"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
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
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
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
Hole diameter is equal to anchor diameter.
The installation of concrete wedge anchors can be completed by following the steps below:
- 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
- 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
- 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.
- Insert the wedge anchor through the fixture hole.
- 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.
- 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.
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.
As with any anchoring project, it is important to keep safety in mind and follow instructions carefully. Always remember to wear safety goggles, handle all tools with extra care and follow all technical specifications. This article is meant to serve only as a basic explanation of concrete fasteners. Always refer to manufacturer's instructions or consult a contracting expert during any anchoring project.
Article Written By:
Bob Carlisle, President of Concrete Fasteners
Concrete Fasteners, Inc. has over 40 years of experience selling concrete fasteners. We can ship out one box or a whole pallet of concrete anchors. Our products are of the highest quality... "your satisfaction is guaranteed". We ship all orders the same day the order is received.