anchor bolt spacing

What is the Maximum Spacing for Anchor Bolts?

Typically, information on anchor bolt spacing will be contained within your construction drawing set. If you don’t have a set of drawings to work from, you’ll want to reference your local building code for information.

According to the International Building Code (IBC), the maximum spacing allowed is 6 feet (1829 mm) on center for exterior wall wood sole and sill plates supported directly on masonry or concrete foundations. For buildings over two stories in height, the maximum spacing decreases to 4 feet on center (1219 mm).

Two bolts are required per plate section. One bolt needs to be located no more than 12 inches (305 mm) or less than 4 inches (102 mm) from each end of the plate section. Additionally, bolts should be placed within the middle third of the width of the plate.

Keep in mind that these are minimum requirements, that is, not to exceed spacing. It’s generally good practice to allow for some margin of error and increase the number of anchor bolts used (decrease the spacing) from these figures. This is particularly true in seismic regions.

How Anchor Bolts Work

Anchor bolts are used when attaching structural elements to concrete or masonry grout. They can either be set in place prior to concrete pour, or they can be installed after the concrete has cured to at least 60 percent of its designed strength.

If they are installed after the concrete pour, they are typically set into a hole which is drilled into the concrete or masonry grout. They are then installed by hammering them into the holes.

Once the anchor bolts are set into the holes, they are tightened by turning the nut at the top end to secure them in place. They work through a process of friction between the anchor bolt and the wall surface of the hole drilled into the concrete or masonry grout.

How Long Should Concrete Cure Before Setting Anchor Bolts?

The time it takes for concrete to cure will depend on a few factors including the type of concrete used, the region you are located in, ambient temperature and relative humidity. Additionally, admixtures, if added to the concrete, can also affect the cure time.

Generally speaking, however, concrete should cure within 3 to 7 days. At this point, the concrete should be at about 60 percent of it’s designed strength. It should be OK to drill into the concrete to set your anchor bolts in place after this period of time.

How Deep Should Anchor Bolts Be in Concrete?

Bolts are also required to extend a minimum of 7 inches (178 mm) into concrete or grouted cells of concrete masonry units. This minimum length needs to be fully embedded in the concrete or grouted cells.

In addition to embedded depth, the minimum diameter required for anchor bolts is 1/2 inch (12.7 mm) per the International Building Code (IBC). As with maximum spacing, this is a minimum requirement. Increasing the diameter to 5/8 inches is usually a good way to allow for some margin of error, particularly in seismic regions.

Anchor Bolt Types

Anchor bolts come in a variety of shapes and sizes. They also vary in the way they function to secure structural elements to concrete or masonry grout. Some of the more commonly used types of anchor bolts include:

  • Wedge Anchors
  • Sleeve Anchors
  • Concrete Screw Anchors
  • Drop-In Anchor
  • Expansion Anchors
  • Undercut Anchors
  • Bonded Anchors
  • Lag-Shield Anchors

The type of anchor used depends on whether it is cast-in-place during concrete or grout pour, or whether it is fastened after the concrete has cured. For example, J-Bolts which have a 90-degree bend at the bottom can only be installed in wet concrete.

Also, the material that it is being installed on can determine the type used. Concrete is the strongest and most versatile material. That is, most types of anchor bolts will work. On the other hand, in masonry, block or mortar settings, wedge and drop-in anchors cannot be used.

How to Set Anchor Bolts in Wet Concrete

When setting anchor bolts in wet concrete, cast-in-place anchor bolts are typically used. Spacing can be marked on concrete forms with a pencil and tape measure prior to pouring the concrete.

Next, nail anchor bolt holders to the concrete forms at the marked locations. Screw the anchor bolts into the holders using a wrench. Set them at the required height and depth.

Once all anchor bolts are positioned in place, pour the concrete into the concrete form. Allow the concrete to flow into the anchor bolts, stopping at the right level set by the concrete form.

Once the concrete has cured, the anchor bolt holders and concrete form can be removed. It’s important to allow the concrete to dry before removing so the anchor bolts do not shift. The holders can be removed by unthreading the cap of the concrete bolt.

How to Install Anchor Bolts in Existing Concrete

The most effective way to drill a hole in cured concrete is to use a hammer drill. Be sure to set the drill to hammer mode. The diameter of the hole should match the diameter of the fastener. For example, if using a 5/8″ anchor bolt, use a 5/8″ diameter drill bit.

The depth of the drill-hole should be a bit longer than the length of the anchor bolt to allow for some leeway. Use the depth gauge of the hammer drill to set the correct length of the hole. Alternatively, you can wrap tape around the drill bit to use as a reference.

Be sure to use ear and eye protection during drilling. After drilling the hole, dust off all the debris creating during the drilling. Additionally, it’s important to keep the hole dust-free. You can use a wire brush or vacuum to remove the debris that is trapped inside the hole.

Once this is completed, attach the nut and washer to the head of the anchor bolt and use a hammer to insert the anchor bolt into the hole. Finish the installation by turning the nut finger tight, and then an additional 3 or 4 turns using a wrench.

Closing Notes

Anchor bolt spacing is typically spelled out in drawing documents and/or specifications. The maximum allowable distance, diameter, and required embedded depth are determined by local building codes.

Using the International Building Code (IBC) as a point of reference, the maximum spacing for a building of two stories or less is 6 feet (1829 mm) on center between anchor bolts. The minimum embedded depth is 7 inches (178 mm) and the minimum diameter required is 1/2″ (12.7 mm).

These are minimum requirements, however, which is why designers often will decrease the spacing requirement or increase the diameter of the bolt in order to allow for a margin of error. This is particularly true in seismic regions.

At the same time, spacing anchor bolts too close together can damage the concrete or grout material that the anchors are being attached to. It’s important, therefore, not to veer too far from the maximum spacing requirements spelled out by code.

References:

International Building Code (IBC) 23-1, 2308.3.1, 2308.3.2, International Code Council, Inc.

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