Patent Publication Number: US-2010115860-A1

Title: Girder element for concrete formwork comprising a structure for automatically compensating bending strains

Description:
The invention relates to a girder element for a concrete formwork, in particular a ceiling girder element, having a girder for accepting external forces, a traction band, which runs behind the girder, and at least one length-adjustable tensioning device, which is situated between the girder and traction band, using which the local spacing between the girder and traction band is settable. 
     Such a girder element is known, for example, from the brochure “Column Hung System” of HI-LITE Systems, division of JASCO Sales Inc., Mississauga, Ontario, Canada, 2001. 
     Formwork technology is frequently used for manufacturing buildings. A building structure to be manufactured, such as a wall, column, or ceiling, has formwork elements built around it (“erecting formwork”) and is filled with liquid concrete. The concrete is subsequently permitted to harden. 
     The formwork elements, which have a formwork skin subjected directly to the liquid concrete, have a girder construction built behind them, which keeps the formwork elements in a desired position. A girder construction typically comprises a plurality of girder elements, which are partially fixedly connected to one another (longitudinal girders and crossbeams), and are partially fastened or supported on fixed structures (such as already finished building parts, for example, a story column or a story floor). 
     Girder elements can be deformed by the intrinsic weight of girder elements, and above all by the weight of further girder elements and liquid concrete resting or pressing thereon. Deformed girder structures fundamentally result in undesired dimensional deviations of the manufactured concrete structure from the desired concrete structure. 
     An example in this regard: during manufacturing of a story ceiling having an area of 10 m×10 m in formwork technology, weights of approximately 10 tons for the formwork and approximately 75 tons for liquid concrete typically occur. Upon fastening of the girder structure to four corner story columns, with typical girder elements, a sag of approximately 7 cm occurs under load. 
     Through a sufficiently large number, i.e., density, of fastening and support points, deformations in the girder construction may be reduced. However, a high density of support points is associated with a high outlay for work and material during construction of the formwork. Furthermore, adequately solid structures for supporting or fastening a girder construction are simply not available to a sufficient extent for many concrete structures to be manufactured. For example, during manufacturing of story ceilings, the manufacturing of the next higher story ceiling is often already to have begun before the story ceiling underneath is completely hardened; the next higher story ceiling and/or the girder construction thereof must then exclusively be fastened to already hardened story columns. 
     In order to be able to construct a desired concrete structure, even with a low density of fastening and support points of a girder construction, the most rigid possible girder elements are used. Girder elements for a ceiling formwork construction are known from the cited brochure “Column Hung System”, which have a framework-like frame having two parallel chords and partially inclined spokes running between the chords. The upper chord is used as the support for other girder elements and/or formwork girders and formwork elements. A traction band runs below the lower chord. A tensioning device is situated between the lower chord and the traction band (trussed framework girders). These known girder elements are very rigid, but are very heavy and are therefore difficult to handle on the construction site and are expensive to produce because of the large steel consumption. In addition, the deformation problems can be reduced, but not completely eliminated using these girder elements. 
     Attempts have also been made to initially calculate the bending deformation of girder elements under the expected load. The girder elements are prepared (for example, using cambering brackets) so that without load they have a curvature which is initially undesired for the structure to be manufactured, but a desirable shape for the concrete structure to be manufactured results under load. However, prior calculation of the bending deformation is time-consuming and difficult and must be performed for each concrete structure to be manufactured and also individually and newly for each girder element. A specially prepared girder element is typically only usable for a single concrete structure to be manufactured at a specific point of the girder construction. Furthermore, maintaining the desired ceiling thickness is difficult, because the formwork sag changes during the concrete casting procedure. 
     OBJECT OF THE INVENTION 
     It is the object of the present invention to propose a girder element with which arbitrary concrete structures may be exactly and cost-effectively manufactured. 
     BRIEF DESCRIPTION OF THE INVENTION 
     This object is achieved by a girder element of the type above-mentioned type which is characterized in that the length-adjustable tensioning device is implemented as a length-adjustable compensation device having a motorized drive for setting the length of the compensation device and a measuring unit is provided, by which the position of the girder in the area of the compensation device relative to a target position can be determined, and with a control unit by which the length of the compensation device can be regulated as a function of the position of the girder. 
     It is the fundamental idea of the present invention to also determine and set the shape of a girder element, and in particular the curvature of the girder of the girder element, through a length-adjustable compensation device. The length of the compensation device is selected in dependence on the external forces which act on the girder so that the desired shape (typically a linear shape) of the girder results. According to the invention, the length of the compensation device can also be set under load by the motorized drive, in particular readjusted. 
     The girder has a (typically linear) front side, upper which the external forces can act. For example, other girder elements or formwork girders rest or press on this front side. A traction band runs above the opposite side of the girder, typically slightly inclined with respect to the girder. The outer ends of the traction band and girder are typically connected to one another; the outer ends of the girder are typically also fastened on fixed structures. There is usually a further connection between the girder and traction band at the compensation device. The traction band is under tensile stress, while the girder and optionally further compression rods are under compressive stress. If an external force acts on the girder, which attempts to deform it (push-in), a spreading force can be exerted between the girder and the traction band using the compensation device, which keeps the girder in the present shape. The compensation device causes a redistribution of the elastic deformation in the girder element under load, namely from the girder to the traction band. 
     The deformation of the girder by external forces, such as the weight force of liquid concrete, and also by the intrinsic weight of the girder element, thus has a superimposed deformation of the girder by the compensation device. The shape of the girder is monitored in that a measured actual position of the girder is compared to a target position. The target position of the girder is predetermined absolutely by the concrete structure to be constructed; the actual position of the girder, in contrast, is a function of the applied load, the intrinsic weight of the girder element, and the length setting of the compensation device. A measuring unit is used to determine the position of the girder and relays its measuring results (typically in electronic form) to a control unit (also usually electronic). If the actual position of the girder deviates from the target position, a length change of the compensation device is activated to bring the girder closer to its target position. The girder can always be kept at the target position completely automatically by continuous regulation. 
     A desired shape of the girder of the girder element can be maintained at fundamentally arbitrary loads by setting the shape of the girder using a measuring and regulation system. Prior calculations of the load are not necessary (within a maximum strain of the girder element). The girder element does not have to be particularly rigid as a whole, because a deformation of the girder is prevented by the length-adjustable compensation device. The girder element according to the invention can thus be relatively light and have little material, whereby it is easy to handle during the construction and teardown of formwork. In comparison to typical girder elements, weight reductions of up to 50% may be achieved. 
     It is typically sufficient to describe the formwork deformation of a girder by the position of one measuring point of the girder and to compensate for the deformation of the girder using one compensation device (per girder element). However, multiple measuring points and/or multiple compensation devices per girder/girder element may also be provided within the context of the invention in order to increase the compensation precision. One measuring point is then preferably provided for each compensation device. A measuring point is preferably as close as possible on the girder/girder element to the associated compensation device. 
     An additional advantage of a girder element according to the invention is simplified stripping of formwork elements. The girder element can be lowered and/or retracted by actuating the compensation device (lowering the pressure). 
     It is typically sufficient within the context of the invention, if the length-adjustable compensation device can build up pressure (for example, can apply a spreading force between girder and traction band) in one direction (piston side), for example, a plunger cylinder can be used here. However, in specific cases, double-acting cylinders can also be useful, for example, for compensation of deformations of taller formwork in strong wind. 
     PREFERRED EMBODIMENTS OF THE INVENTION 
     In a preferred embodiment of the girder element according to the invention, the motorized drive comprises an electric drive, in particular a linear motor. Such a drive is simple and easy to maintain. 
     In an alternative embodiment, which is also preferred, the motorized drive comprises a hydraulic drive or a pneumatic drive. Particularly large forces may be applied using a hydraulic drive. Water and oil can preferably by used as hydraulic media. Water does not cause any damage on the construction site in the event of leaks. Typical high-pressure cleaning devices (and/or the assemblies thereof) can be used for providing and maintaining pressure with water because of the slow pace of deformation pathways during concrete casting. A joint provision of pressure for a plurality of compensation devices, in particular also of various girder elements, can be performed with hydraulic or pneumatic drives; the force regulation at the individual compensation devices is performed by locally controllable valves, either in the supply lines of the compensation devices or directly at the particular compensation device. 
     It is to be noted that the motorized drive can comprise a mechanical adjustment unit (for example, having gear wheels, spindles, or wedges), which is driven by the motorized drive. 
     An embodiment is also preferred in which the measuring unit comprises a spacer bar or a cable having a deflection roller and ballast or a laser distance meter. A spacer bar and a cable having ballast are very simple measuring units. A laser distance meter is particularly simple to install. 
     An embodiment of a girder element according to the invention in which the compensation device is length-adjustable in a direction which extends essentially perpendicular to the girder is particularly preferred. This ensures effective and uniform force introduction into the girder. 
     In another preferred embodiment, the girder element has fasteners for the girder element at two opposing ends of the girder, in particular for fastening on story columns. This embodiment is suitable above all for ceiling formwork, with greater distances (typically 7 m to 10 m) being spanned by a girder construction. In this embodiment, no further fastening or support points are provided except at the opposing ends of the girder. Alternatively, the fasteners may also be implemented for fastening on other girders. 
     An embodiment in which the girder is implemented as telescoping is very particularly preferred. The girder element can thus be set to a length to be spanned. 
     An embodiment which provides that the girder has fasteners for fastening a plurality of formwork girders on the girder is also preferred. This increases the safety of the overall construction. 
     In another advantageous embodiment, multiple length-adjustable compensation devices are provided, which are distributed over the length of the girder. A more precise compensation of a deformation of the girder can thus be achieved, in particular if an asymmetrical deformation (for example, with one-sided fastening of the girder element) is to be compensated for. Each compensation device preferably has a separate measuring and control unit. Furthermore, an essentially uniform distribution of the compensation devices over the girder element is preferred. 
     An embodiment is also advantageous in which a signal device is provided, which outputs a warning signal if a limiting value for the position of the girder is exceeded, in particular an acoustic, optical, or electronic warning signal. Through the warning signal, in the event of overload of the compensation device (i.e., deformation compensation is not possible or is no longer entirely possible), safety measures may be initiated. 
     The use of girder elements according to the invention for constructing concrete formwork, in particular for implementing ceiling formwork, is also within the context of the present invention. This concrete formwork has a high manufacturing precision and, in particular, can be used free of formwork deformation. Because girder elements of lighter construction may be used universally, the construction and teardown of the formwork according to the invention requires little work and is thus cost-effective. 
     A use of a girder element according to the invention for the continuous compensation of bending deformations of the girder, a changeable external force acting on the girder, in particular the external force on the girder being caused by the weight of concrete, is also within the context of the present invention. 
     Further advantages of the invention result from the description and the drawing. According to the invention, the above-mentioned features and the features listed hereafter may also be used individually or in multiples in arbitrary combinations. The embodiments shown and described are not to be understood as an exhaustive list, rather have exemplary character for description of the invention. 
    
    
     
       DRAWING AND DETAILED DESCRIPTION OF THE INVENTION 
       The invention is illustrated in the drawing and is explained in greater detail on the basis of embodiments. In the figures: 
         FIG. 1  shows a schematic side view of an embodiment of a girder element according to the invention for a ceiling formwork; 
         FIG. 2   a  shows a girder construction for a ceiling formwork having girder elements according to the invention, in a schematic perspective view; and 
         FIG. 2   b  shows the girder construction of  FIG. 2   a  with applied formwork girders and formwork elements, in a schematic perspective view. 
     
    
    
       FIG. 1  shows an embodiment of a girder element  1  according to the invention, which is used for the construction of a ceiling formwork (compare also  FIGS. 2   a ,  2   b  in this regard) and is fastened on two story columns  2 ,  3 . A level, horizontal ceiling is to be manufactured, for example. 
     The girder element  1  has an upper girder  4 , on the front side  14  of which (on top in  FIG. 1 ), further girder elements or also formwork girders or formwork elements may be laid and/or fastened (not shown). The girder  4  is preferably implemented as telescoping and comprises one or more steel profiles, for example. A traction band  5 , which is stretched in a V-shape, runs behind the girder  4  (below the girder  4  in  FIG. 1 ). The traction band  5  is fastened in the middle to a lower end of a length-adjustable compensation device  6 . The outer ends of the traction band  5  are connected via fasteners  7   a,    7   b  to the outer ends of the girder  4 . The traction band  5  (and/or each section of the traction band) is implemented as a wire cable or as a solid rod and/or as a steel pipe, for example. The fasteners  7   a,    7   b  are additionally also connected to one another using a pressure rod  8 . The pressure rod  8  can be implemented as a solid steel rod or as a steel pipe, for example (it is to be noted that the girder  4  itself can also or does also act as a pressure rod). Furthermore, the girder  4  is connected in the middle to the upper end of the compensation device  6  via a connection element  13  (e.g., rod, bar, plunger of a cylinder). The compensation device  6  is length-adjustable in the vertical direction by a force-driven drive in  FIG. 1 . 
     The girder element  1  is fixedly connected to the story columns  2 ,  3  via the fasteners  7   a,    7   b  (i.e., the outer ends of the girder  4  are also fixed under load). There is no direct fastening or support of the girder element  1  on an already manufactured story ceiling (i.e., the floor)  9 , for example, because the story ceiling  9  has not yet sufficiently hardened. In addition, the girder element  1  may be supported close to a story column  2 ,  3  via a support on the floor, because a premature strain of a story ceiling is possible in these areas. The girder element  1  spans the intermediate space between the story columns  2 ,  3 . 
     If external forces  10  are introduced into the girder  4  (from above in  FIG. 1 ), in particular by the weight force of formwork girders, formwork elements, and liquid concrete resting thereon, the middle of the girder  4  begins to sag downwardly. In other words, the spacing x between the girder  4  and the floor  9  decreases. 
     The distance x of the girder  4  to the floor  9  (i.e., the position of the girder  4 ) is monitored using a laser distance meter  11  and compared to a target distance xs in a control unit  12 . The target distance xs corresponds to the position of the non-deformed, linear girder  4 . The laser distance meter  11  is situated in an area of the compensation device  6 . 
     If the distance x falls below the target value xs, the control unit  12  orders an enlargement of the length L of the compensation device  6 . This results in lifting of the girder  4  and pressing down of the traction band  5  (more strongly or weakly depending on the modulus of elasticity of the traction band) in the middle area in proximity to the compensation device  6 . The girder  4  can thus be kept at a uniform level. 
     The position of the girder  4  is typically kept at the target position during the entire concrete casting and hardening of the ceiling to be manufactured, i.e., the distance x is kept at the target distance xs. The girder  4  thus always remains nearly linear, and the ceiling receives the desired, level shape. 
       FIGS. 2   a  and  2   b  illustrate the use of girder elements according to the invention in a girder construction for ceiling formwork for manufacturing a story ceiling. 
       FIG. 2   a  shows an already finished, but not yet completely hardened story ceiling (floor)  9 , from which four story columns  2 ,  3 ,  51 ,  52  project. The story columns  2 ,  3 ,  51 ,  52  are already completely hardened. 
     The new story ceiling to be manufactured is to be erected, the associated ceiling formwork only being fastened and/or supported on the story columns  2 ,  3 ,  51 ,  52 , but not on the floor  9 , which cannot yet be fully loaded. For this purpose, a girder construction having a total of five girder elements  53 - 57  according to the invention and a further girder  58  is used. 
     Only the two girder elements  53 ,  54  are fastened at the outer ends of their girders  4  to fixed structures, namely the story columns  51 ,  2  and/or  52 ,  3  (and thus with these outer ends always fixed, even under load). The fastening is performed using anchors in the particular column body and a support on a frame  62 , which encloses the particular column base. The girders  4  of the girder elements  53 ,  54  are also referred to as yoke girders. 
     According to the invention, the girders  4  of the three further girder elements  55 ,  56 ,  57  are laid (supported) and fastened on the yoke girders of the girder elements  53 ,  54 . The girders  4  of the girder elements  55 - 57  are also referred to as crossbeams; they are implemented as telescoping (i.e., changeable in their length). The girders  4  of the girder elements  53 ,  54  and  55 ,  56 ,  57  intersect at right angles. 
     According to the invention, each girder element  53 - 57  has a length-adjustable compensation device  6  with a motorized drive (not shown in greater detail) for setting the local spacing of a traction band  5  from the girder  4 , i.e., the distance (measured perpendicular to the girder  4 ) in the area of the compensation device  6 . The girders  4  and the traction bands  5  of each girder element  53 - 57  are fastened to one another in the middle via the particular compensation device  6 , a connection element  13  (bar, rod), and optionally (only for the girder elements  55 - 57 ) the installed, intersecting girder  58 . 
     A spacer rod  59  is provided at the lower end of each of the compensation devices  6  for measuring the position of the girder  4  of the particular girder element  53 - 57 . The spacing of the lower end of the length-adjustable compensation device  6  from the floor  9  is measured directly. Together with the current length setting of the compensation device  6  (and the dimension of connection element  13  and optionally the girder  58 ), the current position and thus the degree of sag of the particular girder  4  can thus be concluded. By adjusting the length of the compensation device  6 , the position of the girder  4  under load can be regulated separately on each girder element  53 - 57  (as described in greater detail under  FIG. 1 ). 
       FIG. 2   b  shows the girder construction of  FIG. 5   a  in a later stage of construction. Formwork girders  60  have been laid on the girders  4  of the girder elements  53 ,  54  and on the girder  58 . These formwork girders  60  have a slightly greater overall height than the adjacent girders  4  of the girder elements  55 - 57  (alternatively, the overall height of the girders  4  of the girder elements can also be equal to the overall height of the formwork girders  60 ). Ceiling formwork plates  61  (alternatively ceiling formwork elements having formwork skin directed upward) are situated on the formwork girders  60  and optionally the girders  4  of the girder elements  55 - 57 , on which liquid concrete is poured in the context of the concrete casting of the story ceiling to be erected. Only a part of the formwork elements  61  is shown in  FIG. 2   b  for simplification. 
     It is to be noted that the formwork area, defined by the story columns in  FIGS. 2   a ,  2   b , typically has an edge length of approximately 7 m to 10 m. 
     In summary, the present invention describes a compensation device for a girder or bolt, the sag of the girder being settable by the compensation device by tension against a traction band running behind the girder. The traction band is spaced apart from the girder in an area of the compensation device, the outer ends of the traction band engaging directly on the girder. Using the invention, a sag of the girder under intrinsic weight and/or external load can be counteracted. Load deformations of the girder which arise through the concrete load and through the intrinsic weight may be compensated for using the means according to the invention.