How to Design Anchor Bolts

Designing anchor bolts in concrete can be a complex process with many failure modes to check. However, with a systematic approach, it becomes much more manageable. Follow these steps to design most anchor bolt configurations according to ACI 318.

  1. Geometry - Define the concrete geometry and anchor layout. This includes edge distances, embedment depth, and anchor spacing.
  2. Materials - Select the concrete and anchor material properties.
  3. Loads - Apply tension and shear loads to the anchor or anchor group.
  4. Design Method - Choose the design code and whether to consider seismic effects.
  5. Review Results - Check all potential tension and shear failure modes to ensure the anchor design is adequate.

In this tutorial, we'll use the WebStructural Anchor Bolt Designer to walk through each of these steps. By the end, we'll have designed a four-bolt anchor group subject to shear and tension, checking all the required strength calculations per ACI 318-14.

If you'd like to follow along, open the Anchor Bolt Designer in a separate tab. You'll notice the interface is organized around the key design steps described in this tutorial.

Let's get started

Step 1: Configure Anchor Bolt and Geometry

The first step is to configure the anchor bolt, define the geometry of the concrete slab and layout the anchors. This is critical because the capacity of the anchors is highly dependent on the distance to concrete edges and the spacing between anchors.

Click the Anchor Bolt card to launch the Anchor Bolt Dialog. Select Threaded Rod and Square as the anchor type and nut. Select a diameter of 1/2" for the threaded rod. Set the embedment depth, hef, to 8 inches. The embedment depth is the distance from the surface of the concrete to the bottom of the anchor. A deeper embedment generally leads to higher capacity.

For our example, we will design a group of four anchors with 6" spacing in each direction, located 12" from two edges of a large concrete slab. Concrete edge distances and anchor spacing significantly influence the anchor capacity, so it's important to define these accurately. These values can be modified in the Geometry section of the interface, just below the drawing.

Step 2: Choose Materials

Next, we'll define the concrete properties.

Click the "Concrete" button. For this example, we'll use a concrete strength of 4000 psi and a slab thickness of 15 inches.

Step 3: Apply Loads

Now we need to apply loads to our anchor group. Loads are typically broken down into tension (pulling the anchor out of the concrete) and shear (pushing the anchor sideways). We will apply both.

Click the "Loads" button. For this tutorial, we'll apply a tension load, Nua, of 10 kips and a shear load, Vua, of 5 kips.

Step 5: Review the Results

Once all the inputs are set, WebStructural automatically performs the calculations and presents the results. The results are shown as a series of capacity ratios for each potential failure mode in tension and shear. A ratio less than 1.0 means the demand is less than the capacity for that failure mode, and the check passes (indicated in green).

The primary failure modes checked are:

  • Tension: Steel Strength, Concrete Breakout, Pullout, and Side-Face Blowout.
  • Shear: Steel Strength, Concrete Breakout, and Concrete Pryout.

Finally, an interaction check is performed for combined shear and tension.

If you entered everything correctly, your design should look like this

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