Patent Application
20070039276
Embodiments of the present invention relate generally to reinforcing concrete, and more particularly to reinforcing concrete slab floors subject to shear stress.
Constructing multi-level structures using concrete slab floors is common, and becoming more common. However, the concrete slabs may experience stress concentrations at certain points of support, particularly at columns.
There are a number of methods in use to reinforce the concrete slabs against shear failure. They may include making the slab thicker around the column, or flaring the column top by, for example, adding a column capital which includes additional reinforced concrete formed around the column to support a portion of the concrete slab. The additional reinforcing requires additional form work, more concrete, and the end product of the building may have a lower ceiling in the area around the column, and may require the whole building to have a greater distance between floors.
There are a number of techniques being used wherein the additional form work and additional concrete is avoided by adding metal reinforcing into the slab itself in the area of the slab adjacent to each column. One technique is to use metal reinforcing having a number of bends therein to shape each bar into a square wave-type pattern, sometimes called a shear stirrup, and sometimes referred to as a galloping stirrup. The shear stirrups may be made from concrete reinforcing bars, or rebar. A number of the shear stirrups are placed parallel to one another in a spaced-apart configuration. Several stringers, typically straight pieces of rebar, may be placed perpendicular to the shear stirrups and tie wired in a twist-tie fashion to each of the shear stirrups to form a shear stirrup unit. Typically, at least two spaced-apart parallel stringers are wired at or near corresponding bends of the shear stirrups, and a third or fourth stringer is tied at or near diagonally opposed bends in an attempt to provide three-dimensional stability for the shear stirrup unit.
Despite the three or four stringers and the tie wires, the shear stirrup units are prone to moving, shifting, or falling over. In particular, busy construction sites will have many electrical cords and pneumatic lines, and the like, pulled around and over the shear stirrup units, which cause the units to shift, tip, and/or collapse. In addition, the tie wires may tend to scratch and/or cut the hands and arms of workers.
To best resist shear stresses in the slab it is important for the parts of the stirrups intended to be vertical. i.e., the vertical parts of each stirrup, in the final concrete pour and the resultant slab, to be positioned and to stay as close to vertical as possible. However, verticality of the vertical members of the shear stirrup when tie-wired is not guaranteed. Therefore, engineers designing shear stirrup units may allow for an average number of the shear stirrups being off vertical and may compensate when selecting a factor of safety when specifying the shear stirrup unit characteristics to be used in any given application such as: thickness of stirrup; the spacing of each stirrup; the number of bends in a stirrup; the number of verticals; and the like.
Included among the activities that may go on in and around the metal reinforcing, and form work, in a concrete slab construction, is installation of post-tensioning cables. Post-tensioning cables are steel cables, housed in a sheathing or a duct, to prevent the steel from bonding to the concrete, which are placed in a rough grid configuration within the slab form. After the concrete is poured, and cured, covering the reinforcing work and the post-tensioning cables, the steel cables are tensioned by pulling on, and anchoring, each end. The strands of post-tensioning cables can have a diameter of one-half inch and may be stressed to a force of 33,000 pounds using a hydraulic jack. The post-tensioning strands are typically placed between the vertical elements of the shear stirrups. They are typically maneuvered and moved around on the job site, also tending to knock over the tie-wired shear stirrup units.
One attempt to avoid the hassle and management and re-management of the shear stirrup units is to use so-called stud rails instead. Stud rails are prefabricated metal studs attached at one end to a spacing bar. The other end of each metal stud has a circular disk attached to it. The metal studs are oriented vertically in the concrete slab to resist shear stresses near columns, and the spacing bars extend from the column. Stud rails, may not tend to sag or collapse like tie-wired reinforcing units may, however, they may still be knocked from a vertical plane during normal construction activity. Stud rails also tend to be expensive.
Embodiments of the present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings. To facilitate this description, like reference numerals designate like structural elements. Embodiments of the invention are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings.
Embodiments of the invention may provide a low maintenance reinforcing unit to be used in reinforcing concrete. Such unit may not need maintenance, repositioning, and repair during the course of a construction project once such a unit is in place, and prior to the concrete pour. Embodiments may provide a greater degree of positional reliability of shear stress-resistant reinforcing members affording structural designers and engineers, and the like, greater confidence to avoid “overdesigning” by specifying more metal be used than is absolutely necessary, and instead allowing them to conserve material. Embodiments also may provide reinforcing units that may be easily stacked and unstacked in a nesting fashion, which may save space and may make the units easy to transport.
In the following detailed description, reference is made to the accompanying drawings which form a part hereof wherein like numerals designate like parts throughout, and in which is shown by way of illustration embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made in alternate embodiments. Therefore, the following detailed description is not to be taken in a limiting sense, and the scope of embodiments in accordance with the present invention is defined by the appended claims and their equivalents.
The following description may include terms such as inner, outer, under, between, upward, downward, outward, inward, top, bottom, above, below, and the like. Such terms are used for descriptive purposes only and are not to be construed as limiting in the description or in the appended claims. That is, these terms are terms that are relative only to a point of reference and are not meant to be interpreted as limitations but are, instead, included in the following description to facilitate understanding of the various aspects of the invention.
The phrase “in one embodiment” is used repeatedly. The phrase generally does not refer to the same embodiment; however, it may. The terms “comprising,” “having,” and “including” are synonymous, unless the context dictates otherwise.
Embodiments of the invention may use a plurality of shear stirrup straps and may include galloping stirrups, arranged parallel to one another, and may include a straight rebar member fixed to each of the stirrups. Embodiments may include a number of bent rebar shear stirrups which may be shear stirrups, and two spaced-apart stringer elements extending perpendicular to each of the shear stirrups and tack-welded to each stirrup forming a rigid frame which resists, or is prevented from, tipping over.
In one embodiment only one stringer is used. In one embodiment two or more stringers are used. In one embodiment a stringer may be discontinuous. In another embodiment, a stringer may be cut to allow something, for example, one or more post-tensioning cables, to pass through it, and/or to be positioned below it. In one embodiment a stringer may be cut and something may be passed through the opening, and another stringer may be secured in place with a rigid joint. In one embodiment one or more horizontal portions may be cut, for example, to allow something to pass through.
In one embodiment, a plurality of the reinforcing devices 50 may be configured to be stacked in a nesting fashion.
In the embodiment illustrated, three concrete reinforcing devices 116 substantially surround the column location which may be the case for an outside column, i.e., at an outside edge of a building. In the case of an interior column four reinforcing devices 116 may substantially surround the column and may extend outwardly therefrom. A corner column may have two reinforcing devices 116 extending therefrom.
In one embodiment according to the invention a concrete reinforcing device may be used to reinforce a concrete wall. In one embodiment according to the invention a concrete reinforcing device may be used to reinforce a portion of a concrete slab not near a column.
The method may include:
specifying the shear stirrup reinforcing units using a factor of safety determined from considering that each of the shear stirrups are to be rigidly connected to at least one stringer at respective substantially horizontal parts of each of the shear stirrups, 200; and
based on the factor of safety specifying the characteristics of the shear stirrups, 202.
a shear stirrup material thickness;
a shear stirrup spacing;
a number of shear stirrups;
a number of first parts in each shear stirrup;
a spacing of the first parts in each shear stirrup; and
a number of bends in each shear stirrup, 204.
Preparing a specification may include providing instructions which may be written in various degrees of formality or may include providing verbal instructions to instruct workers on how reinforcing is to be placed in a concrete form.
The factor of safety may be an estimate based on a tendency of another shear stirrup reinforcing unit not made according to the present invention to move relative one another, and/or to tip. For example, other shear stirrup reinforcing units that may include shear stirrups not fixed to at least one stringer at respective horizontal portions.
One embodiment of the invention may include a method of specifying by, for example, preparing a specification, which may include estimating an increased level of strength for the shear stirrup reinforcing unit which includes shear stirrups being rigidly fastened to at least one stringer at respective horizontal portions as compared to another shear stirrup reinforcing unit not including shear stirrups fixed to at least one stringer at respective horizontal portions.
One embodiment of the invention may include a method which includes requiring the stringers be fastened to the second parts of the shear stirrup unit with a rigid joint which may be of a weld; an adhesive joint such as an epoxy joint; a cemented joint; or a fastener joint such as with screws or nuts and bolts.
positioning two or more shear stirrups substantially parallel to one another such that a first part of each of the two or more shear stirrups are parallel to one another, 210;
securing a stringer to each of a second part of the two or more shear stirrups with a rigid joint such that the first part of each shear stirrup remains oriented parallel to one another, 212. Securing may include securing more than one stringer to each of more than one second part of the shear stirrups by, for example, welding, adhering, cementing, fastening. The method may further comprise positioning each of the shear stirrups in a fixture, or a jig, such that each of the first parts are substantially parallel, and welding the at least one stringer to each of the second parts.
In one embodiment the method may further comprise placing a shear stirrup unit made from the positioning and the securing within a concrete slab form such that the first part of each of the shear stirrups are substantially vertical. The shear stirrup unit may be placed adjacent to a column location.
Although certain embodiments have been illustrated and described herein for purposes of description of the preferred embodiment, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent embodiments or implementations calculated to achieve the same purposes may be substituted for the embodiments shown and described. Those with skill in the art will readily appreciate that embodiments in accordance with the present invention may be implemented in a very wide variety of ways. This application is intended to cover any adaptations or variations of the embodiments discussed herein. Therefore, it is manifestly intended that embodiments in accordance with the present invention be limited only by the claims and the equivalents thereof.
