IMPROVEMENTS IN OR RELATING TO REINFORCED CONCRETE STRUCTURAL ELEMENTS

Abstract: A shear failure reinforcing system for structural elements, in which thin elongate strips of high stiffness material are anchored around a layer of conventional reinforcement, and/or are anchored around a plurality of layers of conventional reinforcement, such that the strips tie the element and improve its resistance to shear failure.

Description: IMPROVEMENTS IN OR RELATING TO REINFORCED CONCRETE STRUCTURAL ELEMENTS This invention relates to reinforced concrete structural elements, and more particularly to a reinforced concrete structural element having improved resistance to shear failure and to a method of providing shear reinforcement for a reinforced concrete structural element. BACKGROUND TO THE INVENTION Thin reinforced concrete elements, for example flat concrete slabs, provide an elegant form of construction, which simplifies and speeds up site operations, allows easy and flexible partitioning of space and reduces the overall height of buildings. Reinforced concrete flat slab construction also provides large uninterrupted floor areas within a minimum construction depth, and is used extensively for a wide range of buildings such as office blocks, warehouses and car parks. One design problem associated with this form of construction is punching failure, which occurs as a result of high point loads or high shear stresses around the supporting columns. In punching failure, the failed surface of the slab forms a truncated cone or pyramid. This problem has in the past often lead to the use of mushroom heads or local thickening of the slab, but these solutions increase costs and slow down the rate of construction. As the spans become larger and the slabs become thinner the increased stresses around the critical shear perimeter have created even greater problems for the structural engineer. A variety of design solutions have been proposed, of which the most commonly used are as follows: 1. Conventional shear reinforcement This solution is very labour-intensive and requires extra work both in the design and on site. 2. Use of a larger column and/or a thicker concrete slab These solutions increase the deadload of the building and reduce the available space. 3. Use of a column head This requires more complicated formwork, slows down the rate of construction, and interferes with the installation of building services. 4. Use of slab drops These are a modified form of column head. Shear reinforcement, when required, is normally accomplished by providing reinforcing members either at an angle or laterally to the main flexural reinforcement. In thin structural elements, such as flat slabs, anchoring of short lengths of shear reinforcement is a major design problem. The problem is aggravated by the fact that normal shear reinforcement cannot be placed above the top layer of flexural reinforcement without reducing either the durability, or the efficiency, of the flexural reinforcement. In addition, there is the practical problem of supporting the shear reinforcement during the construction stages. Recently a new system has been introduced by Square Grip Limited, designated the Shearhoop system, which consists of an assembly of specially shaped links (shear leg bobs) and hoop reinforcing bars. The hoops are available in a range of sizes and can be combined to form a complete system extending outwards from the column to the zone where the shear resistance of the concrete slab alone is adequate. In the construction of a slab using Shearhoops, bars B1,B2 for the bottom layer of reinforcement are first laid down and the Shearhoops placed over them in the appropriate location. Top reinforcement T2 is then positioned on chairs and the bars overlapping the Shearhoops fully located under the ends of the shear leg bobs extending from the Shearhoops. Finally the top reinforcement Tl is placed over the entire structure. Whilst the Shearhoops are an improvement on previous arrangements, they still cannot be anchored above the top layer of reinforcement Tl and thus do not provide the best possible shear reinforcement. From the above, it is apparent that, although much effort has gone into the design of reinforcing systems that address some of the above mentioned problems, none of them provide a complete solution. Although pre¬ packaged reinforcing systems offer some time savings over the in-situ steel fixing solutions, they are nevertheless more expensive in terms of materials and other resources, such as labour and crane time. Some of the other prior art proposals are also of questionable effectiveness, or produce an unquantifiable increase in flexural capacity. There is a need, therefore, for an improved reinforcing system to impart better shear resistance, without increasing the thickness of the slab. An additional advantage would be to provide a shear reinforcement system enabling thinner slabs to be used. US 4854106 describes foundations for buildings and like structures employing steel reinforcement. A hook leg has an elongate member bifurcated at each end longitudinally of the member to form a pair of extensions with a slot therebetween, the distal portion of the extensions being bent into a curved form extending transversely of the member to form hooks adapted to resiliently engage a pair of reinforcing rods in the reinforcement, the slots in the unbent portions of the extensions being adapted to receive a second pair of reinforcing rods extending transversely of the first pair, whereby to fix the rods in spaced alignment. There is no mention of shear reinforcement. US 4472331 describes a reinforcing framework for a concrete building structure in which column and beam reinforcing bars are inserted into holes in reinforcement frames disposed at predetermined intervals. Shearing reinforcement bands, formed by bending a steel strip into a rectangular frame shape, are disposed between adjacent reinforcement frames and secured to wooden sheathing boards by nails. The construction requires access to all sides of the column or beam, and the protruding nails would give rise to potential corrosion problems. DE 3331276 describes shear reinforcement elements for column supported flat slabs or beams of reinforced or prestressed concrete, which consist of flat steel strips which are undulating in at least two dimensions and transverse to the main surface of the flat slab or beams. The shear reinforcement elements are used in place of conventional round reinforcing bars. SUMMARY OF THE INVENTION The present invention provides a shear failure reinforcing system for structural elements, in which thin elongate strips of high stiffness material are anchored around a layer of conventional reinforcement, and/or are anchored around a plurality of layers of conventional reinforcement, such that the strips tie the structural element and improve its resistance to shear failure. In preferred embodiments, the strips are anchored around the outermost reinforcing members of a layer or layers of reinforcement, to give improved shear resistance. In one aspect, the invention provides a method of providing shear reinforcement for a reinforced structural element having reinforcing members located adjacent its major surfaces, which comprises disposing a plurality of thin elongate strips of high stiffness material such that they anchor around one or more of the reinforcing members adjacent one major surface, and/or around one or more reinforcing members adjacent each major surface, such that the strips tie the structural element and improve its resistance to shear failure. In another aspect the invention provides a reinforced structural element having reinforcing members located adjacent its major surfaces, wherein shear reinforcement is provided by a plurality of thin elongate strips of high stiffness material disposed such that they anchor around one or more reinforcing members adjacent one major surface, and/or around one or more reinforcing members ad acent each major surface, such that the strips tie the structural element and improve its resistance to shear failure. DETAILED DESCRIPTION OF THE INVENTION The reinforced structural element may be cast in- situ or precast, and may be provided with any suitable longitudinal reinforcement comprising elongate reinforcing members, which may be initially unstressed, pre-stressed, or post-tensioned. The invention finds particular application where the reinforced structural element is a slab structure especially a flat slab, although it can also be a waffle or ribbed slab, a slab with downstands, a foundation slab or footing, or a staircase slab. Other possible uses may be in a wall, a wide band beam, or normal beam, a normal or extended column, a box or other hollow shape, or a shell or other three dimensional shape. The element may be with or without openings, as desired. The reinforced structural element may have any suitable thickness, depending upon the application. Henceforth the invention will be more particularly described with reference to thin reinforced concrete structural elements, for example flat slabs, having a thickness of from 10 to 80cms, more particularly from 10 to 30cms, but it is to be understood that although the invention has particular advantages when applied to such structures, it is not limited thereto. The thin reinforced concrete structural element may have any desired length and width, but reinforced flat slabs used in conventional building construction are often of a size of from 1 to 10 metres in length and from 1 to 10 metres in width. The reinforcing members will usually be elongate rods or bars embedded in the structural element and lying parallel to the major surfaces of the element. The reinforcing members can have any suitable cross-section, for example round, square, or rectangular. Typically, the reinforcing members lie adjacent one or more of the major surfaces of the structural element, and comprise series of reinforcing bars laid at right angles to each other. The major surfaces of the structural element will normally be the top and bottom surfaces, where the element is a slab, but they could also be the side surfaces of a wall. The material of the reinforced concrete structural element may be normal concrete, high strength concrete, light weight concrete, concrete with special cements and aggregates, polymer modified concrete, special cement mortar, special polymer mortar. Elements formed from other suitable materials able to be cast which require strengthening in shear, such as, for example, fibre reinforced plastics and ceramics can also be used. The thin elongate strip of high stiffness material preferably has dimensions such that it will not radically change the overall thickness of the structural members to which it is anchored, and such that it will not break when bent to the required shape, which could be around tight corners. Preferably the strip has a thickness of from 0.5 to 1.0mm and a width of from 10 to 30mm. The material of the strip is preferably a high tensile, high stiffness material, such as, for example, high tensile steel, although other high stiffness materials, for example structural polymers such as polypropylene and fibre reinforced plastics comprising, for example, carbon fibre, glass fibre and aramids, are not excluded. The material is required to have high stiffness in order to be able to arrest the development of shear cracks at low strains, and, for example, a material of stiffness of from lOOKN/mm 2 to 210KN/mm 2 is preferred. High strength material is preferred for the strips because a lower volume of strip material can be used. A typical strength for a high tensile steel used for the strip can be, for example, from 460N/mm 2 to 1500N/mm 2 . Special hardness strips may be useful when dealing with walls in safe ■ areas. The durability of the strip may be improved by adequate cover, by special surface protection, or by using non-corrosive materials such as stainless steel, or fibre reinforced plastics. Where the strip is metallic, it may also be charged to provide cathodic protection. Punched holes, embossments and indentations in the strip, as well as special bending, twisting or surface treatment of the strip, can help the overall bond characteristics of the strip to the material of the structural element, although a right angle bend may be sufficient to anchor the strip where concrete is used as the material for the reinforced structural element. In use, the strip may be disposed in a vertical, horizontal, or inclined direction, and may be bent or clipped around the reinforcing member to which it is anchored, or tied thereto. In a preferred aspect of the invention, the strip is anchored around one or more of the outermost reinforcing members, that is, those members closest to the major surfaces of the structural element. Since the reinforcing bars are often of significant thickness, for example, around 20mm diameter, this provides shear reinforcement to a point closer to the surface than has been possible hitherto. Bending and shaping of the strips to the desired shape may be readily accomplished by hand, or by the use of specialised automated or semi-automated equipment. The strips may be preformed before conveying to the site, and use, if desired. The strips may be anchored in the material of the structural element by providing an appropriate extra strip length beyond a bend around a structural element, or alternatively ends of the strip may be secured together by metal clips, rivets or other fixing means. It is particularly preferred for the strip to be so shaped that it can be positioned from one side of the structural element, without the need to obtain all round access. The strip can, for example, be bent into a zig- zag shape, a castellated shape, a sine wave curved shape, or other repeating straight sided or curved shaped and then dropped into position on the reinforcing members. This greatly facilitates assembly, where it is often difficult to obtain all round access to the structural element. Preferably the strips are arranged such that they are totally enclosed within and not exposed at any point on the surface of the structural element, and are not connected to any metal fixing, for example, a nail or screw, which is exposed on the structural element surface. This is to avoid the risk of corrosion or deterioration of the strips in service. Structural elements reinforced by the method of the invention can have improved strength and substantially improved ductility, imparting improved resistance to shear failure. In addition, structural elements reinforced in accordance with the invention can have a thinner section then those hitherto specified because of their improved resistance to shear failure. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be better understood, preferred embodiments thereof will now be described in detail, by way of example only, with reference to the accompanying Drawings in which: Figure 1A shows schematically a sectional side elevation of a reinforced flat structural element according to the invention; Figure IB shows a sectional side elevation of a reinforced curved structural element according to the invention; Figure 1C shows a sectional side elevation of a reinforced flat structural element according to the invention in which the strip is anchored to both top and bottom reinforcing members; Figure ID shows a sectional side elevation of a reinforced flat structural element according to the invention reinforced with single spacing inclined strip; Figure IE shows a sectional side elevation of an inclined reinforced structural element according to the invention; Figure IF shows a sectional side elevation of a vertical reinforced structural element according to the invention; Figure 2 shows examples of punched and pre-formed steel strips for use in the invention; Figure 3A shows a perspective view from the top and one side of the reinforcing formwork of a flat reinforced concrete structural slab in accordance with the invention, reinforced with inclined metal strips with punched holes; Figure 3B shows a perspective view from the top and one side of the reinforcing formwork of a reinforced flat concrete structural slab in accordance with the invention, having inclined metal strip shear reinforcement, but without punched holes in the strips; Figure 3C shows a perspective view from the top and one side of the reinforcing formwork for a reinforced flat concrete slab in accordance with the invention, having shear reinforcement comprising vertically arranged metal strips with punched holes; Figure 4A shows the load versus deflection curves for the slabs of figures 3A to 3C (PPSB to PPSD) in comparison with an unreinforced control slab (PPSA); and Figure 4B shows the load versus strain in the flexural reinforcement for the slabs of figures 3A to 3C (PPSB to PPSD) in comparison with an unreinforced control (PPSA) . Referring now to figure 1, in figure 1A there is shown a flat element 1, supported on a column 7 about a centre line C L , having upper reinforcing bars, 2, 3, arranged at right angles to each other, and lower reinforcing bars 4, 5 similarly arranged. U-shaped strips 6 of thin, elongate high stiffness steel are arranged between the upper and lower reinforcing bars in order to provide double spaced vertical shear reinforcement. In figure IB there is shown a curved reinforced concrete element 10, supported on columns 16, having upper reinforcing bars 11, 12 and a lower reinforcing bar 13. A thin strip of 14 of high stiffness steel is bent around the upper reinforcing bars 12 and the lower reinforcing bar 13 to provide single spacing vertical strip shear reinforcement. The strip 14 is bent at its ends 15 around the lower reinforcing bar 13, leaving a substantial length of the strip for anchoring in the concrete. Figure 1C shows a flat concrete structural slab 20, supported on a column 21 about a centre line C L , and having upper reinforcing bars 22, 23, and lower reinforcing bars 24, 25. In this case the thin, high stiffness metal strip 26 is bent around both upper and lower reinforcing bars. In figure ID there is shown a flat reinforced concrete slab 30, supported upon a column 31, and provided with upper reinforcing bars 32, 33 and lower reinforcing bars 34, 35. Shear reinforcement is provided by the metal strip 36 which is bent around upper and lower reinforcing bars so as to provide inclined shear reinforcement. Figure IE shows an inclined concrete reinforcing slab 40, supported on a column 41, and provided with upper reinforcing bars 42, 43 and lower reinforcing bars 44, 45. Shear reinforcement is provided by the high stiffness metal strip 46 which is bent around both upper and lower reinforcing bars to form a single spaced shear reinforcement. Figure IF shows a vertical concrete structural slab 50 having right side reinforcing bars 51, 52 and left side reinforcing bars 53, 54. Shear reinforcement is provided by the high stiffness metal strip 55 which is bent around both left and right side reinforcing bars to provide inclined shear reinforcement. The invention will now be illustrated by the following examples: Example 1 This example describes the enhancement of shear capacity of a flat slab with inclined metal strip reinforcement having punched holes. Steel strips are produced having a series of punched holes as shown in figure 2, and are preformed to the castellated shape shown therein. The strips are arranged in the formwork for a concrete slab in locations determined by using British Standard BS8110 (1985), as illustrated in figure 3A. It will be noted that it is only necessary to have access to the top side of the formwork in order to place the strips in position. Concrete is then poured to produce a slab of thickness 175mm which is below the 200mm limit imposed by BS8110 on the thickness of flat slabs. The slab (B) was tested with an eight-point load arrangement, simulating loading typical of flat slabs in buildings of conventional construction. The load versus deflection curves and the load versus strain in the flexural reinforcement curves for this slab and others tested for comparison are shown in figures 4A and 4B respectively. Slab (A) was unreinforced and failed in abrupt punching shear mode at a load of 460kN. Slab (B) deflected considerably more, developed very large strains in the longitudinal reinforcement and failed in a ductile mode at a maximum load of 560kN, in the fashion desired in practice by structural engineers. Example 2 This example demonstrates the increase in load and ductility of a flat slab reinforced with inclined steel strip. Steel strips without the punched holes are preformed as shown in figure 2 and arranged in the metal formwork for a concrete slab in locations determined by using BS8110 (1985) as illustrated in figure 3B. Concrete is then poured to produce a slab of thickness 175mm. The slab (C) was tested with an eight-point load arrangement, making extra allowance for anchoring the strip at its ends. The load versus deflection curves and the load versus strain in the flexural reinforcement curves for this slab and others tested for comparison are shown in figures 4A and 4B respectively. Slab (C) deflected considerably more than slab (A), and developed very large strains in the longitudinal reinforcement, failing in a ductile mode at a maximum load of 560kN. Example 3 This example demonstrates the increase in load and ductility of a flat slab reinforced with vertical steel strip reinforcement anchoring both layers of longitudinal -reinforcement. Steel strips, punched and pre-formed as shown in figure 2, are inserted into the form work of a concrete slab as shown in figure 3C and anchored to the upper and lower layers of longitudinal reinforcing bars. The strips are arranged in locations determined by using BS8110 (1985). Concrete is then poured to produce a slab of thickness 175mm. The slab (D) was tested with a eight-point load arrangement, simulating loading typical on flat slabs in buildings. Extra allowance was made for anchoring the strip at its ends. The load versrs deflection curves and the load versus strain in the flexural reinforcement curves for this slab and others tested for comparison is shown in figures 4A and 4B respectively. Slab (D) deflecting considerably more than slab (A), developed very large strains in the longitudinal reinforcement, and failed in a ductile mode at a maximum load of 560kN. The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings) , and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) , may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims: CLAIMS 1. A method of providing shear reinforcement for a reinforced structural element having reinforcing structures located adjacent its major surfaces, which comprises disposing a plurality of thin elongate strips of high stiffness material such that they anchor around one or more of the reinforcing members adjacent one major surface, and/or around one or more reinforcing members adjacent each major surface, such that the strips tie the structural element and improve its resistance to shear failure. 2. A method according to Claim 1, in which the strips are anchored around the outermost reinforcing members of a layer or layers of reinforcement. 3. A method according to Claim 1 or 2, in which the reinforced structural element is a flat slab. 4. A method according to any of the preceding claims, in which the structural element is a reinforced concrete element. 5. A method according to any of the preceding claims, in which the structural element has a thickness of from 10 to 30cms. 6. A method according to any of the preceding claims, in which the structural element has a length of from 1 to 10m and a width of from 1 to 10m. 7. A method according to any of the preceding claims, in which the reinforcing members comprise a series of reinforcing bars laid at right angles to each other. 8. A method according to any of the preceding claims, in which the elongate strips of high stiffness material have a thickness of from 0.5 to 1.0mm and a width of from 10 to 30mm. 9. A method according to any of the preceding claims, in which the material of the strips comprises high tensile steel. 10. A method according to any of the preceding claims, in which the material of the strips has a stiffness of from lOOKN/mm 2 to 210KN/mm 2 and a strength of from 460N/mm 2 to 1500N/mm 2 . 11. A method according to any of the preceding claims, in which the elongate strips are provided with holes along the lengths thereof to assist the overall bond characteristics of the strips to the material of the structural element. 12. A method according to any of the preceding claims, in which the strips are bent or clipped around the reinforcing members to which they are anchored, or tied thereto. 13. A method according to any of the preceding claims, in which the strips are preformed before use. 14. A method according to Claim 13, in which the strips are preformed into a castellated shape. 15. A method according to any of the preceding claims, in which the strips are anchored in the material of the structural element by providing an appropriate extra strip length beyond a bend around a structural element, or by securing ends of the strip together by metal clips, rivets or other fixing means. 16. A method according to any of the preceding claims, in which the strips are placed in position from one major surface of the structural element. 17. A method according to any of the preceding claims, in which the strips are totally enclosed within the structural element and are not connected to any exposed metal fixing. 18. A method according to any of the preceding claims, substantially as hereinbefore described with reference to the Examples. 19. A method according to any of the preceding claims, substantially as hereinbefore described. 20. A reinforced structural element having reinforcing members located adjacent its major surfaces, wherein shear reinforcement is provided by a plurality of thin elongate strips of high stiffness material disposed such that they anchor around one or more reinforcing members adjacent one major surface, and/or around one or more reinforcing members adjacent each major surface, such that the strips tie the structural element and improve its resistance to shear failure. 21. A reinforced structural element according to Claim 20, produced by a method according to any of Claims 1 to 19. 22. A reinforced structural element substantially as hereinbefore described with reference to and as illustrated in the accompanying Drawings. 23. A reinforced structural element substantially as hereinbefore described. 24. A reinforced structural element produced by a method according to any of Claims 1 to 19. 25. Formwork for a reinforced structural element having shear reinforcement substantially as herein before described.

Inventors: PILAKOUTAS KYPROS, PILAKOUTAS, KYPROS
Applicants: PILAKOUTAS KYPROS, UNIV SHEFFIELD, PILAKOUTAS, KYPROS, THE UNIVERSITY OF SHEFFIELD
Application year: 1996
Main classification: E04C5
IPC codes: E04B5, E04C5