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While Cast-in-place anchors have been used in the past to attain high load capacities, positioning them into form work is difficult and alignment errors are common. Typically, 25% to 30% of cast-in-place anchors are either misplaced or omitted completely. Forced-entry fasteners driven by hand, pneumatic tool or powder-actuated tools are driven directly into the base material without pre-drilling. However, they should use only for light duty, static load applications. |
Drilled-in, or post-installed, anchors provide the user flexibility to install an anchor exactly where it is required without the intricate advanced planning associated with cast-in-place anchors. Although drilled-in anchors are supplied in many styles, they can be grouped into two categories--
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Anchors of this type can be distinguished by their style and method of expansion. One-step anchors do not require layout or hole-spotting since the drilled hole required is the same size as the anchor diameter. Bolt or screw anchors are installed flush with the surface of the base material and have internal threads to accept a bolt, screw or threaded rod. Hollow wall anchors are specifically designed for applications in hollow base materials such as wallboard, concrete plank, block and structural clay tile. Mechanical expansion anchors achieve their load capacities based on friction, clamping, compression, undercutting or a combination of methods. Friction anchors such as concrete screw anchors do not expand but develop load capacity by creating a friction force between the shank of the anchor and the base material as they are driven into an undersized hole. Anchors of this type are generally suitable for light-duty, static load conditions. |
Clamping anchors such as toggles achieve their capacities by clamping the fixture to the base material. Tension loads are developed by spreading the load over the back face of the hollow material such as concrete block. Most drilled-in mechanical anchors can be described as compression anchors. They have an expansion device-such as a sleeve, slotted shell or wedge assembly-that, when mechanically expanded, compresses against the wall of a drilled hole. Some types of compression anchors, pre-expanded to eliminate the need for secondary setting operations, decreasing the total installation time. The one-piece anchor is installed as follows: Drill a hole, place the anchor through a fixture hole and drive it in. |
As Undercut anchors expand at the anchor is driven, the mechanism compresses to the size of the hole. Bottom of the anchor hole, similar to a standard compression anchor, except that the diameter of the expanded area is wider than the drilled hole. Anchors of this type can be self-undercutting or may require a second operation to form the undercut. During installation, the expansion mechanism undercuts the base material to form a large bearing area. This allows higher loads to be transferred to the base and permits undercut anchors to be used in cracked concrete. For increased performance and resistance to vibratory loads, some styles of undercut anchors expand at two levels, both in the anchor sleeve and cone.
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