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BACKGROUND 
       [0001]    The present invention relates to structural engineering and more specifically to a structural member capable of use in the construction of structures such as concrete slabs and elements. The invention further relates to methods of use of the structural member including its application in preparatory formwork and as an element in a composite structure including in layered concrete. The invention also relates to structures which employ the structural member. The invention further relates to lightweight structural members used in a variety of concrete structures. 
       PRIOR ART 
       [0002]    Slab beam concrete constructions is widely used in civil and structural engineering. The typical structure will comprise columns of a size dictated by applied such as dead loads and self weight and live loads and of a spacing dictated by loadings and slab spans. Reinforced and pre stressed concrete floors for buildings are generally made in one of two methods. The floors are either cast in situ using supporting temporary formwork, or are formed from pre-cast concrete planks supported on beams or walls which are then typically covered in a relatively thin in situ layer of concrete. Most major construction work of concrete buildings typically relies on the first cast in situ method in which formwork is constructed as a temporary support for structural reinforcing steel over which is poured structural concrete. Cast in situ reinforced concrete floors, require extensive formwork, are relatively time consuming and labour intensive particularly with respect to the assembly and dismantling of formwork and the time required for the in situ concrete to achieve the required strength. Existing construction systems using pre-cast elements have significant cost and other disadvantages including poor underside surface finish, ribbed profiles on the underside necessitating separate ceilings in many applications, difficulty of running services through and lack of flexibility of the type of structures which can be built. These disadvantages render in situ casting construction the preferred method of slab and floor construction. 
         [0003]    There are currently three different types of pre-cast floor systems which are in common use in the building industry. 
         [0004]    The first system often known as “Hollow Core” relies on the use of extruded, pre-tensioned, concrete generally rectangular planks which include a series of cylindrical holes or voids extending longitudinally along the plank. The planks are laid on the top of beams or walls and concrete is laid in situ over the top of the planks. This construction system can span relatively long distances, but has the disadvantage that it typically has a very poor surface finish on the underside necessitating in many applications a false ceiling or cladding over the concrete finish. The structural planks are typically produced in quite narrow strips requiring many joints and it is difficult to put services through the floor, as it is very difficult to access the voids. Also the planks are relatively thick and the services typically have to be either hung on the underside, also necessitating false ceilings in some applications or the services may be hidden in thick topping concrete. 
         [0005]    A second type of system commonly known as “Ultrafloor employs pre-cast ribs in the shape of an inverted T and which are supported on walls or beams, and these ribs support a thin fibre cement panel such as a “Hardie panel” or the like extending between the ribs. A reinforced concrete floor is then laid over the ribs and panel. This floor system produces a ribbed soffit which necessitates the provision of a cover ceiling in most applications, but it does have the advantage that it is relatively easy to run services through, prior to casting the in situ layer. A further disadvantage of Ultrafloor is that it has a limited span both during concrete pouring and as a finished floor. Ultrafloor is limited in the types of floor structure which can be made from the basic panel and from the panel used in conjunction with the shell beam. 
         [0006]    Another prior art system of floor construction is known as “Transfloor” in which a relatively thin 50 mm thick plank of concrete includes longitudinally extending steel reinforcing bars in triangular arrangements of groups of three, with one bar forming an ‘apex’ of the triangle spaced above the upper surface of the concrete plank and joined to the other two bars with steel rods. The planks are placed on top of walls or beams and void/void formers are placed on the concrete plank between the reinforcing and a concrete layer floor laid in situ on top of the plank. In the structural engineering industry the term void typically means an absence of concrete rather than an absence of material. 
         [0007]    Void formers are most commonly formed by polystyrene blocks although other non cementitious materials such as pipe clay or the like can be used to form voids in concrete members. This system has the disadvantage that it is limited to relatively short spans of about 7 m or so. Also there is a requirement to support the floor with props and bearers at relatively close spacings of between 2 and 4 m while insitu concrete is being poured and is gaining strength. Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. 
       INVENTION 
       [0008]    The present invention seeks to provide an improved plank for use in forming concrete flooring which addresses and at least partially alleviates some of the problems of the prior art assemblies as discussed above. 
         [0009]    The present invention provides a structural member capable of use in the construction of structures such as floor assemblies, concrete slabs and structural elements. The invention further provides methods of uses for the structural member including its application in preparatory formwork and as an element in a composite structure including in situ layered concrete. The invention also provides structures which employ lightweight structural concrete members. 
         [0010]    In a first aspect of the present invention, there is provided; 
         [0011]    a pre-formed structural element for use in forming a concrete floor of a building or the like, the plank comprising: a generally planar base portion; and a series of formations extending from the base portion, defining voids there between and wherein the upper portion of the formation is generally thicker than the lower portion of the formations at the junction with the base. 
         [0012]    Preferably the element is manufactured from concrete cast in a mould and the formations are generally parallel spaced apart ribs. The formations have sides which are inclined relative to the planar base. 
         [0013]    In its broadest form the present invention comprises: 
         [0014]    a generally elongated pre cast structural element for use in the construction of a composite floor and beam slab construction, the structural element comprising; 
         [0015]    a base and an upper surface, 
         [0016]    at least one formation extending from the upper surface and including a plateau and side walls each defining, with an opposing side wall of an adjacent formation, a recess; the side walls disposed for at least part of their length at an angle other than normal to the to the upper surface. 
         [0017]    In another broad form the present invention comprises: 
         [0018]    a generally elongated pre cast structural element for use in the construction of a composite floor and beam slab construction, the structural element comprising; 
         [0019]    a base and an opposing upper surface, 
         [0020]    at least two spaced apart formations extending from the upper surface and including a plateau and side walls each defining, with an opposing side wall of an adjacent formation, a tapered recess. 
         [0021]    According to a preferred embodiment the tapered recess has a wide portion at the upper surface of the base of the structural element and a narrow portion at or near the plateau of the formations. 
         [0022]    Preferably the formations form longitudinal ribs along the length of the element. 
         [0023]    Preferably each longitudinal rib is parallel to each other rib with even spacing therebetween. In an alternative embodiment, the element may be tapered along its longitudinal axis such that the ribbed formations converge in the direction of one end and diverge in the direction of an opposite end. This embodiment might be used in a case where the elements are placed in a horizontal curve. Preferably, each rib includes an outward taper such that the plateau of the formation is wider than a junction between the formation and the upper surface of the element. In one embodiment, the taper extends from the plateau of each formation at least part way towards the junction between the formation and the upper surface of the element. In another embodiment, the taper extends the full distance from the plateau to the upper surface of the base of element. In another embodiment the taper is terminated short of the plateau. In a further embodiment there is provided a shoulder associated with the plateau which receives a cover over the recess thereby maintaining a void space in the element. 
         [0024]    Preferably each formation has a generally dovetail geometry with a narrow portion at the junction between the upper surface of the element and the formation tapering out to a wide portion at the plateau. 
         [0025]    In another broad form the invention comprises: a construction system using a generally elongated pre cast structural element comprising; 
         [0026]    a base having a lower underside surface and an opposing upper surface, at least two spaced apart formations extending from the upper surface and including a plateau and side walls each defining, with an opposing side wall of an adjacent formation, a tapered recess; wherein the system employs at least one said elements as part of a composite concrete slab, wherein the slab is formed by said at least one element and an overlay layer which abuts said plateau of each said formations. 
         [0027]    In another broad form the present invention comprises: 
         [0028]    a composite structural floor comprising; 
         [0029]    at least one pre cast structural element having 
         [0030]    a base having an underside surface and an opposing upper surface, 
         [0031]    at least two spaced apart formations extending from the upper surface and each including a plateau and side walls each defining, with an opposing side wall of an adjacent formation, a recess, the side walls disposed at an angle other than normal to the upper surface of the base of the element; 
         [0032]    an overlay layer which engages the at least one element via the formations. 
         [0033]    In another broad form the present invention comprises: 
         [0034]    a composite structural floor comprising; 
         [0035]    at least one pre cast structural element having 
         [0036]    a base having an underside surface and an opposing upper surface, 
         [0037]    at least one spaced apart formation extending from the upper surface and each including a plateau and side walls each defining, a recess, the side walls disposed at an angle other than normal to the upper surface of the base of the element; 
         [0038]    an overlay layer which engages the at least one element via the at least one formation. 
         [0039]    According to one embodiment when two elements having one formation are abutted the upper surfaces and adjacent walls of each element combine to define a void recess which receives either a void former or overlay concrete. 
         [0040]    According to one embodiment, the overlay layer spans between the plateaus of each said formations closing said recess thereby forming voids in said slab. The system is preferably used in the construction of a composite suspended beam and floor slab assembly. The voids improve the structural performance of the element both during construction carrying wet concrete and in the permanent composite structure. They also provide through passages for services. Preferably, the structural elements are formed in a mould which includes a steel base which imparts a smooth high quality surface finish to the element soffit. The voids reduce the weight of the element. The structural geometry of the formations allow more efficient use of concrete in that the so formed composite has a large compression flange at the top of the formations imparting to the composite a high strength to weight ratio for a given span. The elements may therefore be much thinner for a given span than a prior art conventional slab. In one embodiment the side walls of the formations are generally planar and are inclined at an angle less than 90 degrees and around 40° to 70° to the upper surface of the element. The structural element has a versatility allowing the voids to be filled with polystyrene, cement or concrete Alternatively the voids may be retained with empty spaces. 
         [0041]    The base portion is preferably reinforced with fabric or steel rods and/or reinforcing fibres and may be pre-stressed respectively by pre tensioning or post-tensioning. Alternatively, the element may be non stressed. In one embodiment the top of the formations receive and support a sheet of material. In use, the building slab elements may be supported on walls or transverse beams arranged to define a floor. Gaps between adjacent like elements are sealed with part of a composite layer. The void spaces in each element are sealed and an in situ layer is poured over the plateaus of each formation. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0042]    The present invention will now be described according to a preferred but non limiting embodiments and with reference to the accompanying illustrations in which: 
           [0043]      FIG. 1  shows an end view of a pre-cast concrete element incorporating an intermediate formation and adjacent voids filled with a polystyrene filler. 
           [0044]      FIG. 2  shows an end view of a pre-cast concrete element of  FIG. 1  incorporating an intermediate formation and adjacent empty voids typically used as a spine beam. 
           [0045]      FIG. 3  shows an isometric view of the spine beam of  FIG. 2 ; 
           [0046]      FIG. 4  shows a cross sectional view of a composite beam including the element of  FIG. 1  and including an overlay layer. 
           [0047]      FIG. 5  shows a cross section of a composite beam comprising an abbreviated element supporting associated elements and an overlay layer disposed over formation plateaus and associated elements. 
           [0048]      FIG. 6  shows a perspective view of a banded beam flooring system having two columns with drop panels and two columns without drop panels and including elements arranged for co operation with support columns. An arrangement of temporary propping for this floor system is also shown. 
           [0049]      FIG. 7  shows an enlarged abbreviated end view of a portion of a pre-cast concrete element with alternative formation geometry including shoulders. 
           [0050]      FIG. 7   a  shows an enlarged abbreviated end view of a portion of a pre cast concrete element with alternative formation geometry including radiused walls. 
           [0051]      FIG. 8  shows an enlarged cross sectional view of a portion of a pre-cast concrete element with alternative formation geometry including an abbreviated taper. 
           [0052]      FIG. 9  shows an enlarged cross sectional view of a portion of a pre-cast concrete element with alternative formation geometry including an abbreviation in the taper near its plateau. 
           [0053]      FIG. 10  shows an enlarged cross sectional view of a portion of a pre-cast concrete element with alternative formation geometry including an abbreviation in the taper near its plateau and radiused junction. 
           [0054]      FIG. 11  shows an enlarged cross sectional view of a portion of a pre-cast concrete element with alternative formation geometry including a radiused junction. 
           [0055]      FIG. 12  shows an enlarged cross sectional view of a portion of a pre-cast concrete element with alternative formation geometry including abutment shoulders and a radiused junction. 
           [0056]      FIG. 13  shows a cross sectional elevation of a composite slab including a structural element and a reinforced overlay layer and including an edge profile on a formation which transmits shear to an adjacent member at right angles to it. 
           [0057]      FIG. 14  shows a sectional view through the end of the perpendicular member of  FIG. 13  showing the method of transmission of shear at an undercut to the ribs of this member. 
           [0058]      FIG. 15  shows a perspective view of a flooring assembly which allows production of a flat plate structure or a flat slab with drop panels including an array of structural elements supported by columns according to one embodiment. In this arrangement the soffits of all the precast elements are in the same plane. 
           [0059]      FIG. 16  shows a sectional elevation view of a composite slab flooring assembly of the type shown in  FIG. 15  including structural elements and composite slab finish regime about support columns. 
           [0060]      FIG. 17  shows an enlarged sectional elevation view of the composite slab flooring assembly including structural elements and composite slab finish regime of  FIG. 16 . 
           [0061]      FIG. 18  shows according to an alternative embodiment a perspective view of a composite slab flooring assembly including precast structural elements and composite slab finish regime with cast in situ band beams about support columns. 
           [0062]      FIG. 19  shows a sectional elevation view of the composite slab flooring assembly including structural elements and composite slab finish regime of  FIG. 18 . 
           [0063]      FIG. 20  shows an enlarged sectional elevation view of part of the assembly of  FIG. 19  including structural elements and composite slab finish regime about a support column. 
           [0064]      FIG. 21  shows an enlarged sectional elevation view of the composite slab flooring assembly including structural elements and composite slab finish according to the regime of  FIG. 6 . 
           [0065]      FIG. 22  shows an enlarged view of a part of the assembly of  FIG. 21 . 
           [0066]      FIG. 23  shows a cross sectional view of a shear junction between the side of a composite slab and support column or precast concrete wall. 
           [0067]      FIG. 24  shows a sectional view of a shear junction  0  between a composite slab assembly and a support wall/column with alternative orientation of structural element. 
       
    
    
     DETAILED DESCRIPTION 
       [0068]      FIG. 1  shows an end view of a pre-cast concrete element  1  comprising a base  2  having an underside surface  3  and upper surface  4 . Element  1  further comprises formations  5 ,  6  and  7  which define void spaces  8  and  9 . Void  8  is defined by upper surface  4  and side walls  10  and  11 . Void  9  is defined by upper surface  4  and side walls  12  and  13 . In the embodiment of  FIG. 1  voids  8  and  9  are filled with polystyrene or similar lightweight material which maintains a lighter weight than an equivalent element with voids filled with concrete or cement. Element  1  typically includes reinforcing (not shown) in base  2 , typically reinforced with steel bars or prestressed reinforcement above which two or more polystyrene void formers  14  and  15  preferably in the shape of an isosceles trapezium are located. 
         [0069]    Formations  5 ,  6  and  7  comprise ribs with longitudinal extent and whose width increases as the distance from surface  4  increases so that there is more material at the top of the formations  5 ,  6  and  7 . The embodiment of  FIG. 1  shows a symmetrical intermediate formation  6  which is dove tail (or inverted trapezoidal) creating voids which are trapezoidal. Increase in material at the top of the rib plateaus  16 ,  17  and  18  improves the performance of the element in bending in that it creates a compressive flange of higher capacity and which is more eccentric to the tensile reinforcement. This increase in bending capacity is in comparison to a prior art element having a rectangular formation were employed. Ideally walls  11  and  12  of formation  6  for instance will be disposed at an angle to surface  4  of base  2  of other than 90 degrees. In the example of  FIG. 1  the side walls of the ribs extend at an angle of about 50° although the angle could ideally fall within the range of about 45 to 70°. 
         [0070]      FIG. 2  shows an end view of the pre-cast concrete element  1  of  FIG. 1  with corresponding numbering and incorporating an intermediate formation  6  and adjacent empty voids  8  to  9  with no void formers. The arrangement of  FIG. 2  would typically be used as a spine beam.  FIG. 3  shows an isometric view of the spine beam of  FIG. 2  with corresponding numbering. 
         [0071]      FIG. 4  shows a cross sectional view of a composite beam assembly  20  including an element  21  which is similar to element  1  of  FIG. 1 . Element  21  comprises formations  22 ,  23 ,  24  and  25  which define void spaces  26 ,  27  and  28 . Void  26  is defined by upper surface  29  and side walls  30  and  31 . Voids  27  and  28  are similarly defined. In the embodiment of  FIG. 4  voids  26 ,  27  and  28  are filled with polystyrene which maintains a lighter weight than an equivalent element with voids filled with concrete or cement. Element  21  includes tensile reinforcing comprising a series of longitudinally extending reinforcing steel rods  32  which may, as required, be pre-tensioned, post-tensioned or unstressed depending on the application and the requirements for element  21 . Element  21  includes three polystyrene void formers in respective voids  26 ,  27  and  28 . Formations  22 ,  23 ,  24  and  25  comprise ribs of longitudinal extent and whose width increases as the distance from surface  29  to respective plateaus  33 ,  34 ,  35  and  36  so there is more material at the top of the formations  22 ,  23 ,  24  and  25 . The embodiment of  FIG. 4  shows symmetrical intermediate formations  23  and  24  which are dove tail (or inverted trapezoidal) creating voids which are trapezoidal. Also in base  37  of element  21  is a sheet of mesh, loose reinforcement or fibre reinforced concrete  39  to provide resilience to handling of the element  21  and help resist cracking and breaking of the element. Typically element  21  would be manufactured in a mould or extruded. Where the element is moulded, it is preferred that a mould having a steel floor is used so that the underside or soffit  40  of base  37  remains smooth. Typically, steel bars  32  will be pre-stressed along with an untensioned fabric  39 . An approximately 20 mm to 80 mm layer of concrete is poured into the base of the mould so as to cover the reinforcing steel bars  32 . Void formers are then put into position on the top of the base in voids  26 ,  27  and  28  and the remaining concrete is poured to bring the height of the rib formations ribs up to the top of the void formers. The concrete is then allowed to set before the composite is removed from the mould. Should reinforcement  20  be pre stressed, then it is either pre tensioned before the casting of elements  37  and  22 ,  23 ,  24  and  25  or post tensioned after the concrete achieves sufficient strength. Once element  21  is erected in its final position in the structure, a relatively thin overlayer  42  is poured over element  21  evenly supporting the overlayer which adheres to plateaus  33 ,  34 ,  35  and  36 . As an alternative embodiment overlay layer  42  may be factory cast prior to site installation of the composite. 
         [0072]    Alternatively, element  21  may be extruded through a die using a relatively stiff concrete mix. Extrusion is the preferred method where polystyrene void formers are not used, although either method may be used. In use, with reference to  FIG. 4 , a plurality of concrete elements  21  are placed on top of beams or walls (not shown) and a layer of reinforcement  43  is placed on top of the elements  21  as required. The element  21  is then covered with a relatively thin in situ layer of concrete  42 . Because of the design of the elements  21  and in particular, the thickening of the ribs distal from the base  37 , element  21  performs well in bending and can be much lighter than other known pre-formed elements. Thus, the system uses less concrete which reduces materials cost. Also, for a building of given height, the building will weigh less and this allows the columns and footings to be less extensive and consequently cheaper. Also as the floors are thinner, the space saved may be equivalent to one or more extra floors in a building. 
         [0073]      FIG. 5  shows a cross section of a composite beam assembly  50  including element  51  and an overlay layer  66  disposed over formation plateaus  53 ,  54  and  55  of formations  56 ,  57  and  58  which define voids  59  and  60 . Located and bearing on plateau  53  is a beam element  63 . Located and bearing on plateau  55  is a second element beam  64 . 
         [0074]    Because the base  49  of the element  51  is relatively thin, it is possible to place reinforcing  67  inside the voids close to the base  49  of the resultant spine beam (element  51 ) to resist bending of the beam. It is also possible, to place reinforcement  65  at the top of the beam when concrete overlay layer  66  is poured in situ into the spine beam  51  and over adjacent elements  63  and  64 . 
         [0075]    In a variant of the element cross-section shown in  FIGS. 1 to 5 , the rib/formation shape of the elements may be varied. Also, the representations shown in  FIGS. 1-5  are of indefinite width and it will be appreciated that the elements may include more or less than the numbers of formations/ribs illustrated. 
         [0076]      FIG. 6  shows an abbreviated section of a flooring assembly including elements employed as formwork prior to pouring of an overlay layer (not shown) but analogous to overlay layer  66  of  FIG. 5 . Shown a perspective view of a banded beam flooring system  70  having two columns with drop panels and two columns without drop panels. The system shown includes elements arranged for co operation with support columns. An arrangement of temporary propping  99  for this floor is also shown. Banded beam flooring system  70  includes elements arranged for co operation with support columns  71 ,  72 ,  73  and  74 . The arrangement of  FIG. 6  provides formwork of elements which will provide a base for a composite slab and band beam system similar to the arrangement of  FIG. 4  in the slab spanning direction and  FIG. 5  in the band spanning direction. System  70  comprises transverse elements  75  of a first span length determined according to structural design requirements. Elements  76  on the outside of columns  71  and  72  and columns  73  and  74  are abbreviated. Transverse elements  75  are supported at their ends on longitudinal spine beam elements  77  and  78 . Elements  78  on the outside of columns have been abbreviated for clarity.  FIG. 6  shows the assembly of panels prior to the placement of reinforcement along the spine beam elements  77  and  78  and over the entire assembly including elements  75  and  76  and the placement of a concrete layer over the entire assembly. In this arrangement, conventional formwork is used to form the drop panel  79 . 
         [0077]    It should be noted that Elements  77  and  78  may or may not incorporate void formers. There are two different junctions shown between elements  77  and  78  and the columns  71 ,  72 ,  73  and  74 . Columns  71  and  72  are either cast with the floor or are precast and are provided with shear keys and the spine beams  77  and  78  abut the columns. In the second form there is a drop panel  79  formed by conventional formwork which connects the spine beams and adjacent slab beams to the column. 
         [0078]    The in situ panel  79  produced with conventional formwork may be terminated at the underside plane of the precast panels  77  and  78  or may project below the general floor soffit. Throughout the specification the term soffit will betaken to mean an underside surface of a structural member. Temporary supports  99  may be required as shown to support the whole floor assembly while concrete is being poured and until it acquires sufficient strength. 
         [0079]      FIG. 7  shows an enlarged abbreviated end view of a portion of a pre-cast concrete element  80  comprising a base  81  having an underside surface  82  and an upper surface  83 . Extending from upper surface  83  are dove tail formations  84  and  85  which define void space  86 . Wall  87  of formation  84  terminates at upper plateau  88  in shoulder  89 . Likewise wall  90  of formation  85  terminates at upper plateau  91  in shoulder  92 . A sheet  93  of fibre cement or the like can be rested on shoulders  89  and  92  spanning void space  86 . This obviates the need to include a void former in void space  86 . Formations  84  and  85  are generally in the shape of an inverted trapezium. 
         [0080]      FIG. 7   a  shows an enlarged abbreviated end view of a portion of a pre cast concrete element with alternative formation geometry including radiused walls. In this embodiment, element  94  comprises a base  95  having an underside surface  96  and an upper surface  97 . Extending from upper surface  97  is formation  98  including walls  98   a  and  98   b  which are substantially S shaped each with opposing radii of curvature. 
         [0081]      FIG. 8  shows an enlarged abbreviated end view of a portion of a pre-cast concrete element  100  with alternative formation geometry. In this embodiment, element  100  comprises a base  101  having an underside surface  102  and an upper surface  103 . Extending from upper surface  103  is formation  104  including walls  105  and  106 . Walls  105  and  106  each have a first portion  108  disposed at an angle normal to the plane of surface  103  and a portion  107  at an angle to surface  103  other than normal. 
         [0082]      FIG. 9  shows an enlarged abbreviated end view of a portion of a pre-cast concrete element  110  with alternative formation geometry. In this embodiment, element  110  comprises a base  111  having an underside surface  112  and an upper surface  113 . Extending from upper surface  113  is formation  114  terminating in plateau  115  and including walls  116  and  117 . Walls  116  and  117  are disposed at an angle less than normal to surface  113  and terminate in a perpendicular abbreviation  118 . 
         [0083]      FIG. 10  shows the embodiment of  FIG. 9  with a radiused junction  119  between surface  113  and formation  114 . 
         [0084]      FIG. 11  shows an enlarged end view of a portion of a pre-cast concrete element  120  with alternative formation  121  geometry including a radiused junction  122  between base  123  and formation  121 . 
         [0085]      FIG. 12  shows an enlarged end view of a portion of a pre-cast concrete element with alternative formation geometry including abutment shoulders and a radiused junction. Element  130  comprises a base  131  having an underside surface  132  and an upper surface  133 . Extending from upper surface  133  are dove tail formations  134  and  135  which define void space  136 . Wall  137  of formation  134  terminates at upper plateau  138  in shoulder  139 . Likewise wall  140  of formation  135  terminates at upper plateau  141  in shoulder  142 . A sheet  143  of fibre cement or the like can be rested on shoulders  139  and  142  spanning void space  136 . This obviates the need to include a void former in void space  136 . Wall  137  terminates in a radiused portion at the junction of formation  134  and base  131 . Likewise wall  140  of formation  135  terminates in a radiused portion  144  at the junction of formation  135  and base  131 . 
         [0086]    An advantage of the above elements is that where a floor is required to resist bending in a lateral as well as a longitudinal direction, and/or to locally enhance the elements shear capacity, it is possible to remove portions  143  of fibre reinforced cement formwork where present and simply fill the voids with concrete in those areas where such lateral resistance to bending and/or shear capacity, is required. Similarly it is possible, though not as convenient to remove the void formers of  FIG. 4  in order to allow the abovementioned local improvements of transverse bending and/or shear capacity to be implemented. 
         [0087]      FIG. 13  shows a cross sectional elevation of a composite slab assembly  150  including a structural element  151  and a reinforced overlay layer  152  and including an edge profile  153  on a formation  154  which transmits shear to an adjacent abutment member  155 . The arrangement of  FIG. 13  is an example of one form of engagement between element  151  and an abutting support. Element  151  includes dovetail formations  156  as described earlier defining voids  157 . Edge profile  153  of formation  154  opposes abutment  155  and is arranged to transmit shear forces between element  151  and abutment element  155 . Overlay layer  158  is laid over plateaus  159  of formations  154  and is preferably reinforced with a reinforcing steel  160 . Element  155  has its void formers terminated a short distance from its end to allow overlay in turn a shear connection with the edge profile  153  of Element  151 . In this way a concrete layer  158  to be poured around the dovetail ribs  144  of element  155  and to thus create a shear connection between the overlay concrete  158  and the dovetail ribs  144  and in turn a shear connection is made between elements  155  and  151  as indicated by arrows  161  and  162 . 
         [0088]      FIG. 14  shows element  155  rotated  90  degrees from its orientation in  FIG. 13 . Element  155  is incorporated with overlay layer  158  which forms a composite beam structure. Layer  158  co operates with element  155  via dove tail formations  144  which define trapezoidal voids  147 . Void  147  includes walls  145  and  146  which receive shear forces transmitted by undercasting via overly layer  158  as shown by arrows  148  and  149 . This structural effect is repeated in each void between formations  144 . 
         [0089]      FIG. 15  shows a perspective view of a flooring assembly including an array of structural elements supported by columns according to one embodiment. Shown is a flooring system  170  including elements arranged for co operation with support columns  171 ,  172 ,  173  and  174 . The arrangement of  FIG. 15  provides a formwork of elements which will provide a base for a composite slab similar to the arrangement of  FIGS. 4 and 5 . System  170  comprises transverse elements  175  of a first span length determined according to structural design requirements. Elements  176  on the outside of columns  171  and  172  and columns  173  and  174  are abbreviated for clarity. Elements  178  on the outside of columns have been abbreviated for clarity Transverse elements  175  are supported at their ends next to and with their soffits (underside surface) in the same plane as the soffits of the longitudinal spine beam elements  177  and  178 . Elements  175  may be temporarily supported independently of the spine elements  177  and  178  or may be supported by temporarily connecting them to spine elements  177  and  178 .  FIG. 15  shows the assembly of panels prior to the placement of reinforcement along the spine beam elements  177  and  178  and over the entire assembly including elements  175  and  176  and the placement of a concrete layer over the entire assembly. In this arrangement, conventional formwork is used to form the drop panel  179 . It should be noted that Elements  177  and  178  may or may not incorporate void formers. The structure produced by this assembly of panels has a flat and planar soffit over the entire underside of the floor. The in situ panel  179  produced with conventional formwork may be terminated at the underside plane of the precast panels  175 ,  176 ,  177  and  178  or may project below the general floor soffit. 
         [0090]      FIG. 16  shows a sectional elevation view of a column and composite slab flooring assembly of the type shown in perspective view  FIG. 15  taken perpendicular to the spine beams  177  and  178 . Assembly includes support columns  190  and  191  each supporting respective spine elements  192  and  193 . Spanning therebetween are elements  194 . On opposite side of column  190  and extending from spine beam element  192  is element  195  abbreviated for clarity. On opposite side of column  191  and extending from spine beam element  193  is element  196  abbreviated for clarity. This arrangement shows the versatility and inter engagement of structural elements which on one hand may be used as a spine beam and on the other hand as transverse span beams. This also demonstrates how the elements can be arranged as formwork in advance of preparation of a composite structural slab. This also demonstrates how all the precast element may be arranged with their soffits co-planar to produce a flat soffit. Elements  195 ,  192 ,  194 ,  193  and  196  are overlaid with overlay layer  197  which completes the slab composite and floor assembly. Reinforcement has been omitted for clarity but it will be appreciated by persons skilled in the art that each representation of floor assembly shown herein would normally include design reinforcement in tensile regions of the composite and to control shrinkage cracking and to enhance the structure&#39;s shear capacity. 
         [0091]      FIG. 17  shows with corresponding numbering for corresponding parts an enlarged sectional elevation view of the composite slab flooring assembly including structural elements and composite slab finish regime of  FIG. 16 . This view also shows overlay layer  197 . Void formers  187  have been terminated a short distance from the respective ends  185  and  186  of the panels  194  and  195  to allow the overlay concrete to flow around the dovetail ribs  188  and thus form a shear connection with the overlay concrete  197  Spine beam element  192  includes an end formation  198  having an outer profile  199  which co operates with element  194  to establish a shear connection therebetween. Overlay layer  197  locks element  192  to element  194  and assists in transmission of loads. Overlay layer  197  is in one embodiment supported by spine element  192  and covers the void formers or penetrates the voids (not shown) when the void former is absent in elements  194  and  195  thereby completing the layered composite floor structure. Voids  189  of spine element  192  will receive concrete from overlay layer  197  but in a case where void formers are used, overlay layer will sit over (bridge) voids  189 . 
         [0092]      FIG. 18  shows a perspective view of a flooring assembly  180  including an array of structural elements supported by columns. Flooring assembly  180  includes transverse elements  240  arranged for co operation with support columns  181 ,  182 ,  183  and  184 . The arrangement of  FIG. 18  provides formwork for concrete to be supplied and a base for a composite slab similar to the arrangement of  FIG. 15 . Assembly  180  comprises transverse elements  240  of a first span length determined according to structural design requirements. Elements  241  on the outside of columns  181  and  183  and elements  242  on the outside of columns  182  and  184  are abbreviated for clarity. Elements  240  are supported at their ends by longitudinal elements  243  and  244  which are cast in situ on conventional formwork. Longitudinal beams  244  and  243  provide an abutment to receive elements  240 ,  241  and  242 . 
         [0093]      FIG. 19  shows a sectional elevation view of a composite column slab flooring assembly of the type shown in perspective view  FIG. 18 .  FIG. 19  shows according to an alternative embodiment, a sectional elevation view of a composite slab flooring assembly  200  including structural elements and composite slab finish regime about support columns. Shown are support columns  201  and  202  each supporting respective cast in situ spine beams  203  and  204  which are formed with conventional formwork. Spanning between columns  201  and  202  the supply are elements  205 . On opposite side of column  201  and extending from spine beam element  203  is element  206  abbreviated for clarity. On opposite sides of column  202  and extending from spine beam element  204  is element  207  abbreviated for clarity. 
         [0094]      FIG. 20  shows with corresponding numbering an enlarged sectional elevation view of the composite slab flooring assembly  200  of  FIG. 19  including structural elements  205   206  and  203  and composite slab finish regime of  FIG. 19 . 
         [0095]      FIG. 21  shows a sectional elevation view of a completed composite column slab flooring assembly  210  of the type shown in the perspective view of  FIG. 6 , when a section is taken through spine beams  77  and  78 . Composite slab flooring assembly  210  includes structural elements and composite slab retained about support columns. Banded beam flooring system  210  shows two columns  211  and  212  with drop panels arranged for co operation with the support columns. Flooring system  210  includes transverse elements  215  of a first span length determined according to structural design requirements. Elements  216  on the outside of columns  211  and elements  217  outside column  212  are abbreviated for clarity. Transverse elements  215  are supported at their ends on longitudinal beam elements  213  and  214 . Overlay layer  218  is placed over element  215  and beam elements  213  and  214  to complete the floor slab composite. 
         [0096]      FIG. 22  shows an enlarged sectional elevation view of the composite slab flooring assembly  210  of  FIG. 21  with corresponding numbering. 
         [0097]      FIG. 23  shows a cross sectional view of a shear junction  220  between a composite slab assembly  221  and support wall/column  222 . Column includes a recess  223  which provides a key in lock for shear transmission at the junction  220 . Composite assembly  221  includes structural element  224  having a base  225  and extending therefrom formations  226  defining voids  227 . A reinforcing ferrule  235  is embedded in column/wall  222  and engages reinforcing steel  228  which is embedded in overlay layer  229  which lies over plateaus  230 . Overlay layer  229  also fills recess  223  and gap  232  between recess  223  and outer profile  233  of formation  234 . The co operation between profile  233  and recess  223  when gap  232  is filled in with concrete from overlay layer  229  results in transmission of shear between precast members  224  and column  222  as indicated by arrows  235  and  236 . 
         [0098]      FIG. 24  shows a sectional view of a shear junction  250  between a composite slab assembly  251  and support wall/column  252 . Column includes a recess  253  which provides a key in lock for shear transmission at the junction  250 . The void formers of composite assembly  251  are terminated a short distance from the end to facilitate the undercasting of concrete around the ribs  256  of assembly  251  to facilitate the transmission of shear in a manner alike to that demonstrated in  FIGS. 13 and 14 . Composite assembly  251  includes structural element  254  having a base  255  and extending therefrom formations  256  defining voids  257 . A reinforcing ferrule  258  is embedded in column/wall  252  and engages reinforcing steel  259  which is embedded in overlay layer  260  which lies over plateaus  261 . Overlay layer  260  also fills recess  253  and gap  262  between recess  253  and the void around the outer profile  254  at the end of element  251  and outer profile of formation  263 . The co operation between recess  253  and profile formation  263  when gap  262  is filled in with concrete from overlay layer  260  results in transmission of shear between pre cast members  254  and column  252  as indicated by arrows  264  and  265 . 
         [0099]    The versatile use of the structural elements described above provides distinct advantages over existing pre-formed concrete elements. The first advantage is that it is relatively easy to put services through the floor in voids between the formations/ribs of the elements. Secondly, the elements can be formed in a mould having a steel base which allows a high quality finish to the soffit of the element. 
         [0100]    Thirdly, the provision of the voids reduces the weight of the element and the shape of the formations/ribs provides more concrete at the upper reaches of the composite thereby providing a large compression flange at the top of the ribs where it is required which allows the elements to be much thinner for a given span. 
         [0101]    Fourthly, the void formers may be removed to allow overlay concrete to flow around (undercast) the dovetailed formations and engage them for shear connection. This allows these units to be readily joined to adjacent structural elements with in situ concrete producing both neat appearance and a joint which is readily fire rated as opposed to the external steel connections often employed which need to be separately fire protected. 
         [0102]    The structural elements which form the composite floor slab have the capacity for long span without intermediate support both during construction when supporting wet concrete and when integral with the completed composite structure. Element dimensions including depth, rib shape, rib spacing, panel width, and the plan shape of the panel may be varied according to design requirements. For instance, wide panels are not restricted by fixed extrusion equipment allowing quick erection of floors with fewer joints. 
         [0103]    The elements may be tapered relative to their longitudinal axis, for instance in a case where the elements form a horizontal radiused corner. Reinforcement in both the tensile and compression regions may be varied according to design requirements. No extrusion tools are needed to fabricate panels and the formation/ rib shape and height is largely determined by the void former shape and size which may be readily changed. The elements may be fabricated as plain reinforced, pre tensioned reinforced or post tensioned reinforced members allowing for flexibility of manufacture dictated by design requirements. Since the elements are lightweight pre cast elements, this allows economic transport and efficient lift by crane. 
         [0104]    The use of lightweight elements allows for more lightly loaded columns and consequently smaller footings. Shallow structural depth allows more efficient buildings saving on the lengths of services, facades, and allows for more usable building space in areas where there is a height restriction. 
         [0105]    Each element has a smooth flat soffit over whole panel width which can be treated as a final finish with no mandatory need for separate suspended ceilings are claddings. The flat soffit combined with shallow structural depth and lack of ceiling space realizes economic operation of air conditioning with no wasted “dead air” between ribs or in ceiling spaces. A further advantage of the element is the access to voids during construction allowing the installation of services in the void areas and through the relatively thin base slab of the composite. The dovetail formations with void blockouts removed provide shear connections to adjacent elements which are both neat, easily made and fire resistant as opposed to the conventional methods of other pre cast systems which either require bulky expensive and unsightly corbels or exposed steelwork which requires fire protection. Very little tooling required for the manufacture of the elements which means a low cost set up, manufacture. Also mobile manufacturing plants are economically feasible. The elements may also be manufactured on the construction site. Finally, irrespective of whether the elements are manufactured with air voids or voids filled with an insulating polystyrene, a floor is created which has optimal sound, heat and fire separation properties. 
         [0106]    It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Summary:
A pre-formed structural concrete element for use in the formation of a composite concrete floor of a building or the like, the element comprising: a generally planar base portion having opposing faces; a series of generally parallel spaced apart formations extending from one said faces of the base portion each defining along with an adjacent formation a void space therebetween and wherein the formations terminate in a plateau and have at least a narrow portion and a wide portion between the plateau and the one said faces of the base portion.