Concrete Anchor Design Considerations

When designing anchored concrete connections, more anchors won’t always make for a stronger connection. Anchor material, design, strength, and connection style will vary between manufacturers, but there are some simple principles that all anchor design relies on that can dramatically affect the resultant strength of your connection.

When an anchor is installed, its goal is to “grab onto” as much of the host material as possible and hold on when forces are applied to the anchored part. Whether via mechanical connection like threads of a screw or a chemical connection like an adhesive glue, the goal of the anchor is to grab onto as much of the material as possible around itself. The area of the host that the anchor is using to create its strength is called its “cone of influence”:

The cone of influence is a visual way to represent how much of the host concrete contributes to the anchor’s strength in tension. Imagine that as tension is applied to the anchor, the anchor resists from the bottom of the concrete up to the surface. As the anchor grabs onto the concrete at its deepest embedment point, that concrete grabs onto whatever it can around and above it. In this way, the anchor is supported not only by the concrete is directly in contact with, but also much of the concrete in its general area.

Shear and tensile strengths for various anchors are detailed in the manufacturer’s technical specifications. These documents provide base strength values (whether they be ultimate or design strength is dependent on the manufacturer), as well as reduction factors for suboptimal conditions. The most common installation conditions that may affect anchor strength are Embedment depth, Concrete Edge Distance, and Anchor Spacing:

Reducing an anchor’s embedment will reduce the total volume of concrete the anchor can affect. With a shorter anchor, there is less concrete above the deepest point of the anchor to resist tensile forces. When reviewing manufacturer anchor strength charts, be sure to note the required embedment depth for the selected anchor at the required strength.

Placing an anchor close to the edge of the slab will reduce its tensile strength by removing additional concrete that would have otherwise been used for support. Additionally, placing the anchor too close to the edge may reduce the connection’s overall strength in shear or compression, as the concrete itself may crack or split along the edge. Watch a video of this happening as an anchor too close to the concrete’s edge is overloaded and fails: Engineering Express Test Video of Anchor Into Concrete Edge Distance Failure

Sometimes, there isn’t enough concrete for each anchor to grab onto. When multiple anchors are spaced too closely, the cone of influence can overlap, representing that both anchors are drawing their tensile strength from the same area of concrete. This concrete may be able to share, but it cannot provide its strength twice. As a result, the overall connection strength is reduced by a factor, which is dependent on how closely spaced the anchors are.

These anchor conditions represent the most common ways anchored connection strength can be reduced without changing the physical anchor or concrete being used in the connection. When finding anchor strength from manufacturer spec sheets, be sure to consider reductions and minimum allowances for embedment, edge distance, and spacing.

Of course, there are many other factors that contribute to anchored connection strength. The host structure plays a huge role in strength calculation. Check your anchor’s technical report for requirements of concrete condition (cracked or uncracked), concrete strength, concrete thickness, reinforcement, and any admixtures that may affect chemical adhesive bond strength. Atypical loading conditions such as sustained loading or significant seismic conditions will also play a part in designing the connection.

For simple conditions, using manufacturer-provided strength and reduction tables may be sufficient for design. But for more complex projects, many anchor manufacturers provide finite element analysis design software which calculate better, more accurate results.

For help designing and certifying your anchored concrete connections, contact Engineering Express