Trying to determine what type of anchor to use for your project?
If you are unsure of where to begin – equip yourself with
the following three pieces of information to aid you as you are
narrowing down your choices.
The base material that you are anchoring into
The load capacity required
Whether you are working with a static or dynamic load
Base Materials
Base materials have their own limitations and advantages when it
comes to load capacity and strength. Because of this, the base material
that you are anchoring into plays a huge role in determining what
type of anchor is required to secure an applied load. Common base
materials include concrete, substrates that have the properties
of concrete, brick, tile and stone.
Anchors should be installed perpendicular to the surface of the
base material. It is commonly recommended when anchoring into a
solid base material that the thickness of the base be 125% of the
anchor embedment depth.
Cast-In-Place and Precast Structural Concrete
Excellent compressive strength
Low Tensile Strength
Normal Weight and Lightweight Selections
Attains desired properties within 28 days of being cast
The compressive strength of structural concrete, ranging from
2,000 psi to 20,000+ psi, is determined by looking at the proportions
of the components, specifically the amount of water, in the mixture.
In addition, structural concrete, whether cast-in-place or precast
often includes steel bars, cable, wire mesh or random glass fiber
which are designed to provide reinforcement.
The difference between cast-in-place and precast concrete has
to do with the way that the concrete is delivered to the work
site. Cast-in-place concrete is brought to the work site in liquid
form and poured into structural forms that have been erected at
the site. Precast concrete can be made off site or on site, but
the concrete is first formed and then moved into its final place
within the structure.
In many cases precast concrete has thinner cross sections than
cast-in-place concrete and can also be made with hollow cores.
Concrete Masonry Unit (CMU) or Concrete Block
Concrete block has the same properties as concrete, but it is
cast with a large core. Typically, it is required that reinforcing
rods be used to with concrete blocks and that the hollow cores
be filled with grout.
*Although it is no longer practice, concrete used to be mixed
with coal cinders and then used to form “cinder blocks”.
Use caution if you are working with cinder block rather than concrete
block. The base material of a cinder block tends to soften, hence
lose strength, with age.
Clay Brick
Hard and Brittle
Difficult to anchor into
Available either solid or with a hollow core
Clay brick is scarcely used in structural applications. It’s
most common applications today is in building facades (curtain
wall or brick veneer). In these applications, brick ties are usually
used to attach the brick to the structure walls.
Clay Tile
Hollow core blocks
Narrow cavity wall cross sections
Brittle
Difficult to anchor into
It is recommended that caution be used when anchoring into clay
tile.
Load Information
Classifications
To understand the type of load that you are supporting using an
anchor, familiarize yourself with the following terms.
Tension Load – A load that is applied
parallel to the length of the anchor. A tension load will stress
the fastener and the base material.
Shear Load – A load that is applied perpendicular
to the length of the anchor. A shear load stress the anchor, except
at limited edge distances.
Combination Load – A combination load apply
stress with the properties of both tension and shear loads.
Static Load – A load that remains constant,
without moving.
Dynamic Load – A load that is created by
movement or vibration. A load can be made dynamic by vibration,
wind or seismic activity. Keep in mind that a dynamic load may
cause loosening of a mechanical anchor, pulverization of the base
material and a loss of load resistance.
Anchor Load Capacity
Capacity
The load capacity of both mechanical and chemical anchors is determined
by a variety of variables.
Anchor Spacing – The distance between
anchors.
Edge Distance – The distance between the
centerline of the anchor to the nearest edge of the base material.
Base Material Compressive Strength
Hole Sizing – The relationship between
the hole diameter and the anchor size.
Anchor Diameter
Embedment Depth – The distance from the
surface of the base material to the embedded end of the anchor.
Static or Dynamic Load Properties
Resistance
The resistance of any anchor, mechanical or chemical, is determined
by evaluating the expansion force or adhesive bond or the anchor
itself and the base material being anchored into.
When a load is applied to an anchor it is transferred directly
to the base material. Because of this, base material plays a huge
role in determining the overall resistance and holding power or
the anchor.
When the base material is solid, the resistance of the anchor usually
increases as the depth of the embedment increases. The resistance
stops increasing only at the point where the maximum load capacity
of either the anchor of the base material is reached.
Always be aware of the edge, corner and anchor spacing, as well
as recommended hole size because of their effect on the base material
and consequently load capacity. If any of these requirements are
changed, the load capacity reacts accordingly. For example, the
load resistance of an anchor reduces linearly as the edge spacing
is reduced past its minimum requirement.
Anchor Failure Modes
Concrete Cone Failure
Concrete Split
Anchor Split
Bond Failure
Steel Failure
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