There are many choices for anchoring into concrete, masonry and other base materials. It is often difficult to determine which type of anchor is best suited for the application. In some cases, there may be more than one type of anchor that will work well. The following information is intended to guide the designer / end user toward the anchor best suited for the specific application.
There are three major criteria to be considered in anchor selection:
- The type of base material to be anchored to
- Desired load capacity
- Type of load: static or dynamic
Note: Throughout this Web site, this symbol indicates important information that should be given special attention.
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MECHANICAL ANCHOR INSTALLATION GUIDELINES
Base material is a generic industry term that refers to the element or substrate to be anchored to. Base materials include concrete, brick, concrete block (CMU) and structural tile, to name a few. The base material will determine the type of fastener for the application. Generally, solid base materials like concrete and stone have the greatest load carrying capacity due to both the strength of the material and its mass. Hollow base materials have much less mass to anchor into and thus do not provide the load carrying capacity of solid base materials.
The most common type of base material where adhesive and mechanical anchors are used is concrete.
CONCRETE
Concrete can be cast in place or precast concrete. Concrete has excellent compressive strength, but relatively low tensile strength.
Cast in place (or sometimes called poured in place) concrete is placed in forms erected on the building site. Cast in place concrete can be either normal-weight or lightweight concrete. Lightweight concrete is specified when it is desirable to reduce the weight of the building structure. Lightweight concrete differs from normal-weight concrete by the weight of aggregate used in the mixture. Normal-weight concrete has a unit weight of approximately 150 pounds per cubic foot compared to 115 pounds per cubic foot for lightweight concrete.
The type of aggregate used in concrete can affect the tension capacity of an adhesive anchor. Presently, the relationship between aggregate properties and anchor performance is not well understood. A recent study based on a limited test program has shown that in relative terms, concrete with harder and more dense aggregates tend to yield greater anchor tension capacities. Conversely, use of softer, less dense aggregates tends to result in lower anchor tension capacities. Research in this area is ongoing. Test results should not be assumed to be representative of expected performance in all types of concrete aggregate. Relatively large factors of safety are applied to anchors for this, among other reasons.
Prefabricated concrete is also referred to as precast concrete. Precast concrete can be made at a prefabricating plant or site-cast in forms constructed on the job. Precast concrete members may be solid or may contain hollow cores. Many precast components have thinner cross sections than cast in place concrete. Precast concrete may be either normal or lightweight concrete.
Reinforced concrete contains steel bars, cable, wire mesh or random glass fibers. The addition of reinforcing material enables concrete to resist tensile stresses which lead to cracking.
The compressive strength of concrete varies according to the proportions of the components in the mixture. The desired compressive strength of the concrete will be specified according to the application. Water and cement content of the mix is the main determinant of the compressive strength. The compressive strength of concrete can range from 2,000 psi to over 20,000 psi, depending on the mixture and how it is cured. Most concrete mixes are designed to obtain the desired properties within 28 days after being cast.
CONCRETE MASONRY UNITS (CMU)
Block is typically formed with large hollow cores. Block with a minimum 75% solid cross section is called solid block even though it contains hollow cores. In many parts of the country building codes require steel reinforcing bars to be placed in the hollow cores, and the cores to be filled solid with grout.
In some areas of the eastern United States, past practice was to mix concrete with coal cinders to make cinder blocks. Although cinder blocks are no longer made, there are many existing buildings where they can be found. Cinder blocks require special attention as they soften with age.
BRICK
Clay brick is formed solid or with hollow cores. The use of either type will vary in different parts of the United States. Brick can be difficult to drill and anchor into. Most brick is hard and brittle. Old, red clay brick is often very soft and is easily over-drilled. Either of these situations can cause problems in drilling and anchoring.
The most common use of brick today is for building facades (curtain wall or brick veneer) and not for structural applications. Brick façade is attached to the structure by the use of brick ties spaced at intervals throughout the wall.
In older buildings, multiple widths, or "wythes" of solid brick were used to form the structural walls. Three and four wythe walls were common wall thicknesses.
CLAY TILE
Clay tile block is formed with hollow cores and narrow cavity wall cross sections. Clay tile is very brittle, making drilling difficult without breaking the block. Caution must be used in attempting to drill and fasten into clay tile.
It is always recommended to thoroughly evaluate the condition of the base material before attempting to select an anchoring system.
Values listed for Simpson Anchor Systems products are for sound base materials with known compressive strengths. When the strength of the base material is not known, or its load carrying capacity is questionable, it is always recommended to perform onsite testing to determine that the required load capacities can be obtained.
Many factors influence the load carrying capacities of mechanical and adhesive anchors installed in concrete or masonry, including:
- Anchor Spacing- The distance between anchors, measured centerline to centerline.
- Edge Distance- The distance from the centerline of an anchor to the nearest free edge of concrete or masonry.
- Base Material Compressive Strength- f'c, f'm
- Hole Sizing- The relationship between the hole diameter and the anchor size.
- Anchor diameter and mechanical properties.
- Embedment Depth- The distance from the surface of the base material to the embedded end of the anchor.
ANCHOR SPACING AND EDGE DISTANCE
Spacing and edge distances listed for products in this catalog have been determined by testing under controlled conditions or engineering analysis. For proper selection of the correct spacing and/or edge distances, consult the appropriate product data table(s).
Where multiple anchors are used to support a load, the capacity of the anchor group is calculated by multiplying the number of anchors within the group and the lowest (minimum) tension (or shear) value for a single anchor within the group. At the critical anchor spacing, the efficiency of each anchor is 100%. As anchors are spaced closer than the critical spacing, their efficiency is reduced and their load carrying capacities are diminished. Refer to the appropriate product specific load adjustment factor tables for guidance. The spacing shall not be less that the minimum spacing listed.
Where anchors are installed close to a free edge, the effect of edge distance needs to be considered. At the critical edge distance, the efficiency of each anchor is 100%. As the anchors are spaced closer to the free edge than the critical edge distance, their efficiency is reduced and their load carrying capacities are diminished. Refer to the appropriate product specific load adjustment factor tables for guidance. The edge distance at which the anchors are installed shall not be less than the minimum edge distance listed.
The failure modes for both mechanical and adhesive anchors depends on a number of factors including the anchor type and geometry, anchor material mechanical properties, base material mechanical properties, loading type and direction, edge distance, spacing and embedment depth.
Six different failure modes are generally observed for mechanical and adhesive anchors installed in concrete under tension loading: concrete cone breakout, concrete edge breakout, concrete splitting, anchor slip, adhesive bond, and steel fracture. Three failure modes are generally observed for mechanical and adhesive anchors installed in concrete under shear loading: concrete edge breakout, pryout and steel failure.
CONCRETE EDGE BREAKOUT FAILURE
This failure mode is observed for both mechanical and adhesive anchors installed at less than critical edge distance under either tension or shear loading. For this failure mode neither the adhesive nor mechanical anchor fail, but rather the concrete fails. According to Simpson's testing, the tension load at which failure occurs is correlated to the concrete aggregate performance. Other factors may also influence tension load.
CONCRETE CONE BREAKOUT FAILURE
This failure mode is observed for both mechanical and adhesive anchors installed at less than critical edge distance under either tension or shear loading.
CONCRETE SPLITTING FAILURE
This failure mode is observed for both mechanical and adhesive anchors installed in a "thin" concrete member under tension loading.
ANCHOR SLIPPING FAILURE
This failure mode is observed for mechanical anchors under tension loading in which the anchor either pulls out of the member (e.g. - a Drop-In Anchor installed through metal deck and into a concrete fill) or the anchor body pulls through the expansion clip (e.g. - a Wedge-All Anchor installed at a deep embedment depth in concrete).
ADHESIVE BOND FAILURE
This failure mode is observed for adhesive anchors under tension loading in which a shallow concrete cone breakout is observed along with an adhesive bond failure at the adhesive/base material interface. The concrete cone breakout is not the primary failure mechanism.
STEEL FRACTURE
This failure mode is observed for both mechanical and adhesive anchors under tension or shear loading where the concrete member thickness and mechanical properties along with the anchor embedment depth, edge distance, spacing, and adhesive bond strength (as applicable), preclude base material failure.
HOLE SIZING
In order for mechanical anchors to perform optimally the proper drilled hole size is important. The above table provides the dimensional specifications of carbide tipped drill bits that conform to the ANSI B212.15 standard. Rotary hammer drills with light, high frequency impact are recommended for drilling holes. When holes are to be drilled in archaic or hollow base materials, the drill should be set to rotation only mode.
Finished Diameters for Rotary and Rotary Hammer Carbide tipped Concrete Drills per ANSI B212.15
BUILDING CODES
Local and/or regional building codes may require meeting special conditions. Building codes often require special inspection of anchors installed in concrete and masonry.
For compliance with these requirements, it is necessary to contact the local and/or regional building authority. Except where mandated by code, Simpson's products do not require special inspection.
There are three basic classifications of loads that can be applied to an anchor: tension, shear and oblique.
TENSION LOAD
A load that is applied parallel to the length of the anchor.
SHEAR LOAD
A load that is applied perpendicular to the length of the anchor.
OBLIQUE LOAD
A load that is applied to an anchor which can be resolved into tension and shear components. In the case of an oblique load, calculations using the appropriate interaction equation should be performed to ensure that the anchor is not overstressed.
In addition, the preceding loads may be further classified as either static or dynamic.
STATIC LOAD
A load whose magnitude does not vary appreciably over time. An example of this classification of loading includes the self weight of a supported fixture.
DYNAMIC LOAD
A load whose magnitude varies over time. seismic, vibratory and fatigue loads are examples of dynamic loads.
LOAD RESISTANCE
The load resistance of an anchor is influenced by both the expansion force of the anchor (or adhesive bond) and type of base material. In some cases, even the smallest anchor can exceed the capacity of the base material. An example of this is an anchor installed in gypsum drywall.
Generally, when anchoring into solid base material, the load carrying capacity of the anchor increases with a corresponding increase in embedment depth up to a point where either the ultimate load carrying capacity of the base material or anchor is reached.
Loads are transferred from the anchor to the base material. The base material has to have the necessary strength to resist the applied load. This makes edge distance and anchor spacing important considerations for anchoring. Maintaining prescribed edge distances and/or anchor spacing are critical to achieving the required load resistance of anchors. The closer together the anchors and/or the closer to the edge of the base material, the lower the load resistance due to the stresses placed on the base material. For most anchors, critical edge and spacing distances may be reduced as long as the load values are reduced accordingly.
BENDING STRESS
Anchors subjected to shear loads with large eccentricities may undergo high bending stresses that have not been considered in normal testing. The designer should consider these additional stresses when designing an anchorage.
BASE MATERIAL THICKNESS AND STRENGTH
Unless stated otherwise in the load tables, tests were performed in members with minimum thickness equal to 1.5 times the anchor embedment depth. Anchoring into thinner members requires the judgement of a qualified designer. In all conditions, members anchored to must have sufficient strength to resist the internal stress (axial, torsional, bending, shear, etc.) created by the anchor load. When in doubt, consult a qualified designer.
PRE-LOAD RELAXATION
Expansion anchors that have been set to the required installation torque in concrete will experience a reduction in pre-tension (due to torque) within several hours. This is known as pre-load relaxation. The high compression stresses placed on the concrete cause it to deform which results in a relaxation of the pre-tension force in the anchor. Tension in this context refers to the internal stresses induced in the anchor as a result of applied torque and does not refer to anchor capacity. Historical data shows it is normal for the initial tension values to decrease by as much as 40-60% within the first few hours after installation. Retorquing the anchor to the initial installation torque is not recommended, or necessary.
ANCHOR INSTALLATION
Anchors are intended to be installed perpendicular to the surface (±6 degrees from vertical). Deviations can result in anchor bending stresses, thereby reducing the ultimate load carrying capacity of the anchor.
