Standard DTI FAQs
What is the FHWA DTI Installation Verification Procedure?
The complete text of this procedure is currently listed in the AASHTO Specification and in many State DOT specifications. It involves using samples from a single DTI lot with bolts from another lot, and tightening them in a Skidmore first to 5% over minimum tension (see bolt tension), and at that point checking that the DTI gap is sufficiently "open" to a .005" feeler gage.
At this load, the DTI is generally completely open, that is all gaps will accept a .005" feeler gage. The test continues. Further tighten the bolt and DTI to "approximately flat but not quite flat", and, after removing the bolt from the Skidmore, check that the stretch in the first few threads of the bolt is such that the nut can be run down to the thread root. If it can, fine. The bolt and DTI combination pass the test. Even if the bolt stretches just a little too much to allow free run-down of the nut, if the bolt tension at the point where the DTI was just about flat is less that 95% of the maximum bolt tension that the same lot of bolts achieved in the Rotational Capacity test, the assembly including the DTI passes.
In the rare event of failure of the bolt to be sufficiently strong to just about completely flatten the DTI's with out stretching the bolt to much, the bolt assemblies can be relubricated and/or a different lot of DTI's can be selected for retesting.
