Post-installed mechanical anchors come in several styles: wedge, sleeve, drop-in, stud, and undercut. The undercut anchor can sometimes require additional drilling, and often can be three times the cost of other post-installed mechanical anchors. So why would you choose to use one?

It all comes down to strength. Undercut anchors are the strongest you can buy, and you put them in places where you want to avoid problems no matter what the cost. That is why they are commonly used on roller coasters, nuclear power plants, and other structures where human safety is paramount. They are well suited for dynamic loads and are frequently employed in seismic retrofitting. The anchorage is so strong that the steel in the anchor will often break before the concrete. From an engineering point of view, steel failure is more predictable and reliable than concrete failure, so it is preferred by engineers, says Bob Sayer of Liebig International, a German manufacturer of anchors for the past 30 years.

Undercut anchors are not your typical anchors. The bottom of the hole has a notched opening, or undercut, made either with a special drill bit or with an anchor that has a drilling device at the end of the bolt. When in place, the anchor uses the compressive strength of the concrete to resist the load, unlike a wedge anchor, which relies on the concrete's tensile strength.

There are two types of undercut anchors. The standard type requires drilling the hole, followed by drilling the undercut with the special bit. The second type is called a self-drilling anchor, which doesn't require the additional bit.

The American Concrete Institute has added new testing requirements for post-installed mechanical anchors, which is to say all anchors that are not cast into place in the concrete. The requirements are found in ACI 355.2. One of the concerns is how these anchors perform in cracked concrete. Fortunately, undercut anchors tend to fare well in these stringent tests.

Here is a short list of manufacturers who make undercut anchors. Please circle the number on the reader service card and mail it in for more information.

Self-cutting mechanical anchor

The HDA Undercut Anchor is a heavy-duty self-cutting mechanical anchor for hardened concrete. As preset (HDA-P) or throughset (HDA-T) anchors, they are available as part of a system that comes with stop drill bits, setting tools, and a rotary hammer drill with four metric bolt sizes. The anchors function well in cracked concrete, including tension zones and earthquake-resistant structures. Hilti, 800-879-8000,

Anchor provides continuous interlock

The Titen HD screw anchor features cutting threads that undercut concrete or masonry during installation, resulting in a continuous mechanical interlock between anchor and base material. The thread design requires less installation torque for quick and easy installation. Its non-expansion characteristics make the high-strength anchor ideal for minimum edge distances or reduced spacing conditions. Simpson Strong Tie Co., 800-999-5099,

Anchor provides mechanical interlock

The Ultraplus undercut anchor provides mechanical interlock with the concrete, rather than frictional grip inside the hole typical of expansion anchors. Spring-loaded steel segments snap out into the undercut at the bottom of the hole, and the steel will remain clamped for the whole of its working life. In addition, the design allows reduced edge distances and center spacings without losing performance. The anchors are available in lengths up to 1.5 meters. Liebig International, 800-247-2652,

Undercut anchor designed for high strength

The S9 undercut concrete anchor was designed to eliminate the direct lateral stress found with conventional anchors and bring its characteristics closer to those of cast-in-place anchors. With the use of the undercutting tool, the conical shape of the anchor fits into the conical cut of the hole, developing the tensile capacity of the bolt without slip or concrete failure. Williams Form Engineering Corp., 616-365-9220,