Modular Composite Floor Units

Abstract

The invention provides a modular composite floor unit and a method for its manufacture. The floor unit is factory-made. An edge frame (10) is provided from cold-rolled sheet metal members (24 and 32) welded or brazed together to create edge shuttering. A cast concrete ceiling slab (12) is cast within the edge frame (10) over a smooth casting surface. The cast ceiling slab (12) encases a first inturned lip of the edge frame (10), a first lattice (26) of reinforcing rods or wires anchored at their ends to opposite sides and ends of the edge frame (10), and the bottom edges, or hangers (70) suspended below the bottom edges, of an array of mutually parallel spaced metal joists (18) which are welded or brazed to the edge frame (10) at their opposite ends. An infill layer is then created from blocks (16) or particulate material filling most of the height of the exposed portions of the array of mutually parallel spaced joists (18). A concrete floor slab (14) is cast within the edge frame (10) over the top of the infill layer, encasing an upper inturned lip of the edge frame (10), a second lattice (28) of reinforcing rods or wires anchored at their ends to opposite sides and ends of the edge frame (10), and the top edges, or anchorage members (60) secured to the top edges, of the mutually parallel spaced joists (18). The top surface of the cast floor slab (14) is float-finished to create a final floor unit that requires no screeding. The bottom surface of the cast ceiling slab (12) has a finish defined by the surface on which it was cast, and is visible without further treatment as the ceiling of the room below the floor unit when the unit is used in the construction of a multi-storey building.

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. 13a is a plan view of a sheet metal blank which can be folded to form an alternative connector cradle;

FIG. 13b is a perspective view of the alternative connector cradle created by folding the blank of FIG. 13a;

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 24A and to what will ultimately become the lower surface 30a 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, 3A 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, 2A 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 18A 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 18A by means of a scribe mark (not shown) or an aperture (not shown) punched through the vertical wall of the joists 18A 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 18A.

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 18A does act to reduce the sound transmission laterally along the floor unit, but the joists 18A 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 18B anchored at their ends in the top concrete slab 14 as shown in FIG. 6, and joists 18C anchored at their lower ends in the bottom concrete slab 12. In FIG. 6 those joists 18B and 18C 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 18B and 18C 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 18B and 18C can be enhanced significantly by using wider joists as shown in FIG. 7, and staggering them so that the joists 18B are offset on one side of the joists 18C. 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 18B and 18C 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 18C bedded in the bottom concrete slab 12 alternating with a pair of joists 18B 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 18B 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 18C, and taller blocks of expanded polystyrene 92 are placed on the inturned and upturned bottom edges of the joists 18B. 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 18C, and runners 54 welded to the joists 18B. 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 18B and 18C. 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 18B and 18C, but those holes are of different sizes so that the conduit contacts and is supported by the joists 18B as illustrated in FIG. 10, whereas it makes no contact at all with the joists 18C. Equally, the relative sizes of the holes punched in the joists could be reversed so that the conduit is supported by the joists 18C and makes no contact with the joists 18B. 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 18B and supported by holes punched in the joists 18C. 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 18A or 18b which support the top slab across its width. Use of separate boards 52, one between each pair of adjacent supporting joists 18A or 18B, 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 52A as shown in FIG. 12. That board 52A is placed directly over the top of joists 18D which support the top slab 14 across its width. Those joists 18D 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 18A of FIGS. 5 to 8. The very fact that the rigid board 52A rests on the hollow section joists 18D means that the joists 18D 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 18D by means of self-tapping screws 62 which pass through the solid board 52A. 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 18D through the rigid board 52A, 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 60a as illustrated in FIGS. 13a and 13b can be used. FIG. 13a illustrates a sheet metal blank 60b from which the cradle 60a of FIG. 13b can be formed by bending. Rows of oval cut-outs in the blank 60b are separated by relatively narrow metal webs 63 so as to define fold lines enabling the sheet metal blank of FIG. 13a to be easily bent by hand to the shape of FIG. 13b. A pre-formed hole 65 is provided in the flange which becomes the base of the final cradle 60a to receive the screw 62 of FIG. 12, and slots 68a and 68b receive the longitudinal and lateral reinforcing rods respectively of the reinforcing lattice 28. The slots 68a and 68b may be at the same distance from the base as shown in FIG. 13b, in which case the cradle 60a 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 68a and 68b may be at mutually different heights to reflect the different levels of the longitudinal and lateral reinforcing rods.

FIG. 12 shows that the joists 18C 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 18D are considerably stronger than the separate J-section joists 18C even when those joists 18C 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 18E, similar to the hollow joists 18D supporting the top slab. The support between the hollow joists 18E 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 18E by wires. Depending on the length and diameter of the supporting wires, this will provide very limited sound transmission between the hollow beams 18E 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 18D and 18E 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 18D and 18E of FIG. 14 being formed as hollow box sections as illustrated, an improvement in strength, as compared with the simple J-section joists 18B and 18C 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 18D 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, 30a and 30b, with an outer side plate 30A 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, 18A, 18B, 18C, 18D and 18E of the various embodiments to the edge frame by welding or brazing. The joists 18 to 18E 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 18F 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 18F are welded or brazed to the edge frame 30 at their ends. Hangers 70 shaped as in FIG. 16 straddle the joists 18F 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 18F 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 18F, and one or more solid boards 95 placed over the tops of the joists 18F. 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 18F at intervals along the length of the joists 18F. 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 18F 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 60a, as already illustrated in FIG. 13 or in FIGS. 13a and 13b, are secured at intervals over each joist 18F and are secured to the joist 18F 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 60a 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 30a and 30b 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 30b of the edge frame 30 by strips 30c of expanded polystyrene inserted into the C-section component 30b 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 18F but isolated from the edge frame 30 by the expanded polystyrene strips 30c. 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 30c 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/m² 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 18F, 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.

Claims

1. A modular composite floor unit for an above-ground-level floor of a building, comprising:

an edge frame made from cold-rolled sheet metal edge members welded or brazed together to form an accurately sized and proportioned edge shuttering for the floor unit:

a cast cement-based or gypsum-based ceiling slab cast within the edge frame and encasing a first lattice of reinforcing rods or wires; and

a cast cement-based flooring slab spaced from the ceiling slab and cast within the edge frame, encasing a second lattice of reinforcing rods or wires which are welded or brazed at their ends to opposite edge members of the edge frame;

each of the ceiling and flooring slabs being supported across its width by an array of mutually parallel spaced metal joists which extend across the floor unit between the cast slabs and are welded or brazed at their opposite ends to opposite edge members of the edge frame; and

the space between the ceiling and flooring slabs containing a sound insulating material of lower density than that of the cast ceiling and floor slabs.

2. A floor unit according to claim 1, wherein the sound insulating material completely or substantially completely fills the inter-joist space between the cast ceiling and floor slabs.

3. A floor unit according to claim 2, wherein the sound insulating material comprises an array of blocks of lower density than that of the material of the cast ceiling and floor slabs.

4. A floor unit according to claim 3, wherein the blocks are blocks of a cinder-based or porous aggregate-based cement walling block material, expanded polystyrene, rockwool, compressed straw or balsa wood; or are plastic or cardboard boxes filled with loose particulate material such as rockwool, shredded newspaper, paper mache, chopped straw, glass fibre matting or reclaimed particulate rubber.

5. A floor unit according to claim 3, wherein the cast floor slab has been cast directly over the tops of the array of blocks and has been allowed to flow around and between the blocks of the array.

6. A floor unit according to claim 2, wherein the sound-insulating material comprises a layer of a lightweight sound absorbing material laid over the top of the cast ceiling slab and between the joists, and a solid board or an array of solid boards placed over the sound absorbing material, the board or boards being supported by the sound absorbing material or by the joists.

7. A floor unit according to claim 6, wherein the cast floor slab has been cast over the top of the solid board or boards.

8. A floor unit according to claim 1, wherein the sound insulating material only partially fills the space between the cast ceiling and floor slabs.

9. A floor unit according to claim 8, wherein the sound insulating material comprises a layer of lightweight sound absorbing material laid over the top of the cast ceiling slab and between the joists, and a solid board or an array of solid boards supported by the joists at a level spaced from the top of the layer of sound-absorbing material.

10. A floor unit according to claim 9, wherein the cast floor slab has been cast over the top of the solid board or boards.

11. A floor unit according to claim 1, wherein each joist has one longitudinal edge embedded in the material of the cast ceiling slab and an opposite longitudinal edge embedded in the material of the cast floor slab.

12. A floor unit according to claim 11, wherein the joists are made from cold-rolled C-section steel.

13. A floor unit according to claim 1, wherein each joist has one longitudinal edge embedded in the material of one of the cast slabs and an opposite longitudinal edge in the space between the cast slabs, in an alternating sequence of joists or pairs of joists across the floor unit.

14. A floor unit according to claim 13, wherein the joists are made from cold-rolled J-section steel, the inturned longitudinal edge of each section being that which is located in the space between the cast slabs.

15. A floor unit according to claim 13, wherein the joists having a longitudinal edge embedded in the material of the cast ceiling slab are offset laterally from those having a longitudinal edge embedded in the material of the cast floor slab, the wall portions of the respective joists extending more than half way across the space between the cast ceiling and floor slabs.

16. A floor unit according to claim 15, wherein pairs of adjacent joists, one having an edge embedded in the material of the cast ceiling slab and the other having an edge embedded in the material of the cast floor slab, are closely adjacent one another and separated by a greater distance from adjacent similar pairs of joists.

17. A floor unit according to claim 16, wherein free edges of adjacent joists having edges embedded in the material of the same cast slab, those free edges being the edges located in the space between the slabs, are joined together at intervals along the length of the joists by straps which transfer buckling loads between the joists.

18. A floor unit according to claim 10, wherein the joists are hollow box section joists or hot rolled parallel flanged channel joists.

19. A floor unit according to claim 18, wherein the cast ceiling slab is supported by the joists across its width by hangers suspended from the joists or from some of the joists and supporting the first lattice of reinforcing rods or wires across the width of the cast ceiling slab.

20. A floor unit according to claim 19, wherein the hangers are wire hangers.

21. A floor unit according to claim 19, wherein the hangers are metal straps which pass over the joists from which they are suspended and hang down on opposite sides of those joists, having transverse slots in lower ends of the hangers which hook around and support the reinforcing rods or wires of the first lattice.

22. A floor unit according to claim 18, wherein the cast floor slab is supported by the joists across its width by being cast on a solid board resting directly on the top of those joists.

23. A floor unit according to claim 22, wherein the cast floor slab is anchored to the joists which support it across its width by an array of anchorage members which are connected to the second lattice of supporting rods or wires and are screwed to the joists which support the cast floor slab through the solid board.

24. A floor unit according to claim 23, wherein each of the box section joists supports both the cast ceiling and floor slabs.

25. A floor unit according to claim 23, wherein alternate ones of the box section joists across the width of the floor unit support the cast ceiling slab and intermediate ones of the box section joists support the cast floor slab.

26. A floor unit according to claim 18, wherein the box section joists contain a sound-absorbing material.

27. A floor unit according to claim 10, wherein embedded in the cast ceiling slab and/or the cast floor slab are reinforcing diagonal cross-struts welded at their ends to the edge frame.

28. A floor unit according to claim 27, wherein the reinforcing diagonal cross-struts are made from ribbons of sheet metal that have been unrolled from a roll and prevented from curling by imparting a longitudinal crease thereto.

29. A floor unit according to claim 10, wherein each of the cast ceiling and floor slabs has a thickness of from 50 to 100 mm, and the space between the cast ceiling and floor slabs is from 150 to 300 mm.

30. A floor unit according to claim 29, wherein each of the cast ceiling and floor slabs has a thickness of about 65 mm.

31. A floor unit according to claim 29, wherein the space between the cast ceiling and floor slabs is about 225 mm.

32. A floor unit according to claim 10, wherein the surface finish of the underside of the cast ceiling slab is a paper or fabric material that has been laid over the casting surface on which the ceiling slab has been cast.

33. A floor unit according to claim 10, wherein the surface finish of the underside of the cast ceiling slab is the surface finish of a board or plate that has been covered with a mould release agent before casting the ceiling slab.

34. A floor unit according to claim 10, wherein the surface finish of the top surface of the cast ceiling slab is a power float finish.

35. A method for the manufacture of a floor unit, which comprises:

forming an edge frame by welding or brazing together cold-rolled sheet metal edge members;

welding or brazing to the edge frame an array of mutually parallel spaced metal joists;

welding or brazing to the edge frame the ends of the reinforcing rods or wires of the first lattice;

casting the ceiling slab by pouring wet concrete or gypsum-based plaster into the shuttering created by the edge frame, to a depth sufficient to encase a first inturned lip of the edge frame, to encase the first lattice of reinforcing rods or wires, and to embed lower longitudinal edges of some or all of the parallel spaced joists of cold-rolled sheet metal or of hangers suspended from those joists;

placing between the parallel spaced joists of cold-rolled sheet metal the sound insulating material of lower density than the concrete of the cast slabs and optionally the array of solid boards to form a base for the cast ceiling slab;

welding or brazing to the edge frame the ends of the reinforcing rods or wires of the second lattice;

pouring wet concrete over the top layer of sound insulating material or over the tops of the solid boards to a depth sufficient to encase a second inturned lip of the edge frame, to encase the second lattice of reinforcing rods or wires and optionally to embed upper longitudinal edges of some or all of the parallel spaced joists of cold-rolled sheet metal; and

providing a smooth surface finish to the top of the cast floor slab using a power float.