Can a threaded rod with a welded nut be substituted for a bolt with a forged head?
When a threaded rod with a nut is substituted for a bolt with a forged head, two issues come into play. When in-house mechanical testing was performed by Portland Bolt on headed bolts versus rods with a nut, both from the identical lots of steel, the rod with a nut developed up to 12% less strength than the comparable headed bolt. The reason for this reduction in strength on the rod with nut is that the stress area at the junction of the rod and nut (which is substituting for the forged bolt head) is significantly reduced. Because the minor diameter (root) of the threads is significantly less than the full-size diameter of the unthreaded shank on a headed bolt, the rod with nut often breaks at a much lower strength than a headed bolt. More importantly, even if the rod with nut does develop enough strength to meet the specification, it will often break at the junction of the nut which is acting as the head of the bolt when wedge tested per ASTM F606. For this reason, technically a rod with a nut in lieu of a headed bolt will frequently fail mechanical testing because the head (nut in this case) comes off before the bolt breaks in the body or threaded section of the fastener which constitutes an automatic failure.
We often see fastener distributors and manufacturers without forging capabilities talking customers into switching from a headed bolt to a rod with a nut since they are unable to provide the forged product that was engineered into the structure. We see this most frequently with regard to headed anchor bolts. Making the decision to switch from a bolt with a forged head to a rod with a nut should always be approved the Engineer of Record to prevent any liability in making such a substitution should a failure of the fasteners occur.
Wedge Testing per ASTM F606
Note: Wedge testing as shown above is required to obtain mechanical properties for most ASTM high strength fasteners. A greater difference was found for high-strength fasteners than mild steel fasteners during Portland Bolt in-house testing of headed bolts versus rods with a nut. The reduced cross-sectional area at the junction of the nut as compared to a bolt with a forged head caused the fasteners with a nut acting as a bolt head to break at lower strengths than the bolts with forged heads. Additionally, the high strength rods with a nut acting as a head broke just below the nut which would automatically constitute a failure since the wedge test requires the bolt to break either in the body of the bolt or in the threaded portion, not at the junction of the head.