A multi-layer prefabricated wall panel for modular building and a method for manufacturing the same. The panel comprises a load-bearing and vapor barrier core layer; an interior insulating and utility installation layer bonded to the inner face of the core layer; an exterior sheet of a first rigid building material bonded to the interior insulating layer; and an interior sheet of a second building material bonded to the outer face of said core layer. The method for manufacturing the wall panel comprises the steps of forming a stack of the panel layers with adhesive coating between successive layers and bonding the layers to each other by uniform compressing forces.

BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention generally relates to modular construction and more specifically to prefabricated wall panels for use in modular buildings, to a method for fabricating the panels and to building components including the same. 
         [0003]    2. Discussion of the Related Art 
         [0004]    Industrialized building methods are becoming more popular over the last decades in the construction industry for both domestic and public buildings, Also referred to as large-panel construction, such methods utilize high degree of factory prefabrication in order to reduce site work and improve the quality and speed of construction. The large prefabricated panels are transported from the factory to the construction site where they are assembled in a modular manner to form walls, ceiling and/or floors. Optionally, the panels may be assembled, at least partially, at the factory site to form larger building components, where limitation to the size of pre-assembled components is mainly due to transportation possibilities. In either case, work and debris at the construction site is reduced significantly as compared with traditional and/or conventional construction methods. 
         [0005]    Various prefabricated large panels for modular building are known in the art, including multi-layer sandwich panels. However, available building panels and the manufacturing methods for fabricating such panels still suffer from a number of drawbacks. In particular, fabrication of known large building panels involves mechanical fastening for joining the different components and/or layers of the panel to each other. Thus, not only the assembling of the panels to form larger building components walls requires manual work, but in many cases the fabrication of the panel themselves require manual work which slows down the manufacturing and increase costs. Furthermore, the panels so produced may suffer from uneven fastening and consequently from insufficient flatness and from weak points or seaming lines susceptible to instabilities. Moreover, many times known prefabricated panels do no provide all the properties required from building components in terms of stability, insulation, easiness of utility installation, etc. 
         [0006]    Therefore there still exists a need for improved prefabricated building panels and for an improved process for manufacturing the same. 
         [0007]    Accordingly, it is the object of the present invention to provide a building panel which is fabricated as one solid integral piece, which is structurally strong and dimensionally stable, which provides high level of thermal and acoustic insulation and is moisture and vapor resistant as well as fire resistant. 
         [0008]    A further object of the invention is to provide a building panel having the above features which allows for easy installation of utility lines such as electricity wiring and plumbing and which provides flexibility at the planning and manufacturing stage so that it can be easily tailored to specific needs and allows for future changes. 
         [0009]    An additional object of the invention is to provide prefabricated panels having the above features, which allow for joining individual panels to each other as well as to floor and ceiling by welding rather than by mechanical fasteners. 
         [0010]    Yet, a further object of the invention is to provide a method for fabricating large-size and extra-large-size panels having the above features, which minimizes manual work at the fabrication site as well as reduces assembling and finishing work at the construction site. 
         [0011]    Other advantages of the invention will be apparent from the following description. 
       SUMMARY OF THE PRESENT INVENTION 
       [0012]    The present invention provides improved prefabricated wall panels for modular building and an improved method for fabricating the same. 
         [0013]    One aspect of the present invention is a multi-layer prefabricated wall panel having an interior planar surface and an exterior planar surface. The panel comprises: a load-bearing and vapor barrier core layer having an inner face and an opposite outer face; an interior insulating and utility installation layer bonded to the inner face of the core layer; an exterior sheet of a first rigid building material bonded to the interior insulating layer; and an interior sheet of a second building material bonded to the outer face of the core layer. 
         [0014]    The core layer preferably comprises at least two tubular metal members and one or more interior sandwich panels extending between the at least two metal members, wherein the one or more sandwich panels preferably comprise thermal insulating material, selected from the group consisting of mineral wool, polymer foam and timber, sandwiched between two flat metal skins. The wall panel further comprises a top frame member and a bottom frame member welded to the least two metal tubular member for forming a frame around the panel. 
         [0015]    The interior insulating and utility installation layer comprises a plurality of channels extending from top to bottom thereof for accommodating utility lines. In accordance with one embodiment of the invention the interior insulating and utility installation layer comprises a plurality of spaced apart elongated blocks of insulating material. In accordance with another embodiment, the insulating and utility installation layer comprises a mattress-like body made of insulating material provided with a plurality of channels extending the entire length of the mattress-like body and having openings at the top and bottom edges of the body. The one or more sandwich panels and the metal members are having substantially the same length and thickness so as to form in combination a solid layer with two opposite flat faces. Preferably, the exterior sheet is selected from the group consisting of a cement board, a timber board, a metal sheet and a reinforced plastic sheet and the interior sheet is selected from the group consisting of a gypsum board, a cement board and a timber board. The thickness of the wall panel, defined as the distance between interior and exterior planar surfaces of the wall panels is preferably in the range of 140 to 260 mm. 
         [0016]    A second aspect of the invention is a method for fabricating a building panel, the method comprising the steps of: forming a stack of horizontally placed building layers in a successive manner; coating the upper surface of each layer with an adhesive before the next layer is placed thereon; and subjecting said stack to uniform compression forces. The method may further comprise the step of incorporating a frame metal into said stack of building layers. The step of subjecting the stack to uniform compression forces may be performed by means of a vacuum manifold or alternatively by means of a compression plate. The stack preferably comprises a first sheet of a building material, a core layer, an insulating and utility installation layer and a second sheet of building material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: 
           [0018]      FIGS. 1 and 2  are a partial isometric view and a partial frontal view, respectively, of a wall panel according to one embodiment of the present invention, illustrating the multi-layer structure of the panel; 
           [0019]      FIG. 3  is a horizontal cut of the wall panel of  FIGS. 1 and 2  taken along line  3 - 3  of  FIG. 1 ; 
           [0020]      FIG. 4  is a vertical cut of the panel of  FIGS. 1 and 2  taken along line  4 - 4 ; 
           [0021]      FIG. 5  is an isometric view of the metal frame in accordance with the embodiment of  FIG. 1  showing the upper and lower framing and the vertical load-bearing metal members; 
           [0022]      FIG. 6A and 6B  illustrate two embodiments of the interior sandwich panels of the core layer; 
           [0023]      FIG. 7  is a vertical cross sectional view of a building comprising the wall panel of  FIG. 1 , showing connections of the wall panel to floor and ceiling and a utility line running through the inner insulating layer; 
           [0024]      FIG. 8  is a horizontal cut through a wall panel of the invention in accordance with a second embodiment; 
           [0025]      FIG. 9  is a partial isometric view of the inner insulating layer in accordance with the second embodiment depicted  FIG. 8 ; 
           [0026]      FIG. 10  is a vertical cross sectional view through a building comprising of wall panels of  FIG. 8 ; 
           [0027]      FIGS. 11A and 11B  are frontal and side cross-sectional views, respectively, illustrating the formation process of a panel in accordance with the novel method of the invention; 
           [0028]      FIG. 12A  is a frontal view of a wall panel of the invention comprising pre-designed a window opening; 
           [0029]      FIG. 12B  is a horizontal cross section through line B-B of  FIG. 13A . 
           [0030]      FIG. 13A  is a horizontal cut through a wall in accordance with the invention, showing two adjacent panels joined together to form a wall; 
           [0031]      FIG. 13B  is a horizontal cut through a building corner in accordance with the invention, showing perpendicularly joined panels. 
       
    
    
       [0032]    It will be realized that the drawings are not drawn to scale and that the aspect ratio of the elements illustrated, as well as the dimensional ratios between different elements, are distorted in order to better demonstrate various features of the invention. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0033]    The present invention provides improved wall panels and improved methods for manufacturing the same. The panels of the invention comprise multiple layers, each designed to impart the panel a particular functionality and/or benefit, while the method of the invention for fabricating the multi-layer panel as one integral unit with no mechanical fasteners between elements, further imparts the panel structural stability and enhanced flatness and smoothness. In accordance with the novel method of the invention, the different layers of the panel are bonded to each other under pressure by one compression step rather than being fastened to each other by mechanical fasteners such as screws and bolts. Besides enhancing stability and appearance, this allows for manufacturing extra large panels which significantly reduces the number of joints and consequently reduces site work and cost as well as the amount of defects that might be introduced during joints assembling. 
         [0034]    Referring to the  FIGS. 1-4 , there is shown a wall panel, generally designated  10 , in accordance with one embodiment of the invention. Wall  10  comprises a core layer  20 , an interior insulating and utility installation layer  30 , an exterior sheet  40  and an interior sheet  50 . Layers  20 ,  30 ,  40  and  50  are bonded to each other to form one integral panel having two opposite smooth planar surfaces defined by the outward faces of sheets  40  and  50 . Panel  10  further comprises a top frame member  64  and a bottom frame member  62  which together with members  25  of core layer  20  form a peripheral metal frame  60  that encompasses panel  10  to enhance the panel structural stability and to allow weld-joining to adjacent panels as well as to ceiling and floor. Frame  60  is illustrated in  FIG. 5 . 
         [0035]    Core layer  20  comprises at least two rectangular, preferably square, tubular metal members  25  extending about the full length L of panel  10  and one or more interior sandwich panels  22  extending between members  25  and in contact therewith to fill the space therebetween. Members  25  and panels  22  are of the same length and thickness so as to form a mattress-like layer having two opposite flat faces, a flat top edge and a flat bottom edge. The outward sides  26  of members  25  define the side edges of layer  20 . Members  25  constitute the main load-bearing construction elements of panel  10  and therefore should be distributed at appropriate intervals. Thus, depending on the size of panel  10  and on the total construction requirements, one or more additional members  25  may be incorporated into layer  20  between panels  22 . In practice, frame members  62  and  64  are welded to members  25  to form structural metal frame  60 , which is incorporated into panel  10  at the manufacturing process as explained below. 
         [0036]    Interior panels  22  are sandwich panels comprising an insulating core material  24  sandwiched between opposite skins  26  and  28 . Preferably, skins  26  and  28  are metal sheets, preferably 0.2-0.8 mm thick steel sheets. Skins  26  and  28  serve as vapor barrier between the interior and exterior of panel  10 . Insulating material  24  may be any insulating material and may be in the form of prefabricated blocks or as a bulk material. Possible materials for insulator  24  include mineral wool, expanded or extruded polymer foam or polymer fibers, timber blocks or wood fibers and the like. Preferably, insulator  24  is mineral wool of 100-140 kg/m 3  density. However, insulator  24  may be selected in accordance to the thermal and acoustic insulation requirements at the particular location where the building is to be built. Thus, for rough weather conditions where thermal insulation is crucial, insulator  24  is preferably polyurethane or polystyrene foam while under milder weather conditions insulator  24  is preferably mineral wool, being a better acoustic insulator. Sandwich panels  22  may be prefabricated off-the-shelf panels or may be especially fabricated to suit particular insulation and dimensional requirements. Alternatively, when insulating material  24  is in the form of blocks, panels  22  may be formed during the manufacturing process of panel  10 . The width (horizontal dimension) of panels  22  can vary and is mainly determined by the width of available metal sheets. When layer  20  comprises more than one panel  22 , panels  22  are abutted against each other to form continuous insulating layer between the two metal skins.  FIGS. 6A and 6B  illustrate two possible embodiments for abutting and joining sandwich panels  22  to each other to form a continuous layer. According to the embodiment illustrated in  FIG. 6A , the exterior skins  26  and  28  of panel  22   a  extend to some extent  26   a  and  28   a,  respectively, beyond insulator  24  so that when the panels are abutted against each other, portion  26   a  overlap skin  26  of adjacent panel and portion  28   a  overlap skin  28  of adjacent panel, in a slate-like manner. According to the embodiment illustrated in  FIG. 6B , additional skins  23  are placed against both skins  28  and  26  along the seam line between adjacent panels  22   b.  In accordance with both embodiments, the continuous overlapping exterior contact between adjacent panels reinforces layer  20  and enhances its structural stability. It will be realized that since the metal skins of panels  22  are only a fraction of a millimeter thick, the double-skin overlapping areas at the vicinity of seam lines do not affect the face smoothness of layer  20  to any significant extent. 
         [0037]    Next to core layer  20  toward the interior face of panel  10 , is insulating and utility installation layer  30 . In accordance with the embodiment illustrated in  FIGS. 1-7 , the interior insulating layer  30  comprises a plurality of spaced-apart elongated insulating blocks  32 , preferably of a rectangular cross section, disposed between core layer  20  and interior sheet  40 . Blocks  32  are preferably made of a water-proof closed-cell polymer foam such as expanded polystyrene. However, at areas where additional strength is required, such as for example where cupboards are to be suspended from the wall, polymer blocks may be replaced by structured wood blocks in metal profiles for enhancing anchoring force of cupboard to wall. Elongated blocks  32  of length Li extend longitudinally between bottom frame member  62  and horizontal beam  67  of top frame member  64 . Length Li corresponds to the interior height of the building. Blocks  32 , preferably about 40 to 100 mm thick and about 100 to 300 mm wide, are equally spaced, leaving elongated channels  33  therebetween. Channels  33  are of preferably narrower dimensions than that of blocks  32  so that layer  30  comprises of about 75% solid and about 25% space. Channels  33  allow for installation of utility lines such as electrical wiring, plumbing pipes, communication lines etc., as best seen in  FIG. 7 . A plurality of openings  65   a  and  65   b  provided at bottom frame  62  and beam  67 , respectively, in alignment with channels  33  allow for threading the utility lines through the frame for connection to utility hubs installed under floor and/or above ceiling. 
         [0038]    The interior faces of blocks  32 , opposite the faces in contact with panels  22 , are covered by interior sheet  50  of length Li. Interior sheet  50  may be of any building material suitable as interior wall including a gypsum board, a cement board, a timber board and the like. Preferably sheet  50  is an off-the-shelf gypsum board of 9 to 32 mm thickness. It will be appreciated that panel  10  requires no further finishing on the interior side of the building as it is well known in the art to cover inner surfaces with gypsum boards. Exterior sheet  40 , of length L, bonded on the outward surface of core layer  20  may be of any durable building material suitable for withstanding the climate conditions where the building is to be located, including cement, timber, metal, reinforces polymer sheets and the like. Preferably, sheet  40  is a cement board of 7.5 to 20 mm thick. It will be appreciated that although not necessary, any type of cladding (i.e. siding, stucco, EIFS, brick, stone) may be applied to the interior and/or exterior faces of the panel similar to traditional construction methods. The cladding may be applied at the manufacturing site or may be applied later at the construction site after the building is erected. It will be appreciated that the structure of wall  10  is designed such that there is minimum continuous metal thermal conductive path from one face of the wall to opposite face. It will be further appreciated that the interior sandwich panel of the core layer serve as vapor barrier between inside and outside. 
         [0039]    Referring to  FIG. 5 , there is illustrated metal frame  60  that encompasses the peripheral edges of layers  20  and  30  of panel  10 . Layers  40  and  50  are bonded to the opposite outermost surfaces of frame  60  as best seen in  FIGS. 4 and 7 . Frame  60  comprises a bottom frame member  62 , an upper frame member  64  and two vertical load-bearing members  25 . Frame members  62  and  64  extend the full width of panel  10  and are each having a profile comprising of vertical and horizontal sections configured to receive the layers of panel  10  and to allow metal welding to corresponding metal frames in floor and ceiling. Thus, top member  64  comprises upper and lower L-shaped profile sections  66  and  67 , respectively, directed at opposite directions and connected by vertical section  61 . Similarly, bottom frame member  62  comprises two L-shaped profile sections  63  and  68  connected by vertical section  69 . Core layer  20  of length L is accommodated between the horizontal sections of sections  66  and  63  while blocks  32  of layer  30 , having length Li, are inserted between L-shaped sections  67  and  68  and are positioned between openings  65   a  and  65   b  to provide openings into the channels  33  that are formed between the blocks as best seen in  FIG. 1 . 
         [0040]    The overall combined thickness T of panel  10  is preferably in the range of 120 to 300 mm, where the core layer  20  is about 80-140 mm thick, the interior insulating layer  30  is about 40-100 mm thick, the interior sheet  50  is about 9-32 mm thick and the exterior sheet is about 7.5 to 20 mm thick. The vertical dimensions of panel  10 , L and Li, correspond to the exterior and interior heights of the building, respectively, and are determined according to construction plan. Preferably, L is in the range of 3 to 4 m, while Li is 20 to 60 cm shorter. The horizontal dimension of panel  10  can be of up to 15 m, meaning that for some buildings, depending on the building size, a complete wall can be prefabricated as one integral piece having continuous smooth flat surfaces. It will be appreciated that the possibility to provide an extra-large multi-layer wall panel significantly reduces assembling work and cost. It will be also appreciated that as no mechanical fasteners are required for joining the multiple layers to each other or for joining adjacent portions of the same layer in order to form a larger component, the structural integrity and stability of the panel as well as surface flatness and smoothness, are significantly enhanced compared with prior art panels. Furthermore, the unique multi-later structure of panel  10  provides high level of thermal and acoustic insulation, vapor barrier properties, easiness of installation of utility lines and enhanced flexibility in tailoring the wall panels to fit specific construction requirements. 
         [0041]    Referring to  FIG. 7 , there is depicted a vertical cut through a building having walls made of panels  10 , showing panel  10  joined to floor  80  and roof  90 . As can be seen bottom profile  62  of panel  10  is welded to foundation frame  82 , which also supports the floor reinforcing beams  84 . At its upper end, panel  10  is welded to reinforcing roof beams  92 . A utility line, designated  70 , running through channel  33  of layer  30 , may connect to a central utility line  71  that runs under the flooring  86  through opening  65   b  in frame  60  and/or to utility line  72  running above ceiling  96  through opening  65   a.  Utility line  70  may be an electrical wiring, a water or a heating pipe, a communication line such as an optic fiber or a telephone line, etc. It will be appreciated that panel structure allows for easy installation of such utility lines to be connected to central utility hubs under floor or above ceiling, by providing prefabricated infrastructure channels at a relatively high density. Layer  30  further facilitates guiding the utility lines and keeping them separated from each other. 
         [0042]    An alternative embodiment of panel  10 , generally designated  110 , is illustrated in  FIGS. 8-10 . In accordance with this embodiment, the insulating utility-installation layer  30  of panel  10  is replaced by layer  130 . Layer  130  comprises a solid body  131  of insulating material provided with a plurality of prefabricated utility channels  132  that run the full length of body  131  between top and bottom edges, extending between top openings  135  and bottom openings (not shown). Layer  130  is preferably made of expanded polystyrene. Channels  132  are preferably of oval cross section and are located closer to the inner face of layer  130 . The other layers of panel  110  are similar to layers  20 ,  40  and  50  described above in association with  FIGS. 1-6 . However, in accordance with this embodiment, upper and lower frame members  164  and  162  are simpler in shape than frame members  62  and  64  of panel  10  and do not include openings. Referring to  FIG. 10 , unlike frame members  62  and  64 , frame members  162  and  164  end toward the interior face of the panel with horizontal sections  161  and  163 , respectively, and do not include a vertical section. Sections  161  and  163  extend up to openings  135  in layer  130  so as not to cover the openings. It will be realized that since layer  130  comprises one integral piece, there is no need to provide further vertical elements in frames  162  and  164 . Embodiment  110  has the advantage of reducing panel assembling time as compared with panel  10  since layer  130  is placed as one piece instead of placing a plurality of separated blocks. Layer  130  also has the advantage of continuous and larger contact surfaces with adjacent layer, thus increasing the panel structural stability. Furthermore, in accordance with the structure of panel  110 , sheet  50  is supported by layer  30  only and is not in contact with metal frame  60 , such that there is no metal continuity between outer and inner sheets  40  and  50 . This prevents any thermal conductivity between interior and exterior faces and provides higher level of thermal isolation. 
         [0043]    Turning now to  FIGS. 11 , the present invention provides a novel method for fabricating multi-layer building panels by forming a horizontal stack of the multiple layers with intermediate layers of adhesive therebetween, and subjecting the stack to pressure, thereby bonding the layers to each other in a single operation. Compression may be applied either mechanically by a compression plate or by means of a vacuum device. In either case, the panels are uniformly pressurized.  FIGS. 11  demonstrate the fabrication process of a panel in accordance with embodiment  10 . It will be easily realized that the fabrication of a modified panel, such as panel  110 , is performed in a similar manner. In accordance with the panel fabrication method of the invention, the multiple layers are orderly placed horizontally on a working table  200  comprising a horizontal working plate  205  supported on legs  204 . The layers are placed one above the other wherein the yet-free upper surface of each layer is sprayed to be covered by a layer of adhesive before the next layer is placed over it. The steel frame, consisting of the two tubular columns and the top and bottom frame members, is incorporated into the panel at the appropriate stage in accordance with the specific structure of the panel in hand. Thus, referring to  FIGS. 11 , demonstrating fabrication of panel  10 , the first layer to be placed on working surface  205  is interior sheet  50 . The sheet is sprayed with adhesive layer and frame  60  is placed over its periphery. Two vertical supporting beams  208  and  210  configured to conform with the dimensions and with the upper and lower profiles of the multi-layer panel, are mounted along opposite sides of table  200  to support the panel during fabrication process and to facilitate alignment of the layers. Beams  208  and  210  are preferably removably mounted to plate  205  such as to allow the selection of beams in accordance with the panel in hand. After frame  60  is appropriately placed over sheet  50 , supported on beams  208  and  210 , the plurality of insulating blocks  32  are placed over sheet  50  to form insulating layer  30 . Blocks  32  are inserted between sections  68  and  67  of frame  60  which guide appropriate placing and help to align the blocks. Next, blocks  32  are sprayed by adhesive and core layer  20  is placed over layer  30  and over sections  61  and  69  of frame  60 . The upper surface of layer  30  is then sprayed to be coated by an additional adhesive layer and exterior sheet  40  is placed over layer  20 , peripherally supported on and in alignment with the outermost surface of frame  60 . A pressure P is then uniformly applied on the multiple layers until the adhesive is cured for reinforcing bonding between layers, forming one integral piece. Preferably the pressure applied is in the range of 0.2 to 0.6 Kg/cm 2 . It will be easily realized that a panel of structure  110  is similarly fabricated with the exception of mounting frame  160  onto layer  30  after the later is already placed over sheet  50 . It will be also realized that layers  20 ,  40  and  50 , as well as layer  130  in case of embodiment  110 , may consist of one piece or may consist of a number of portions abutted against each other to form a continuous layer when placed over table  200 . It will be appreciated that the dimensions of such portions is mainly determined by market availability. The adhesive used to bond the layers to each other is preferably sprayable one-component or tow-component polyurethane adhesive such as polyurethane adhesives distributed by Sika AG. 
         [0044]    As mentioned above, pressure P may be applied by a compression plate  125  pressed from above, as illustrated in  FIG. 11 , or alternatively may be applied by means of a vacuum manifold (not shown) coupled to table  200 . In the later case, the vacuum manifold may be coupled to peripheral channels that circumferences plate  205  and open inwardly. A flexible air-impermeable cover is then used for entirely covering the table, including the table channels and the pre-assembled layers laying on the table, in an air-tight manner. As the vacuum manifold is activated, the cover is evacuated to form sub- atmospheric pressure under the cover to apply uniform pressure on the pre-assembled panel. 
         [0045]    It will be appreciated that the method of the invention allows for enhanced flexibility in designing a wall panel in terms of the panel dimensions and the panel specific structure, to be tailored to specific requirements depending on location of the building and the location of the specific panel in relation to the building. It will be further realized that the fact that during assembling, the layers of the panel are horizontally displayed one following the other, enhances the easiness by which different materials may be selected for specific zones within the same panel in order to optimize the panel functionality. For example, when knowing in advance where cupboards are to be installed, the insulator material of interior insulating layer  30  (or  130 ) at the known locations may be specifically selected as wood blocks, instead of the polystyrene foam, for enhancing connection strength between cupboard and wall. Further, threading of utility lines may be performed while the panel is still in horizontal position or even before completion of the assembling process. 
         [0046]      FIG. 13  illustrates a wall panel provided with a prefabricated opening adapted to receive a window frame. Panel  310  is a composite panel of substantially the same multi-layered structure as of panel  10  or panel  110  described above. Portions of core layer  20  and insulating layer  30  (or  130 ) are cut-out to form an opening  350 . Two vertical metal studs  328  extending the full length of the panel are added to metal frame  360  for reinforcing the panel around the opening. It will be realized that the portions of layers  20  and  30  need not actually being cut out but instead layers portions of appropriate size may be placed above and below the opening during fabrication. A window frame  352  is already incorporated into the panel. In order to protect frame  352  during transportation, inner and outer sheets  50  and  40  fully cover the panel when fabricated. After installation of the panels at the construction site, portions  41  and  51  (shown in broken lines in  FIG. 13B ) are cut out to expose the opening and for mounting the window on window frame  352 . It will be easily realized that the particular size and location of the window opening may varied and that a door opening may be similarly pre-prepared. 
         [0047]      FIGS. 13A and 13B  are horizontal cuts through a wall and a wall corner, respectively, of a building made of the panels of the invention, showing the joints between panels. Panels  10   a  and  110   b  and  110   b  are joined to each other by welding tubular members  25   a  and  25   b  of adjacent panels either in a parallel for forming a continuous wall or perpendicularly for forming a corner. During fabrication, core layer  20  at the vicinity of tubular members  25  as well as members  25  themselves, is left exposed, namely it is not covered by the other layers, in order to allow accessibility of the welding device to members  25  during weld-joining. After the panels are joined, complementary layer pieces  38 ,  48 , and  58  for a continuous wall joint and pairs  34 ,  44  and  54  for a corner joint, are added for covering the joints. 
         [0048]    It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims which follow.