What do I do if I poured my anchor bolts too low or too high?

The answer can be found in AISC Design Guide 1,Section 2.11.3:

“Anchor rod projections that are too short or too long must be investigated to determine if the correct anchor rods were installed. If the anchor rod is too short, the anchor rod may be projecting below the foundation. If the rod projection is too long, the embedment may not be adequate to develop the required tensile strength.”

Too Low

Bolt extensions
Bolt extensions bought the anchor bolts to the correct height in this Radio Tower in Honolulu

If your anchor bolts are poured too low, the AISC offers five options.

  1. Partially engage the nut

    “Often, when the anchor rod is short, it may be possible to partially engage the nut. A conservative estimate of the resulting nut strength can be made based on the percentage of threads engaged, as long as at least half of the threads in the nut are engaged. Welding the nut to the anchor rod is not a prequalified welded joint and is not recommended.”

  2. Redrill and use an epoxy anchor

    “If the anchor rod is too short and the rods are used only for column erection, then the most expedient solution may be to cut or drill another hole in the base plate and install a drilled-in epoxy-type anchor rod.”

  3. Extend using a coupling nut

    “When the rods are designed for tension, the repair may require extending the anchor rod by using a coupling nut…”

  4. Butt weld extensions to the existing, already in place bolts if they are weldable anchors.

    “When the rods are designed for tension, the repair may require extending the anchor rod by (using a coupling nut) or welding on a piece of threaded rod. ASTM F1554 Grade 36 anchor rods and ASTM F1554 Grade 55 with supplement S1 anchor rods can be extended by welding on a threaded rod. Butt welding two round rods together requires special detailing that uses a run out tab in order to make a proper groove weld. The run out tab can be trimmed off after welding, if necessary, and the rod can even be ground flush if required.”

  5. Extend the anchor by welding on splice bars to add on a threaded rod

    “It is also possible to extend an anchor by using splice bars to connect a threaded rod extension.”

Too High

If your anchor bolts are poured too high, the AISC only has one recommendation: use plate washers to make up the difference.

“When anchor rods are too long, it is easy to add plate washers to attain an adequate thread length to run the nut down to the base plate. As noted previously, anchor rod details should always include an extra 3 in. or more of thread beyond what the detail dimension requires to compensate for some variation in anchor rod projection.”

More detailed information and diagrams can be found in section 2.11.3 of AISC Design  Guide 1.  Your project engineer should also be consulted before any of these repairs are undertaken to make sure the best method for the specific application is chosen.

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4 comments

    What is the proper procedure for a sheared off anchor bolt that is securing a metal frame pillar for a metal building? Is it common for anchor bolts to shear off ( we had three failures). My concern is that there was a bad batch of bolts from the manufacturer.

    @Daniel- No, it is not common for the anchor bolts to shear off. There are several possibilities for why this is happening, but to be certain you might need to contact a failure analysis engineer.

    My house is currently elevated. The anchor bolts are too short so the mason added threaded rod to extend their length. Most of the house has 3 pieces of 2 x 6 pressure treated wood for the sill plate. Only the sun porch needed 1 piece of pressure treated wood. The bolts were extended on most of the house. Is this ok? Also, do holes need to be drilled into the house itself to accommodate these bolts? I also see metal straps to bolt to the house along with universal service plates. Maybe that’s all I need.

    @Patty- Anchor bolts are commonly extended in the manner you are describing. As for whether or not this is the appropriate fix in your situation, I cannot say. You would need to consult an engineer.

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