Concrete anchors can fail catastrophically, and many failure mechanisms are possible
between shear failures, tension failures, concrete failures, and steel failures.
To understand why concrete anchors fail, in this video we'll cover the basics types of
anchors and their failure modes, as well as dive into the design concepts.
Concrete anchors generally fall in two categories, cast-in or post-installed. Cast-in anchors are
laid out and placed before the concrete is poured around them, and post-installed anchors are laid in
and placed after the concrete has been poured. Some examples of cast in anchors are shown
here. They generally involve a bolt or piece of rebar with some bearing
element such as a nut, head, or hook, embedded in the concrete to grab onto more material.
Some examples of post-installed anchors are shown here. Post-installed anchors can
be either adhesive anchors (basically glued in) or mechanical anchors (basically wedged,
torqued, or screwed in). Mechanical anchors have many types, common ones are undercut,
wedge, drop in, and screw. Depending on the type of
concrete anchor and complexity of anchor groups, there can sometimes be a dozen or
more failure modes for one connection! The failures can generally be categorized
between steel failures and concrete failures. Focusing on the steel first, failure can occur under
tension loading, or under shear loading. Failure in the steel generally occurs when
the anchor bolt diameter is too small for the loads being applied.
In more complicated embeds such as anchor channels,
the steel flanges of the channel could also fail by prying open as shown here.
This is a bending or flexural failure of the flanges caused by high tension loads locally.
Often, tension and shear loads are combined to produce the stresses which the anchor feels.
Prying effects can amplify these combined loads too which we'll touch on later in this video.
Now, let's look focus on the failures that can occur in the concrete,
Under tension loads we can have a concrete breakout failure
or, the anchor can pullout from a failure at the concrete-to-anchor interface.
And under shear loads, the concrete can pryout like this,
These types of concrete failures generally occur when the anchor embedment is too low,
or if the concrete has insufficient strength, reinforcing or vibration or other defects; or
if the type of anchor is simply not strong enough for the application.
These concrete failures also illustrate the way that anchors stresses are moving
through the concrete the forces generally spread in a cone outwards from the anchor
point. Imaging the failure surface is an important step in the design process
which we'll touch on towards the end of the video.
Tension and shear loads also cause combined stress in the concrete.
To further complicate the matter, anchors are often required near edges. This reduces the volume
over which anchors can distribute their stresses, and can result in edge breakout as shown here.
Additionally, concrete anchors are typically used in groups for stability. If groups of anchors are
spaced to closely, again the volume over which the anchors distribute their stresses can overlap to
cause a grouped anchor failure like this. This is not an exhaustive list of all the concrete
anchor failure modes but it's the main ones and is enough to convey the general concept of these failures
How can we avoid these failure modes? This is the number one question engineers work to answer when
designing concrete anchors. Although a full design walkthrough is beyond the scope of this video,
we'll still introduce you to the general concepts needed for the design of concrete anchors.
The primary design criteria for concrete anchors are
- embedment - diameter
- edge distance - spacing
type-specific parameters such as post-installed wedge size
or post-installed epoxy adhesion strength and prying, which we'll illustrate here shortly
These design criteria are often interconnected, which creates
a multi-variable problem for engineers to solve, even for a simple little base plate!
Prying is one aspect that often causes concrete anchor connections to become overloaded.
Let's examine a base plate for a post.
A typical design may look like this, with a post centered on a base plate and anchors on all sides.
Let's take 500lbs (2.22 kN) as the design load applied at 4ft (1.22m) up the post.
When loads are applied to the post in this case, the prying distance is 1ft (0.3m).
That means that the load in the anchor will be 500lbs * 4ft / 1ft = 2000lbs (8.9 kN). So
prying has now amplified our forces by a factor of four.
Now, what if the design is revised to give additional clearance on one side of the post?
Say the base plate is shorted and the anchors are all moved to the other side of the post.
The prying distance is now just 3 in or 0.25ft (7.6cm). That means that the load on the anchor will now be
500lbs * 4ft / 0.25ft, this is 8,000lbs. So prying has now amplified our forces by a factor of 16!
This is why prying is such an important consideration in the design of concrete anchors.
If you know your design loads, product manufacturers will have data catalogues for
you to compare your loads to, and see what anchors work. More interestingly, certain manufacturers
also have design software which allows you to iterate many possible configurations.
Some examples of this are Hilti Profis, Dewalt Design Assist, and Jordahl Expert.
These softwares allow you to enter your loads and geometry to see what anchors can work.
The calculations performed in these softwares combine engineering statics calculations,
building code calculations, and test data from manufacturers.
We have no affiliation with these companies, they're just the ones we are familiar with,
so if there are others you know of and use and enjoy, let us know in the comments!
We hope this video gave you some insight into the engineering considerations that go
into designing concrete anchors. Engineers spend unthinkable amounts of time choosing anchors and
base plate sizes for their connections so that failure events can be as rare as possible.
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