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CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/784,615 filed Mar. 14, 2013, herein incorporated by reference in its entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates, generally, to materials used in construction. More specifically, the present invention relates to steel joist structures used in building construction. 
     BACKGROUND OF THE INVENTION 
     Steel joists have been used to structurally support building roofs and floors throughout the United States for the better part of a century. An exemplary array of conventional joists forming a support for a deck or roof is depicted in  FIG. 1 . The term “joist”, as used herein, indicates a closely spaced, repetitive member that directly supports (and in combination directly supports) a relatively flat structural element such as a roof deck or floor slab or the like. A steel joist, as opposed to a common truss, is defined by the U.S. Department of Labor in OSHA 29 C.F.R. §1926.751, incorporated fully herein by reference. Joists of identical properties are commonly found in a building in relatively large numbers, and as a result, such joists are currently manufactured in mass quantities. In contrast to the joist, a “girder” is a relatively heavier member that are fewer in number and that directly supports the joists. 
     The conventional steel joist used today consists of a top chord, a bottom chord, and multiple diagonals. As  FIG. 2  indicates, the top chord is a horizontal (or slightly sloped) member that in typical conditions fastens directly to the corrugated metal roof or floor deck that is being supported. The bottom chord is a horizontal member that is beneath and parallel (or nearly parallel) to the top chord. The diagonals (also known as web members) are inclined members arranged in a zig-zag pattern to join the top chord to the bottom chord. All of these members lie in, or nearly in, a common vertical plane. 
     The top chord of today&#39;s conventional steel joist consists of a pair of steel angles, parallel to one another, and positioned in a “back-to-back” orientation. See  FIG. 3 . The bottom chord also uses this same configuration. The web members are typically fabricated from steel angles or steel rods and are frequently welded in the gap between the parallel steel angles of the top (and bottom) chord. 
     Well known problems associated with present conventional steel joist constructions include: 1.) the need for erection bracing, also known as erection bridging as defined by OSHA; 2.) poor aesthetics; 3.) potential for corrosion of untreated areas; 4.) proclivity to top and/or bottom chord local bending; 5.) poor power actuated fastener penetration due to top chord local bending; 6.) inability to properly support/distribute and/or aesthetically conceal electrical and plumbing lines and HVAC ductwork. A need, therefore, exists for a steel joist assembly which resolves or greatly reduces these known problems. 
     SUMMARY OF THE INVENTION 
     The present invention is a substantially hollow tubular joist structure, a joist assembly including a plurality of aligned repetitive tubular joist structures, and a method of constructing this joist assembly. The tubular joists are preferably steel. Tubular joists offer several advantages over conventional steel joists. The tubular joists of the present disclosure are designed to fully comply with OSHA 29 C.F.R. §1926.757(a)(3), incorporated fully herein by reference. 
     Steel joists have never been fabricated exclusively from hollow steel tubes. These hollow steel tubes may include, by way of example and without limitation, a square, rectangular, round, oval, diamond shape, or hexagonal cross-section, however, it is understood that any suitable geometry could be employed as may be suitable for a particular application or known or developed by one of skill in the art. Preferred geometries may include round, square (including substantially square such as square with rounded or truncated corners), or rectangular (also perhaps with rounded or truncated corners) with rectangular or substantially rectangular being the most preferred cross-section. These hollow tubes (most preferably steel but may be constructed of any suitable material) shall be referred to herein as “tubular.” Joists constructed using tubular chords which may also include tubular diagonals shall be referred to herein as “tubular joists”. 
     The joist structure of the present disclosure includes a tubular top chord; a tubular bottom chord; and, a plurality of diagonals extending between the tubular top chord and the tubular bottom chord. The diagonals are also, in a preferred arrangement, tubular in construction. The diagonals are preferably arranged in a zig-zag formation between the tubular top chord and the tubular bottom chord. 
     The tubular top chord may be capable of receiving a power actuated fastener (PAF). The tubular top chord and the tubular bottom chord are capable of receiving a utility conduit. A utility conduit may include an electrical conduit or cable, a plumbing conduit, or it may receive a HVAC duct or may even itself act as an HVAC duct to convey conditioned air. 
     A method of constructing a tubular joist includes arranging a tubular top cord and a tubular bottom chord in a nearly or substantially parallel relationship. The tubular top chord and tubular bottom chord support one another through a plurality of diagonals which extend between the tubular top chord and tubular bottom chord in a preferred, substantially zig-zag manner. The diagonal are fastened to the tubular top chord and the tubular bottom chord preferably by welding or using fasteners or by any other means or as known in the art. 
     A method of constructing a tubular joist assembly of the present disclosure includes assembling a plurality of tubular joist structures each including a top chord, a bottom chord, and a plurality of diagonals extending between the top chord and bottom chord; and, wherein a plurality of the joist structures include a tubular top chord and a tubular bottom chord. This method of construction allows for the joist to be set in place with a substantially reduced requirement and in many instances without requiring a crane to support the joist while the erection bridging is installed since in most practical cases the erection bracing can be eliminated. By way of example, however, a tubular joist structure, as disclosed herein, could also be fabricated so as to be longer than conventional joists. In such longer structures, it is contemplated that erection bracing or the use of a crane for support during installation of the erection bracing may be preferred. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a typical prior art floor or roof plan view showing joists, girders, and columns. 
         FIG. 2  depicts a prior art joist top chord, joist bottom chord, and joist diagonals. 
         FIG. 3A  depicts a conventional steel top chord construction. 
         FIG. 3B  depicts a conventional steel bottom chord construction. 
         FIG. 4A  is a perspective view of a prior art joist assembly requiring erection bracing. 
         FIG. 4B  is a perspective view of the tubular joist assembly of the present disclosure requiring only horizontal bracing. 
         FIG. 5A  is a partial side view of a conventional steel joist construction illustrating the need for vertical web members to locally support the top chord to reduce bending stresses. 
         FIG. 5B  is a partial side view of a tubular joist assembly of the present disclosure which illustrates the benefits of the top chord local bending strength that allows vertical web members to be eliminated. 
         FIG. 6A  is a partial side view of a conventional steel joist construction assembly illustrating the need for additional bracing against bottom chord local bending. 
         FIG. 6A  is a partial side view of a tubular joist assembly of the present disclosure requiring less bracing due to the fact that tubular constructed bottom chords can support heavier local loads. 
         FIG. 7A  depicts a partially cut away view, taken along line  7 A- 7 A of  FIG. 6A  of a conventional steel joist construction illustrating a common problem associated with failure of a power actuated fastener (PAF) to penetrate the top chord of the joist causing local top chord bending. 
         FIG. 7B  depicts a partially cut away view, taken along line  7 B- 7 B of  FIG. 6B , of a tubular joist assembly of the present disclosure receiving an exemplary power actuated fastener. 
         FIG. 8A  depicts exemplary wall penetrations of the top chord and bottom chord of a conventional steel joist construction assembly. 
         FIG. 8B  depicts exemplary wall penetrations of the top chord and bottom chord of a tubular joist chord assembly of the present disclosure. 
         FIG. 9  depicts exemplary electrical and plumbing lines inside a tubular joist chord of the present disclosure. 
         FIG. 10  depicts an isometric view of a tubular joist assembly of the present disclosure. 
         FIG. 11  depicts a partially cut away view of a tubular joist assembly of the present disclosure depicting a substantially round cross-section. 
         FIG. 12  depicts a partially cut away view of a tubular joist assembly of the present disclosure depicting a substantially oval cross-section 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processes and manufacturing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the invention herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the claimed invention. 
     With reference to  FIG. 2  in combination with  FIGS. 3A and 3B , a conventional steel joist  10  generally includes a top chord  20 , a bottom chord  22  and multiple diagonals  24 . A plurality of joists  12 ,  14 , and  16  identical to joist  10  are depicted in  FIG. 2  supporting a corrugated metal roof deck  18 . Top chord  20  is a horizontal (or slightly sloped) member that in typical conditions fastens directly to corrugated metal roof  18  or to a floor deck in an alternate application.  FIG. 3A  depicts top chord  20  which includes two opposed steel angles  28  and  30 . Diagonal  24  extends between steel angles  28  and  30 . Diagonal  24  is depicted to include a crimped end  32  which is sandwiched and welded between opposed angles  28  and  30 . 
     Bottom chord  22  is a horizontal member that is beneath and parallel (or nearly parallel) to top chord  20 . With reference to  FIG. 3B , bottom chord  22  is depicted. Bottom chord  22  is comprised commonly of up to two steel angles  32  and  34 . Diagonal  24 , as with top chord  20 , frequently includes a crimped end which is sandwiched between steel angles  32  and  34  and typically welded therein. 
     The diagonals  24  ( FIG. 2 ) are also commonly referred to as web members and are inclined members arranged in a zig-zag pattern to join top chord  20  to bottom chord  22 . The diagonal members  24  are typically fabricated from steel angles or steel rods and welded between the steel angles of the top chord  20  and the bottom chord  22 . Top chord  20 , diagonals, collectively  24 , and bottom chord  22  are typically configured to be in a common vertical plane. 
       FIG. 1  depicts a conventional array of conventional open-web joists  10  forming a support for a deck or roof  11  shown partially cut-away. Vertical building columns  36  support a plurality of girders  38 . Girders  38 , in turn, support joists  10 . In the exemplary array depicted in  FIG. 1 , nine building columns  36  support six girders  38  to which thirty-four joists  10  are secured. 
       FIG. 10  depicts a tubular joist construction of the present invention which is contemplated to replace joists  10  in applications such as depicted in  FIG. 1 . With reference to  FIG. 10 , tubular joist  100  includes a tubular top chord  102  and a tubular bottom chord  104  connected by diagonals  106 . In the preferred embodiment depicted in  FIG. 10 , top chord  102  includes a length of tubular steel, preferably high strength (HSS) with a substantially rectangular cross section. In this embodiment top chord  102  is oriented such that the longer sides  108  of the rectangular cross section are oriented substantially vertically while the shorter sides  110  are oriented substantially horizontally. 
     Bottom chord  104  includes a length of tubular steel the same construction as top chord  102  and positioned parallel to top chord  102  and separated by diagonals  106 . In the preferred arrangement depicted in  FIG. 10 , bottom chord  104  includes substantially the same rectangular geometry in cross section as is top chord  102 . However, in this embodiment, the longer sides of the rectangular cross section  112  are positioned horizontally while the shorter sides  114  are positioned vertically. It should be understood that the embodiment depicted in  FIG. 10  is exemplary such that tubular top chord  102  and tubular bottom chord  104  could have the same or different cross sectional geometries or orientations from one another or could be oriented in any desired manner. Alternatively, it is conceivable that top chord  102  could be replaced with a conventional top chord design, such as  20  of  FIG. 3A  such that only bottom chord  104  is tubular. Likewise bottom chord  104  could alternatively be replaced with a conventional bottom chord design, such as  22  of  FIG. 3B  such that only top chord  102  is tubular. 
     Diagonals  106  connect tubular top chord  102  and tubular bottom chord  104 . In the preferred arrangement, diagonals  106  are also steel tubular construction also with a rectangular cross section but of a smaller size than tubular top chord  102  and tubular bottom chord  104 . However, it is understood that diagonals  106  could be constructed of any suitable geometry. Alternatively, diagonals  106  could be of a conventional construction and not tubular. Diagonals  106  in the preferred arrangement are oriented in a zig-zag pattern to join tubular top chord  102  and tubular bottom chord  104 . Diagonals  106  are welded to top chord  102  and bottom chord  104 , thus forming a rigid open web tubular joist design. Tubular top chord  102 , tubular bottom chord  104  and diagonals  106 , when constructed lie in, or nearly in, a common vertical plane. 
     Tubular joists offer several advantages over conventional steel joists. Specifically, nine such advantages have been identified and are set forth herein. For example, with regard to fabrication, tubular joists have several advantages. Tubular joists have half the number of chord pieces, and one-third fewer web member pieces (no verticals) to handle and cut in the shop. Tubular joists will have less than half the surface area that must be coated. All web-to-chord tubular connections are simple gapped joints with small fillet welds made on the flat area of the HSS tube wall. 
     Advantage 1: Erection Bracing: 
     With reference to  FIG. 4A , conventional joist chords  20 ,  22 , consisting of a pair of steel angles, offer relatively little resistance against torsion (i.e., twist). The chord&#39;s resistance to torsion, or lack thereof, heavily influences a joist&#39;s tendency to laterally buckle under the weight of an iron worker. Consequently, since conventional joists  10  lack torsional resistance they are prone to lateral buckling. As a result, the United States Occupational, Health, and Safety Administration (OSHA) has strict rules, for joists exceeding certain lengths, that require the crane lifting assembly (e,g., the crane hook) to remain connected to the joist until after “erection bridging” is installed. “Erection bridging”  40  typically consists of bracing members that laterally support the joist  10  and prevent lateral buckling under the weight of an iron worker. It is typically provided in a “X” brace configuration ( FIG. 4A ). As elaborated below, a comparable tubular joist offers superior torsional resistance, leading to greater stability against lateral buckling. 
     The torsional constant “J”, which is a property of the member cross section, directly impacts the member&#39;s effectiveness in resisting torsion: the greater “J”, the greater the resistance against torsion. The following comparison contrasts a conventional top chord  20  ( FIG. 3A ) consisting of ¼ ″ thick angles with 4″ long legs and a ¾″ gap between the angles, and a comparable tubular chord:
         Conventional chord  20 , J=0.088 in 4 .   A Square tubular chord  118  ( FIG. 4B ) of the present disclosure, having equivalent weight (4″ square, 0.2586″ thick): J=13.54 in 4 .       

     Hence, the tubular chord  118  ( FIG. 4B ) offers a torsional constant that is 150 times greater than the conventional joist chord  10 . The same would be true for a comparison of a conventional bottom chord  22  ( FIG. 4A ) and a square tubular chord  120  ( FIG. 4B ). The efficiency offered by tubular joist  118  dramatically reduces the joist&#39;s tendency to buckle and can reduce, and in most cases, eliminate the need for erection bridging ( 40  of  FIG. 4A ). This allows the erection bridging to be replaced by simple horizontal bridging  120  ( FIG. 4B ) that is installed after the crane has released from joist  116 . The assembly benefits are two-fold:
         workers will be supported by more stable joists, and   the erection bridging (bolted X bridging) installation operation will be reduced or eliminated.       

     According to the erection stability equation that is behind the OSHA erection bridging span tables, an unbraced conventional design (32LH06) joist performs unfavorably compared to an unbraced tubular joist of the present disclosure of equivalent weight &amp; load carrying capacity: 
                                                           Conventional   Tubular           Joist   Joist                                    Allowable span without    40 feet    90 feet           erection bridging                   Weight of erector that   100 lbs   3300 lbs           causes a 40′ span to buckle                    
This is because the torsional constant of the tubular joist is 130 times greater than that of the conventional joist. As a result, the tubular joist design of the present disclosure would be the first joist to be manufactured in compliance with OSHA 29 C.F.R. §1926.757(a)(3).
 
     The cost benefits are also two-fold:
         crane rental cost savings will accrue from the additional speed of erection that comes from avoiding the delay caused by the crane holding the joist while erection bridging is installed, and       

     Example Crane Savings from Eliminating Bolted X Bridging (BXB): 
     
       
         
           
             
               330 
               ⁢ 
               
                   
               
               ⁢ 
               joists 
               * 
               
                 
                   2 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   BXB 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   sets 
                 
                 joist 
               
               * 
               
                 
                   3.7 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   min 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   of 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   crane 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   time 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   per 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   set 
                 
                 
                   60 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   min 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   per 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   hr 
                 
               
             
             = 
             
               40.6 
               ⁢ 
               
                   
               
               ⁢ 
               crane 
               ⁢ 
               
                   
               
               ⁢ 
               hours 
             
           
         
       
       
         
           
             
               40.6 
               ⁢ 
               
                 
                     
                 
                 ⁢ 
                 
                     
                 
               
               ⁢ 
               crane 
               ⁢ 
               
                   
               
               ⁢ 
               hours 
               * 
               
                 $285 
                 
                   crane 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   hour 
                 
               
             
             = 
             
               $11 
               , 
               568 
             
           
         
       
         
         
           
             reducing/eliminating the erection bridging will reduce the number of bracing members that must be installed. The example in  FIG. 4B  shows replacing the erection bridging  40  ( FIG. 4A ) with horizontal bridging  120  ( FIG. 4B ) affords the following quantity reductions:
           the number of bracing members is reduced by a factor of 3, and   the number of bolts is cut in half.   
         
           
         
       
    
     Example Labor Savings Form a Typical 150,000 sq. Ft. Building Replacing Bolted X Bridging (BXB) with Horizontal Bridging 
     
       
         
           
             
               1680 
               ⁢ 
               
                   
               
               ⁢ 
               sets 
               ⁢ 
               
                   
               
               ⁢ 
               of 
               ⁢ 
               
                   
               
               ⁢ 
               BXB 
               * 
               2 
               ⁢ 
               
                   
               
               ⁢ 
               men 
               * 
               
                 ( 
                 
                   
                     6 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     min 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     saved 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     per 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     set 
                   
                   
                     60 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     min 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     per 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     hr 
                   
                 
                 ) 
               
             
             = 
             
               336 
               ⁢ 
               
                   
               
               ⁢ 
               manhours 
             
           
         
       
       
         
           
             
               336 
               ⁢ 
               
                   
               
               ⁢ 
               manhours 
               * 
               $46 
               ⁢ 
               .31 
               ⁢ 
               
                   
               
               ⁢ 
               per 
               ⁢ 
               
                   
               
               ⁢ 
               hr 
             
             = 
             
               $15 
               , 
               562 
             
           
         
       
     
     Advantage 2: Aesthetics: 
     Conventional steel joists  10  ( FIG. 4A ) are typically used in areas where aesthetic considerations are secondary. Architecturally, tubular steel joists  116  ( FIG. 4B ) would usually be preferred over conventional steel joists. Readily available tubular steel joists would increase the market available for steel joist construction. 
     Advantage 3: Corrosion Reduction: 
     Conventional steel joist fabrication utilizing a pair  28 ,  30  and  32 ,  34  ( FIGS. 3A and 3B ) of steel angles for each chord  20 ,  22  results in tight spaces where it is very difficult to adequately weld, leading to rough welds creating water traps. Experience has shown that this difficulty leads to localized areas that are susceptible to corrosion. Consequently, engineers generally do not use conventional steel joists if those joists will be exposed to outside air or otherwise corrosive environments. A tubular joist  100  ( FIG. 10 ) avoids this since all exposed surfaces are accessible to welding and painting. Hence, this attribute of the tubular joist would further increase the market available for steel joist construction. 
     Advantage 4: Top Chord Local Bending: 
     With reference to  FIGS. 5A and 5B , the top chord of a tubular joist  116  ( FIG. 5B ) offers greater strength against local bending than that of a comparable conventional joist  10  ( FIG. 5B ). The section modulus is a property of the member cross section that is a direct measure of the allowable weight a member can support. If the section modulus is doubled, the allowable supported weight is doubled. Using the same comparison as was done for the torsional constant:
         Conventional chord  20  ( FIG. 5A ), S=2.06 in 3      Tubular chord  118  ( FIG. 5B ) of equivalent weight (4″ square, 0.2586″ thick); S=2.5 in 3 .       

     Hence, an equivalent square tubular chord  118  offers a 21% increase in bending strength over the conventional chord  20 . This efficiency offers two cost benefits:
         Uniformly distributed roof/floor loading on the top chord  20  of a conventional joist  10  is typically carried by adding a vertical web member  26  to the joist during fabrication ( FIG. 5A ). This provides support to the otherwise unsupported top chord  20  between the panel points where diagonals  24  attach to chords  20  and  22 . The tubular joist  116  ( FIG. 5B ), since it is stronger in bending avoids this, resulting in fewer web members,   Concentrated floor or roof loads often fall on the joist top chord between the panel points. Roof top HVAC units are an example of this. Such conditions will typically require a supplemental reinforcing member to be installed, usually in the field, to support the top chord beneath the concentrated load, A tubular top chord will reduce the number of instances where this reinforcement is required.       

     Advantage 5: Bottom Chord Local Bending: 
     With reference to  FIGS. 6A and 6B , with regard to a conventional steel joist, concentrated hanger loads often fall on the joist bottom chord  22  between the panel points where the diagonals  24  attach to bottom chord  22 . HVAC ductwork is an example of this. Such conditions will typically require a reinforcing member  42  to be installed to support the otherwise unsupported length of bottom chord  22  between diagonals  24 ′ and  24 ′ ( FIG. 6A ) because double angle chords are relatively weak in regard to their ability to withstand bending stresses/forces. 
     Similar to the top chord comparison, the additional bending strength of an equivalent tubular bottom chord  120  ( FIG. 6B ) reduces the number of instances where this reinforcing member is (shown in phantom) needed between diagonals  122 . 
     Advantage 6: Local Bending Preventing PAF Penetration: 
     Attention is next directed to  FIGS. 7A and 7B . First with reference to  FIG. 7A  a conventional joist construction, power actuated fasteners (PAF)  44  are a relatively new addition to the various alternatives for fastening a corrugated metal deck  18  to the top chord  20  of a joist. PAF&#39;s are a fast and often preferred means of attaching the corrugated metal deck  18  to the supporting joists. Conventional joists have been known to bend locally as shown in  FIG. 7A , preventing the PAF  44  from penetrating steel angle  30  of steel top chord  20 . Because of this, engineers sometimes prohibit the use of PAF&#39;s on projects. 
     Referring to  FIG. 7B , since the top face  110  of tubular chord  102  is supported by both sidewalls  108  of the tube, a tubular chord would likely eliminate this problem, opening the door to the cost savings that comes with the speed of construction associated with PAF&#39;s. Re-work costs related to this problem would also be avoided, and the risk of a poorly fastened metal deck would be reduced. This latter benefit is also a structural stability benefit since buildings frequently depend on the corrugated metal deck for overall building stability, and proper fastening of the deck is critical to that function. 
     Advantage 7; Wall Penetrations: 
     Reference is next made to  FIGS. 8A and 8B . When joist chords or diagonals in a conventional joist design ( FIG. 8A ) must pass through a wall  45 , “L” shaped wall cutouts  46  shown in  FIG. 8A  are often made to accommodate the wall penetration. These cutouts  46  are expensive relative to the cutouts  126  in wall  125  required for a tubular member as depicted in  FIG. 8B . Simplifying these cutouts will result in construction labor cost savings. 
     Advantage 8: Electrical and Plumbing Lines: 
     When electrical and plumbing lines run parallel to the conventional joists that support them, clips and hangers must be used to attach those lines to the joist chord(s). A tubular joist chord provides a ready conduit for these lines  128 ,  130  ( FIG. 9 ), and in a large building it would eliminate significant quantities of clips and hangers resulting in labor and material cost savings. Such an arrangement also provides the aesthetic benefit of concealing lines  128  and  130 . 
     Advantage 9: Conditioned Air Delivery 
     Similar to electrical and plumbing lines  128  and  130  ( FIG. 9 ), HVAC ductwork often runs parallel to the joists supporting it. In such cases, the tubular chord  102  is available for distributing air and if utilized, may substantially reduce the quantity of ductwork needed for the building. Again, this would lead to construction labor and material cost savings, and the aesthetic benefit of less visible ductwork. 
     An example calculation of estimated cost savings for the different one-story “Big Box” type buildings resulting from the use of the tubular steel joists of the present disclosure over a conventional steel joists are set forth in Table I. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 One Story “Big Box” Type Bldg 
               
               
                 Cost Benefit From Using Tubular LH Joists 
               
               
                 Metal Deck Roof: 1.5B. 22 GA with 5⅝″ Puddle Welds &amp; 8 -#10 TEK Sidelap Screws 
               
             
          
           
               
                 Measure 
                 Joists Spanning 60′ 
                 Joists Spanning 75′ 
                 Joists Spanning 90′ 
               
               
                   
               
               
                 Building Site 
                 153,600 SF 
                 157,500 SF 
                 162,000 SF 
               
               
                 Tonnage 
                 310 tons (181 tons of joists) 
                 434 tons (260 tons of joists) 
                 525 total tons (322 tons of joists) 
               
               
                 Schedule Reduction (days) 
                 26 days reduced to 20 ==&gt; 6 days 
                 44 days reduced to 38 ==&gt; 6 days 
                 45 days reduced to 39 ==&gt; 6 days 
               
               
                 Field Savings ($) 
                 $50,015 
                 $48,989 
                 $47,979 
               
               
                 Add&#39;l Mat&#39;l Cost of HSS ($) 
                 $24,678 
                 $29,122 
                 $34,223 
               
               
                 Net Benefit ($) 
                 $25,337 
                 $19,867 
                 $13,756 
               
               
                 Net Benefit ($/lb of joists) 
                 $0.07  
                 $0.04  
                 $0.02  
               
               
                   
               
               
                 Notes: 
               
               
                 1) Field savings reflect steel erection bid prices based on generally accepted labor productivity rates as compiled by the software program “Steel Erection Bid Wizard”, This program has been the subject of a Steel Erectors Association of America (SEAA) newsletter, and is used by Granau Metals, Panther City Ironworks, WhaleySteel, Harris County Ironworks, and 71 other domestic Steel Erectors for producing steel erection bids. 
               
               
                 2) Material costs assume $40.00/cwt for rolled angle iron and $50.43/cwt for HSS tubing. 
               
             
          
         
       
     
     Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those skilled in the art. Such changes and modifications are encompassed within the spirit of this invention as defined by the appended claims.

Summary:
A hollow tubular joist structure, a joist assembly including a plurality of aligned repetitive tubular joist structures, and a method of constructing this joist assembly. The tubular joist structure may include any suitable cross-sectional geometry. The joist structure includes a tubular top chord; a tubular bottom chord; and, a plurality of diagonals extending between the tubular top chord and the tubular bottom chord. The diagonals may also be tubular. The diagonals are arranged in a zig-zag formation between the tubular top chord and the tubular bottom chord. The tubular top chord may be capable of receiving a power actuated fastener (PAF). The tubular top chord or the tubular bottom chord may also be capable of receiving a utility conduit. A method of constructing a joist assembly of the present disclosure includes assembling a plurality of joist structures each including a top chord, a bottom chord, and a plurality of diagonals extending between the top chord and bottom chord; and, wherein a plurality of the joist structures include a tubular top chord and a tubular bottom chord.