CALIBRATED WRENCH BOLTING METHOD
AISC & RCSC Calibrated Wrench or Torque Installation
As with every bolt pretensioning method, Calibrated Wrench may only be performed after all steel plies in a connection have been drawn into firm contact, i.e. snug-tightened. Failure to do so will result in inadequate bolt pretension and loose connections.
The Calibrated Wrench pretensioning method is performed by applying a calculated average torque value to fasteners based on daily testing representative fastener assembly samples using the exact configuration of installation tools and power supplies under as nearly identical conditions as possible as actual installation conditions, within one day or less. The representative fastener assembly samples are primarily defined by production lot combination of each assembly and further divided by fastener condition, individual component state, jobsite environment, time removed from protective storage and every arrangement thereof. The use of formulas or charts with specific values is not permitted. Counting seconds, particular sounds, vibrations, or lack thereof, while tightening are not acceptable substitutions for actual daily testing.
Except for projects having very few bolt combinations that will be tensioned in a minimal number of days, calibrated wrench is the most expensive method to tension bolts. Torque testing will be nearly constant since all test results apply only to a specific tool, a single unique bolt assembly, fully tensioned on the day of testing. Torque charts and equations are expressly forbidden by the RCSC/AISC specification:
RCSC/AISC 8.2.2. Calibrated Wrench Pretensioning: The pre-installation verification procedures specified in Section 7 shall be performed daily for the calibration of the installation wrench. Torque values determined from tables or from equations that claim to relate torque to pretension without verification shall not be used.
The phrase “without verification” means a tension calibrator must be employed to verify enough torque has been applied. Since DuraSquirt DTIs are tension calibrators1, an alternative to nonstop testing is possible if DuraSquirt DTIs are used in conjunction with Calibrated Wrench.
DuraSquirt DTIs provide verification necessary regardless of tool, torque applied or bolt condition. If A DuraSquirt DTI squirts completely, enough torque has been applied. There is no need to determine an actual torque value. If the DTI fully squirts, it is properly tensioned.
For a full description of this procedure follow the link below:
Day 1 Fastener Sampling:
Collect 3 samples of all complete fastener assemblies based on each combination of diameter, grade, length and production LOT and collect additional complete sample assemblies when:
- The lot of any component of the fastener assembly is changed;
- The finish and tolerance on the bolt and nut threads changes;
- The uniformity, degree and condition of lubrication changes;
- The shop or job-site conditions that contribute to dust and dirt or corrosion on the threads changes;
- The friction that exists to a varying degree between the turned element (the nut face or bearing area of the bolt head) and the supporting surface changes;
- The lot of any component of the fastener assembly is relubricated;
- When significant differences are noted in the surface condition of the bolt threads, nuts or washers;
Collect every permutation of items 1 through 7 daily adding additional 3 complete sample assemblies and tests when the following factors are encountered
- The variability of the air supply parameters on impact wrenches that results from the length of air lines or number of wrenches operating from the same source;
- The condition, lubrication and power supply for the torque wrench, which may change within a work shift;
- The repeatability of the performance of any wrench that senses or responds to the level of the applied torque.
- When any major component of the wrench including lubrication, hose and air supply are altered.
- Every tool and supply and/or combination of tools and supply
- For every permutation of items 8 through 12.
Day 1 of Testing & Bolting Operations:Conduct the Preinstallation Verification procedure described in section 7 of the Research Council on Structural Connection's (RCSC) Specification for Structural Joints Using High Strength Bolts on all samples collected per the previous section.
- Place each fastener assembly of each 3 piece sample group into a Tension Calibrator device and tighten until the appropriate pretension required per the 7.1 of the RCSC Specification is reached (for reference table 7.1 of the 2009 is shown below).
- Record the torque required to achieve step 1 above and average the 3 values.
- Snug the steel plies of the connection whose sample group has completed testing.
- Using the average Torque value recorded for a specific sample group; apply recorded torque to all fasteners' representatives defined by the sample group within a reasonable time* and while all conditions remain similar to that observed during testing or repeat steps 1 through 3 if conditions are not similar. NOTE: the unturned element must be restrained from rotating during this operation.
- Repeat number 1 - 4 for every combination of installation equipment components (wrenches, hoses, compressors etc.) that will be performing the bolting operations for Day 1.
Day 2 and every day thereafter:
- Disregard all torque values calculated from previous Day of Fastener Sampling described here if applicable.
- Repeat Day 1 of Fastener Sampling described here.
- Repeat Day 1 of Testing & Bolting operations described here.
- Repeat Step 1 above of this series of instructions (i.e. Day 2 and every day thereafter).
Observe that all sampling and testing described here has been conducted. Routinely confirm specific torque results for precise hardware combinations are only applied using the exact equipment that was used during testing.
Inspection of installed bolts must be conducted by watching installation personnel apply the predetermined torque to bolts in the steelwork. The use of any type of torque inspection wrench is not permitted:
AISC/RCSC 16.2-9.2.2: …it shall be ensured by routine observation that the bolting crew properly applies the calibrated wrench to the turned element.
Time elapsed between unpacking, sampling, testing, snugging and pre-tensioning to be minimal or all affected hardware must be re-tested. Fasteners installed, not but not fully pretensioned within a reasonable time*, must be removed from the work, separated according to Day 1 Sampling described earlier and fully tested per Day 1 Testing & Bolting Operations described here.
Warning:Calibrated Wrench or torque installation is only suited for small projects having few production LOTs, lengths, and diameters of components, to be installed using the minimum number electric torque wrenches (or similarly reliable) possible that can complete all bolting operations (unpacking, testing, stuffing, snugging, pre-tensioning & inspecting) in a single day or less. Furthermore, Calibrated Wrench or torque installation is specifically prohibited for any project that references the Canadian Institute of Steel Construction's (CISC) Handbook of Steel Construction.
1RCSC/AISC-348-15 Section 7.1 “Direct tension indicators (DTIs) may be used as tension calibrators”