How do I know how much thread length I need on my anchor rods?

There are no standard thread length calculations for anchor rods.  Anchor bolt thread length will depend on the required projection for the protruding end and the required hardware or configuration for the embedded end.  It is very common for structural drawings to detail the required thread lengths for anchor bolts.

Bottom Threads

Bottom threads will not be required if the anchor is a 90° bent, swedged, or headed configuration.  If the anchor has bottom threads, the length will usually be determined by the thickness of material that is being assembled onto the threads plus a small amount of additional thread for welding, peening (damaging threads), or adjustment purposes.

Top Threads

Top threads on anchor rods are longer than the bottom threads in normal circumstances. The thread length at the top of the anchor will almost always be determined by the projection from the top of concrete.  It is normal for about 1” to be added to the projection to equal the top thread length.  The added length can helpful in providing a slight adjustment to aid with installation.

When the projection is unknown, the thread length will be calculated by adding the thickness of the nut(s), jamb nut (when applicable), washer(s), and the thickness of the base plate of the structure being bolded to the foundation.  Typically, an inch or two is added to the sum of these components to determine the projection and subsequent top thread length.

To assist inspectors and installation crews in the field, the threaded end of all F1554 anchor bolts intended to project from the concrete should be color coded or permanently marked with the grade symbol and/or manufacturer’s identifier.

In most situations, the thread lengths for anchor rods will be determined by the engineer and detailed on the construction plans.  If the anchor rod thread lengths are not provided, using the information listed above should help in determining the required top and bottom thread lengths.

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