal to 0.70 times the minimum tensile strength of bolts as specified in ASTM F3125/F3125M for grades A325 and A490 bolts, with UNC threads, rounded to the nearest kip.

Tension Control Advisories

Non-powered manual TC tools cannot achieve the minimum required pretension. This limitation is clearly stated in the tool’s manual, and no solution is currently available to address this issue. As a result, manual tools are suitable only for snug-tight joints.

Research conducted by the University of Toronto in 20044 and 20075 revealed the following findings:

1. Behavioral Differences of TC Bolts: Tension Control (TC) bolts behave differently in Hydraulic Tension Calibrators (e.g., Skidmore devices) compared to their performance when installed in steelwork. Testing TC bolts with hydraulic load cells typically yields results 10% higher than what is achieved in actual steelwork installations.

2. Impact of Moisture: Exposure to moisture, whether from rain or other sources, reduces the effective pretension of TC bolts by approximately 10%.

3. Effect of Temperature Changes: Fluctuations in temperature can negatively impact the pretension of TC bolts, reducing it by another 10%.

4. Cumulative Impact: These factors compound one another. For instance, a production batch of ¾” A325 TC bolts tested on a Skidmore device showing 29 kips will only achieve approximately 26 kips when installed in steelwork. If these bolts are exposed to rain, their effective pretension drops further to around 23.5 kips. Additionally, a decrease in ambient temperature would reduce the effective pretension even further to approximately 21 kips.

In 2005, a study funded by the RCSC led to a presentation at the 2007 RCSC Annual Meeting in Cleveland, Ohio, by Professor Emeritus Peter Birkemoe from the University of Toronto. During his presentation, Professor Birkemoe made several recommendations that were not incorporated into the RCSC Specification:

Revise the Specification to require bolts to be removed from steelwork for verification purposes.

Update the Specification to mandate TC bolts achieve 1.1 times the minimum required pretension in their as-received condition and modify the current pre-installation verification requirements.

Include cautionary notes in the commentary about the requirements for verification and installation in cold weather.

Add the “wet” condition as part of the verification requirements.

These recommendations, copied verbatim from Professor Birkemoe’s RCSC presentation, can be reviewed in full by accessing the 2007 report and meeting minutes available on the RCSC website: Final Report on TC Bolts Phase 1.

The presentation to the RCSC committee is documented in the 2009 RCSC Main Minutes Package, specifically on pages 36 to 50 of the PDF.

Additionally, ASTM 3125, which governs the manufacturing and testing of TC bolts (previously covered by standards F1852 and F2280), requires assembly testing using a “tension measuring device,” typically a hydraulic load cell like a Skidmore Wilhelm. Consequently, the pretension values reported on a Material Test Report (MTR) for a TC bolt lot should be interpreted with the findings of the U of T study in mind, as steelwork results may be up to 10% lower than the values listed on the MTR.

 

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DTIs and TC/Twist Off bolts

Although it is uncommon to use DTIs with TC bolts, it is allowed. The DTI is incorporated into the assembly, the TC shear wrench is used to tighten the fastener, and the DTI verifies whether the TC bolt has been properly tightened. Additionally, combining Squirter® DTIs with TC bolts meets AISC/RCSC PIV testing requirements, provided that the first three assemblies include the Squirter DTI on the head side, with the DTI protrusions flattened below the calibrated gap. Once the initial three bolts are tightened, Squirter DTIs can be placed on either the nut side or head side for subsequent assemblies.

 

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In conclusion, the proper use, testing, and installation of Tension-Control (TC) bolts, or "twist-off" bolts, demand strict adherence to AISC/RCSC specifications and thorough Pre-Installation Verification (PIV). While these bolts offer convenience and unique advantages for single-side installations, their performance is highly sensitive to factors such as tool calibration, environmental conditions, and bolt storage practices. TC bolts require specialized equipment, precise handling, and cannot be reworked on-site, making compliance with specification requirements critical for ensuring structural integrity.

The limitations of TC bolts—such as sensitivity to weather, prohibitions on re-lubrication, and variable performance in field conditions—underscore the importance of rigorous onsite verification. PIV testing ensures that the fasteners, tools, and installation crew are adequately prepared for the bolting process, identifying potential issues before construction begins. By following the guidelines outlined in AISC/RCSC specifications and accounting for environmental impacts, project teams can optimize the performance of TC bolts while minimizing risks associated with improper tensioning and environmental degradation. Ultimately, informed practices and adherence to standards ensure the reliability and safety of bolted connections in steel structures.