Tightening of High Strength Metric Bolts

Gilroy, Michael J.


The mandated use of metric dimensions and designs for new highway construction requires the use of metric dimension fasteners. Even though the ASTM Specifications A325M and A490M for metric high strength bolts have been available for almost 20 years, they have not been used in any U. S. construction and have only been manufactured in a test lot by one U. S. manufacturer.

This research examines the influence of bolt length, grip length, bolt strength, and thread lubrication on the relationship between nut rotation and bolt tension. Two different diameters of A325M metric bolts were tested: 24 mm bolts with lengths of 70, 100, and 120 mm, and 20 mm bolts with lengths of 50, 60, and 70 mm. The performance results for the two sizes of fastener are evaluated and compared with the present nut rotation requirements for inch fasteners. The results are used to set installation requirements for the turn-of-the-nut procedure and make all required changes to the rotational capacity test to accommodate metric fasteners.

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