Reinforcing Assemblies and Reinforced Concrete Structures
Reinforcing assemblies for use in structural concrete members, as well as reinforced concrete structures, are provided that are uniquely suited to deal with punching shear failure in structural concrete members such as slabs, beams footings and flat foundations. The reinforcing assemblies include a support base, comprising two elongate rebar members disposed substantially parallel to each other, and several shear-resisting rebar elements having two opposing ends and cast, or bent, in hairpin shape on a common plane in generally parallel orientation to each other and secured to the base. The reinforced concrete structures include a concrete member, having a first face and a generally opposed second face, and at least four reinforcing assemblies that are embedded and retained at prescribed locations within the concrete member by their base and by the opposed faces of the concrete member, and which comprise a support base and multiple hairpin-shaped rebar elements secured to the support base.
This invention relates to reinforcing assemblies for use in structural concrete members. In particular, this invention is concerned with a reinforcing system that is uniquely suited to deal with shear failure and with punching shear failure in structural concrete members such as slabs, beams, footings and flat foundations. More particularly, the invention relates to a structural concrete reinforcing assembly that may be made entirely of rebar. Specifically, the invention relates to a novel technique for eliminating or minimizing shear failure and punching shear failure in structural concrete members such as slabs, beams, footings and flat foundations by means of a unique reinforcing assembly made up of hairpin-shaped rebars, which novel technique allows the user to fabricate reinforced concrete structures with enhanced punching-shear-resistant capabilities.
BACKGROUND OF THE INVENTIONCommercial concrete is a mixture of cement, sand and stone aggregate held together as rigid structures by the action of small amounts of water. While concrete made in this fashion, usually referred to as “unreinforced concrete”, has fairly good resistance to compressive stresses, any significant tension will tend to break the rigid structure and cause undesirable cracking and separation of the concrete. For practical reasons, most commercial and industrial structural concrete members are made of “reinforced concrete”, that is, from a mixture of cement, sand and stone aggregate where a solid member made of a material with high strength in tension, such as steel, is placed and made to remain embedded. Reinforced concrete sections where the concrete is capable of resisting the compression and the steel placed inside is designed to resist the tension are often made into many shapes and sizes for the construction industry. Commercial and industrial structural concrete members such as slabs, beams, footings and flat foundations, even when made with reinforced concrete, are susceptible to shear forces that create tensile forces along them and often result in structural failure, that is, undesirable cracking and/or breaking of the structural concrete members. The type of failure caused by these shear forces and the attendant incline cracks that tend to propagate throughout the concrete, usually from the area under tension towards the area under compression, are not always easy to detect and correct since they are often not visible when they occur. One-way shear failure, often referred to as “shear failure”, usually occurs in beams and, occasionally, in walls, slabs, footings and other vertical members. Two-way shear failure, often referred to as “punching shear failure”, tends to occur in horizontal concrete members such as slabs, footings and flat foundations. Various methods and techniques exist for reinforcing structural concrete members so as to prevent or minimize the undesirable cracking and/or breaking of concrete structures caused by shear forces. Some of the methods and techniques that are commonly employed with various degrees of success include the use of open or closed stirrups that are strategically placed in designated areas of the concrete structures. A particular type of situation that is often encountered, for example when a load-bearing concrete slab is supported by a column, is what is commonly referred to as “punching shear” failure. Shear strength of flat slabs in the vicinity of columns or concentrated loads is often controlled by the two-way shear known as “punching shear”. Shear considerations can therefore be the controlling factor in determining the required slab thickness or increasing column size, especially of post-tensioned (“PT”) flat-plates. Punching shear failure is usually a substantial inclined cracking that occurs at about a 19-to-34-degree angle with respect to the top surface of a PT slab and extends from the edge of the load being applied. Post-tensioned slabs and slabs that make use of high-strength concrete are particularly susceptible to punching shear failure. Conventional solutions to shear problems are not always satisfactory when dealing with punching shear problems. To prevent punching shear failure at slab-column connections, conventional solutions usually provide for the use of stirrups, structural steel shearheads or studrail reinforcements. Each of these techniques, while adequate in many cases, also has its share of disadvantages.
Stirrups, with longitudinal bars or with vertical bars, are difficult to place in the concrete structures and often present anchorage slip problems. Increasing the strength of concrete slabs with conventional stirrups is common, but the anchorage of stirrups is difficult to provide in thin slabs (e.g., less than about 6 inches in height) and therefore should be used only if the stirrups are closed and contain a longitudinal bar at each corner. The use of the stirrups as shear reinforcements in slabs is practicable only if the effective depth of the slab is greater than about 6 inches, but not less than 16 times the shear reinforcement bar diameter. So-called “shearheads” are structural steel shapes such as “I” shapes or channel sections. Shearheads are rarely used because their installation is very expensive and because they often interfere with the placement of flexural reinforcing bars and post-tensioned cables. So-called “studrails” consist of headed studs in the form of vertical bars mechanically anchored at each end by a plate or head capable of developing the yield strength of the bars. Studrails are often used to increase the shear strength in flat slabs, but in order to develop the full yield strength of the studs the area of the anchor head must be a minimum of 10 times the cross sectional area of the stud stem. Also, because of their anchor heads, studrails do not always provide adequate confinement of the concrete where the punching shear failure tends to occur.
It is apparent that a need exists to provide a solution to the problems associated with punching shear failure that does not suffer from the shortcomings attendant the use of stirrups, shearheads, studrails and similar conventional devices currently in commercial use. The present invention provides one such solution.
SUMMARY OF THE INVENTIONIt is an object of the present invention to provide a practicable and economical solution to the problems associated with shear failure and punching shear failure in concrete structures. Another object of the present invention is to provide a safe and cost-effective reinforcing assembly for use in structural concrete members that is uniquely suited to deal with shear failure and punching shear failure in structural concrete members such as slabs, beams, footings and flat foundations. It is also an object of this invention to provide a structural concrete reinforcing assembly that may be made entirely of rebar members. An additional object of the invention is to provide a safe and low-cost reinforced concrete structure that is particularly resistant to punching shear forces and that is uniquely suited for commercial use with structural concrete members such as slabs, footings and flat foundations. A further object of the invention is to provide a structural concrete reinforcing assembly that is particularly effective at eliminating or minimizing shear failure in structural concrete members while affording better anchorage than the conventional devices used in preventing or minimizing shear failures. A particular object of the invention is to facilitate an efficient technique for eliminating or minimizing shear failure in structural concrete members such as slabs, footings and flat foundations by means of a unique reinforcing assembly that may be made up entirely of hairpin-shaped rebars, which novel technique permits the fabrication of reinforced concrete structures with enhanced punching-shear-resistant capabilities. These and other objects of the invention will be apparent from the disclosures that follow.
The reinforcing assembly of the present invention involves a support base that comprises two elongate rebar members disposed substantially parallel to each other, and several shear-resisting rebar elements having two opposing ends and a hairpin shape on a common plane in generally parallel orientation to each other and secured to the base. The rebar elements may be cast in the hairpin shape, or they may be reformed by bending; and they may have a smooth surface, which affords cost savings, or be ribbed or corrugated in order to improve cohesion between the steel and the concrete. The reinforcing assemblies of the present invention are best used as sets of four units that are strategically placed at or near the areas of high shear failure propensity. The reinforced concrete structure of the invention includes a concrete member, having a first face and a generally opposed second face, and at least four reinforcing assemblies that are embedded and mechanically retained in a prescribed location within the concrete member by their base and by the opposed faces of the concrete member. Each of the four reinforcing assemblies of the reinforced concrete structure comprises a support base and multiple hairpin-shaped rebar elements secured to the support base.
A clear understanding of the key features of the invention summarized above may be had by reference to the appended drawings, which illustrate the system of the invention, although it will be understood that such drawings depict preferred embodiments of the invention and, therefore, are not to be construed as limiting its scope with regard to other embodiments which the invention intends and is capable of contemplating.
Accordingly,
The invention will now be described with reference to its application to a post-tensioned slab structure that is supported by a column, which may be a steel supporting column or a concrete supporting column.
In using the reinforcing assembly system of the invention it is often best to place several, e.g., four, of these assemblies, or “PTE-PSRs”, right next to the loaded or reaction area, arranged in the manner depicted in
Following is a detailed description of tests conducted in order to confirm the efficiency of the present invention and compare it with prior art techniques for reinforcing concrete members.
The prestressing strands used were 0.5 in. (12.7 mm) in diameter, seven wire strands conforming to ASTM standard A421, with a specified ultimate strength of 270 ksi (1861 Mpa). The tendons were protected with a plastic sheathing to prevent the “cable-concrete” bond and to reduce friction at the time of stressing. #3 bars (10 mm) were used for fabricating hairpin assembly reinforcements conforming ASTM standard A421, with a specified yielding strength of 60 ksi (413.5 Mpa). The edge forms for the slabs were constructed using ¾ in. (19 mm) thick and 4 in. (102 mm) high plywood which were braced using 2 in. (51 mm)×6 in. (153 mm) wood studs. The edge forms were carefully aligned, leveled and nailed to the floor to help prevent movement during casting. The slabs were cast on the floor over plastic sheets. Prior to casting the slabs, the plastic sheets were oiled for ease of removal of the specimens. Chairs were used to ensure that the desired tendon profile was attained. Ready-mix concrete was delivered and pumped to the location of the specimens. The concrete was vibrated and the final slab finish achieved using steel trowels and wooden floats. A plastic cover was placed over the slabs for a 15-day curing period.
The specimens were tested in an elevated position using a steel test frame. Application of an upward load allowed for the observation of the punching shear failure on the top surface of the slab. The reaction frame was designed and fabricated of structural steel. The central loading was applied upwards and the concrete slab was held down at four locations which were 3 ft. (915 mm) apart. The slab also had a 1.5 ft. (457 mm) overhang on each side. The tendons were stressed up to 33 kips (146.5 KN) after the slab was positioned on the test frame. The loading was accomplished by using a hydraulic jack. The hydraulic jack was previously calibrated to yield the applied load. A dial gauge was mounted at the center of the slab to measure the displacement of the slab. Load cells were also placed between the edge of the slab and the anchoring mechanism of the prestressing tendons to measure the force in the four central tendons. The strain-gage based load cells were manufactured by Honeywell Sensotec and had a capacity of 50 kips (222 KN) each. These load cells were connected to a 4-channel digital indicator which displayed the tendon force.
The tests were carried out by increasing the pressure in the hydraulic jack and recording the central deflection and the tendon forces at each increment of the load. The data for one of the slabs with hairpin assembly reinforcements (PT-1) are provided in Table 2. The first crack in PT-1 was observed at a load of 30.5 kips (135.7 KN). The second perpendicular crack in PT-1 appeared at a load of 43.2 kips (192.2 KN) in PT-1. The maximum load carried by PT-1 was 63.8 kips (283.7 KN). Both specimens PT-1 and PT-2 failed in the flexural mode. The results presented in Table 2 indicate that the tendon force increases linearly with the load up to the point of failure. The maximum stress in the tendon at failure was 190 ksi (1308 Mpa). The use of the hairpin assembly reinforcements (the PTE-PSRs) caused flexural failure to occur before punching shear failure in both specimens PT-1 and PT-2. The central load vs. deflection plots for PT-1 and PT-2 are provided in
The data obtained for one of the slabs with the studrails (PT-3) is provided in Table 3. The first crack in PT-3 was observed at a load of 30.5 kips (135.7 KN) and the second perpendicular crack appeared at a load of 39.9 kips (177.5 KN). The maximum load reached was 57.3 kips (255 KN). The central load vs. deflection plots for PT-3 and PT-4 are provided in
The ACI approach for punching shear capacity with headed-stud reinforcements is based on the nominal punching shear stress of a critical section located at a distance d/2 from the column face and the length of the shear reinforcement as described earlier. The test results are compared with the values obtained from the ACI provision of headed-stud reinforcements in Table 4. The ACI-421.1 R recommendation for nominal punching shear stress using headed-studs is 8√{square root over (f′c)} at a distance d/2 from the column face. For hairpin assembly reinforcements, or PTE-PSRs, (PT-1 & PT-2), the average punching shear strength was 62.2 Kips (276.7 KN) which is equal to a nominal punching shear stress of 10.2√{square root over (f′c)}. For PT slabs reinforced with headed-studs (PT-3 & PT-4), the ACI predictions were found to be almost equal or slightly conservative. The average capacity was 50.2 Kips (223.3 KN) which corresponds to a nominal shear stress of 8.2√{square root over (f′c)}. The results obtained for punching shear using the PTE-PSRs were about 20% higher than those obtained for punching shear using the headed-studs.
The tests demonstrated that the PT slabs with headed-stud reinforcements failed in punching shear, while PT slabs with hairpin assembly reinforcements (PTE-PSRs) failed in flexure. The test results with the headed-stud reinforcements were almost equal to those predicted by the ACI equation. The tests with hairpin assembly reinforcements had the shear failure prevented, and almost complete redistribution of bending moments were achieved prior to collapse. The PT slab capacity with hairpin assembly reinforcements were about 20% higher than the PT slab capacity with the headed-stud reinforcements. The nominal shear stress for a PT slab with hairpin assembly reinforcements was equal to 10.2√{square root over (f′c)} and could actually be higher as the slabs failed in flexure. The test results show that, in terms of shear reinforcement capability, concrete structures using hairpin assembly reinforcements in PT slabs provide superior confinement of the concrete where punching shear failures tend to occur and behave structurally better than concrete structures that use headed-stud reinforcements.
The hairpin assembly reinforcements described herein may be set in the concrete structure by placing them in a light-weight plastic support base, such as a stool, or “chair”, and setting the support base and the hairpin assembly reinforcements on wood forms of the type commonly used commercially to set concrete and concrete reinforcements. Normally, the assembly can be secured to the support base by means of wires and the support based nailed or otherwise fastened to the wood forms before pouring the concrete. The techniques for embedding reinforcing devices in concrete are well known to those skilled in art of concrete pouring and handling, and any of a number of such techniques may be used for this purpose. The reinforcing assemblies of the present invention may also be placed in the post-tensioned anchorage zone of the reinforced concrete structure.
While the present invention has been described in terms of particular embodiments and applications, in both summarized and detailed forms, it is not intended that these descriptions in any way limit its scope to any such embodiments and applications, and it will be understood that many substitutions, changes and variations in the described embodiments, applications and details of the method and system illustrated herein and of their operation can be made by those skilled in the art without departing from the spirit of this invention.
Claims
1. A reinforcing assembly for use in structural concrete members, comprising:
- (a) a base, comprising a first elongate rebar member and a second elongate rebar member, said first and second elongate rebar members disposed substantially parallel to each other; and
- (b) a plurality of shear-resisting rebar elements having two opposing ends and cast or reformed in a generally hairpin shape, said shear-resisting rebar elements disposed on a common plane in generally parallel orientation to each other and bridging said first and second elongate rebar members comprising said base, each said rebar element having its first opposing end secured to said first base elongate rebar member and its second opposing end secured to said second base elongate rebar member.
2. The reinforcing assembly of claim 1, wherein said first and second elongate rebar members comprising said base and disposed substantially parallel to each other are spatially disposed on substantially the same plane with respect to each other.
3. The reinforcing assembly of claim 1, wherein said generally hairpin-shaped rebar elements bridging said first and second elongate rebar members comprising said base have a substantially smooth surface.
4. The reinforcing assembly of claim 1, wherein said generally hairpin-shaped rebar elements bridging said first and second elongate rebar members comprising said base are ribbed or corrugated.
5. The reinforcing assembly of claim 1, wherein each said rebar element has its first opposing end welded to said first base elongate rebar member and its second opposing end welded to said second base elongate rebar member.
6. The reinforcing assembly of claim 1, wherein all of said generally hairpin-shaped rebar elements bridging said first and second elongate rebar members are spaced at the same distance from each other on said base.
7. The reinforcing assembly of claim 1, wherein the cross-sectional area of each of said generally hairpin-shaped rebar elements is substantially round.
8. The reinforcing assembly of claim 1, wherein the shape and the height of each of said generally hairpin-shaped rebar elements are substantially the same.
9. The reinforcing assembly of claim 1, wherein each of said generally hairpin-shaped rebar elements is made of steel.
10. The reinforcing assembly of claim 1, wherein said first and second elongate rebar members and said generally hairpin-shaped rebar elements are made of round steel rebars having the same cross-sectional area and wherein all said generally hairpin-shaped rebar elements have the same shape and height.
11. A reinforced concrete structure, comprising:
- (a) a concrete member having a first face and a generally opposed second face; and
- (b) at least four reinforcing assemblies embedded within said concrete structure at prescribed locations, each said reinforcing assembly comprising (i) a base, comprising a first elongate rebar member and a second elongate rebar member, said first and second elongate rebar members disposed substantially parallel to each other; and (ii) a plurality of shear-resisting rebar elements having two opposing ends and cast or reformed in a generally hairpin shape, said shear-resisting rebar elements spatially disposed on a common plane in generally parallel orientation to each other and bridging said first and second elongate rebar members comprising said base, each said rebar element having its first opposing end secured to said first base elongate rebar member and its second opposing end secured to said second base elongate rebar member, wherein each of said at least four reinforcing assemblies is retained at said prescribed locations by said base and said opposed concrete member faces.
12. The reinforced concrete structure of claim 11, wherein said first and second elongate rebar members comprising said reinforcing assembly base and disposed substantially parallel to each other are spatially disposed on substantially the same plane with respect to each other.
13. The reinforced concrete structure of claim 11, wherein said generally hairpin-shaped rebar elements bridging said first and second elongate rebar members comprising said base have a substantially smooth surface.
14. The reinforced concrete structure of claim 11, wherein said generally hairpin-shaped rebar elements bridging said first and second elongate rebar members comprising said base are ribbed or corrugated.
15. The reinforced concrete structure of claim 11, wherein each said rebar element has its first opposing end welded to said first base elongate rebar member and its second opposing end welded to said second base elongate rebar member.
16. The reinforced concrete structure of claim 11, wherein all of said generally hairpin-shaped rebar elements bridging said first and second elongate rebar members are spaced at the same distance from each other on said base.
17. The reinforced concrete structure of claim 11, wherein the cross-sectional area of each of said generally hairpin-shaped rebar elements of said reinforcing assemblies is substantially round.
18. The reinforced concrete structure of claim 11, wherein the shape and the height of each of said generally hairpin-shaped rebar elements of said reinforcing assemblies are substantially the same.
19. The reinforced concrete structure of claim 11, wherein four reinforcing assemblies are embedded within said concrete structure and arranged in cross-like fashion at substantially right angles with respect to each other on substantially the same plane and each of said generally hairpin-shaped rebar elements is made of steel.
20. The reinforced concrete structure of claim 11, wherein said first and second elongate rebar members and said hairpin-shaped rebar elements of said reinforcing assemblies are made of round steel rebars having the same cross-sectional area, and wherein the shape and the height of each of said generally hairpin-shaped rebar elements of said reinforcing assemblies are substantially the same.
21. A reinforced concrete structure, comprising:
- (a) a concrete member having a first face and a generally opposed second face;
- (b) a column supporting said concrete member; and
- (c) at least four reinforcing assemblies embedded within said concrete member at prescribed locations around said supporting column, each said reinforcing assembly comprising (i) a base, comprising a first elongate rebar member and a second elongate rebar member, said first and second elongate rebar members disposed substantially parallel to each other; and (ii) a plurality of rebar elements having two opposing ends and cast or reformed in a generally hairpin shape, said generally hairpin-shaped rebar elements spatially disposed on a common plane in generally parallel orientation to each other and bridging said first and second elongate rebar members comprising said base, each said generally hairpin-shaped rebar element having its first opposing end secured to said first base elongate rebar member and its second opposing end secured to said second base elongate rebar member, said at least four reinforcing assemblies placed within said concrete member in cross-like fashion at substantially right angles with respect to each other on substantially the same plane, and wherein each of said at least four reinforcing assemblies is mechanically retained at said prescribed locations by said base and by said opposed faces of said concrete member.
22. The reinforced concrete structure of claim 21, wherein said first and second elongate rebar members comprising said reinforcing assembly base and disposed substantially parallel to each other are spatially disposed on substantially the same plane with respect to each other.
23. The reinforced concrete structure of claim 21, wherein each of the said at least four reinforcing assemblies embedded within said concrete member in cross-like fashion is placed around said supporting column at a distance which is equal to or less than one-half of the effective depth of said concrete member.
24. The reinforced concrete structure of claim 21, wherein four reinforcing assemblies are embedded within said concrete member.
25. The reinforced concrete structure of claim 21, wherein eight reinforcing assemblies are embedded within said concrete member.
26. The reinforced concrete structure of claim 21, wherein the shape and the height of each of said generally hairpin-shaped rebar elements of said reinforcing assemblies are substantially the same.
27. The reinforced concrete structure of claim 21, wherein four reinforcing assemblies are embedded within said concrete structure and arranged in cross-like fashion at substantially right angles with respect to each other on substantially the same plane and each of said generally hairpin-shaped rebar elements is made of steel.
28. The reinforced concrete structure of claim 21, wherein eight reinforcing assemblies are embedded within said concrete structure and arranged in cross-like fashion at substantially right angles with respect to each other on substantially the same plane and each of said generally hairpin-shaped rebar elements is made of steel.
29. The reinforced concrete structure of claim 21, wherein said first and second elongate rebar members and said hairpin-shaped rebar elements of said reinforcing assemblies are made of round steel rebars having the same cross-sectional area, and wherein the shape and the height of each of said generally hairpin-shaped rebar elements of said reinforcing assemblies are substantially the same.
30. The reinforced concrete structure of claim 21, wherein said supporting column is made of concrete.
31. The reinforced concrete structure of claim 21, wherein said supporting column is made of steel.
Type: Application
Filed: Apr 27, 2007
Publication Date: Oct 30, 2008
Inventor: Zaher Ali Abou-Saleh (Margate, FL)
Application Number: 11/741,295
International Classification: E04C 5/06 (20060101); E04C 5/01 (20060101);