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What is a Drop-In Anchor?

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  • Written By: Paul Scott
  • Edited By: R. Halprin
  • Last Modified Date: 31 August 2016
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A drop-in anchor is part of the fastener family that used to attach a variety of decorative, utility, or construction elements to concrete or masonry surfaces. It consists of a hollow sleeve that features a slotted, expandable skirt at one end, an internal expander plug, and a fastener thread at the other end. To use the drop-in anchor, a suitable hole is first drilled into the concrete surface and the drop-in anchor inserted into the hole skirt end first. A specially designed expansion tool expands the slotted skirt to firmly grip the sides of the hole. A range of fasteners can then be screwed into the anchor to attach the relevant accessory.

The drop-in anchor is one of the most simple and easy-to-apply concrete and masonry fastener systems available. The only structural intervention necessary is a suitably sized hole drilled into the brickwork or concrete. Drop-in anchors posses fairly high levels of structural integrity and can carry considerable loads, especially when applied to vertical surfaces. In domestic applications, they may be used to attach gutters, pipes, flower pots, hanger brackets, and window boxes to walls and concrete ceilings. Industrial uses of the drop-in anchor include attachments for fire sprinkler pipes, cable trays, signage, and suspended ceiling grids.

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Drop-in anchors consist of a hollow carbon steel tube that features a series of slots cut longitudinally up from one end to form an expandable skirt. The tube is cut with a length of thread at the other end to accept a bolt. Located inside the tube above the slots is a movable, tapered expansion plug that serves to force the tabs between the slots outwards to lock the anchor in place. The tube may feature a lip at the threaded end to support the anchor on the rim of the mounting hole. The tubes are generally zinc plated to discourage corrosion.

Installing a drop-in anchor is a fairly simple and quick process which requires only the anchor, a suitably sized masonry drill bit, an impact capable drill, and the anchor's expansion tool. Once the site for the anchor has been established and marked off, a hole is drilled slightly deeper than the length of the anchor. It is important to ensure that the drill bit is the correct diameter because an oversize hole will compromise the load bearing capacity of the anchor. Once the hole has been thoroughly cleared of drilling debris, the anchor is simply “dropped” into the hole with the threaded end towards the mouth of the hole. If the anchor fit is a little snug, it can be tapped into place using a mallet or hammer.

Once seated, the expansion or setup tool is used to drive the expansion plug to the bottom of the tube. This forces the tabs on the slotted skirt to expand and firmly grip the walls of the hole. Friction keeps the plug in place and maintains the anchors lock in the hole. A suitable bolt is screwed into the anchor to affix the desired accessory. Drop-in anchors are available in a variety of designs including standard hex head bolts, ring bolts, pipe clamps, and hooks.

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