Field of the invention

The present invention relates generally to a novel building structure including a novel type of building element. In particular, the invention preferably relates to building structures wherein adjoining precast concrete floor elements are interconnected and supported by peripherally located columns spaced apart by a distance which may be greater than a width of one of the building elements.

Description of the prior art

The floors of concrete building structures are generally expected to cover as large areas as possible with a minimum of structural height, to possess proper load ca- pacity, to be economical as regards material consumption, to exhibit low deformations and deflections under working loads, and to be adapted for prefabrication.

The floor constructions of known prefabricated building structures that cover large areas typically consist of main beams that join supporting columns, as well as floor elements extending perpendicularly to the beams. One such example is disclosed in US 4 366 655 where several cross-ribbed floor elements are joined by tensioning cables embedded in ribs running at a distance from the sides of the elements. Still, it is a problem with the prior art floor elements and building structures including such floor elements that the design freedom is rather limited in requiring sup- porting beams or supporting columns located below each floor element.

US 2005/1 15195 A discloses a prestressed or post-tensioned composite structural system for bridge floors etc. The structural system comprises a composite structure comprising an unfilled grating as a base component, and a prestressed, post- tensioned reinforced concrete slab as a top component. The base grating component preferably constitutes a plurality of main bearing bars without any distribution bars or tertiary bars. The upper portions of the main bearing bars are embedded in the concrete component permitting horizontal shear transfer and creating a composite deck structure which maximizes the use of tensile strength of steel and the compressive strength of concrete.

GB 1402259 A disclose post-tensioned concrete floors.

Summary of the invention

The invention permits the construction of building structures with floor building elements spanning relatively large distances and with relatively few supporting structures, with some floor building elements supported by adjoining ones and with an imaginary or virtual beam being defined by an integral edge-portion of the building elements and extending along the full distance between supporting columns.

According to the invention each floor element, though being adapted to be fabri- cated singly and each forming a load bearing unit when mounted, permit spanning over large distances by being capable of being restrained against rotation by the supporting structure along at least one of their sides, along which a material portion forms the aforementioned virtual beam.

Specifically, the above object is achieved in that the concrete material building element is slab-shaped and has an upper face and a lower face, a first side defining a first extension of the building element and a second side defining a second extension of the building element, elongated metal torsion reinforcement for limiting or preventing concrete cracking due to torsion being at the first side, embedded in the concrete material to extend along the first side. Elongated metal tension reinforcement for resisting or preventing concrete cracking due to tension is also embedded in the concrete material, transversally to extend inwards into the building element from the first side, and a duct suitable for receiving a tendon for post- tensioning the building element is embedded in the concrete material at the first side to extend along the length thereof. Preferred embodiments are defined in the dependent claims; in particular, shear connectors may be provided along the second side, and also along a third side opposite the second side, to allow for interconnection of adjoining building elements, i.e. to transfer vertical forces between adjoining elements, in particular to transfer a turning couple in the area of the virtual beam. Even where the building element has no shear connector structure along the side the building element may be used by itself, supported only at corners thereof, with a post-tensioned tendon preventing cracking due to tension in the direction of the first side of the building element.

Conveniently, the building element may be a cross-ribbed structure, possibly having a light-weigth material, such as light-weight concrete, filling open recesses at the lower side thereof, with the aforementioned reinforcement being located embedded in a rib defining the first side of the element. Conveniently, a cage structure may form the metal torsion structure, which may be integral with the transverse tension reinforcement, allowing for simple procedures when casting the building element. Prestressing tendons running perpendicular to the first side may be included as the tension reinforcement, to introduce compressive stresses in the building element thereby resisting or preventing concrete cracking when the building element is subjected to vertical loads in use. Preferably, the concrete material is molded around the prestressing tendons which then apply compressive stresses to the concrete by friction between the tendons and the concrete material.

Description of the drawings

A presently preferred embodiment of the invention will now be described with reference to the appended drawings; the following description is not intended to limit the scope of the present invention which is defined by the claims.

Fig. 1 a shows a partial, schematic and perspective view of a precast slab-shaped concrete material building element,

Fig. 1 b shows a part of a reinforcement for the concrete material,

Fig. 1 c shows an embodiment of the building element with cross-ribs,

Fig. 2a shows another partial, schematic and perspective view of the building element of fig. 1 a, Fig. 2b is a partial end view of two adjoining building elements as shown in fig. 1 a, Fig. 3 shows a specially designed building structure incorporating the building elements of fig. 1 a, and

Fig. 4a and 4b show a completed building structure according to the invention.

Fig. 1 a shows a partial, schematic and perspective view of a precast slab-shaped concrete material building element 10. A normal use of the building element 10 would be as a horizontal floor element or as a roof element, in a building structure comprising a plurality of such precast concrete elements. The shown embodiment of the building element 10 of the invention has an upper face 1 1 and a lower face 12, a straight first side 14 defining a first extension of the building element 10 and a straight second side 18 perpendicular thereto defining a second extension of the building element 10.

Opposite the first and second sides 14, 18 are third and fourth sides 15, 16 which, however, in other embodiments may not be parallel with the first and second sides 14, 18; the building element 10 may by way of example also have a triangular shape, with the first side 14 defining the base of the triangle. Also, the first and second sides 14, 18 may not extend along straight lines nor extend perpendicularly to each other. Often, where the building element 10 is rectangular, the first side 14 will define a width W of the building element while the second side 18 will define the relatively longer length L of the building element 10, the length L corre- sponding to a span between supporting structures, as discussed further below. The thickness D of the building element, i.e. the distance between the upper and lower face 1 1 , 12, may be in the order of eg. 18cm-28cm.

In fig. 1 a are also shown various components of the building element 10, depicted in solid lines although it will be understood that they are embedded within the con- crete material mass C forming the building element 10 and, hence, generally are not visible. An imaginary vertical plane 80' delimits an end portion of the building element 10, generally identified by numeral 80, of a width such as in the order of eg. 40cm-80cm and a height in the order of eg. 16cm-28cm and extending along the full width W of the building element 10. The end portion 80 may for the pur- pose of the following discussion be thought of as a virtual beam which is an integral part of the building element 10; in some embodiments the end portion 80 may assume a well-defined rib-like geometrical form, such as by defining the outermost rib of a cross-ribbed building element with further ribs parallel with the end portion 80 as well as secondary ribs 90 perpendicular thereto. Such a cross-ribbed con- figuration is illustrated in fig. 1 c, which also shows that the building element 10 as such may be prestressed by prestressing tendons 95 embedded in the concrete material to introduce compressive stresses in the precast building element 10 in the direction of the length L. The concrete element 10 may have a plurality of recesses formed in the lower face 12, optionally including blocks of a light-weight material 300 as shown in fig. 1 c. The blocks of light-weight material 300 may be of a light-weight concrete material bonded to the concrete material C defining the ribs and the upper face 1 1 .

Among the aforementioned components embedded in the end portion 80 is an elongated torsion reinforcement 30, normally of metal, for limiting or preventing concrete cracking due to torsion and extending along the length of the first side 14. An elongated tension reinforcement bar 40, normally of metal, for resisting or preventing concrete cracking due to tension is also embedded in the solid concrete material mass C in the end portion 80, so as to extend across the imaginary plane 80' away from the torsion reinforcement 30, normally to a distance of 2%- 10% of the length L from the first side 14. The tension reinforcement 40 is arranged closer to the upper face 1 1 since this is the area where tensile stresses are expected in a specially designed building structure, also to be discussed below, wherein the building element 10 may find primary use. The aforementioned prestressing tendons 95 extending along the full length L of the building element 10 may as such constitute the elongated tension reinforcement, supplementing or replacing shorter length plain and non-prestressed bars 40 illustrated in fig. 1 a.

Preferably, the torsion reinforcement 30 is a preformed cage-structure with stirrups 32 and longitudinal bars 34, and the metal tension reinforcement 30 is integral with the stirrups 32, as shown in fig. 1 b. In another embodiment the metal tension rein- forcement may comprise elongated bars 40 separate from the stirrups 32.

Also, a duct 50 suitable for receiving a tendon for post-tensioning the building element 10 is embedded in the concrete material mass C in the end portion 80 near the first side 14; the duct 50 extends along the width W of the building element 10, opening up at the second side 18 and at the fourth side 15 opposite the second side 18. The opposite duct 50 openings will typically be the only outside indication of the various components embedded in the concrete material C.

Referring now to fig. 2a it will be seen that the building element 10 also has a shear connector structure 60 formed at least on the second side 18, at the end portion 80; normally there will be a similar shear connector structure 60' located opposite, on the fourth side 15. The shown shear connector structure 60 includes a pair of shear keys 60' formed as recesses in the concrete material C and located on a respective side of the duct 50 opening. The recesses may also be defined by embedded open-ended metal boxes connected to the end of the longitudinal bars 34 of the cage-structure 30. Often, there will be similar shear keys 60, 60' formed along the full length L of the building element 10. Fig. 2b is a partial end view of two adjoining building elements 10, 10' of the aforementioned type, placed with the shear keys 60, 60' opposite each other and filled with grout or cement G such that shear forces will be transferred from one element 10 to the other 10' via this shear connector structure 60, 60'. By way of example, a torsional moment M illus- trated in fig. 2a may be balanced by an opposite couple arising from vertical forces in the shear-keys 60' at the end portion 80.

Shown in fig. 2a is also a through-going passage 19 formed in the building element 10 and extending from the upper face 1 1 to the lower face 12, in the area of end portion 80. Turning now to fig. 3 showing an exemplary specially designed building structure 1 incorporating the aforementioned building elements 10, a basic principle thereof is to establish a frame-like structure with rigid joints resisting or limiting any free load- induced rotation of the building elements 10 at their ends while at the same time allowing for some of the building elements to be non-supported at their ends, i.e. along their first sides 14. Through their design the building elements 10 allow load- generated tensile stresses near the upper face 1 1 to be resisted by the tension reinforcement 40, and torsional moments resisting or restricting the aforementioned free rotation to be transferred from one building element 10 to a neighboring building element 10' via the shear key 60, 60', concrete cracking due to tor- sional moments M in the end portion 80 being resisted by the torsion reinforcement 30. The duct 50 is used for receiving a stretched tendon 52 to hold adjoining building elements 10, 10' together by a compressive force, thereby maintaining the effectiveness of the interconnection between neighboring building elements 10, 10' by the shear connectors 60. Fig. 3 shows a first step in a process for making the exemplary building structure 1 according to the invention, using a supporting structure comprising on the one hand two columns 5, 5' and on the other hand an opposite wall 8. Shown in this example are three building elements, namely a middle one 10 and two adjoining outermost ones 10', 10". Columns may replace the wall 8, and the building ele- ments 10, 10', 10" may also at their right hand or upper end as shown in fig. 3 be supported in the manner discussed below where rotation is restricted or limited. For the purpose of the present discussion it is assumed that load-induced turning moments M _T are only transferred between the two outermost building elements 10", 10' and the two supporting columns 5, 5'.

In the first step the supporting structure 5, 5', 8 is established and a temporary beam support T is arranged to span between the two columns 5, 5'. Next, using a crane the building elements 10, 10', 10" are lowered in correct position onto the supporting structure and the temporary beam support T next to each other and such that there is an alignment of the respective duct 50 of each building element 10, 10', 10". The process also involves establishing a rigid, such as monolithic lo- cal, connection between the outermost building elements 10', 10" and their supporting columns; in the example this is done by passing an upwardly projecting part of the columns 5, 5', such as bolts 4, through the through-going passages 19 in the building elements 10', 10" and then filling the through-going passages 19 with cement. Alternatively, a separate clamping structure may be used; clamping may even arise following a subsequent positioning or casting on the building elements 10', 10" of a superposed column or wall structure above those areas where the building elements 10', 10" are locally supported. Generally, the purpose of this rigid connection is to establish a balancing of turning moments M _T at the level of the end portion 80 transferred from the middle building element 10 to the outer- most building elements 10', 10" and then to the columns 5, 5'.

Next, a tendon 52 is passed through the aligned ducts 50, tension is applied to stretch the tendon 50 in the aligned ducts 50, and the stretched tendon 52 is then anchored to the building elements, possibly to the outermost building elements 10', 10" only, by suitable anchors or even by grouting the ducts 50, thereby trans- ferring prestressing forces to the concrete of the building elements 10, 10', 10" at the level of the aligned end portions 80, as shown in fig. 4a and 4b. Before or after this step the building elements 10, 10', 10" are interconnected via the shear connector structures 60, such as in the manner discussed with reference to fig. 2b. The temporary support T is then removed to finalize the building structure 1 . Fig. 4b shows a completed building structure 1 of the invention incorporating building elements as described above, with curved first sides 14.

It is noted that the concrete elements 10, 10 ^' , 10" discussed above may be modified to include also along any of the other sides 15, 16, 18 reinforcement and a duct as discussed for the first side 14, to allow for the building element to be sup- ported in a manner similar to that described above for the first side 14. By way of example, in some cases it may be relevant to allow for the third side 16 opposite the first side 14 to also be supported in a manner resisting rotation.