Weathering Steel DTIs on Bridges
Problem: Type 3 DTI's installed on bridges are often installed under bolt heads to a residual gap of .005" by order of state DOT bridge engineers and AASHTO. This can lead to unnecessarily high bolt tension.
Here's Why: When DTI's are manufactured, ASTM F959 requires that the testing is done at a residual gap of .015". Consequently, in bridge applications, the bolt tension will be about 20% higher than the mean compression strength achieved in the (.015") DTI certification tests. This high bolt tension will sometimes cause distress in the bolt during tightening. (Note that in buildings, a residual DTI gap of .015" is used, so the problem does not exist)
Solutions: There are several solutions, any of which must be checked by means of the AASHTO "DTI Pre-Installation Verification Test":
1. Put the DTI on the nut end under a hard flat washer (check for flat washer fit over DTI bumps). This will result in an apparent .005" DTI gap at a lower bolt tension.
2. Leave the DTI under the bolt head, keep the bolt VERY well lubricated, but tighten it in one pass right up to the correct DTI compression. Restarting tightening when the DTI is almost, but not quite, compressed to .005", may cause a torque spike which unnecessarily breaks the bolt in combined stress. If that occurs, first back up the nut, then drive it down in one go.
3. Leave the DTI under the bolt head, but use a slightly different (bigger) DTI gap, such as .010". This will result in a bolt tension about 10% over the mean compression strength of the DTI, and is easily accommodated by Type 3 bolts even when marginally lubricated.