Concrete Anchors and Fasteners
General:
Anchors and fasteners are available in a wide variety of types
and sizes. The type and size needed for a particular application
depends on a variety of factors including load level, direction
of load (tension, shear, or a combination of the two). Other factors
are environment (shock and/or vibration) and characteristics of
the material into which the fastener is to be imbedded (new concrete,
old concrete, concrete or cinder block, plaster, dry wall, etc.)
Fastening - is when a fixture needs to be connected
to another, but subject to minimal amount of stress - for example,
hanging a small picture, towel rack or mirror.
Anchoring - is when a fixture needs to be connected
to another, but subjected to appreciable stress and vibration -
for example, steel beam to concrete or guard rail to concrete.
Determining Correct Fastener:
The following questions must be answered before a correct
fastener can be selected.
1. What is the base material and its density?
There are two types of base material;
Hollow Core - drywall, plaster board. sheet rock,
particle board, wafer board,
plywood, plaster and lath, tile, glass, metal, fiber-glass, plastic
and concrete block.
- Drywall: A low density material formed between sheets of
paper and used primarily for covering studded walls and ceilings.
Exhibits relatively low pull-out resistance.
- Plaster and Lath: Thin wooden strips (lath) were nailed to
the studs or ceiling beams and low density plaster was applied
over them. Exhibits relatively good pull-out resistance.
- C. Hollow Core Concrete Block: Commonly used in basements and
commercial properties. Exhibits relatively high pull-out depending
on compression strength of the block,
Solid Core - concrete, brick and mortar, stone,
solid cinder block and wood.
- Solid Concrete: Exhibits the highest pull-out resistance, but
requires a suitable carbide bit than meets ANSI standards.
- Stone: Since there are a large variety and different types,
first identify the hardness and characteristics. Match up with
the type of anchor needed.
- Brick or Mortar: like stone, it will vary from hard (brittle)
to soft. In hard brick anchors with low-impact and/or low- torque
are recommended. In soft brick, a draw-up anchor is recommended,
i.e. sleeve anchor, machine screw anchor.
There are three degrees of density and strength of base materials;
Low Density -
weak resistance to stress, i.e. drywall and cinder block. Medium
Density - moderate
resistance to stress, i.e. plaster, tile fiber board and particle
board. High Density -
significant resistance i.e. concrete, concrete block, stone.
Resistance to stress will vary within density and thickness of
the base material. Age and manufacturing standards for material
will result in different failure points for each anchor.
2. What is the thickness of base material?
How thick the base material is, will help identify the type of
anchor as well as the
embedment allowable.
For Hollow Core base material it is critical to have the right length
anchor so as to
be able to secure tightly. For Solid Core base material the embedment
of anchor can
not be closer than five (5) anchor diameters, because its an unsupported
edge and
holding values will be effected.
3. What is the size and weight of the fixture to be fastened?
If a fixture weighting 100 pounds needs to be fastened/anchored
and the safe working load of the anchor selected is 50 pounds, at
least 2 anchors will be needed to safely secure the fixture. Additional
anchors may be warranted; not because of weight, but because of
the bulkiness or size of fixture. When in doubt, always use more
anchors.
4. What is the location of the fixture to be fastened?
Will the fixture be outside and subjected to the elements, inside,
on a wall, suspended from a ceiling or fastened to a floor. Will
the fixture be close to an unsupported edge. The distance between
anchors is critical. If installed to close to each other, the interaction
of forces between fasteners will reduce the holding power of the
anchor in the base material. As a general rule, the anchor &
fastener industry has established the following minimum spacing
standards: Ten (10) anchor diameters between anchors, five (5) anchor
diameters from the edge.
5. What kind of stresses may be on fixture once fastened?
The type and size of anchor or fastener suitable for a specific
application depends
in part on the type and level of stress imposed on it. Five stress
factors, illustrated
below, should be considered.
Tension:
Where a force acts in line with a Fastener, tending to pull it
out. Ceiling mounted fasteners, as shown are subjected to tension.
But so sometimes are fasteners that are mounted on walls and floors
Shear:
Where a force acts at right angles to a fastener, tending to shear
it off. Wall mounted fasteners, as shown are subjected to shear
stress. But so sometimes are fasteners that are mounted on ceilings
and floors.
Impact:
Occasional or repetitive impacts subject fasteners to stress over
and above that resulting from normal tension and shear forces. The
railing at right would undergo such impact loads.
Vibration:
Over time vibration will stress fasteners and the base material;
Vibration stress can be induced by near by heavy machinery, railroad,
automobiles or truck traffic and wind.
Combination:
In some cases, fasteners are subjected to two or more of the forgoing
stresses. The example at the right fasteners supporting drapery
rods. The fasteners are in both tension and shear as well as vibration
and shock stress.
Types of Anchors and Fasteners
1. Mechanical Fasteners: Best for hollow core materials.
Mechanical fasteners open on the back side providing the greatest
holding
power for hollow wall applications such as drywall, plaster and
lath, and hollow
core concrete blocks.
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2. Expansion Anchors: For solid core material.
Expansion anchors produce a friction hold when expanded or wedge
inside a hole.
Their holding power is enhanced in concrete because the concrete
will not “give”
like drywall or plaster. Typically employed in solid-core materials--
concrete, brick
and mortar, stone, solid cinder block. Heavy to light duty.
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