Compression Anchors

Compression anchors depend on a screw, lag, or bolt to expand the anchor, causing it to squeeze against the sides of a pilot hole. The most common example of this is the simple plastic shield. While it's obvious that plastic shields could never be used to connect structural elements or anchor heavy mechanical fixtures, the variable loads exerted in many non-structural applications may also be too extreme. Plastic shields do not react well in the long term to changes in humidity and temperature. As the anchor dries and shrinks, play between screw and anchor rattles the anchor loose. Even in light-duty applications, such as anchoring a toilet paper holder or towel bar, intermittent vibration from using the fixture can lead to the demise of a shrunken plastic shield. Nylon plugs have been used in Europe for over a decade. Fischerwerke of Germany is the world leader in sales of nylon plugs, and Fischer plugs are widely distributed through U.S. contractor supply houses; more recently, Driltec has introduced its Mungo anchor (see ""). For light- and medium-duty applications, nylon plugs are extremely versatile, offering reliable holding power in concrete and masonry. Nylon plugs are almost always a better choice than plastic shields, because the material is more durable over time, even with extreme changes in moisture and temperature. In many cases, nylon plugs are a better choice than concrete screws, especially in weak or lightweight concrete and old masonry. Nylon absorbs vibration much better, and the material draws tight to the sides of the hole, expanding into irregularities to form a mechanical key. Nylon plugs also retain most of their holding power in applications that require removal and reinstallation of the fixture. While most nylon plugs are non-structural, the Mungo Frame Anchor exhibits holding values comparable to small sleeve and wedge anchors for medium-duty structural applications, such as attaching ledgers or stair stringers. The long bearing range makes this anchor a good choice in hollow-wall masonry. Frame Anchors are supplied with lags with either hex-drive or flush Posi-drive heads. Metal shields. Don't confuse modern steel drop-ins with the old-style lead or zinc shields. The newer fasteners are made for holding heavy pipes and other fixtures to concrete, primarily in overhead applications. While steel drop-ins have relatively high holding values, they must be installed with a setting tool that expands the plug inside a predrilled hole, leaving a threaded hole to accept a bolt or threaded pipe hanger. Pay close attention to edge spacing and distance requirements; otherwise, the drop-in can crack the base material when it expands. Sleeve and wedge anchors are the heavyweights of mechanical anchors. Both types come pre-assembled. After dropping the anchor in the hole, tightening the nut draws the bolt together, expanding the anchor until it's locked tight against the sides of the hole.

Sleeve anchors.Tightening the nut of a sleeve anchor, forces the outer sleeve down, causing it to expand against the sides of the predrilled hole. Sleeve anchors have a wider bearing than wedge anchors, making them a better choice in weaker substrates.

Wedge anchors have the highest holding values — sizes 1/2-inch in diameter and larger can serve as shear wall anchors in sound base materials, provided you’re not too close to an edge. Metal sleeve anchors exert compression over a larger area of the hole as the fastener is tightened, making them better for weak base materials.

Wedge anchorsexhibit the highest holding values among mechanical anchors, but need a strong base material to prevent the concentrated compression forces from pulverizing or cracking the concrete.

Neither fastener, however, is a particularly good choice in hollow-wall masonry. Wedge anchors exert enough localized force at the bottom of the hole to compress the concrete (called undercutting), creating a mechanical key. But this concentrated force has a greater chance of overstressing the base material. Most sleeve and wedge anchors must be tightened to a specific torque; too tight, and the anchor can easily overexpand, pulverizing or cracking the base material.