DuraSquirt® DTI Bolting Method
DuraSquirt® DTIs, are enhanced versions of ASTM F9591 DTIs that render feeler gauges unnecessary. Like Squirter DTIs, DuraSquirt® DTIs expel orange indication media when a bolt has been properly pretensioned in accordance with AISC/RCSC specifications. The difference with DuraSquirt® DTIs, is the indication media is permanent, unless actively removed.
This picture illustrates how permanent. Each assembly has been fully tensioned with a DuraSquirt® DTI. The galvanized bolt on the left is a freshly tightened assembly. The plain bolt on the right, was tightened in November 2014. It is rusty since it was washed in Applied Bolting’s lunchroom dishwasher every working day for a year. Afterwards, it was carried loose in a toolbox for another 2 years, unprotected, as our technical personnel traveled around the USA to conduct structural bolting seminars. While the fastener underwent serious abuse and neglect, the DuraSquirt® DTI can still be inspected visually.
Like all DTIs, DuraSquirt® DTIs are single use mechanical load cells that function independent of torque or turn. While a DuraSquirt® DTI is a smart device, they are dumb about how much it will take to tighten a bolt. If a bolt is tightened enough, the DTI will show it, if not, the DTI will show that too. While current specifications considered DTIs an installation method, in reality, they are actually an inspection tool. They indicate success of pretensioning, or the lack of it.
DuraSquirt® DTIs are the only DTIs that can be used as Bolt Tension Measurement Devices, per RCSC section 72 because they are the only “verification lot” of DTIs available. Per AISC/RCSC:
A Bolt Tension Measurement Device is a calibrated device that is used to verify that the bolting assembly, the pretensioning method, and the tools used are capable to achieve the required tensions when a pretensioned joint or slip-critical joint is specified. (AISC/RCSC 16.2-x)
Pre-installation Verification Testing is essential for:
- (1) Evaluating the suitability of the bolting assembly, including the lubrication that is applied by the Manufacturer or specially applied, to develop the specified minimum pretension;
- (2) Verifying the adequacy and proper use of the specified pretensioning method to be used;
- (3) Determining the installation torque for the calibrated wrench method of pretensioning;
- (4) Verifying the initial torque applied achieves at least the required initial tension when using the combined method of pretensioning; and
- (5) Demonstrating the suitability of the bolt tightening equipment to be used during installation.
Further reading AISC/RCSC section 7 describes how verification lot DTIs with Calibrated Gaps are to be used:
Direct tension indicators (DTIs) may be used as bolt tension measurement devices, except in the case of the turn-of-nut method and the combined method. This method is especially useful for, but not restricted to, bolts that are too short to fit into a hydraulic bolt tension measurement device.
Using a verification lot DTI as a tension calibrator has been included in the AISC/RCSC specification since 2009 but the U.S. Department of Transportation’s Federal Highway Administration has permitted the procedure since at least 19913.
Once the DTIs are calibrated, they are used on short bolts to set the installation wrenches at the required bolt tension. (FWHA 10-6)
Similarly, the AISC/RCSC states:
The bolting assembly may then be installed in a standard size hole with the additional verification DTI. The prescribed pretensioning procedure is followed, and it [the bolting assembly] is verified…4
To date, DuraSquirt® DTIs are the only DTIs manufactured as verification DTIs per AISC/RCSC5. The AISC/RCSC explains tightening with an assembly sufficiently, with a verification lot DTI, can replace Preinstallation Verification testing with a hydraulic tension calibrator. In other words, tightening bolts with verification lot DTIs satisfies section 7 bolt verification testing. Preinstallation testing is conducted “on the fly” as bolting commences. As each verification DuraSquirt® DTI is installed, with enough torque to make the DTI squirt completely, the fastener has passed field testing requirements.
Such robustness is not possible with other styles of DTIs because they rely on feeler gauges for pretension confirmation and feeler gauging is susceptible to false positive readings. Feeler gauges will erroneously indicate tension when a hole exceeds standard hole dimensions (oversized, reaming, etc.), flat washer problems, as well as improper DTI orientation i.e., upside down (yes, it happens).
With poor quality flat washers, the DTI’s bumps are not compressed but feeler gauges indicate they have been. Poor quality washers will imprint themselves onto the DTI. The two images below illustrate this. Obviously, the DTI washer combination shown is not “tight” since there is no bolt, but the feeler gauge is refused entry anyway.
The image on the left is the same washer/DTI combination separated. Observe the cavities imprinted in the flat washer. DTIs are hardened, structural flat washers are supposed to be as well. This one is not heat treated so it formed cavities where it contacted the DTI bumps.
In the case of holes outside of the standard hole dimensions and tolerance, the flat washer will push (not compress) the DTI bumps and force the DTI into the oversized hole. Also, when the DTI is placed upside down, i.e. bumps against the steel work, the DTI will “appear” flat against the adjacent washer, since the hardened bumps imprint into the softer the steel. In both cases, the DTIs’ bumps are intact, and will pass feeler gauge inspections.
DuraSquirt® DTIs solve these problems and more since bump compression is verified by the DTI expelling DuraSquirt® media. If the DTI does not squirt, it triggers the installer/inspector that something is out of spec and can be corrected immediately. Poor quality flat washers can be replaced or an additional flat washer against the oversized hole (as required by code) can be added to the assembly.
Simply put, a fully deployed DuraSquirt® DTI (i.e. orange visible at all bump locations), indicates adequate bolt tension and confirms holes and washers are per modern construction standards.
- Snug the steel plies, in as many steps as necessary, to bring the steel into firm contact as required by AISC/RCSC section 8.1, without compressing the DTI to complete indication. If the DTI is compressed to complete indication during the snugging operation, remove and replace the DTI.
- After snugging, apply enough torque to compress the DTI until complete indication is achieved.
The inspector MUST:
- Verify the joint has been snug-tightened.
- Verify the DuraSquirt® DTI bumps have not been compressed to complete indication during the snugging operation.
- Verify the DuraSquirt® DTI exhibits complete Indication.
- Accept any fastener with a pretension greater than the minimum required
The inspector MUST NOT:
- Inspect DTI assemblies with a torque wrench.
- Compare the amount of turn between the nut and bolt with Turn-Of-Nut table 8.1.
- Reject DTIs that have been completely flattened during pretensioning.
1ASTM F959 3.1.1 compressible-washer-type direct tension indicator, n—washer-type element inserted under the bolt head or hardened washer, having the capability of indicating the achievement of a required minimum bolt tension by the degree of direct tension indicator plastic deformation. Hereafter referred to as direct tension indicator.
2AISC/RCSC 16.2-59, “Direct tension indicators (DTIs) may be used as bolt tension measurement devices”
3FHWA-SA-91-031 May 1991.
5AISC/RCSC 16.2-59 DTIs being used for verification testing, termed the “verification lot.”
6AISC/RCSC 9.2.4 “A pretension that is greater than specified in table 5.2 shall not be cause for rejection.”