Concrete Wedge Anchors & Their Many Uses

"Wedge anchor" is a generic term for a type of anchor used to fasten a variety of materials to concrete. Some companies have different brand names for their wedge anchors such as Kwik Bolt, Power Stud, and Thunderstud. The different brands of wedge anchors all work on the same principles, are made from the same basic materials, and provide similar high quality performance.

What is a Wedge Anchor?

Wedge anchors consist of two separate pieces that are permanently pre-assembled into a single unit. The first piece is a carbon steel rod that is threaded for a portion of its length. The opposite/installed end of the anchor has a necked-down diameter, or conical space, that tapers outward back to the rod's full diameter.

The second piece of the wedge anchor is a steel slip, or sleeve, that is permanently assembled around the conical section of the rod. Each wedge anchor also requires a nut and washer to complete installation.

Wedge anchors can be used in numerous applications as long as the base material is solid concrete. Unlike many other concrete fasteners, the wedge anchor cannot be used in brick or block.

The wedge anchor is simple to use and is available in a wide variety of diameters and lengths. Wedge anchors can be used in both light and heavy duty applications. A 1/4" x 1-3/4" wedge anchor, for instance, would be utilized in a light duty application, while a 1-1/4" x 12" anchor would be used in a situation that requires a heavy duty anchor.

How Does a Wedge Anchor Work?

Again, wedge type concrete fasteners are designed to use in solid concrete only. They are designed to go into a pre-drilled hole in concrete. Once installed, these wedge anchors cannot be removed without destroying the concrete. The size of the anchor is always equal to the size of the hole that needs to be drilled into the concrete. The tolerance between the hole and anchor is very tight. As the anchor is hammered into the hole, the clip is recessed in the gap of the conical space, allowing the anchor to penetrate the hole. To set the anchor, the rod must be pulled up (which occurs while the nut is being tightened), making the clip move outward on the tapered space, wedging itself between the rod and the wall of the hole. 

Since the hole tolerance is critical in the performance of the wedge anchor, it is important that the correct drill and drill bits are used. A hammer drill should be used rather than a standard power drill because it creates a better quality hole. A carbide-tipped masonry bit should also be used because they are specifically designed for use in hammer drills and meet ANSI standards.

Wedge Anchor Holding Values

When anchoring into concrete, wedge anchors are an excellent choice because of their holding values. The holding power of the wedge anchor, as with any anchor, is always dependent on the quality of the concrete. As a rule of thumb, a safety factor of 4:1 or 25%, is generally accepted as a safe working load. Holding values are also derived from the depth of the embedment- the deeper the embedment, the better the holding values. The wedge anchor must be embedded into the concrete up to or past the minimum embedment depth to obtain published holding values. It is also essential to ensure that the anchors are not placed too close together or too close to an unsupported edge.

The pressure exerted on the concrete by a wedge anchor can be best described as cone-shaped, with the small end of the cone being the inserted end of the anchor and the large end of the cone being at the surface of the concrete. If two wedge anchors are set too close together, the pressure from the two anchors could intersect, decreasing the holding values of both anchors. These same factors apply when placing an anchor near an unsupported edge. It is possible for the concrete to crumble under pressure if the anchor is placed too close to the unsupported edge.

The general recommendation is that an anchor should be placed no closer than five diameters from any unsupported edge. For example, a 1/2" diameter anchor should be at least 2-1/2" from any unsupported edge of concrete. Also, two anchors should never be set less than ten diameters from eachother. That is, two 1/2" diameter anchors should be placed at least 5" apart. 

Determining the Correct Size Wedge Anchor to Use

The diameter of the wedge anchor required for an application depends on the size of the hole in the fixture to be fastened. The "fixture" could be anything from a 2x4 to a piece of angle iron, bracket, or steel plate. The hole size in the fixture is most often determined by an engineer based on the necessary holding values for each particular application. It is imperative to ensure that the concrete wedge anchor fits through the hole in the fixture. A 1/2" anchor is, in reality, slightly larger than 1/2". The chart below shows the correct diameter to use based on fixture hole diameter:

Thickness of Material + Minimum embedment + Thickness of nut/washer Minimum length of anchor

Each wedge anchor diameter is avaiable in a variety of lengths. The length required for each application is dependent upon the thickness of the fixture to be fastened and the minimum embedment. To determine the length of anchor needed, simply add the thickness of the fixture to be fastened to the minimum embedment for the anchor diameter being used, plus the thickness of the nut and washer (thickness is typically close to the diameter of the anchor):

This will give you the minimum length of anchor required. Using a longer wedge anchor than necessary will increase holding values, but may increase the risk of drilling into rebar embedded in the concrete.

Wedge Anchor Installation

Helpful Hint:
Hole diameter is equal to anchor diameter.

The installation of concrete wedge anchors can be completed by following the steps below:

1. Determine the correct length of wedge anchor to ensure that the minimum embedment is met and make sure the wedge anchor fits through the hole in the fixture. The washer should also be larger than the fixture hole.
2. Using the hole in the fixture as a template, drill the holes using a hammer drill and carbide-tipped masonry bit. The drill bit diameter should be the same as the anchor diameter. It is imperative that the hole be cleared of all dust and debris using a wire brush, compressed air, or vacuum.
3. Place the washer and nut on the anchor, turn the nut a couple turns. Not fully threading the nut protects the threads of the wedge anchor when hammering into the hole.
4. Insert the wedge anchor through the fixture hole.
5. Carefully hammer the anchors into each hole ensuring that the threads go below the surface of the concrete or past the edge of the fixture and that the minimum embedment is met.
6. Tighten the nut- finger tight. Use a wrench to tighten the nut 3-4 turns, or use a torque wrench to ensure they are tightened to the required torque value. Do not over- torque, as the anchor will either spin or pull out of the hole. Below is a chart that outlines what size wrench to use based on the nut size for each diameter:

For more detailed information, please view this Wedge Anchor installation video.

Wedge Anchor Torque Values

Wedge anchors require a minimum torque to set. Do not tighten the nut as tight as possible. When setting a wedge anchor, it is important that the nut only be screwed on finger tight. Each anchor diameter has a required torque value to which it must be tightened.

Length Indentification Chart

Once the anchor is properly set, all that is protruding above the concrete is the nut, washer, and a portion of the threads. The diameter of the embedded anchor can easily be determined by simply measuring the stud. The length of the anchor can be difficult to determine since the depth of embedment may be unknown. To help identify the length of an installed anchor, an identification code has been developed. A letter is stamped on the end of the threaded portion of the anchor so that the length can be easily determined after installation. This letter corresponds to the overall length of the anchor (in inches).

Wedge Anchor Material Specifications

Concrete wedge anchors are available in different types of steel as well as different plating. The type of steel and plating to use is based on the application's environment. Zinc-plated carbon steel wedge anchors are used in interior applications, where corrosion is not a factor. The anchor, nut, washer, and clip are all made of zinc-plated carbon steel. This type of plating is not recommended for use in ACQ treated lumber.

Hot-dipped galvanized wedge anchors are suited for exterior applications where corrosion from water or moisture is a concern. The anchor, nut, and washer are all made of carbon steel and are hot-dipped galvanized. The clip is made of 303 stainless steel. Hot-dipped galvanized wedge anchors are acceptable for use in ACQ treated lumber. 

Wedge anchors are also produced in two grades of stainless steel. 303 stainless steel has excellenet corrosive characteristics for exterior uses and is resistant to many organic and inorganic chemicals, but should not be used in a salt water environment. The anchor is made from 303 stainless steel while the nut, washer, and clip are made of 18-8 stainless steel which is comparable to 303 stainless steel.

The 316 series of stainless steel has the best corrosion resistance and is typically used in harsh environments. 316 stainless steel anchors can also be used in a salt water environment. All of the components in these anchors are made from this resilient 316 steel. 

Overview

Wedge anchors are an excellent choice when anchoring into solid concrete. These anchors have excellent holding values and are available in a range of materials to meet the requirements of many applications. It is important to make certain that the correct diameter and length are used in each application to ensure that the wedge anchor is set properly and safely.