Drywall, comprised of compressed powdered gypsum contained between sheets of thick paper or cardboard, is one of the most common interior-wall construction materials, particularly in the United States. It is easily handled, readily installed and provides an excellent base for paint. It is available in standard 4′×8′ sheets with common thicknesses of ½″ and ⅝″. It is however, a relatively poor material as a base support for supporting items, particularly heavy items, since it has little structural strength and tends to crumble when disrupted. In order to increase supporting strength, sheets of drywall are often doubled, such as in standard hospital wall construction. Nevertheless, drywall, even with double thickness, remains a relatively poor support when compared to masonry, metal, wood or other construction materials.
The common structure of an interior wall constructed of drywall comprises a series of wooden or metal framing studs, generally of nominal cross section 2″ by 4″ dimensions, laterally spaced 16″ apart and attached to an outer wall. A hollow is formed directly behind the drywall, of 3.75″ actual depth, between the drywall and the outer wall, into which insulation is placed and electrical, plumbing, heating and other lines are drawn and concealed. While it is preferred to attach heavy objects directly to the framing studs, it is not always logistically possible, since most of the wall space is unsupported drywall.
Many types of objects are commonly hung on and are supported directly on walls. These items range from those which are decorative and static, such as pictures and minors (which can be light or heavy depending on the size and nature of the picture or minor), to heavy static items such as big screen televisions. Static items are not moved nor is the load on the wall changed. There are, however, numerous non-static items which, in addition to their usual heavy weight, also subject supporting walls and structures to moving stresses as well. Examples of non-static items include televisions on swivel supports, bookshelves, kitchen cabinets and hand railings or grab bars, in particular those used in hospitals. Support offered by drywall structures is problematic especially in applications where support, directly on the framing studs, is not possible or feasible.
In response to the need for reliable fastening and support of items on construction drywall, numerous fastening expedients or anchors have been developed. These fasteners and anchors fall within various categories based on load to be supported. For very light loads, there are adhesive hooks, expansion plugs, nail-in hooks and self-drilling anchors with oversize screw threads, which are supported directly on the surface or within the thickness of the drywall. A class of anchors with greater holding strength is hollow wall expansion anchors, which are inserted within a usually pre-drilled hole and which are expanded behind the wall. These hollow wall anchors include the familiar toggle bolt having spring hinged “toggles” that fold for insertion with an inserted bolt and spring open behind a wall. Another common anchor is the molly bolt anchor which is tubular for insertion into the pre-drilled hole and which is expanded by an inserted bolt that engages and pulls the front of the anchor toward the wall while “expanding” side legs outwardly into contact with the wall. Various plastic expansion anchors such as the TOGGLER® over-center anchor, from Mechanical Plastics Corp., is folded for insertion into the pre-drilled hole and pops into or is popped into an expanded holding position.
The very nature of the structures of these hollow wall anchors, which enables them to be inserted and opened by expansion, is also instrumental in their holding limitations. Thus, the hinge portions of a toggle bolt anchor (enabling the anchor to be folded for insertion into the pre-drilled hole) and the soft bendable metal of the molly bolt (with the inserted bolt serving to pull back and expand side legs) are weak points, as is the relatively low shear strength of the flexible plastic of the plastic expansion anchors. These anchors are used for safely holding intermediate loads, generally up to about 100 pounds and then usually only when safely used in groups of anchors.
There is a further class of anchors with higher holding strength designed for use with hollow wall or drywall structures. These include the SNAPTOGGLE® anchor (also from Mechanical Plastics Corp.) with a non-hinged solid metal channel holding element which is turned longitudinally and inserted edgewise and then rotated back to its original axial position, to engage the rear of the wall. These anchors are capable of supporting several hundred pounds in drywall, depending on wall thickness. However, despite such capability there is a need for increasingly greater supporting strength particularly in a shear downward direction especially for non-static high-stress or high-weight holding applications. However, since the holding failure mode with such anchors in drywall is generally that of wall failure, increasing the strength of the anchor alone is of little or no utility.
There are several competing factors which have constrained increasing the support capability of fasteners or anchors such as by increasing size of the anchoring elements. These factors include insertion aperture size, restricted area in the hollow behind the drywall (limiting the size of an anchor that can be placed behind the wall) and the restricted interface area between anchor element and the supporting wall.
The rear of a drywall is essentially directly inaccessible, and it is necessary to place an anchor through a preformed aperture in the drywall for anchoring deployment at the rear or non-accessible side of the wall. This provides a paradoxical situation. Larger anchors are required for greater holding strength, but these larger anchors also require larger insertion apertures. However, in drywall, the formation of larger apertures serves to weaken the wall, thereby negating the effect of using a stronger anchor, which in turn restricts the width and size of the anchor that can be inserted.
In addition to insertion hole restrictions that limit anchor width, the standard 3.75″ available depth of the hollow between the drywall and a supporting base wall, also limits the length of the anchor that can be inserted into the aperture. Hinged or flexible anchors of longer length are beset by weakness at the hinge points and the very nature of the flexibility, which militates against holding strength.
The strength of the drywall (i.e., the breakage limit, dependent on drywall thickness), as described, is the most common limiting factor in determining the amount of weight safely supportable by a drywall wall, particularly with the heavy-duty type of anchors. In order to increase the weight supportable by the wall (aside from increasing the thickness of the wall, which is not readily feasible with existing walls) it is necessary to increase the amount of wall area that supports the anchor or fastener, i.e., spreading out of the load, in order to allow the wall itself to support more load. However, existing anchors, such as the molly bolt and toggle bolt anchors, have already been respectively configured to provide their maximum radial expansion for the molly bolt, and a maximum spring loaded and hinged expansion for the toggle bolt anchor. In a variation of the molly bolt anchor, an anchor specifically designed for grab bar use comprises a central rod with a conical skirt of free ended metal legs which are circumferentially compressed for insertion in a predrilled aperture and which circumferentially expand and splay after insertion and setting into engagement with the wall surface. Though the legs of the anchor do not bend in a manner similar to a molly anchor, this anchor requires a large insertion aperture of an inch and a quarter, is quite costly, and has weak hinge section area at its distal end where the legs are permitted to splay. In addition, the anchor cannot splay too far (i.e., beyond an angle of 45°) to avoid eversion of the legs.