Can I substitute a rod with a nut for a headed bolt?
We do not recommend allowing this type of substitution without approval from the Engineer of Record.
Based on performing mechanical tests to the ASTM specification F606 which covers the testing of construction fasteners, a rod with a nut would typically not produce the same strength characteristics as a bolt with a forged head. Under F606, headed bolts are pulled until they break, providing minimum tensile and yield strengths for the given fastener. In order for a bolt to meet an ASTM specification, the strength of the tested bolt must exceed the minimum requirements of the specification.
For a hex, heavy hex, or square head bolt, an angled wedge (from 4° – 10° based on diameter) is inserted under the head of the bolt. This wedge puts sideways pressure on the head while the bolt is being pulled to failure. This insures that the forging has been done in such a manner that the junction of the head and the shank of the bolt is not the weak point. For this reason, the ASTM F606 specification also stipulates that when the bolt breaks during testing, it can break only in the threads or the shank of the bolt, not at the junction of the head. If a bolt were to break under the head during testing, this constitutes a failure, even if it develops enough strength to exceed the minimum requirements of the specification.
When a rod with a nut is substituted for a bolt with a forged head, two issues come into play with regard to the ASTM F606 testing. First, in the in-house mechanical testing that we have performed on a headed bolt versus a rod with a nut, both from the identical lot of steel, the rod with a nut develops about 20% 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 bolt head) is significantly reduced. Because the minor diameter (valley) of the threads is significantly less than the diameter of the unthreaded shank on a headed bolt, the rod with nut option 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 typically break at the junction of the nut which is acting as the head of the bolt. For this reason, technically a rod with nut in lieu of a headed bolt will often fail the ASTM F606 mechanical test because the head (nut in this case) comes off before the bolt breaks in the body or threaded section.
Another issue is that many companies that attempt to substitute a threaded rod with a nut for a headed bolt will tack weld the nut to the rod. The problem with this is that the vast majority of nuts that are readily available in the marketplace possess a chemistry that is not weldable. Additionally, many fastener specifications are not weldable due to their chemistry, the fact that they have been quenched and tempered, or both.
The reason that many fastener distributors and manufacturers with limited capabilities encourage the substitution of a rod with nut in lieu of a headed bolt is because they simply do not have the capability to forge heads onto bolts. They also do not have the knowledge of the testing specifications or welding issues to be aware that such a substitution would compromise the strength and structural integrity of the fastener. When an engineer designs a project using headed bolts, their calculations are based on those of a bolt with a forged head, not a rod with a nut that may not develop the same strength as a headed bolt. Our recommendation is that you should never authorize such a substitution without approval from the Engineer of Record.