Patent Publication Number: US-2009217612-A1

Title: Modular Composite Floor Units

Description:
FIELD OF THE INVENTION 
     The invention relates to modular composite floor units, being components of modular building systems or of steel frame building systems for the rapid construction of buildings for use either as industrial or commercial premises or as dwellings. The invention also relates to a method for the manufacture of such modular composite floor units. 
     BACKGROUND ART 
     Modular buildings can be constructed from prefabricated wall panels which are bolted or welded together on site to create the framework of the building. The prefabricated wall panels can include pre-installed window frames, door frames, electrical connections and/or plumbing connections to reduce the building and finishing time on-site, and in a typical modular construction process are assembled on-site by being moved into position by a crane or other lifting equipment before being connected together to create a rigid structure. If the building is a steel-framed building then similarly the girders are lifted into position on-site and connected together to create the rigid framework of the building onto and into which are secured the desired external and internal wall panels. 
     The floors of such buildings can be hollow or solid. By “hollow” floors there is conventionally meant floors created from planks or panels, generally of timber or timber-based composite materials such as plywood, chipboard and oriented particle board, laid over a supporting structure such as timber joists or metal beams. By “solid floors” there is conventionally meant concrete floors. 
     Solid floors are often preferred for their better sound insulation properties, and are often specified for multi-occupancy buildings such as apartments, hotels and student accommodation and for industrial and commercial premises. Generally solid floors are made principally from concrete or reinforced concrete, which may be poured on-site. The edges of the solid floor created by pouring wet concrete are defined by the brick work or block work defining the periphery of the building or the room within the building into which the floor is being laid, or by edge shuttering positioned on-site. That edge shuttering may then be removed once the concrete has set, or may remain in position. 
     Solid floors may alternatively be created by laying pre-cast concrete flooring panels. Those panels are pre-cast off-site in open moulds and generally incorporate metal reinforcement bars. They are often cast with longitudinal holes or channels to reduce the overall weight, and also are often cast with a slight convex shape which assists stress distribution in the final building. Ultimately however each array of pre-cast solid flooring panels is covered with a cement screed to smooth out the surface imperfections and irregularities. The screeded area must be kept clear of construction personnel while the cement screed dries and sets, and this of necessity slows down the construction process requiring work on-site to be stopped or diverted to other areas until the screed is sufficiently hard and durable to accept foot traffic without damage. 
     EP-A-881067 discloses a modular composite wall or floor; unit and a method for its manufacture. In fact the strength requirements and in particular the fire resistance performance specifications for wall and floor units are vastly different, so the teaching of EP-A-881067 should not be misunderstood as being that a single product can be laid vertically as a wall or horizontally as a floor. The wall and floor units are substantially different products but according to EP-A-881067 can share common design concepts. The following summary of the relevant teachings of EP-A-881067 is therefore restricted to its teachings of floor units only. 
     The floor unit of EP-A-881067 is a modular floor unit in the sense that t is cast off-site and then transported to the site of the building under construction. It is a composite floor unit in the sense that it is not a single cast slab of concrete that would typify a solid floor unit. It is cast as two concrete slabs separated by an air space or by a layer of insulation (thermal and/or acoustic insulation). The two concrete slabs are cast one at a time in a metal form which has a base and sides. The base gives a smooth finish to the underside of the first slab to be cast, while the sides of the form create the side shuttering for the wet concrete of that first slab. A corrugated plate or array of metal l-beams is placed over the top of the first slab to be cast, and creates a support surface for the base of the second slab to be cast. The sides of the second cast slab are defined by the same shuttering as that used to define the sides of the first cast slab, namely the sides of the metal form. If desired, an edge detail such as a peripheral recess can be added to the second cast slab by positioning a form liner around the periphery of the form before casting the second concrete slab. After casting, and after the concrete has set, the cast composite floor unit is lifted out of the form and any form liner removed, to obtain the final composite floor unit in which the valleys of the corrugated sheet or the bottom flanges of the I-beam are partially immersed in the set concrete of the first (bottom) cast slab and the peaks of the corrugated sheet or the top flanges of the I-beams are partially immersed in the set concrete of the underside of the second (top) cast slab. The composite structure includes a void between the two cast slabs, although that void may if desired be filled with a thermal or acoustic insulation such as a foamed resin composition. 
     Both the thermal and the acoustic performance of the composite floor unit of EP-A-881067 leaves much to be desired. Acoustically, the I-beams or spans of the corrugated metal sheet connecting the top and bottom cast slabs provide a direct sound path from one cast slab to the other, so the filling of the void with an acoustic insulating material does very little to prevent the transmission of sound from the floor defined by the top face or the top slab to the ceiling defined by the bottom face of the bottom slab. Fire resistance is also very poor. In a first test, the bottom slab would rapidly detach from the corrugated metal sheet or I-beams, and the structural integrity of the composite floor unit would soon be lost. The composite floor unit of EP-A-881067 would therefore fall very far short of compliance with British Standard 476, Part 21: 1987, clause 7. That fire resistance standard requires that the structural integrity of the floor unit should be maintained within specified limits even after exposure of one face of the floor unit to a furnace temperature rising to over 1150° C. over a period of 4 hours, and that the mean temperature rise of the face remote from the furnace should be no more than 140° C., with a peak temperature rise of no more than 180° C. Test results are normally reported in terms of the time duration that elapses before one of the monitored parameters indicates failure of the test specimen, either by some loss of structural integrity or by an unacceptable temperature rise at the face remote from the furnace. 
     It is an object of the invention to create a modular composite floor unit which exhibits both good thermal and good acoustic insulation and is capable of markedly better performance characteristics than that of EP-A-881067. 
     It is desirable that both the upper and lower surfaces of the composite floor unit are smooth. Therefore without on-site screeding the floor unit will present an acceptably smooth finish suitable for tiling or carpeting; whereas the underside is preferably smooth enough or has a sufficiently accurate surface finish to be visible as a decorative smooth or patterned ceiling finish to the room below. 
     Most importantly, however, it is a further object of the invention to create a modular composite floor unit which can meet the fire resistance performance demands of British Standard 476, Part 21: 1987, clause 7. 
     THE INVENTION 
     The invention provides a modular composite floor unit as defined in claim  1 . The invention also provides a method for the manufacture of such a floor unit, as defined in claim  26 . 
     One feature of the floor unit of the invention that is not found in the floor unit described in EP-A-881067 is that according to the invention the edge frame forms a permanent part of the floor unit, whereas according to EP-A-881067 it is a temporary form from which the floor unit is removed prior to use. The edge frame of the floor unit of the invention is welded or brazed to the ends of the lattice of reinforcing rods which ultimately will reinforce the material of the ceiling slab. Also the spaced metal joists which take the weight of the two cast slabs are, according to the invention, welded or brazed at their ends to the metal of the edge frame. The result is a composite floor unit which considerably outperforms that of EP-A-881067 in fire resistance tests, and which can survive the test of BS 476, Part 21: 1987, clause 7 for the full 4 hours of the test duration without failure. At first it appeared desirable to weld or braze to the edge frame the lattice of reinforcing rods which ultimately will reinforce the material of the flooring slab. Surprisingly however it has been found that the above excellent fire resistance is obtained when only the reinforcing rods of the cast ceiling slab are welded or brazed to the edge frame, and the reinforcing rods of the cast flooring slab are free from the edge frame. Freeing the ends of the reinforcing rods of the flooring slab in this way makes it possible for the flooring slab to be constructed as a floating floor, which gives the composite floor unit of the invention really outstanding acoustic insulation properties. Although fire resistance could in theory be improved further by connecting the ends of the flooring slab reinforcing rods to the edge frame, this would be at the expense of increased sound transmission through the composite floor unit, and it has been established that the preferred composite floor unit according to the invention is one with only the reinforcing mesh of the ceiling slab welded or brazed to the edge frame. 
     The supporting joists fulfill two different functions. Support for the second (top) slab must be to building regulation standards for the strength and fire resistance of a load-bearing floor. That may be provided by having the top slab simply rest on the joists, but preferably the top slab is physically anchored to the joists by having the longitudinal top edges of the supporting joists embedded in the material of the top slab or by having anchorage members secured to the longitudinal top edges of the supporting joists and embedded in the material of the top slab. Support for the first (bottom) slab may be to the lower building regulation standard for the strength and fire resistance of a suspended ceiling, although according to the invention it is possible to surpass that standard by a very considerable margin. The required support may be provided by having the longitudinal bottom edges of the supporting joists embedded in the material of the bottom slab or by having suspension members supported by the relevant supporting joists and embedded in the material of the bottom slab. 
     The sound insulating material may wholly or partially fill the space between the two cast slabs, which may be of the same or different materials, and the same thickness as each other or of different thicknesses. The top slab must be of a cement based material, such as concrete. The bottom slab may be of a cement based material such as concrete or a gypsum based material. Typical dimensions are that the individual slabs may be from 50 to 100 mm thick with a separation of from 150 to 300 mm. Preferably each slab has a thickness of about 65 mm and preferably the separation is about 225 mm. Other preferred or optional features of the invention will be apparent from the following description of the drawings. 
    
    
     
       DRAWINGS 
         FIG. 1  is a perspective view of a modular composite floor unit according to the invention, with a generally rectangular periphery; 
         FIG. 2  is a section taken along the line A-A of  FIG. 1 ; 
         FIG. 2A  is an enlarged section of the right hand end portion only of  FIG. 2 ; 
         FIG. 2B  is a section through the cold-rolled sheet metal edge member of  FIG. 2A  illustrating its method of construction; 
         FIG. 2C  is a section through one of the joists of cold-rolled sheet metal visible in  FIG. 2A , illustrating its method of construction; 
         FIG. 3  is a section similar to that of  FIG. 2A , but taken along the line B-B of  FIG. 1  through another of the cold-rolled sheet metal edge members; 
         FIG. 3A  is a section through the cold-rolled sheet metal edge member of  FIG. 3 , showing its method of construction; 
         FIG. 4  is a perspective view similar that of  FIG. 1 , but through the modular composite floor unit before the top layer of concrete is poured; 
         FIG. 4A  is a section, greatly enlarged, through one of the reinforcing cross-straps visible in  FIG. 4 ; 
         FIGS. 5 to 12  are enlarged sections, similar to that of  FIG. 2A , through eight different embodiments of the invention, the sequence of Figures being chosen to illustrate sound proofing considerations and the techniques that can be used according to the invention to decrease the sound transmission in various wavebands through a series of modular composite floor units according to the invention; 
         FIG. 13  is a perspective view of a connector cradle for connecting the hollow beams of  FIG. 12  to the top lattice of reinforcing rods or wires; 
         FIG. 13   a  is a plan view of a sheet metal blank which can be folded to form an alternative connector cradle; 
         FIG. 13   b  is a perspective view of the alternative connector cradle created by folding the blank of  FIG. 13   a;    
         FIG. 14  is an enlarged section, similar to that of  FIG. 2A , through a ninth embodiments of the invention to illustrate another sound proofing technique that can be used according to the invention to decrease the sound transmission in various wavebands through a modular composite floor unit according to the invention; 
         FIG. 15  is a perspective view of a connector hanger for connecting the lower row of hollow beams of  FIG. 12  to the bottom lattice of reinforcing rods or wires; 
         FIGS. 16 and 17  are enlarged sections through sheet metal edge members of an edge frame of a modular composite floor unit according to the invention, being the edge members of respectively a side and an end of the edge frame, and showing an alternative cold-rolled sheet metal profile to those shown in  FIGS. 2B and 3A ; 
         FIG. 18  is a vertical. section though the junction between two floor units according to the invention as installed in a building and two wall panels of the building, to illustrate the support of the floor units by their out-turned flanges; 
         FIG. 19  is an enlarged section, similar to that of  FIG. 2A , through a preferred embodiment of the invention; 
         FIG. 20  is a perspective view of the linking strut XX as used in  FIG. 19 ; and 
         FIG. 21  is a detail illustrating the construction of the metal edge member of  FIG. 19 . 
     
    
    
     The modular composite floor unit of  FIG. 1  comprises an edge frame  10  made from cold-rolled sheet metal edge members brazed or welded together to form an accurately sized and proportioned edge shuttering for the floor unit. Into that edge frame  10  is built up a composite floor assembly comprising two, spaced layers of poured reinforced concrete separated by filler materials, as will be particularly described below. 
     The overall structure of the layered infill for the edge frame  10  is illustrated in  FIG. 2 . A bottom layer of poured concrete  12  and a top layer of poured concrete  14  are separated by a space containing a layer of significantly less dense material such as lightweight walling blocks  16 . The walling blocks  16  are supported and separated by an array of mutually parallel spaced joists  18  of cold-rolled sheet metal, the precise shapes of which are better illustrated in  FIGS. 2A and 2C . The parallel spaced joists  18  are welded or brazed to the edge frame  10  at their opposite ends, and L-section pieces of cold-rolled sheet metal  20  and  22  are welded or brazed to the joists  18  and to the respective edge frame members  24  which make up the edge frame  10 , so as to provide runners for supporting the walling blocks  16 . 
     The bottom slab of poured concrete  12  is poured around a reinforcing lattice of rods or wires  26  which are welded or brazed to the edge frame  10  all around its periphery. A similar lattice of rods or wires  28  provides reinforcement for the top layer of poured concrete  14 . The fact that the rods or wires  28  are secured at their ends to the edge frame  10  by welding or brazing has proved to be of enormous importance in providing the fire resistance of the composite floor unit according to the invention. The ceiling and floor slabs with those rods or wires as internal reinforcement are joined integrally to the edge frame  10  in a row of such welded or brazed connections which preferably extend completely around the periphery of the composite floor unit. Furthermore the anchorage of the cast slabs (ceiling and floor) to the edge frame  10  can be considerably enhanced by allowing the unset material of the cast slabs to flow into an around channel ends of C-shaped cold rolled sections of the edge frame  10 , and preferably through apertures formed in the material of the C-shaped sections. For example the poured concrete of both the bottom and top concrete slabs extends through apertures  25 ,  31  formed in the edge frame members  24  and  30  into the internal cavities of the edge frame members  24  ( FIG. 2A) and 30  ( FIG. 3 ) so that the edge frame becomes an integral part of the composite floor unit. The mutually parallel spaced joists  18  which support the walling blocks  16  are also embedded at their top and bottom edges in the concrete of the bottom and top layers  12  and  14 , which adds to the reinforcement of those concrete slabs and to the strength of the finished floor unit. 
     The edge frame members  24  and  30  ( FIGS. 2A and 3 ) could conceivably have the same section as one another, although the corner joints of the edge frame  10  would then have to be mitered. An alternative is illustrated in  FIG. 3 , in which the edge frame member  30  sits inside the generally C-shaped section of the edge frame member  24  of  FIG. 2A , with an end plate  32  being welded or brazed to the edge frame member  30  to bring it to the full height of the edge frame  10 . 
     The method of construction of the modular composite floor unit of  FIG. 1  will now be described. First of all the edge frame  10  is built up in factory conditions. The edge frame members  24  and  30  can be laser-cut to a very high degree of accuracy. The edge frame members are then preferably set out on a factory floor or work bench and held in a jig while they are welded together to the precise size and proportions of the intended final floor unit. The joints  18  are welded or brazed to opposite edge frame members  30  while the edge frame  10  is held in the jig, and in this way the tolerances to which this work can be completed are vastly superior to those attainable on a building site. The first lattice of rods or wires  26  is then welded or brazed into position. Each of the lattices of rods or wires  26  and  28  may be a mesh of reinforcing rods or wires welded together into a square or rectangular grid of crossing rods or wires, such as the reinforcing mesh sold under the Trade Mark WELDMESH. If desired the top lattice  28  may be of heavier duty than the bottom lattice  26  because the bottom lattice  26  will in the final multi-storey building become a part of the ceiling of the room below, and will therefore be subject to less strict building regulations. The securing of the bottom lattice  26  takes place all around the periphery of the edge frame  10 , and all of the assembly up to and including this stage is carried out with the floor unit under assembly being inverted, so that the lattice of rods or wires  26  is welded to inturned flange portions  24 A and to what will ultimately become the lower surface  30   a  of the edge frame members  24  and  30  respectively. If desired, instead of a pre-welded mesh of rods or wires the lattice  26  could be of pre-tensioned wires  26  as described in GB 0515075.0, the individual wires being drawn through apertures in the outer walls of the edge frame members  24  and  30 , placed under tension, and then welded from the outside of the edge frame  10 . This pre-tensioning of the reinforcing lattice  26  can be repeated for the reinforcing lattice  28 , and is possible because the edge frame  10  is securely held in the jig on the work floor or work bench. The pre-tensioning of the lattice is not, however, essential to the method of construction of the composite floor unit according to the invention, and an alternative or additional method of using pre-tension to create a very stable edge frame structure is to incorporate diagonal cross-braces  32  as illustrated in  FIG. 4 . Each cross-brace  32  is formed by unrolling a strip of sheet metal from a roll. If a slight crease  34  is formed in the strip metal of the cross-brace  32 , by cold-forming the strip into a slight apex along the line  34  as shown in  FIG. 4A  along most of its length, then the tendency of the cross-brace strip to reform into a curl can be largely or completely eliminated. Each cross-brace  32  is welded or brazed at its ends to inturned flange portions of the edge frame  10 , and preferably the cross-braces  32  when cold are under a slight tension to ensure complete stability of the edge frame  10 . The cross-braces  32  may extend generally from corner to corner of the edge frame  10 , or may be arranged in any other pattern of triangulation. 
     When the welding of the edge frame  10 , the lattice  26  and the optional cross-braces  32  is complete, the edge frame  10  is turned over onto a smooth flat casting surface, ready for the casting of the bottom layer  12  of poured concrete. The casting surface (not illustrated in the drawings) may be any smooth flat surface coated with a concrete mould release agent. It may, for example, be a flat metal surface such as the smooth flat surface of a steel plate decking in the factory. Mirror steel may be used to provide an even smoother cast finish to the concrete that is poured. Alternatively, the casting surface may be textured, to give an attractive textured appearance to the underside of the cast floor unit, which will become the ceiling of the room below in the finished building. Clearly any texturing must be carefully regulated so that it does not interfere with the mould release. 
     Alternatively, the casting surface may be covered with paper or fabric that is preferably wetted, for example by spraying, with a bonding adhesive that causes it to adhere to the concrete that is poured into the edge frame  10 . That provides a paper or textured fabric finish to the underside of the resulting floor unit, which provides the best possible paintable surface for ultimate ceiling decoration. 
     The concrete layer  12  may be poured as a single layer of liquid concrete, or it may be built up in layers. For example a first layer, about 5 mm deep, of a grano gel coat may be poured first, followed by 25 mm of C30 grade concrete. Concrete with a lightweight or porous aggregate is preferred, and the depth of the concrete is preferably marginally above the level of the runners  20  and  22 , as shown by a broken lead line  36  in  FIG. 2A  and in  FIG. 3 . It will be noted that the concrete layer  12  flows through the apertures  25  into the edge cavities  38  and  40  created by the shape of the edge frame members  24  and  30  respectively (see  FIGS. 2A and 3 ). Care should be taken to fill those cavities completely for maximum strength. 
     While the poured concrete is still unset, rows of walling blocks  16  are placed on the runners  20  and  22  and between adjacent parallel spaced joists  18 , completely to fill the floor space as defined by the edge frame  10 . The walling blocks  16  are preferably wetted before installation using a water-based bonding agent to ensure good adhesion to the concrete, and are preferably pressed into the unset concrete until they rest on the runners  20  and  22 , so that the displaced concrete is pushed up between adjacent walling blocks, to provide better bonding with the walling blocks  16 . The top edges of the walling blocks  16  create a generally planar surface, indicated in  FIGS. 2A and 3  by the broken lead line  42 , for the pouring of the top layer of concrete  14 . 
     Before the top layer of concrete  14  is poured, however, the second lattice  28  of rods or wires is placed over the protruding tops of the parallel spaced joists  18 , and welded to an inturned flange  44  of the edge frame members  24  and to an inturned flange  46  of the edge frame members  30 . Over the top of the lattice  28  there are then preferably welded diagonal cross-braces  32  as illustrated in  FIGS. 4 and 4A . 
     The top layer of poured concrete  14  is then poured over the tops of the walling blocks  16 . The concrete will settle down into any gaps between the walling blocks, and will flow through apertures in the edge frame members  24  as indicated by the shaded portions  48  of the joist members  18  in  FIGS. 2A and 2C , further to enhance the stability and rigidity of the resulting floor unit. Although the top layer of poured concrete  14  will flow down and around the individual warning blocks  16 , that concrete flow will not be sufficient to fill every void between the top and bottom layers of concrete  14  and  12 , and simply for ease of representation, in  FIGS. 2A and 3  the seepage of the top layer of poured concrete  14  down below the level of the tops of the walling blocks  16  is not shown. The top layer of concrete  14  may be the same thickness as that of the bottom layer  12 , or may be a different thickness. In  FIGS. 2A and 3  the top layer is shown as being of a lesser thickness. Finally, the top surface of the top layer of poured concrete  14  is finished with a power float, to create a final finished floor unit which has a surface finish at least as smooth as the final screeded finish of conventional building techniques. That finish is certainly smooth and flat enough to take carpet, or tiles, or laminate flooring in the final building, without requiring a final top screed. 
     To lift the finished floor unit from the casting surface, lifting apertures or hooks or other handling formations (not shown) are formed around the edge frame  10 , and the finished floor unit can be lifted, after the concrete has set, by suitable handling equipment directly onto a lorry or other transport vehicle, to the final site of the building under erection. The accuracy of the dimensions of the floor unit, made under factory conditions, is such that it can be presented up to pre-established mounting bolts or spigots on or in the building under construction, with a virtual guarantee of accurate alignment 
     Many modifications are possible to the method of construction described above. The function of the walling blocks  16 , being less dense than concrete, is to reduce the overall weight of the floor unit. For this reason, the above description refers by way of example to the use of lightweight walling blocks. Walling blocks made from a cinder or porous aggregate are highly suitable, such as those sold under the Trade Mark THERMALITE™. The blocks  16  are provided for their sound insulation properties and to create additional thickness to the floor unit without adding unduly to the overall weight, and a number of alternative materials may therefore be used. For example, in place of walling blocks there may be used blocks of expanded polystyrene, blocks of balsa wood, sheets of rockwool, sheets of fibreglass matting, or hollow moulded plastic boxes. The blocks  16  could even be replaced by hollow boxes made from waxed cardboard. Plastic or cardboard boxes, when used, are preferably filled with a sound absorbing material such as rockwool, fibreglass matting, shredded newspaper, paper mache, compressed straw, reclaimed particulate rubber or other lightweight products of the rubbish recycling industry. Alternatively the longitudinal spaces between the bottom and top layers  12  and  14  of poured concrete can be filled by a lightweight particulate material such as chopped straw, pelleted newspaper waste, hollow balls or polystyrene beads. Boards of wood or of a wood-based product such as plywood or oriented particle board may then be placed over the fill material to create the generally planar surface  42  onto which the top layer of concrete  14  is to be poured, and the remainder of the method of assembly is exactly as described above. If the loose or particulate fill material is compressible, or if it does not completely fill the space separating the two cast concrete slabs  12  and  14  of the finished floor unit, then it will be preferred to incorporate runners (not illustrated) similar to the runners  20  and  22  of  FIG. 2A , to support the boards. 
     The complete floor units may be transported quite easily and safely and with very little added protection required during transport, because they are protected from accidental edge damage by the edge frame  10  which becomes an integral part of the construction. 
     It will be seen from  FIGS. 1 ,  3 ,  3 A and  4  that the edge frame members  30  are formed with out-turned flanges  33  on their end plates  32 . Similar out-turned flanges could if desired be formed on the edge frame members  24  although they are not illustrated. The function of the out-turned flanges is to support the floor unit during transportation and in the final building, where the floor unit can be laid in position to span an assembly of pre-assembled wall panels suspended initially by the flanges before being screwed, bolted, riveted or welded for final securement. 
     The top surface of the floor unit is as flat and smooth as the power float operator can produce, which is a smoothness equal to that of conventional floors screeded on-site. The under-surface is as smooth as the casting surface on which the floor unit is made which, being in factory conditions, is a very high standard of smoothness. Alternatively it may be paper-covered by casing onto paper as described above. Alternatively it may be textured, by casting onto a textured fabric which adheres to the underside of the floor unit after casting and which thus establishes the texture of the resulting visible ceiling; or by casting onto a textured casting surface. 
     The embodiment of  FIGS. 1 to 4  utilises a sound insulating material shown in  FIGS. 2 ,  2 A and  3  as walling blocks, which fill the full height of the space between the bottom and top cast concrete slabs  12  and  14 . That creates a floor unit which provides good acoustic insulation over a range of wavelengths, but for better sound insulation and also, incidentally, for better thermal insulation the sound insulating material should occupy less than the total space between the first and second concrete slabs.  FIGS. 5 to 11  show seven alternative embodiments of modular composite floor units according to the invention in which the sound insulation material is provided in a layer confined to the bottom portion of the space between the first and second concrete slabs, with an air gap above that insulation. In  FIGS. 5 to 11  the same reference numerals have been used wherever possible to those used in  FIGS. 1 to 4 , and the following description is limited to the differences between the different embodiments. 
     The sound insulating material illustrated in  FIGS. 5 to 11  is represented as a series of mats  50  of a sound insulating material such as rockwool. It will be understood that any alternative particular sound insulating material could be used, or any of the other materials discussed earlier in this specification. In the embodiment of  FIGS. 1 to 4  the joists  18  have a J configuration as shown in  FIG. 2C , the upturned flange at the bottom of the J being used as a ledge on which to locate the walling blocks  16 . A simpler shape of joist  18 A is shown in  FIG. 5 , being of C section. Advantageously the level to which the bottom layer of concrete  12  is to be poured may be marked on the joists  18 A by means of a scribe mark (not shown) or an aperture (not shown) punched through the vertical wall of the joists  18 A before assembly, so that the bottom layer of concrete  12  can be poured until it reaches the scribe marks or the tops or bottoms of the punched apertures. As with the previous embodiment, the first and second lattices of reinforcing rods or wires  26  and  28  are welded to the edge frame, as are the ends of the joists  18 A. 
     After the bottom concrete slab has been cast to the required depth, the insulation mats  50  are laid between the joists, and boards  52  are placed on longitudinal supporting runners  54  which have been welded or brazed to the supporting joists. A similar runner  56  is spot welded or brazed to the inside of the edge frame member  24 . The boards  52  provide a base for the pouring of the second concrete slab  14  which is poured and float-finished as described earlier. 
     The air gap above the mats  50  in  FIG. 5  reduces some of the sound transmission between the top and bottom concrete slabs  14  and  12 , and of course enhances the thermal insulation of the composite floor unit of  FIG. 5 . The longitudinal division of that air gap into relatively narrow horizontal channels, by virtue of the joists  18 A does act to reduce the sound transmission laterally along the floor unit, but the joists  18 A themselves provide a direct linkage and sound transmission path from one floor slab to the other, and therefore provide a path for the transmission of sound of certain frequencies. That sound transmission path can be broken by ensuring that the joists are divided into two. sub-groups of joists, namely joists  18 B anchored at their ends in the top concrete slab  14  as shown in  FIG. 6 , and joists  18 C anchored at their lower ends in the bottom concrete slab  12 . In  FIG. 6  those joists  18 B and  18 C are shown as having a J section, the additional inturned flange portion of the J section as opposed to the simple C section of  FIG. 5  providing the joists with increased stability and strength against buckling along their unsupported edges. Nevertheless the joists  18 B and  18 C of  FIG. 6 , which are shown arranged directly aligned one above the other, necessarily have a wall portion depending from the top slab of concrete  14  or a wall portion upstanding from the bottom concrete slab  12  spanning less than half of the base between the two concrete slabs. The reinforcing effect of the joists  18 B and  18 C can be enhanced significantly by using wider joists as shown in  FIG. 7 , and staggering them so that the joists  18 B are offset on one side of the joists  18 C. By having a relatively small spacing between pairs of adjacent joists as shown in  FIG. 7 , the turning moment transmitted from one joist to the other at the outside edge of the edge frame is maximised, for maximum strength. The number of joists  18 B and  18 C used, and their mutual spacing, is dependent on the width of the floor unit and the length which each joist has to span. 
       FIG. 8  shows an alternative arrangement of joists, with a pair of joists  18 C bedded in the bottom concrete slab  12  alternating with a pair of joists  18 B embedded in the top concrete slab  14  across the width of the floor unit. The advantage of this arrangement is that if desired reinforcing straps  80 , one only of which is shown in  FIG. 8 , can be welded or brazed between the free edges of the pairs of adjacent joists, to strengthen the joist assembly and resist buckling. 
     It will be seen in each of  FIGS. 6 to 8  that the runners  54  supporting the boards  52  are welded or brazed to the joists  18 B which are ultimately to be embedded in the concrete of the top slab  14 . One alternative method of supporting the boards  52  is shown in  FIG. 9 . Blocks of expanded polystyrene  90  are placed on the top edges of the joists  18 C, and taller blocks of expanded polystyrene  92  are placed on the inturned and upturned bottom edges of the joists  18 B. The boards  52  are simply balanced between adjacent pairs of blocks  90  or  92  prior to pouring the concrete of the top layer  14 . Expanded polystyrene is a very poor conductor of sound, so that there is very little sound transmission from the top concrete slab  14  to the bottom concrete slab  12  through the blocks  90  and  92 , which do not play any structural role in the final floor unit once the concrete layer  14  has set. It will be understood of course that a combination of polystyrene blocks and runners could be used. For example  FIG. 10  shows a combination of the polystyrene blocks  90  placed on the tops of the joists  18 C, and runners  54  welded to the joists  18 B.  FIG. 10  also illustrates how service ducts can be incorporated into the floor units of the invention.  FIG. 10  illustrates a service duct  100 , which may be for example a plastic conduit, extending laterally of the joist  18 B and  18 C. The duct  100  is suitable for carrying electrical wiring either completely across the floor unit or from an outside edge to a mid position where it could be taken down through the ceiling, up through the floor, or simply turned at 90° to run parallel with the joists. The conduit  100  passes through holes punched in the joists  18 B and  18 C, but those holes are of different sizes so that the conduit contacts and is supported by the joists  18 B as illustrated in  FIG. 10 , whereas it makes no contact at all with the joists  18 C. Equally, the relative sizes of the holes punched in the joists could be reversed so that the conduit is supported by the joists  18 C and makes no contact with the joists  18 B. By avoiding contact with the joists of one set, it can be ensured that sound transmission through the floor unit does not travel through the conduit  100 . 
       FIG. 11  shows an alternative location for the service conduit  100 , beneath the joists  18 B and supported by holes punched in the joists  18 C. The acoustic insulation mats  50  in  FIG. 11  are shown as thicker than those in  FIGS. 5 to 10 , but that is principally because in this embodiment the mats have to be wrapped up and over the conduit  100 , giving them increased height along the section line of  FIG. 11 . Of course, the acoustic insulation mats  50  of  FIGS. 5 to 11  can be of any thickness, even occupying the full height between the bottom concrete slab  12  and the boards  52  on which the top concrete slab  14  is laid. 
     In  FIGS. 5 to 8  the top slab  14  is cast over an array of discrete boards  52 . These boards  52  are supported on runners  54  secured to the joists  18 A or  18   b  which support the top slab across its width. Use of separate boards  52 , one between each pair of adjacent supporting joists  18 A or  18 B, requires an additional step of cutting the individual boards  52  to size and assembling them one by one between the joists and supported on the runners  54 . A preferred construction is to use a single board  52 A as shown in  FIG. 12 . That board  52 A is placed directly over the top of joists  18 D which support the top slab  14  across its width. Those joists  18 D are shown in  FIG. 12  as being hollow box section joists, although they are made from cold-rolled sheet metal, as are the joists  18  of  FIG. 2C  and the joists  18 A of  FIGS. 5 to 8 . The very fact that the rigid board  52 A rests on the hollow section joists  18 D means that the joists  18 D support the top slab  14  across its width, but that support is advantageously considerably enhanced by a series of anchorage members  60  which are screwed to the hollow joists  18 D by means of self-tapping screws  62  which pass through the solid board  52 A. The anchorage members  60  are cradle-shaped as shown in  FIG. 13 , each comprising a pair of upright sides  64  upstanding from a flat base  66 . Slots  68  are cut in the top portions of the upright sides  64  to straddle the rods or wires of the enforcing lattice  28 . When the anchorage member  60  is screwed to the hollow beams  18 D through the rigid board  52 A, this provides the total support for the reinforcing lattice  28  both in the upward direction and the lateral directions, as well as the main load bearing downward direction. 
     Each cradle  60  of  FIG. 13  supports the reinforcing rods or wires of the lattice  28  running in one direction only, but different cradles  60  can be oriented in mutually perpendicular directions so that together they support both the longitudinal and the lateral reinforcing rods or wires of the lattice  28 . Alternatively cradles  60   a  as illustrated in  FIGS. 13   a  and  13   b  can be used.  FIG. 13   a  illustrates a sheet metal blank  60   b  from which the cradle  60   a  of  FIG. 13   b  can be formed by bending. Rows of oval cut-outs in the blank  60   b  are separated by relatively narrow metal webs  63  so as to define fold lines enabling the sheet metal blank of  FIG. 13   a  to be easily bent by hand to the shape of  FIG. 13   b.  A pre-formed hole  65  is provided in the flange which becomes the base of the final cradle  60   a  to receive the screw  62  of  FIG. 12 , and slots  68   a  and  68   b  receive the longitudinal and lateral reinforcing rods respectively of the reinforcing lattice  28 . The slots  68   a  and  68   b  may be at the same distance from the base as shown in  FIG. 13   b,  in which case the cradle  60   a  is easily twisted along one of the fold lines in use to bring the slots to the mutually different levels of the longitudinal and lateral reinforcing rods; or the slots  68   a  and  68   b  may be at mutually different heights to reflect the different levels of the longitudinal and lateral reinforcing rods. 
       FIG. 12  shows that the joists  18 C supporting the bottom slab  12  are constructed in the same way as those of  FIG. 8 , and connected together at intervals by lateral straps  80 . The box section joists  18 D are considerably stronger than the separate J-section joists  18 C even when those joists  18 C are joined together by straps  80 , and an even stronger construction is therefore that shown in  FIG. 14  in which the joists supporting the bottom slap  12  are hollow box section joists  18 E, similar to the hollow joists  18 D supporting the top slab. The support between the hollow joists  18 E and the bottom slab  12  is provided by a series of hangers  70  which are as shown in  FIG. 15 . Each hanger is a metal strap which passes over the joist from which it is suspended, and hangs down on opposite sides of that joist. Transverse slots in the lower ends of the hangers hook around and support the reinforcing rods or wires of the first lattice  26  to provide the necessary support across the width of the bottom slab  12 . 
     It will be understood that instead of the metal of the strap hangers  70  as shown in  FIG. 15 , the reinforcing lattice  26  for the bottom slab could be supported from the hollow joists  18 E by wires. Depending on the length and diameter of the supporting wires, this will provide very limited sound transmission between the hollow beams  18 E and the lower slab  12 , which gives the possibility of a further embodiment (not illustrated) in which each transverse joist  18  can be formed as a hollow box section joist that both supports the top slab as shown in  FIG. 12  and supports the bottom slab by means of connecting wires. 
     Although not illustrated, the hollow box section joists  18 D and  18 E of  FIGS. 12 and 14  can be wholly or partially filled by a sound-absorbing material. Instead of the joists  18  of  FIG. 12  and the joists  18 D and  18 E of  FIG. 14  being formed as hollow box sections as illustrated, an improvement in strength, as compared with the simple J-section joists  18 B and  18 C of  FIGS. 6 to 11 , can be obtained by forming each joist of  FIG. 12  or  14  from two identical J-section joists placed back-to-back and secured together by spot-welding. 
     Another modification (not illustrated) is to place a layer of acoustic rubber over the tops of the box sections  18 D or the single or back-to-back J-sections, together possibly with an edge trim of acoustic rubber between the cast concrete of the top slab  14  and the edge frame  10 . This gives a floating floor without detracting from the excellent rigidity and acoustic superiority of the modular floor units as described and illustrated. 
       FIGS. 16 and 17  show an alternative section for the edge frame members  24  and  30  of the edge frame  10 .  FIG. 16  shows that the out-turned flange  148  at the top of the edge frame member  30  is slightly lower than the top level of the concrete slab  14 . As with  FIG. 3A  the edge frame member  30  is made in two pieces,  30   a  and  30   b,  with an outer side plate  30 A forming that out-turned flange  148 .  FIG. 17  shows the out-turned flanges being level with the top of the top slab  14  of concrete. The way in which the lowered flange  148  of  FIG. 16  is useful in the actual construction of buildings using floor units according to the invention is illustrated in  FIG. 18. 140  shows the top of a wall of the building, on which two floor units according to the invention are supported.  FIG. 18  shows one floor unit  142  to the right of the wall top  140 , and one floor unit  144  to the left. A rubber sheet  146  is placed over the top cap of the wall top  140  to reduce sound transmission through the final building, before the top floor units are placed in position, suspended on their out-turned flanges  148 . Self tapping screws or anchorage bolts  150  are passed through downwardly extending anchorage plates  152  that are welded or brazed to the side plates  32  of  FIG. 16  to render the assembly rigid. The building is then ready to be increased in height by one further storey. If the flanges were not recessed below the top of the top floor slabs, there would be no positive line along which to locate the next higher wall panel  154 . By virtue of the recessed nature of the flanges  148 , the next wall panel  154  can be positively located in the shallow slot formed between adjacent floor units  142  and  144 , and is preferably protected from direct metal to metal contact with the flanges  148  by another strip of rubber  156 . If desired, filler pieces of rubber, plastic or metal can be placed between the top edges of the adjacent floor units  142  and  144  and the wall. panel  154  being assembled into position, to shim the wall panel  154  into totally accurate alignment. 
       FIG. 18  also shows a pair of flexible hangers  158  of the wall panel  154 , to which plasterboard panels  160  are attached in conventional manner. An intumescent strip  162  is placed along the bottom of each set of plasterboard panels  160 , to fill the gap between the plasterboard and. the floor of the building being constructed. 
     It will be appreciated that the construction detail shown in  FIG. 18  reduces the amount of sound transmission vertically through the building, so that the sound insulation properties of the floor units of the invention are put to very good effect. 
     The most remarkable advantage of all of the embodiments of composite floor unit according to the invention as illustrated in  FIGS. 1 to 18  is however their fire resistance. There is very little distortion of the floor units in the event of a fire, because of the anchorage of the rods or wires of the internal reinforcement of the two cast slabs to the edge frame by welding or brazing, and because of the anchorage of the joists  18 ,  18 A,  18 B,  18 C,  18 D and  18 E of the various embodiments to the edge frame by welding or brazing. The joists  18  to  18 E of the various illustrated embodiments described above have been cold-rolled steel profiles. A further embodiment as illustrated in  FIGS. 19 to 21  uses hot-rolled metal section joists  18 F which are of parallel flanged channel profile. Alternative hot-rolled profiles would be I-beam or hot-rolled box section. A modular composite floor unit as described with reference to  FIG. 19  was extensively tested in a fire resistance test and amazingly survived the test for the full 240 minutes of the BS 476 Part 21: 1987, Clause 7 test. 
     Referring to  FIGS. 19 to 21 , the sides of the edge frame  30  are constructed in two pieces as in  FIG. 21 . The parallel flanged channel joists  18 F are welded or brazed to the edge frame  30  at their ends. Hangers  70  shaped as in  FIG. 16  straddle the joists  18 F and support a welded mesh lattice  26  of reinforcing rods which will provide the reinforcement for the bottom cast slab  12  (the ceiling slab). All ends of the welded mesh lattice  26  are welded or brazed to an upturned and inturned flange portion of the edge frame  30 . The welded mesh reinforcing lattice  26  is therefore supported across its central portion by the hangers  70  and secured firmly to the edge frame  80  all around the periphery. At this stage the ceiling slab  12  is cast, with the cement-based or gypsum-based casting material flowing into the edge channel of the edge frame  30  all around the periphery of the floor unit and around the reinforcing lattice  26  across the centre. A bottom portion of each hanger  70  is encased in the cast slab  12  but the joists  18 F are above the level of the cast slab  12 . 
     Insulation  50  such as high density rockwool insulation matting (for example that sold under the Trade Mark BEAMCLAD) is then packed into the voids above the cast slab and between the joists  18 F, and one or more solid boards  95  placed over the tops of the joists  18 F. A very suitable material for those boards  95  is a fibre board impregnated with bitumen, as sold under the Trade Mark BITROC. If desired, additional support for the boards  95  can be provided by first placing transverse beams  96  between pairs of adjacent joists  18 F at intervals along the length of the joists  18 F. Each transverse beam  96 , of which one is shown in perspective view in  FIG. 20 , comprises a box section support portion for the solid board  95  and a pair of mounting plates  97 , one at each end. The mounting plates  97  overlie the joists  18 F as shown in  FIG. 19 , and can if desired be secured in position by self-tapping screws (not shown) or by spot welds. 
     The solid boards  95  provide a base support for the upper slab of concrete  14  that is to be cast over the top of the composite floor unit. Before that concrete is poured, however, the lattice  28  of reinforcing rods is secured in position. Mesh anchorage members  60  or  60   a,  as already illustrated in  FIG. 13  or in  FIGS. 13   a  and  13   b,  are secured at intervals over each joist  18 F and are secured to the joist  18 F using self-tapping screws  62  which pass through the solid board  95  and into the joist. The lattice  28  of welded reinforcing rods is supported by the slots in the anchorage members  60  or  60   a  and held spaced above the boards  95  across the width of the composite floor unit. The edge frame  30  is itself made from two components  30   a  and  30   b  welded together as illustrated in  FIG. 21 . 
     Although not illustrated in  FIG. 19 , a sheet of polythene is laid over the boards  95 . The edges of the polythene sheet are trapped in the C-section component  30   b  of the edge frame  30  by strips  30   c  of expanded polystyrene inserted into the C-section component  30   b  between its upper and lower flanges. The cast floor slab  14  is therefore effectively a floating floor, supported across its width by the parallel flanged channel joists  18 F but isolated from the edge frame  30  by the expanded polystyrene strips  30   c.  The improvement in acoustic insulation of the resulting composite floor unit is remarkable. There is very little sound transmission from the floor slab  14  to the framework of the building (for example to the wall top  140  of  FIG. 18 ) because of the provision of the expanded polystyrene strips  30   c  and the free floating nature of the floor slab  14 . Fire resistance could of course be improved by welding or brazing the ends of the reinforcing rods of the lattice  28  of the floor slab  14  to the edge frame  30 , just as the ends of the reinforcing rods of the reinforcing lattice  26  of the ceiling slab are so welded or brazed. That would however be at the expense of the sound insulation improvement that is obtained by making the floor slab free-floating. Surprisingly, it has been found that the fire resistance is so outstandingly good when only the bottom reinforcing lattice is welded or brazed to the edge frame  30  that a similar edge connection of the top reinforcing lattice is unnecessary. 
     The floor unit as illustrated in  FIGS. 19 to 21  was tested for fire resistance in accordance with British Standard 476: Part 21: 1987, clause 7. The unit was tested for its ability to comply with the performance criteria for load-bearing capacity, structural integrity and thermal insulation. During the test the specimen floor unit being tested carried a surface load of 2 KN/m 2  evenly distributed over its top surface. Thermocouples were positioned over the top surface of the unit being tested, and the unit was suspended over a furnace which enabled it to be heated from below. The test was continued for four hours as specified in BS476, and the specimen survived the full duration of the test. 
     Even though the furnace temperature was raised to 1152° C. during the test, the maximum temperature of the top surface of the floor unit even after 4 hours was only 68° C., indicating excellent thermal insulation between the top and bottom slabs of the floor unit. Structural integrity and load-bearing capability were maintained for the full 4 hours of the test although there was a slight (but acceptable) bowing or sagging of a part of the bottom slab towards the end of the test. The specimen under test still satisfied the test criteria for upper surface temperature, load-bearing capacity and structural integrity at the end of the 4-hour test, which represents really astonishing performance characteristics, way beyond expectations which were for at most a 90-minute satisfaction of all of the test criteria. 
     In addition to the quite unpredictably high fire resistance of the specimen floor being tested, that same floor unit had previously been subjected to a test for acoustic insulation. It was found to be far superior to conventional solid floors and to conventional hollow floors. The excellent acoustic properties are thought to be a combination of the dense nature of the top and bottom slabs, the fact that those slabs are anchored all round their periphery to the edge frame by virtue of the welded or brazed connections between the reinforcing lattice of rods or wires and the edge frame and between the joists and the edge frame, and the less dense interior of the composite floor unit. The less dense interior, provided by the rockwool  50  and the air gap over the rockwool, provides good acoustic insulation. The direct acoustic paths through the composite floor unit from the top surface to the bottom surface are largely confined to the self-tapping screws  62  linking the top slab  14  to the joists  18 F, and the mesh hangers  70 . By judicious spacing of those hangers  70  the composite floor unit of the invention achieves, in a total thickness or depth of less than 300 mm for the floor unit, a level of acoustic insulation that might be expected of a conventional floor unit at least twice as thick.