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CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 10/693,541, filed on Oct. 24, 2003, the contents of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     This invention relates generally to metal structural members for use in building construction, and more particularly to metal roof trusses for construction of roof framing for supporting roofs. 
     A roof truss generally comprises two or more top chord members and a bottom chord member. The ends of the top chords are secured together, and the ends of the bottom chord are connected to the lower, free ends of the top chords for forming the exterior of the roof truss. One or more web members span between and interconnect the top and bottom chords. The web members are secured at their ends to the top chords and to the bottom chord. 
     In building construction, a plurality of trusses are set out across a building frame. When erected upon the building frame, the bottom chord spans the wall frames of the building and is fixed to the top plate of the wall frames. The sub-roof material is then fastened to the top chords, and ceiling material may be fastened to the bottom chord. The combined load of the roof trusses, and the roofing and ceiling material attached to the trusses, is transferred through the outer edges of the trusses to the top plate of the wall frames. 
     In the past, roof trusses have been constructed of wooden chords and web members. More recently, various types of building systems incorporate metal trusses. 
     Metal trusses include chord members and web members rolled from metal sheets and formed into substantially rectangular U-shaped or C-shaped channels. The open sides of the chord members are adapted to receive the ends of the other chord members and the web members. The ends of the chords and web members are then fastened together for securing the truss elements in position. The materials cost for metal trusses is competitive with other building materials. Using metal as the material of construction also has a number of other advantages, including relatively stable price, strength, flexibility, durability, light weight, reliability, minimum waste in use, and noncombustability. 
     A significant problem with the use of metal trusses is the high installed cost. One factor influencing the installed cost of metal trusses is the thermal performance of metal, which is well below that of lumber framing when using standard framing techniques. This is due to the thermal conductivity of metal and the potential for thermal bridging. For example, steel conducts heat more than 300 times faster than wood. The rapid heat flow through steel reduces the insulating value of cavity insulation between 53 and 72%. With respect to metal roof trusses, heat passing through the ceiling material, if present, migrates into the bottom chord. Usually the bottom chord is covered with insulation spread on the attic floor, but heat can still be transferred into the truss at the points where the web members are fastened to the bottom chord. Heat is then conducted by the web members into the attic area and to the top chord at the underside of the roof. The result is a wicking effect whereby heat is transferred out of the building. Special considerations are necessary to reduce the tendency of metal roof trusses to transfer heat in this manner. 
     As a solution, some builders using metal wall frame construction, but top the building frame with wood roof trusses in order to minimize thermal bridging. However, this defeats the purpose of opting for metal frame construction. Other common solutions to improve energy efficiency include increasing the amount of cavity insulation and applying insulation to the exterior of the metal frame elements to provide a “thermal break” to the heat conducting path. Other means for reducing heat loss include punchouts in the chord members, wide truss spacing, and using thicker gauge steel. All of these approaches add to the cost, installation time and the difficulty of using metal roof trusses. 
     For the foregoing reasons, there is a need to provide a metal roof truss for use in a metal frame building system that is more energy efficient. Ideally, the new metal roof truss should be inexpensive, light weight, and adapted to mass production. 
     SUMMARY 
     According to the present invention, a metal truss is provided comprising a pair of elongated top chord members each having a first end and a second end. The top chord members are connected to each other at the first ends. A first elongated bottom chord member is connected at its ends to the top chord members adjacent the second ends of the top chord members. A second elongated bottom chord member is connected at its ends to the top chord members adjacent the second ends of the top chord members such that the second bottom chord member is spaced below the first bottom chord member. At least one web member is positioned between and interconnecting at least one top chord member and the first bottom chord member. One end of the web member is connected to the at least one top chord member and the other end of the web member is connected to the first bottom chord member. 
     Also according to the present invention, a metal frame building system is provided including a building frame comprising a plurality of wall frames having top ends. The building system includes a metal truss comprising a pair of elongated top chord members each having a first end and a second end. The top chord members are connected to each other at the first ends. A first elongated bottom chord member is connected at its ends to the top chord members adjacent the second ends of the top chord members. A second elongated bottom chord member is connected at its ends to the top chord members adjacent the second ends of the top chord members such that the second bottom chord member is spaced below the first bottom chord member. At least one web member is positioned between and interconnecting at least one top chord member and the first bottom chord member. One end of the web member is connected to the at least one top chord member and the other end of the web member is connected to the first bottom chord member. The plurality of trusses are adapted to be erected upon the building system frame such that the second bottom chord member spans the wall frames and is connected to the top ends of the respective wall frames. 
     Further according to the present invention, a building comprises a frame including a plurality of wall frames, each of the wall frames having a top end. A metal truss comprises a pair of elongated top chord members each having a first end and a second end and connected to each other at the first end. A first elongated bottom chord member is connected at its ends to the top chord members adjacent the second ends of the top chord members. A second elongated bottom chord member is connected at its ends to the top chord members adjacent the second ends of the top chord members such that the second bottom chord member is spaced from the first bottom chord member. At least one web member is positioned between and interconnecting at least one top chord member and the first bottom chord member. One end of the web member is connected to the at least one top chord member and the other end of the web member connected to the first bottom chord member. A plurality of the metal trusses are erected upon the frame such that the second bottom chord member spans at least two of the wall frames and is connected to the top ends of the respective wall frames. Roof material is fastened to the top chord members. 
     Still further according to the present invention, a metal truss is provided comprising a plurality of elongated top chord members connected to each other end to end so that the connected top chord members have two free ends. A first elongated bottom chord member is connected at its ends to the top chord members adjacent the free ends of the connected top chord members. A second elongated bottom chord member is connected at its ends to the top chord members adjacent the free ends of the connected top chord members such that the second bottom chord member is spaced from the first bottom chord member. At least one web member is positioned between and interconnecting at least one top chord member and the first bottom chord member. One end of the web member is connected to the at least one top chord member and the other end of the web member connected to the first bottom chord member. 
     According to another embodiment of the present invention, a metal truss is provided comprising a pair of elongated top chord members connected together at their first ends, a first elongated bottom chord member, and means for connecting the first bottom chord member to the top chord members adjacent the second ends of the top chord members. Means are also provided for connecting a second elongated bottom chord member to the first bottom chord member such that the second bottom chord member is spaced from the first bottom chord member. At least one web member is positioned between and interconnecting at least one top chord member and the first bottom chord member. One end of the web member is connected to the at least one top chord member and the other end of the web member is connected to the first bottom chord member. 
     Also according to the other embodiment of the present invention, a metal frame building system is provided including a plurality of wall frames having top ends. The building system includes a metal truss comprising a pair of elongated top chord members connected together at their first ends, a first elongated bottom chord member, and means for connecting the first bottom chord member to the top chord members adjacent the second ends of the top chord members. Means are also provided for connecting a second elongated bottom chord member to the first bottom chord member such that the second bottom chord member is spaced from the first bottom chord member. At least one web member is positioned between and interconnecting at least one top chord member and the first bottom chord member. One end of the web member is connected to the at least one top chord member and the other end of the web member is connected to the first bottom chord member. A plurality of trusses are adapted to be erected upon the building system frame such that the first bottom chord member spans at least two of the wall frames and is connected to the top ends of the respective wall frames, and the ends of the second bottom chord member extend between the inner surfaces of the wall frames. 
     Further according to the other embodiment of the present invention, a building comprises a frame including a plurality of wall frames, each of the wall frames having a top end. A metal truss comprises a pair of elongated top chord members connected together at their first ends, a first elongated bottom chord member, and means for connecting the first bottom chord member to the top chord members adjacent the second ends of the top chord members. Means are also provided for connecting a second elongated bottom chord member to the first bottom chord member such that the second bottom chord member is spaced from the first bottom chord member. At least one web member is positioned between and interconnecting at least one top chord member and the first bottom chord member. One end of the web member is connected to the at least one top chord member and the other end of the web member is connected to the first bottom chord member. A plurality of trusses are adapted to be erected upon the frame such that the first bottom chord member spans at least two of the wall frames and is connected to the top ends of the respective wall frames, and the ends of the second bottom chord member extend between the inner surfaces of the wall frames. Roof material fastened to the top chord members. 
     Still further according to another embodiment of the present invention, a metal truss is provided comprising a plurality of elongated top chord members, the top chord members connected to each other end to end so that the connected top chord members have two free ends. Means are provided for connecting a first elongated bottom chord member to the top chord members adjacent the second ends of the top chord members. Means are also provided for connecting a second elongated bottom chord member to the first bottom chord member such that the second bottom chord member is spaced from the first bottom chord member. At least one web member positioned between and interconnecting at least one top chord member and the first bottom chord member. One end of the web member is connected to the at least one top chord member and the other end of the web member is connected to the first bottom chord member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference should now be had to the embodiment shown in the accompanying drawings and described below. In the drawings: 
         FIG. 1  is a schematic view of a roof truss assembly according to the present invention; 
         FIG. 2  is an elevational end view of a truss member for use in the truss assembly according to the present invention; 
         FIG. 3  is a schematic view of the roof truss assembly shown in  FIG. 1  positioned on wall frames the bottom portion of which have been cut-away; 
         FIG. 4  is a schematic view of another embodiment of a roof truss assembly according to the present invention; 
         FIG. 5  is a cross-section of a truss member taken along line  5 - 5  of  FIG. 4 ; 
         FIG. 6  is a schematic view of one half of the truss assembly shown in  FIG. 4  positioned on a wall frame the bottom portion of which has been cut-away. 
     
    
    
     DESCRIPTION 
     Certain terminology is used herein for convenience only and is not to be taken as a limitation on the present invention. For example, words such as “upper,” “lower,” “left,” “right,” “horizontal,” “vertical,” “upward,” and “downward” merely describe the configuration shown in the Figures. Indeed, the components may be oriented in any direction and the terminology, therefore, should be understood as encompassing such variations unless specified otherwise. 
     Referring now to the drawings, wherein like reference numerals designate corresponding or similar elements throughout the several views,  FIG. 1  shows an embodiment of a roof truss assembly according to the present invention, generally designated at  10 . The roof truss assembly  10  comprises several structural truss members, including a pair of top, or upper, chord members  12 , a pair of spaced bottom, or lower, chord members  14 ,  16 , and web members  18 . Adjacent upper ends of the top chord members  12  are secured together to form an apex joint. In this embodiment, the ends of both bottom chord members  14 ,  16  are secured adjacent to the lower ends of the top chord members  12 . The top chord members  12  and the lower bottom chord member  14  form a triangle, with the lower bottom chord member  14  as the base and the top chord members  12  forming the sides of the triangle. 
     It is well known in the art that there are a number of roof truss profiles in addition to the triangular truss assembly  10  depicted in  FIG. 1 . We do not intend to limit the application of the present invention to a triangular truss profile. Rather, the present invention is applicable to all such truss profiles. 
     The web members  18  extend between the top chord members  12  and the upper bottom chord member  16 . The opposite ends of the web members  18  are secured to the top chord members  12  and upper bottom chord member  16  for rigidifying the roof truss assembly  10 . Eight web members  18  are shown in  FIG. 1 . It is understood that we do not intend to limit the application of the present invention to a roof truss assembly  10  having a predetermined position and number of web members  18 . The number and the position of web members  18  will vary as necessary depending upon the size of a building and the lengths of the chord members  12 ,  14 ,  16  in order to provide the required structural strength with an acceptable safety factor. 
     Each of the truss members is formed from a strip or sheet of metal. The preferred material of construction is steel. However, the present invention is not limited to steel, and other metals such as aluminum, copper, magnesium, or other suitable metal may be appropriate. The scope of the invention is not intended to be limited by the materials listed here, but may be carried out using any material which allows the construction and use of the metal roof truss assembly  10  described herein. 
     As shown in  FIG. 2 , a truss member  20  which comprises the roof truss assembly  10  of the present invention is substantially C-shaped or U-shaped, having a web  24  spanning opposed side walls  26  defining a channel  22  section. When assembled ( FIG. 1 ), the open channels of the bottom chord members  14 ,  16  face upwardly and the open channels of the top chord members  12  face downwardly. Joints are formed where the chord members  12 ,  14 ,  16  and web members  18  intersect one another. The joints can be secured using fasteners (not shown), such as metal screws, bolts and nuts, rivets, or any combination thereof. For this purpose, aligned holes may be punched or drilled through the truss members during production. A short connecting plate (not shown) may also be fitted to the chord members  12 ,  14 ,  16  and web members  18  on each side of a joint and fastened together with the chord members  12 ,  14 ,  16  and web members  18  to form a reinforced joint. Alternatively, the truss members may be joined by welding, soldering, and the like. 
     The truss members can all be produced on-site from coils of sheet metal using a portable roll forming machine, as is known in the art. Features for joining the truss members may be provided by the forming machine, including holes for fasteners. Notches are cut into the side walls  26  a sufficient distance to accommodate intersecting truss members, depending upon the angle at which the truss members meet each other, allowing a portion of one end of a truss member to be fitted within another truss member. All of the truss members can be formed with a common section to simplify production. Additionally, service holes may be provided in the structural member to accommodate electrical wiring or other utilities. 
     In accordance with the present invention, the lower bottom chord member  14  is separated from the upper bottom chord  16 . As a result of this arrangement, there is no direct thermal path from the lower bottom chord member  14  to the web members  18  of the truss assembly  10 . Moreover, the air space  27  between the bottom chord members  14 ,  16  serves as an insulator. The air space  27  between the bottom chord members  14 ,  16  can be insulated to further enhance thermal performance. 
     In building construction, a plurality of truss assemblies  10  are set out across a building frame. As seen in  FIG. 3 , the lower bottom chord  14  spans the wall frames  30  of the building and is fixed to the top plate (not shown) of the wall frames  30 . Ceiling material (not shown) may be attached directly to the lower bottom cord  14 . Tensile elements  28 , schematically shown in  FIG. 3 , may be provided between the bottom chord members  14 ,  16  where necessary to support the weight of the ceiling material. The tensile elements  28  are spaced from the points on the truss assembly  10  where the web members  18  are fastened to the upper bottom chord  16  to minimize the potential for thermal bridging. Preferably, the tensile elements  28  are formed from a material having a low thermal conductivity. 
     Another embodiment of the roof truss assembly according to the present invention is shown in  FIG. 4  and generally designated at  40 . In this embodiment, the roof truss assembly  40  comprises a pair of top chord members  42 , a bottom chord member  44  and web members  46 . The web members  46  extend between and interconnect the top chord members  42  and the bottom chord member  44 . A vertically-positioned heel truss  48  is fastened between each end of the bottom chord member  44  and the free ends of the top chord members  42 . As noted above, the present invention is not limited to a triangular truss profile, but rather is applicable to all known roof truss profiles. Moreover, the number and position of the web members  46  will vary as necessary depending upon the truss profile, the size of a building, and the lengths of the chord members  42 ,  44 , in order to provide the required structural strength with an acceptable safety factor. Thus, the triangular truss profile and the number and position of the web members  46  depicted in  FIG. 4  are merely exemplary. 
     Spacers  50  are positioned along the length of, and fastened to, the bottom chord member  44 . The spacers  50  are located away from the points on the truss assembly  40  where the web members  46  are fastened to the bottom chord member  44 . A ceiling support  52  is secured to the spacers  50 . As seen in  FIG. 5 , the ceiling support  52  may be slightly wider than the web  24  of the bottom chord member  44 . Ceiling material  54  may be attached to the ceiling support  52 . The spacers  50  and ceiling support  52  can be formed from any material as long as the combination, along with the means for fastening the ceiling support  52  through the spacer  50  to the bottom chord member  44 , is sufficiently strong to support the ceiling support  52  and ceiling material  54 . For example, wood, fiberboard, cardboard, plastic, and the like, are all suitable materials for the spacers  50  and ceiling support  52 . Preferably, the spacers  50  have a low thermal conductivity. In keeping with the invention, the spacers  50  function to provide an insulating air space  58  between the bottom chord member  44  and the ceiling support  52  ( FIG. 3 ), which minimizes the potential for thermal bridging. 
     Referring to  FIG. 6 , one side of a truss assembly  40  according to the second embodiment of the present invention is shown in position on a wall frame  30 . The bottom chord  44  spans the wall frames  30  (only one of which is shown in  FIG. 6 ) of the building and is fixed to the top plate of the wall frames  30 . The ends of the ceiling support  54  extend between the inner surfaces of the wall frames  30 . Ceiling material  54  is attached directly to the ceiling support  52 . Optionally, insulating material  56  may be disposed in the air space  58 . For example, as seen in  FIG. 6 , a length of insulating material  56  is placed between the ceiling support  52  and the bottom chord  44  where the web members  46  attach to the bottom chord member  44 . 
     The thermal performance of the roof truss assembly of the present invention is significantly improved over conventional metal trusses. Separation of the lower bottom chord member or ceiling support from the bottom chord member connected to the web members provides an insulating air space between the ceiling and the bottom chord member and eliminates any direct thermal path from the ceiling to the bottom chord member and the web members of the truss assembly. Although the air space  27  can be insulated to further enhance thermal performance, the improvement in thermal performance can be achieved without the additional insulating material, or the use of insulating material as a thermal break. Moreover, a truss configuration according to the present invention allows the use of light gauge metal, preferably having a thickness of less than about 1.2 mm. For example, standard light gauge metal could be used, such as 12, 14, or 16 gauge. 
     Although the present invention has been shown and described in considerable detail with respect to a particular exemplary embodiments thereof, it should be understood by those skilled in the art that we do not intend to limit the invention to the embodiment since various modifications, omissions and additions may be made to the disclosed embodiments without materially departing from the novel teachings and advantages of the invention, particularly in light of the foregoing teachings. For example, the truss profile and the number and position of the truss members may be any of a number of arrangements known in the art. Accordingly, we intend to cover all such modifications, omissions, additions and equivalents as may be included within the spirit and scope of the invention as defined by the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a crew may be equivalent structures.

Summary:
A metal truss comprises elongated top chord members connected to each other at their ends. A first elongated bottom chord member is connected at its ends to the top chord members adjacent the free ends of the top chord members. A second elongated bottom chord member is connected at its ends to the top chord members, or directly to the first bottom chord member via spacers, such that the second bottom chord member is spaced below the first bottom chord member. At least one web member is positioned between and interconnecting at least one top chord member and the first bottom chord member. One end of the web member is connected to the at least one top chord member and the other end of the web member is connected to the first bottom chord member.