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Loose Bolts with DTI Washers

[Subject Prev][Subject Next][Thread Prev][Thread Next] I have been asked to investigate a very large, 10-year old steel structure with bolted connections (there are approximately 11,000 bolts in the building).  The problem is that some of the bolts are falling out.  The owner hired an engineer who inspected 20% (2236) of the bolts and found that 36% (798) were either loose, missing or not tightened properly.  Direct Tension Indicator (DTI) washers were used and were installed properly, but not necessarily tensioned properly.  All of the bolts are 3/4 inch or 1 inch diamter A325 bolts.  It is believed that the DTI washers were installed on the bolt side of the connection, since the engineer references using a 0.015 feeler gage.  The drawings and specifications call for DTI washers to be installed on all high-strength bolts, regardless of connection type. The building supports overhead cranes and is also subjected to other dynamic loads (buffeting winds).  Here are the questions:

1) Other than improper installation/tightening of the bolts, are there any other reasons or explanations as to why the nuts would be loose ten years later?

2) What is the history of DTI washers?  How reliable are they?  What is the likelihood that the washers are defective?  What tests or methods are employed to determine if a washer is defective?  What are the pitfalls of using these washers?

3) Is it possible that the bolts can be fully tensioned and yet the bumps on the DTI washers not be flattened as specified?

4) On connections that are tight, but the DTI washer  is not properly compressed, are there tests that can be performed to determine if the bolt is adequately tensioned, after ten years?

5) What tests or methods can be employed to determine if the loose bolts were ever tensioned?

6) Any thoughts on re-tightening the loose bolts.

I have studied both the Manual of Steel Construction - ASD (8th Edition) and LRFD (2nd Edition) and am familiar with their requirements.  I am seeking information outside the scope of these references.  Thanks!

Michael D. Gregory
Teton Structural Engineers
Idaho Falls ID