Standard DTI FAQs
Pre-Installation Verification Test Report
using Direct Tension Indicators: 
Inspection Procedure
- You want to do a site verification test in conformance
with the Research Council on Structural connections requirements, but
are not sure how to do it
- You're not sure what feeler gage inspection criteria you should use.
See feeler gage - what thickness
- Your bridge project requires the use of DTI's, but you have been given
five lots of DTI's of one diameter and grade instead of just one. You're
not sure how many FHWA DTI Installation Verification Procedure tests
should you do. See DTI lots
- Your DTI's have been completely flattened so that not even a .005"
thick feeler gage can be inserted. Is this OK? See
failure or stripping of bolts
- You are compressing the DTI's OK but you think you are 'overtensioning'
your bolts. What should you do? See condition of
bolt threads
- You have oversized holes. Can DTI's be used? See
oversized holes
- You have Twist-Off or TC bolts on a job, and yet the engineer has
specified that DTI's must be used. Can both TC bolts and DTI's be used
together? See Twist-off (TC) bolts
- How do you inspect DTI's?
You want to do a site verification test in conformance with the Research Council on Structural connections requirements, but are not sure how to do it:
The problem is the "Council" (called RCSC) only specifies that three bolts and DTI's be tested so that it is ".. demonstrated they they indicate a tension not less than 5% greater than the .." minimum required, see bolt tension chart, but a specific test procedure is not outlined. Our recommendation is to take three bolt assemblies, three DTI's, assemble them in a Skidmore in the way that they are to be used in the steelwork, tighten them to the required tension plus 5% using a manual (non-impacting) wrench*, and then make sure that a feeler gage CAN be inserted into approximately half of the DTI openings. The DTI gap exhibits, then, a "usable gap", because in the steelwork, the inspection procedure should be such that the DTI's must be compressed more (that is, to a smaller gap) than in this RCSC test, thereby ensuring that the bolt tension will be higher than what showed in the RCSC Skidmore test.
Note that the exact number of insertions or refusals and thickness of feeler gages is not specified by the Research Council. It is acceptable to use any thickness feeler gage that ensures a smaller DTI gap after tightening the bolts than what was in evidence in the Skidmore test.
Failing any information from a site RCSC test, use the criteria in feeler gage inspection.
When doing the RCSC test with the DTI on the nut (turning) end with a flat ASTM F436 washer between the nut and the DTI bumps, this test sometimes is difficult to do for two reasons:
- The F436 flat washers have a much larger ID than does the DTI, and this can cause only partial compression of one or two of the DTI bumps. Please see the Application Advisory: Using Nut End Installation with 1 1/4" diameter or Larger DTI's
- The rotating flat washer sometimes grinds off a little of the DTI bumps when it turns.
IF THIS TEST CANNOT BE CORRECTLY DONE WITH THE DTI ON THE NUT END, THE ONLY SOLUTION IS TO DO THE TEST WITH THE DTI UNDER THE BOLT HEAD.
* It must be a manual non-impacting wrench because, in a Skidmore, the
action of impacting "shocks" the compressible hydraulic cylinder, and
the DTI follows it to a higher-than-intended bolt tension, leading to
too much DTI bump compression. This is unlike the action of an impact
wrench when tightening a bolt (and DTI) in a solid steel connection.
Pre-Installation Verification Test Report
using Direct Tension Indicators:
See the Pre-Installation Verification Test Video
See the Job Inspection Gap Video.
How do you inspect DTI's?
DTI's compress directly in proportion to the actual TENSION in the bolt. Inspection is done using feeler gages of specific thickness, .015" or .005" thick, to determine the remaining protrusion height, or "gap" thickness.
When
the feeler gage CANNOT be inserted into the space between the DTI protrusions
and the bolt head (or flat washer) HALF WAY AROUND THE DTI CIRCUMFERENCE,
the compression of the DTI protrusions will have been sufficient to assure
bolt installers that the bolt has been TENSIONED correctly.
Alternatively, if the feeler gage CAN be inserted more than half way around, the DTI has NOT been compressed sufficiently, and therefore the bolt HAS NOT been tensioned correctly.
