Patent Application
20120137618
1. Technical Field
This disclosure relates generally to reinforcing assemblies for use in structural concrete members.
2. Background of the Related Art
Commercial concrete is a mixture of cement, sand and stone aggregate held together in a rigid structure by the addition of water. So-called “unreinforced” concrete has fairly good resistance to compressive stresses, however, any significant tension tends to break the structure and cause undesirable cracking and separation. To address this problem, concrete is typically “reinforced” by embedding in place (within the rigid structure) a solid member made of a material with high strength in tension. Reinforced concrete structures are available commercially in many shapes and sizes, such as slabs, beams, footings and flat foundations.
Commercial and industrial structural concrete members, even when made with reinforced concrete, are highly susceptible to shear forces that create diagonal tensile forces within them, which can result in structural failure. The cracking and/or breaking caused by these shear forces tend to propagate throughout the concrete structure. In horizontal concrete members (such as slabs, footings and flat foundations), this problem is known as “punching shear failure.” The problem is especially acute in concrete members when supported by columns. In this situation, the concrete member is subject to a concentration of stress in a zone near the column, wherein the column tends to “punch” through the member. The resulting shearing force creates diagonal tension stresses within the supported member. The concrete is particularly vulnerable to these tensile stresses and thus must have reinforcement, such as embedded steel members, to prevent tensile failure, crack propagation, and consequent structural collapse.
The prior art has addressed the problem of punching shear failure by providing assemblies and reinforcing techniques such as described in U.S. Pat. No. 4,406,103. According to this patent, shear reinforcement is provided by a plurality of substantially vertical elongate reinforcing elements (smooth shafts) fixedly attached in spaced horizontal relation to support means, wherein each element is provided (at the upper end) with an enlarged (flange) portion, which serves as an anchor when the reinforcement is embedded within the concrete slab. At the lower end is a flat steel bar, which serves as a base structure and as a lower end anchorage. In a preferred form, the element consists of thin transverse sections. A commercial product incorporating this design is known as the “Stud-Rail” system.
While the Stud-Rail system is well-known and widely-used, it is not an optimal solution to the problem of punching shear failure. As noted above, the system uses smooth shafts for reinforcing, but those shafts have no means to grip the concrete in the area of primary crack formation. Thus, in operational mode, the stresses from diagonal tension must stretch the concrete (in the vulnerable crack zone) away from the center of the slab, accumulating in proportion to the load and the thickness of the concrete until restrained by a flange at the top or the bottom shaft, i.e. near the surface of the top or bottom of the slab thickness. This restraining force places the shaft itself in tension from one end to the other, which causes the shaft to undergo significant strain. In addition, there is a compressive strain in the compressed zones under the flange at the top and bottom of the shafts. To maintain equilibrium, the sum of the top and bottom compression strains under the flanges, and the tensile strain in the shaft of the studs must be equal to the tensile strain in the central zone of the slab thickness. But, because the total thickness of the top and bottom compression zones are roughly equal to the thickness of the tension zone in the center, because the strain in the shafts is additive to the concrete compression strains, and further because the tensile strain in the central concrete zone must equal the sum of the above two strains, the tensile stress in the central concrete zone must be commensurately higher. In response to these high stresses, cracks still are able to form and propagate at relatively low loads. Another deficiency of the Stud-Rail system is that the reinforcement does not start working (albeit inefficiently, for the reasons stated above) until a crack has started; the structure and reinforcing technique do not act as a prophylactic to prevent such cracks in the first instance.
There remains a long-felt need in the art to provide enhanced concrete structure reinforcing assemblies that overcome the deficiencies of current state-of-the-art systems for punching shear reinforcement.
The inventor has recognized that current state-of-the-art systems for punching shear reinforcing in slabs are deficient in that they are designed primarily with smooth vertical members that do not provide protection against shear stress in an area of the slab where shear cracks originate, namely, the central zone of the slab. In particular, near the top surface of the slab, known reinforcing assembles often include a horizontal rebar that is placed to withstand tensile bending stresses. This rebar also acts to withstand a horizontal component of the diagonal shear. At the bottom of the slab, there is compressive stress due to bending, and this stress neutralizes the horizontal component of the diagonal tension stress. In the central zone of the slab thickness, however, prior art structures provide nothing to reduce or neutralize the horizontal component of the diagonal tension. There, the magnitude of the tensile stress in that area is magnified, and this is the location where the shear cracks originate and thus where reinforcing is most necessary.
A reinforcing assembly to address this problem comprises a plurality of diagonal working members (preferably formed of #3 rebar) each of which is formed with protruding “ribs” to provide mechanical anchorage for the concrete to grip, and each of which is attached at its bottom end to a horizontal runner independently such that the diagonal working members are variably spaced along the assembly. Relative to the horizontal runner, each diagonal member is “bent” at a given angle, preferably in the range of 45° plus or minus 25°. A preferred orientation is approximately 45°. The reinforcing assembly is designed for use to provide punching shear reinforcement in a reinforced concrete structure such as, without limitation, a slab (in which the assembly is supported) located adjacent to a structural column. The top of each diagonal member in the reinforcing assembly preferably comprises a double hook that is configured to fit within reinforcing bars that run in both directions along the top of the slab. The horizontal runner comprises spaced-apart, longitudinally-extending carrier (or “support”) bars, each of which preferably terminates on one end (namely, at the support column) at one end in a structure that is bent upwards and hooked. Preferably, the hooked structure of each runner bar extends into the structural column, thereby converting the runner bar into a reinforcing member that provides the same function as the other diagonal members, and at a confined location at a highest point of stress (i.e. where the slab attaches to the column) where it would not be possible otherwise to fit a diagonal member.
The diagonal working members are located perpendicular to the zone of nascent diagonal cracking and thus provide significantly greater punching shear reinforcement protection. In particular, the tensile reinforcement provided by the reinforcing assembly aligns in parallel with the stresses it is intended to resist. The ribbed characteristic of each diagonal working member provides maximum surface area for concrete bonding and provide mechanical interlock for the transfer of concrete stresses to the rebar, and the spacing of the working members preferably is such that the members are closer together near the column (where stresses are highest), while transitioning to a progressively wider spacing away from the column where stresses are lower. Further, the preferably small diameter of the rebar provides maximum surface area for bonding interaction per unit weight of rebar.
The foregoing has outlined some of the more pertinent features of the invention. These features should be construed to be merely illustrative. Many other beneficial results can be attained by applying the disclosed invention in a different manner or by modifying the invention as will be described.
For a more complete understanding of the subject matter herein and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
With reference now to
Preferably, each working member 204 is angled with respect to the runner bars 202 at a predetermined angle lying within a range of 45° plus or minus 25°. A preferred bending angle of the working member is 45°. As shown in
Automated machinery may be used to configure the working members, preferably in a single pass.
Each longitudinally-extending support bar 202 preferably terminates on one end in a structure 216 that is bent upwards and hooked. This structure is designed to extend into the column to provide additional support in an area of maximum stress.
As best seen in
The working members and the runner bars on which they are disposed are preferably formed of rebar and, in particular, #3 rebar. Preferably, a smallest diameter rebar is used, and preferably the surface of the rebar is ribbed or knurled along its entire length.
The preferred dimensions of the working members are set forth in the figures, although one of ordinary skill will appreciate that these dimensions are merely exemplary.
The progressive spacing between the diagonal working members will depend on the size of the slab, the size of the column, and the anticipated loads. In one embodiment, a smallest unit-to-unit spacing is on the order of 3 inches, with spacing further out from the column then progressing to 5 inches, then 7 inches, and so forth. In an alternative embodiment, non-uniform spacing between the diagonal members may be used, e.g., with each successive spacing (away from the column) being a fixed percentage larger than a succeeding one (closer to the column), but with no spacing (except possibly the first, or the first and second) being the same. In another embodiment, the spacing varies incrementally in fractional-inch increments.
The reinforcing assembly is sometimes referred to herein as a “bar truss” whose “diagonal working members” may sometimes be referred to bar truss “units.” As shown in the drawings, each of the basic units preferably is bent into six (6) different planes before being welded to the runner bars. The runner (carrier) bars preferably are bent with hooks at their inside (support) ends to furnish additional gripping action at the location of highest stress and of load transfer. The truss bars units are each placed diagonally to engage any nascent cracks at a 90° or near 90° angle (with respect to the crack itself), which provides maximum efficiency in terms of aligning the units to directly oppose the shear (splitting) force. By being placed diagonally, each unit traverses a much higher percentage of the potential crack zone per unit length as compared to a vertical orientation. The diagonal placement further enables each unit to engage up to twice as many crack zones per unit. Preferably, at the top of each unit there is a curved “bridge” section which joins the two upstanding sides of the unit. A central connecting section preferably is bent downward from an uppermost section. This bent section, which is preferably comprised of three (3) curves, has several functions and advantages: it provides a very efficient hooked anchorage for the top end of each unit (without the requirement of any additional material), it minimizes conflict with any horizontally-extending rebar (in the slab, which rebar is distinct from the reinforcing assembly), and it performs a bridging function, connecting the two upstanding sides of each unit. Further, its compact size allows it to penetrate upward between even dense top rebar concentrations, and to engage to full depth of structural slab thickness.
Thus, in a preferred embodiment, each truss bar unit in the reinforcing assembly is oriented diagonally, has a knurled (ribbed) surface (preferably along its full length), and is formed of a small diameter (#3) rebar material. This size of rebar has the highest ratio of surface area to cross-sectional area, thus increasing its ability to bond most effectively to the concrete around it, and yet it is stiff enough to maintain its shape during concrete placement. The use of #3 rebar is not a limitation, however, as other types of materials (including #4 rebar, #5 rebar, and the like) may be used. Moreover, while the drawings illustrate that the preferred reinforcing assembly comprises a plurality of diagonal working members whose spacing from one another varies progressively from an inner to an outer end (relative to the column), this is not a limitation either. A reinforcing assembly having at least one diagonal working member having the structural characteristics described and illustrated above is within the scope of the claims that are set forth below.
The reinforcing assembly described above and illustrated in the figures provides numerous advantages over the prior art. The angled (diagonal) orientation of the truss bar units places them at approximately a perpendicular position to the potential diagonal-stress punching shear cracks. The ribbed surface of the units provides maximum efficiency in engaging the concrete at the points of maximum stress, thus preventing the crack from beginning in the first instance. Because the assembly inhibits any crack at the point of maximum stress, and because the structure provides similar reinforcing crossing the crack zone above and below this point, the crack (if it does occur) cannot propagate (extend). If the crack does not occur, or if it is held to a very narrow width, the very significant strength of the concrete will continue to maintain its integrity, thereby enabling the slab (in which the reinforcing assembly is embedded) to carry shear loads. The rebar and the concrete thus work together, and their respective strengths are additive, rather than (as in the prior art) the concrete failing and transferring all of its load-carrying ability to the smooth steel studs or similar reinforcing.
The ribbed reinforcing provided by each angled truss bar unit develops a bond with the concrete with which it is in contact, thus preventing crack origination and/or propagation, because the unit bonds to the concrete on both sides on the crack zone and prevents those concrete segments from moving apart from one another. Thus, the cracking process cannot begin under usual loads. The rebar works with the concrete and supplements it, rather than simply going to work after the concrete has already failed (as in the prior art).
The assembly is easy to manufacture. Preferably, the entire assembly is made of pieces of available materials (such as #3 rebar), and those pieces are readily bent and welded into the assembly using conventional (and preferably automated) manufacturing techniques.
A reinforced concrete structure according to this disclosure is any structure such as a slab, a beam, a footing, a flat foundation, etc. or the like that includes a reinforcing assembly of the type described above and illustrated in the drawings.
As illustrated in the drawings and as described above, each working member preferably has a downwardly-concave hook at its top end, which hook joins each end of an upwardly-concave, laterally-positioned hook that is centered on a centerline of the assembly. The upwardly concave hook forms a structural lateral link between the two upstanding side portions of the working member, thus forming a pair. The upwardly-concave hook preferably is below the level of the horizontal structure rebar, which is in the top level of the slab. The lower end of each working member, which is an angled terminus placed on an inside face of the lower carrier bar at the bottom of the assembly, preferably is located at a lowest possible portion in the slab depth, thus maximizing its embedded length in the slab and allowing an efficient, automatic, in-line, one-pass weld for the entire assembly.
With reference to the cross-sectional view of the described assembly in
Although the use of small (ribbed) rebar requires more members to be used, this provides an advantage in that it allows a more dispersed distribution of the individual working members in the concrete, thus allowing the steel reinforcing to blend into the concrete material and act more as an integral part of the concrete itself.
