anchors are widely used for securing all sorts of things to
concrete. Available from numerous manufacturers, they all operate on a
principle and are simple and easy to use. Each is applied by
inserting it in a hole drilled in concrete. The hole diameter is
to that of the anchor; thus each must be driven into the hole.
The nut, when tightened against the washer (or a flange on the time being mounted), pulls the anchor up and the tapered
end expands the pronged section, wedging it tightly against the sides of the hole.
There are three basic styles of wedge
anchors, fully threaded, partial threaded and the full-bodied wedge anchor. They all
work and even look somewhat a like, but there are differences, which can be crucial in some applications.
The important point is this. Every wedge anchor has a minimum embedment requirement. For published strength values to be
valid, each anchor must be installed as deep or deeper than the specified minimum embedment.
Lets examine the three basic styles of wedge anchors, fully threaded, partial threaded and the full-bodied.
The wedge anchor
that is called fully threaded is threaded for a major portion of its
length. The diameter of the unthreaded
portion is less than the nominal diameter of the threaded portion.
The long threaded section allows considerable come-and-go for shimming
of the item being secured. But, there is a downside to this
feature. It is possible to install such anchors at a depth less than
embedment requirement. Even when the anchor is inserted in a hole
at a depth somewhat less than the minimum embedment specified, the nut
torqued, fixing the anchor in place. Anchoring strength, however,
This misapplication occurs more often than one might expect.
The reason is that concrete commonly contains steel reinforcing
bars, called rebar. Often an installer, while drilling the hole
for and anchor, will run into rebar at a depth less than minimum
that happens, he ought to drill or cut out the bar, or move the
hole, before continuing to drill the concrete. But when installing
anchor he sometimes won’t take the time and effort to do that.
Here’s an example of what can happen with a 1/2” x 5-1/2” fully
threaded anchor. The lower edge
of the threads is 1-1/2” from the working end of the anchor. Thus
the anchor can be successfully installed in a hole as shallow as 1-5/8”,
appreciably less than the recommended 2-1/4” minimum embedment.
But, when so installed, the pullout strength for. the anchor is
reduced and will not meet the values published by the manufacturer.
In addition, when shear loads are applied through the threads, as they
be in this case, the anchor can withstand less force than otherwise
–approximately 20% less for anchors 5/8” and larger.
When installing this type of anchor, installers must be
particularly alert to achieving the minimum embedment levels specified.
This type of anchor is fabricated from steel rods having a
diameter somewhat smaller than the nominal diameter of the final product
(explaining the necked down portion). The steel employed,
typically, is AISE 1018, a material especially suited for cold heading.
The second anchor the partial threaded is threaded for only
about one third of it length and the diameter of the unthreaded
is less than the nominal diameter. This is partially threaded
anchor cannot be successfully installed in any hole that does not meet
or exceed minimum
embedment requirements. Nevertheless, this type of wedge anchor
can be misapplied. Partially threaded anchors with a smaller unthreaded
can be considered to be properly installed only when some portion
of the threaded section lies with in the hole. The same cannon be said
are installed with the threads completely clear of the hole.
For those installations where the threads extend into and fit
tightly in the hole, the threads support and stabilize the anchor. But
these anchor can be installed in situations that often occurs with a
think mounting flange or where an application calls for heavy shimming.
such cases, there is a space between the shank of the anchor and
wall of the hole. This can be a problem for applications subject to
or shock loads. Here, the shank is free to move inside the hole, a
situation than can, over time, reduce or destroy the anchor’s holding
When using this type of anchor, it is important that the
thickness of the fixture being anchored is not as great or greater than
the length of the threaded section.
Relative to material strength, it should again be noted that this type of anchor also employs AISE 1018 steel to accommodate
the cold heading manufacturing process.
The third anchor the full-bodied is also partially threaded sot
that I cannon be installed at less than minimum embedment. The
diameter of the unthreaded portion of this anchor is identical to
that of the threaded portion. These fully machined anchors fit tightly
mounting hole regardless of whether the threads are in or out of
the hole and the AISE 1215 steel is typically employed in their
shear strength of this material is inherently greater than that of
the AISE 1018 steel employed in cold formed anchors. All of these
judgment calls, errors and uncertainty on the part of installers
and contribute to faster, stronger installations.
Interpreting Technical Data
All manufacturers provide good instructions for selecting and
applying their products, but they differ greatly in the way they present
technical data. This can make it difficult to compare anchors
from one company with those from another. These differences give rise
misunderstandings and leave the door open for improper selection.
For example, some manufacturers list an ultimate-load rating for
each of their anchors while others list a working-load rating. Either
term is legitimate, but when comparing products from one
manufacturer with products from another, comparable values should be
used. The industry
generally recommends that working-load ratings be calculated as 25%
of the anchor’s ultimate load rating. Thus, an anchor having a 9,000
lb. Ultimate-load rating can be assumed to have a 2,250 lb.
Working-load rating. Some manufactures provide tension and shear values
for each anchor based on it being embedded to a specified depth,
that depth is some cases being greater than the required minimum
manufacturers provide tension and shear values for various
embedment depths. In any case, recognize that both tension and shear
on the depth to which the anchor is actually embedded.
Both allowable and working tension and shear values are a
function of the type of concrete in which they are embedded. All
wedge anchors are tested in various types of concrete—typically
2,000 psi, 4,000 psi, and 6,000 psi. Some manufactures do not offer the
and shear values for all these types of concrete; others do. The
important point is to recognize that actual ratings depend on the type
Working tension and shear values also assume the anchors are
properly torqued to fix them in the concrete. Installing them
with either less or more than the recommended torque results in
holding and shear values differing from the published ratings. Some
publish and average torque rating for each anchor. Some publish a
range of torques.
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.