they make concrete anchors similar to drywall anchors.
Fine Points on Selecting and Installing Wedge Anchors
Wedge anchors are widely used for securing all sorts of things to concrete. Available from numerous manufacturers, they all operate on a common principle and are simple and easy to use. Each is applied by inserting it in a hole drilled in concrete. The hole diameter is identical to that of the anchor; thus each must be driven into the hole.
The nut, when tightened against the washer (or a flange on the time being mounted), pulls the anchor up and the tapered end expands the pronged section, wedging it tightly against the sides of the hole.
There are three basic styles of wedge anchors, fully threaded, partial threaded and the full-bodied wedge anchor. They all work and even look somewhat a like, but there are differences, which can be crucial in some applications.
The important point is this. Every wedge anchor has a minimum embedment requirement. For published strength values to be valid, each anchor must be installed as deep or deeper than the specified minimum embedment.
Lets examine the three basic styles of wedge anchors, fully threaded, partial threaded and the full-bodied.
The wedge anchor that is called fully threaded is threaded for a major portion of its length. The diameter of the unthreaded portion is less than the nominal diameter of the threaded portion. The long threaded section allows considerable come-and-go for shimming and leveling of the item being secured. But, there is a downside to this feature. It is possible to install such anchors at a depth less than the minimum embedment requirement. Even when the anchor is inserted in a hole at a depth somewhat less than the minimum embedment specified, the nut can be torqued, fixing the anchor in place. Anchoring strength, however, is diminished.
This misapplication occurs more often than one might expect. The reason is that concrete commonly contains steel reinforcing bars, called rebar. Often an installer, while drilling the hole for and anchor, will run into rebar at a depth less than minimum embedment. When that happens, he ought to drill or cut out the bar, or move the hole, before continuing to drill the concrete. But when installing fully threaded anchor he sometimes won‚Äôt take the time and effort to do that.
Here‚Äôs an example of what can happen with a 1/2‚Äù x 5-1/2‚Äù fully threaded anchor. The lower edge of the threads is 1-1/2‚Äù from the working end of the anchor. Thus the anchor can be successfully installed in a hole as shallow as 1-5/8‚Äù, appreciably less than the recommended 2-1/4‚Äù minimum embedment. But, when so installed, the pullout strength for. the anchor is considerably reduced and will not meet the values published by the manufacturer. In addition, when shear loads are applied through the threads, as they would be in this case, the anchor can withstand less force than otherwise ‚Äìapproximately 20% less for anchors 5/8‚Äù and larger.
When installing this type of anchor, installers must be particularly alert to achieving the minimum embedment levels specified.
This type of anchor is fabricated from steel rods having a diameter somewhat smaller than the nominal diameter of the final product (explaining the necked down portion). The steel employed, typically, is AISE 1018, a material especially suited for cold heading.
The second anchor the partial threaded is threaded for only about one third of it length and the diameter of the unthreaded proportion is less than the nominal diameter. This is partially threaded anchor cannot be successfully installed in any hole that does not meet or exceed minimum embedment requirements. Nevertheless, this type of wedge anchor can be misapplied. Partially threaded anchors with a smaller unthreaded shank can be considered to be properly installed only when some portion of the threaded section lies with in the hole. The same cannon be said when they are installed with the threads completely clear of the hole.
For those installations where the threads extend into and fit tightly in the hole, the threads support and stabilize the anchor. But these anchor can be installed in situations that often occurs with a think mounting flange or where an application calls for heavy shimming. In such cases, there is a space between the shank of the anchor and wall of the hole. This can be a problem for applications subject to vibration or shock loads. Here, the shank is free to move inside the hole, a situation than can, over time, reduce or destroy the anchor‚Äôs holding value.
When using this type of anchor, it is important that the thickness of the fixture being anchored is not as great or greater than the length of the threaded section.
Relative to material strength, it should again be noted that this type of anchor also employs AISE 1018 steel to accommodate the cold heading manufacturing process.
The third anchor the full-bodied is also partially threaded sot that I cannon be installed at less than minimum embedment. The diameter of the unthreaded portion of this anchor is identical to that of the threaded portion. These fully machined anchors fit tightly in the mounting hole regardless of whether the threads are in or ou of the hole and the AISE 1215 steel is typically employed in their manufacture. The shear strength of this material is inherently greater than that of the AISE 1018 steel employed in cold formed anchors. All of these factors minimize judgment calls, errors and uncertainty on the part of installers and contribute to faster, stronger installations.
Interpreting Technical Data
All manufacturers provide good instructions for selecting and applying their products, but they differ greatly in the way they present technical data. This can make it difficult to compare anchors from one company with those from another. These differences give rise to misunderstandings and leave the door open for improper selection.
For example, some manufacturers list an ultimate-load rating for each of their anchors while others list a working-load rating. Either term is legitimate, but when comparing products from one manufacture with products from another, comparable values should be used. The industry generally recommends that working-load ratings be calculated as 25% of the anchor‚Äôs ultimate load rating. Thus, an anchor having a 9,000 lb. Ultimate-load rating can be assumed to have a 2,250 lb. Working-load rating. Some manufactures provide tension and shear values for each anchor based on it being embedded to a specified depth, that depth is some cases being greater than the required minimum embedment. Other manufacturers provide tension and shear values for various embedment depths. In any case, recognize that both tension and shear values depend on the depth to which the anchor is actually embedded.
Both allowable and working tension and shear values are a function of the type of concrete in which they are embedded. All wedge anchors are tested in various types of concrete‚Äîtypically 2,000 psi, 4,000 psi, and 6,000 psi. Some manufactures do not offer the working and shear values for all these types of concrete; others do. The important point is to recognize that actual ratings depend on the type of concrete involved.
Working tension and shear values also assume the anchors are properly torqued to fix them in the concrete. Installing them with either less or more than the recommended torque results in holing and shear values differing from the published ratings. Some manufacturers publish and average torque rating for each anchor. Some publish a range of torques.