Abstract:
The invention concerns a reinforced concrete or prestressed concrete part stressed by shearing forces with layers of reinforcement ( 22, 24 ) provided at its upper and lower sides. For shear protection at least one sheet metal reinforcing part ( 30, 32, 34, 36 ) is provided between these layers of reinforcement which mainly extends at right angles to a surface of the reinforced concrete part and mainly over the entire distance between the layers of reinforcement ( 22, 24 ) and crosswise to at least one crack ( 50 ) occurring in the reinforced or prestressed concrete part under transverse load.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of German patent application number DE20001002383 20000120, publication date Jul. 26, 2001, which is herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention concerns a slab reinforcement with a reinforced concrete column and a slab part made of reinforced concrete or prestressed concrete. The invention further concerns a procedure for the fabrication of such slab reinforcements. 
     2. Description of the Related Art 
     Reinforced concrete or prestressed concrete parts, e.g. of a supported slab require shearing check in the form of shear reinforcement in the area of the columns and in other areas. 
     Known types of shear reinforcement include: shear reinforcement made of reinforcing steel in the form of S-shaped hooks or stirrups, “stud rails”, double-headed studs, stirrup mats, lattice beams, “Tobler” hip, “Geilinger” collar, “Riss” star. 
     Because of the poor anchorage, a shear reinforcement made of reinforcing steel in the form of S-shaped hooks or stirrups must embrace a mostly existing bending longitudinal reinforcement to prevent the shear reinforcement from tearing out. This is very expensive. In the case of high reinforcement ratios of the bending tensile reinforcement and a high shearing reinforcement ratio, conventional stirrups are regarded as unsuitable. 
     Stud rails are mostly placed onto the lower formwork, so that the lower layer of reinforcement is encompassed by its cross-section. Exact position and fixing of the rail is decisive for the load bearing performance. The stud rails are welded made-to-order pieces and therefore expensive. 
     Double-headed studs are usually threaded in from above between the upper and lower layers of the existing longitudinal bending reinforcement. In the case of high reinforcement ratios of the flexural tensile reinforcement and different mesh sizes of the upper and lower layers, this is very difficult and sometimes they cannot be installed. The double-headed studs are made to order and therefore expensive. 
     Stud rails and double-headed studs are very much used, but series production is not economical because of the high storage costs. Another problem is the danger of confusion and storage of different stud rails and double-headed studs on the construction site. 
     Tobler hip and collar are steel mounting parts consisting of steel sections welded together and made to order. The bearings structures are to be installed under steelworks conditions and are therefore expensive and labor-intensive. Due to their weight, the mounting parts need to be placed by means of cranes or other hoisting gear. 
     The functioning of all common solutions depends on concrete as a material. A look at the load paths (path of the shear forces) shows that the load is transferred in and out of the reinforcing elements several times until it reaches the non-critical area. Failure due to shear or compressive fracture, or tearing out of the reinforcing parts can occur. 
     Therefore, it is one of the objects of the invention, to provide a new slab/ceiling reinforcement and a method for its fabrication. 
     SUMMARY OF THE INVENTION 
     In accordance with a first characterizing feature of the invention, this objective is achieved by the subject matter of independent claim  1 . Because of the sheet metal reinforcing part, shear forces and moments can be absorbed and distributed better. If first cracks occur when the concrete&#39;s ultimate tensile strength is reached, the load can be distributed over the reinforcing part in a fan-like way. Participation of the concrete for the ties is not necessary. The loads are carried off directly via the reinforcing part in accordance with the principle of minimum deformation work. As a consequence, cracks due to shear forces remain small and the ultimate strength of the concrete part is maximized. The reinforcing part thus assumes the concrete&#39;s function when the concrete reaches its ultimate tensile strength. The reinforcing part encompasses the continuous bending reinforcement of the reinforced concrete column. In this way the punching shear reinforcement provides structural protection against cracking of the flat slab. A flexural reinforcement in the compression zone running over the reinforced-concrete column, as described in DE-A1-19741509, is thus not necessary. 
     To the best advantage, the invention is further developed in accordance with the characterizing features of claim 2, because the ultimate load of a reinforced concrete part can be improved in a simple way. Reinforced concrete part here always also means prestressed concrete part. 
     In accordance with another characterizing feature of the invention, the objective is achieved by the subject matter of claim 7. The shape allows easy installation of the reinforcing part between the upper and lower layers of the flexural reinforcement. Additional position guards are not required. Once the lower layer of reinforcement is installed, the reinforcing part is placed onto it and can thus serve as an additional spacer for the upper layer. 
     According to one aspect of the invention, there is provided a slab reinforcement comprising a reinforced-concrete column; a slab portion of reinforced concrete or prestressed concrete with an upper layer of reinforcement and a lower layer of reinforcement which transfers loads into the reinforced-concrete column; reinforcing elements provided in the reinforced-concrete column which penetrate the slab part; at least one sheet metal reinforcing part; and anchoring means to anchor the concrete. The at least one sheet metal reinforcing part encompasses a reinforcing element of the reinforced-concrete column and, starting from this reinforcing element, between the upper layer of reinforcement and the lower layer of reinforcement of the slab part, basically extends over the complete distance between these layers of reinforcement, and is essentially perpendicular to a surface of the slab part. 
     The sheet metal reinforcing part in horizontal projection may have the shape of a U, V, hairpin or similar. The sheet metal reinforcing part may be corrugated, bent in the shape of a hat or bent in the shape of a trapezoid. The sheet metal reinforcing part may be made of steel, or alternatively a carbon fiber material or a plastic or a composite material. 
     According to another aspect of the invention, there is provided a method of manufacture of a ceiling reinforcement with a reinforced-concrete column with reinforcing elements and a ceiling portion of reinforced steel or prestressed steel. The method comprises: placing a lower layer of reinforcement; placing at least one sheet metal reinforcing part for shear reinforcement onto the lower layer of reinforcement in such a way that it is mainly at right angles to it and encompasses a reinforcing element of the reinforced-concrete column; placing an upper layer of reinforcement onto this at least one sheet metal reinforcing part in such a way that the latter serves as a spacer between the lower and the upper layer of reinforcement; and pouring concrete over the portion formed of the lower layer of reinforcement, the at least one sheet metal reinforcing part and the upper layer of reinforcement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details and advantageous developments of the invention result from the embodiment described in the following and shown in the drawing and from the subordinate claims 
         FIG. 1  A vertical section of an embodiment of an arrangement in accordance with the invention, looked at along line I-I in  FIG. 2 . 
         FIG. 2  A horizontal projection, looked at in the direction of arrow II in  FIG. 1 . 
         FIG. 3  An enlarged representation of a detail of  FIG. 2 . 
         FIG. 4  A representation of the load paths in a sectional drawing analogous to  FIG. 1 . 
         FIG. 5  A representation of the ties and struts, likewise in a sectional drawing analogous to  FIG. 1 . 
         FIG. 6  An isometric drawing of a reinforcing part used in  FIG. 1 through 3 . 
         FIG. 7  A side view of a reinforcing part. 
         FIG. 8  A section, looked at along line VIII-VIII in  FIG. 7 . 
         FIG. 9  A section, looked at along line IX-IX in  FIG. 7 . 
         FIG. 10  A section, looked at along line X-X in  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a detail of a building with a vertical element (column or wall)  10  of reinforced concrete. In this vertical element  10  are reinforcing elements  12 ,  14  in the form of reinforcing bars. The bearing surface of column  10  is secured by means of steel stirrups  16 . 
     Connected to the vertical element  10  is a reinforced concrete slab  20 . (Alternatively this might be a beam system  20 .) Floor  20  has an upper reinforcement  22  and a lower reinforcement  24  with a concrete covering  26  and  28 , respectively, over each. Only part of floor  20  is shown. 
     Between the reinforcements  22  and  24  and preferably as spacers for these are sheet metal reinforcing parts which in  FIG. 1  are marked as  30  for the left part of the floor  20  and with  32  for the right part of the slab. In the preferred embodiment such a reinforcing part  30 ,  32  is V-shaped in horizontal projection, see  FIG. 2  where two additional reinforcements  34  and  36  are shown. Alternatively the shape could be that of a U or a hairpin. 
     The points of the reinforcements  30  and  32  each project into the border area of the vertical element  10  and encompass a reinforcing element  12 ,  14 , assigned to them, see  FIG. 1  and  FIG. 3 . Thus each sheet metal reinforcing part  30 ,  32  is horizontally anchored in the vertical element  10 , engaged in it and can transfer its vertical force component into the bearing area secured by the stirrups  16 . 
     The reinforcing parts  30 ,  32 ,  34 ,  36 , preferably are made of sheet steel, usually between 2 and 6 mm thick. The thickness depends on static requirements. If and when required, the reinforcing parts can also be made of carbon fibers, suitable plastics or a composite material. 
     The reinforcing parts  30 ,  32 ,  34 ,  36 , are thin and flat. For example, reinforcing part  32  stands on the lower reinforcement  24  which is located within the concrete floor  20 . The upper reinforcement  22  lies on reinforcing part  32  and is located in the upper concrete covering  26 . Reinforcing part  32  has recesses (holes)  40  in its upper border. It also has recesses  42  at its lower border area with diameters usually greater than 32 mm. The recesses  40 ,  42 , which could also be called openings, are preferably circular and in this embodiment are arranged vertically one above the other. When the concrete  29  is placed, concrete  29  extends through each of these recesses  40 ,  42 , forming “concrete dowels”, i.e. anchorages, which transfer the shear forces from the concrete  29  into the respective sheet metal reinforcing part  30 ,  32 ,  34 , or  36 . 
     Furthermore, the reinforcing elements  30 ,  32 ,  34 ,  36 , are preferably provided with beads  44  ( FIG. 8 ) in their middle section to improve anchoring in the concrete  29 . Also, the reinforcing elements preferably have recesses  46  at the upper border and recesses  48  at the lower border. This makes these borders look toothed. The recesses  46  and  48  improve the transfer of forces into the respective reinforcing element. 
       FIG. 1  also shows a shearing force Q acting on the slab  20  from the left and right sides. A counterforce F acts against these forces Q from below. Furthermore, a clockwise moment M acting on the right side and a counterclockwise moment M′ of the same amount acting on the left side, along with the forces mentioned, result in tensile and compressive stresses in the slab  20 . 
       FIG. 4  shows the load paths in a radial cut in the usual way of representation. The reference marks are the same as in  FIG. 1 through 3 .  50  identifies a zone in which one or more cracks occur in the concrete  29  under high load and where the floor  20  would usually break when the load becomes too high. In this case the surface of the fracture has roughly the shape of a funnel or cone, therefore the zone  50  is also called “punching shear cone”. It can be seen that a large number of load paths  52  exist which are at angles and sometimes roughly perpendicular to this zone  50  and thus act against fracture in this place. 
     The struts starting at the column  10  are compressive struts. They are anchored in the inner area of the “punching shear cone” at the upper concrete dowels, i.e. the concrete dowels in the recesses  40 . This is the load transfer into the sheet metal reinforcing part  32 . From this anchorage, the struts, as shown, only run in the sheet metal reinforcing part  32  and a shear field is formed which effects a planar load path in the reinforcing part  32  up to the non-critical area outside the zone  50 . 
       FIG. 5 , likewise in a usual way of representation, shows the ties and struts in a section. Here, too, it can be seen that the ties run at angles and roughly perpendicular to the zone  50 , i.e. at angles and sometimes perpendicular to the “punching shear cone” and that therefore they act against fracture in this place because there are many possibilities of anchoring in the area of the “concrete dowels” mentioned (at recesses  40 ,  42 ). If first cracks appear in the concrete  29  when the ultimate tensile strength is reached, the load is distributed to the “concrete dowels” over the entire sheet metal reinforcing part  32  in a fan-like way, as shown in  FIGS. 4 and 5 . Participation of the concrete  29  for the ties is not necessary. The loads are carried off directly via the sheet metal reinforcing element  30 , 32 , in accordance with the principle of minimum deformation work. As a consequence, the cracks  50  due to shear forces remain small and the ultimate strength of the slab  20  is maximized. 
     When the ultimate tensile strength of the concrete  29  in the tensile truss bars is reached, the sheet metal reinforcing part  32  assumes the function of the concrete. 
     If a rigid body mechanism is assumed in the ultimate load state, i.e. the remaining slab  20  is separated from the punching shear cone  50 , then the shear forces are exclusively transferred via the sheet metal reinforcing part  32 . Flexural and shear reinforcements are not decoupled. 
     When the ultimate limit state is reached, there should be early warnings that the arrangement shown is about to fail. The ductility of the sheet metal reinforcing part  30  and  32  is important for this, because in the case of such an arrangement, the shearing forces are transferred via the sheet metal reinforcing part  30 ,  32 . So, when the ultimate limit state is reached, the sheet metal reinforcing parts  30  and  32  will fail, which are preferably made of steel, and such failure is a ductile steel failure and not a non-ductile concrete failure in the form of a shear-compressive fracture, i.e. there are warning signs and the failure will not be sudden. This is also important with regard to earthquakes. 
     The behavior of the “concrete dowels” in the recesses  40 ,  42 , is sufficiently elastic and if one of them fails, the adjoining dowels will take up the load, i.e. the load is just relocated. The recesses  40 ,  42 , and the beads  44  support the concrete dowels in the anchoring of the inclined compressive struts. 
     Reinforcement bars can be placed through the recesses  40 ,  42 , and they can also be attached at these recesses by means of tie wire. This would be a further improvement. 
       FIG. 6  shows an isometric drawing of the reinforcing part  32  of  FIG. 1 through 3 . The same reference marks are used. 
       FIG. 7 ,  8 ,  9  and  10  show details of the embodiment in accordance with  FIG. 1 through 3  in different cutting planes. 
     Naturally, the invention presented allows a large number of variations and modifications. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.