Reinforcement for a concrete slab

Abstract: The invention relates to a reinforcement for a slab made of concrete or corresponding material for carrying point loads, e.g. reactions of abutment of a column (12). The reinforcement of the invention comprises a dome (8) to be cast in the slab, the dome essentially having the shape of a cone, a calotte or a polygon, and a bottom flange (9) formed as an integra part of the dome, the circumference of the bottom flange being larger than that of the load-carrying area (B) of the column.

Description: The invention relates to a reinforcement for a slab made of concrete or corresponding material for carrying point loads, e.g. reactions of abutment of a column. Reinforcements of this kind are known from DE-A-2,221,549 and consist of radially positioned, welded special profiles, which in this structure are subjected to both bending force and a shear resistance. Radial plate sections are welded to a strong pipe or a massive cylindrical block in the middle of the product. The punching resistance of a concrete slab to a point load, e.g. the reaction of abutment of a column, depends on the dimensions of the support and the slab and on the shear strength of the concrete. Punching resistance can be improved by conventional reinforcements but concrete standards set an upper limit to the calculatory strength of a shear reinforced structure. According to the standards currently applied in Finland, the upper limit is twice as high as the strength of an unreinforced structure. Elsewhere in Europe a smaller coefficient is used to determine the upper limit and in the Eurostandard (EC2) that is being established the limit is 1.6. The problem is that in conventional slab arrangements subjected to a point load, punching resistance is a factor that affects the dimensions, whereby it restricts the usefulness of the applications in question. Insufficient punching resistance of a slab to the reaction of abutment of a column causes a brittle fracture about the column in the slab, whereby the slab may collapse. For architectural and spatial reasons, thin column dimensions are desirable in buildings, wherefore a slab needs to be reinforced against punching. A prior art solution is based on steel structures cast in the slab, whereby the critical cross-section of the slab can be enlarged sufficiently and, correspondingly, the shearing stress of concrete can be reduced. The solutions used comprise a plurality of steel parts that are joined to one another usually by welding to provide e.g. a grate-like structure. Thus, the known slab reinforcements are relatively heavy, laborious to manufacture and expensive. Another known solution is to provide more supporting surface by shaping the upper end of the column to have the form of a mushroom by means of a special casting mould. The problem with this solution is the unpopular conspicuous shape of the column, which also complicates the use of space at least in respect of the adjoining structures and installations. Yet another known solution is to use stronger concrete in the critical area than elsewhere in the slab. The solution is not favoured in the construction sites since it slows down the casting and brings about additional costs of quality control. The object of the invention is to manufacture a reinforcement for a slab made of concrete or corresponding material by which reinforcement the disadvantages can be eliminated. In order that this might be achieved, the reinforcement of the invention is characterised in that it comprises a steel dome to be cast in the slab, the dome essentially having the shape of a cone, a calotte or a polygon, and a bottom flange formed as an integral part of the dome, the circumference of the bottom flange being larger than that of the loaded area or the column. The column reinforcement for a concrete slab according to the invention can be manufactured e.g. by deep drawing from one moderately thin steel plate with constant thickness, whereby the reinforcement weighs clearly less than the conventional solutions and can be manufactured easily without expensive machine tools or manual labour. Furthermore, the reinforcement according to the invention can be used with columns of all types, e.g. precast, composite and concrete columns. The other advantageous embodiments of the invention are characterised by what is stated in the attached claims. In the following, the invention is described by examples with reference to the attached drawings, wherein Fig. 1 shows an embodiment of the reinforcement according to the invention, Fig. 2 shows a known reinforcement, Fig. 3 shows another embodiment of the reinforcement according to the invention, Fig. 4 shows a cross-sectional view of a reinforcement according to the invention embedded in a concrete slab. Fig. 1 shows a reinforcement for a concrete slab according to the invention, the reinforcement being made by deep drawing from a single moderately thin high-tensile steel plate with unvarying thickness such that the reinforcement has the form of e.g. a cone or a calotte. The light weight of the reinforcement is due to the fact that the internal forces of the reinforcement are primarily sheet or annular forces that cause tension only in the direction of the plane of the plate. Deep drawing is cold working, which enhances the strength of the material. The idea of the invention is that the area of the surface of the punch-through cone (cf. Fig. 4) is enlarged by the reinforcement according to the invention, whereby the resistance of the slab against this kind of stress is improved. The reinforcement comprises a dome-shaped part 1 seamlessly joining to an octagonal bottom flange 2. Advantageously an essentially vertical zone E is provided between the dome 1 and bottom flange 2 of the reinforcement, the zone forwarding the horizontal compression forces caused by the bending moments that the slab is exposed to. In the upper part of the reinforcement is a hole 4 for concrete casting and lead-through of the iron mountings. In addition, holes 3 can be made onto the side surface of the reinforcement for the lead-through and to facilitate the inlet of the concrete casting into the reinforcement. It has been found that with the reinforcement, the punching resistance of a slab can be improved well over the upper limit set by the concrete standards. On account of its lightness, the reinforcement can be fitted without using power engines; the reinforcement according to the figure weighs about 70 kg if it is made e.g. of a 10 mm steel plate and its diameter is 1100 mm. E.g. with a 280 mm column, the calculatory design punching resistance of a concrete slab with a thickness of 250 mm (K35-1) reinforced with this kind of reinforcement is about 1 MN. Without a reinforcement the punching resistance of this kind of slab is less than 0.4 MN. A corresponding prior art reinforcement according to Fig. 2 that is made of welded I-shaped beams 5, 6 and 7 weighs more than twice as much. In Fig. 3, which shows another embodiment of the reinforcement according to the invention, the reinforcement comprises a polygonal part 8 and an adjoining bottom flange 9. Here the dome 8 is made of planar surfaces forming a polygon with e.g. eight faces. The bottom flange 9 is shown as being circular unlike in Fig. 1 where it is octagonal only to illustrate different possibilities of forming reinforcements according to the invention. Even here an essentially vertical zone F is provided between the dome 8 and the bottom flange 9 of the reinforcement, the zone allowing the horizontal compression forces of the slab to pass through without that the reinforcement is inclined to slide in relation to the slab on account of the vertical force components. In the upper part of the reinforcement is a through hole 11 for the casting and the iron mountings, and through holes 10. Through holes may be provided e.g. for electric cables or water-supply pipes. Here the holes are triangular with rounded ends, which is advantageous to the strength of the reinforcement. The appropriate shape of the dome of the reinforcement (cone, calotte, or polygon) and the size, shape and distribution of the possible holes vary and they are selected on the basis of the structure, the lead-throughs needed, etc. Fig. 4 shows a cross-sectional view of a reinforcement according to the invention embedded in a concrete slab 14 in an extended column structure 13a, 13b. At the column there is a reinforcement 15 according to the invention and ribbed bar iron mountings 16 for the column passing through the centre hole of the reinforcement 15. Without the reinforcement 15 the column 13a is inclined to push itself through the slab 14 such that the slab fractures along the conical shearing surface C (dashed lines). The angle of the surface of fracture in relation to the horizontal direction has in practice proved to be close to 30°. With a reinforcement according to the invention, which comprises a bottom flange having a larger circumference than that of the load-carrying area B of the column 13a, the conical shearing surface of the slab (cf. dashed lines D) moves beyond the reinforcement. This eliminates the risk of punching since the area of the surface of the punch-through cone is larger than the original and therefore its shear strength is also greater. The conventional design standards of a concrete slab apply outside the range of the reinforcement. It is obvious to one skilled in the art that the different embodiments of the invention are not limited to the above-mentioned examples but that the embodiments of the invention may vary within the scope of the attached claims.

Claims: A reinforcement (15) for a slab (14) made of concrete or corresponding material for carrying point loads, e.g. reactions of abutment of a column (12, 13a), characterised in that the reinforcement (15) comprises a steel dome (1, 8) to be cast in the slab, the dome essentially having the shape of a cone, a calotte or a polygon, and a bottom flange (2, 9) formed as an integral part of the dome, the circumference of the bottom flange being larger than that of the loaded area (B) or the column. A reinforcement according to claim 1, characterised in that between the dome (1, 8) and the bottom flange (2, 9) of the reinforcement is provided a cylindrical essentially vertical zone (E, F). A reinforcement according to claim 1 or 2, characterised by a lead-through and casting hole (4, 11) in the upper part of the dome (1, 8) of the reinforcement. A reinforcement according to claim 1, 2 or 3, characterised by holes (3, 10) made into the side surface of the dome (1, 8) of the reinforcement. A reinforcement according to any one of claims 1 to 4, characterised in that it is made of high-tensile structural steel by deep drawing.

Inventors: ALANDER CASPER, ALANDER, CASPER
Applicants: FUNDIA BETONITERAEKSET OY AB
Application year: 1993
Main classification: E04B5
IPC codes: E04C5, E04B5