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FIELD OF THE INVENTION  
       [0001]     This invention relates broadly to the art of truss and joist construction. More particularly the invention relates to trusses which combine the benefits of open web truss construction with closed web joist construction, there by providing construction site variable length adjustment while conserving the factory built structural integrity.  
       BACKGROUND OF THE INVENTION  
       [0002]     Current truss designs envision a variety of broad design types, but they generally fall into two broad categories, open or closed web design. Both current designs have their problems.  
         [0003]     The closed truss is the most common variable length truss. Closed truss designs suffer from several construction problems. First, the design requires a great deal more material, having a closed volume. Second, the design has difficulty with respect to the passage of ducting, plumbing, wiring, etc. within the area defined by the interior of the truss since it is solid and must be cut. This provides for two problems. First, the strength of the truss may be affected by the cutting. Second, a great deal of time and equipment may be necessary in order to manufacture this space for utility passage. Other related problems may exist as a result of these general problems including the need for engineering analysis to determine where holes may be located and where web reinforcements are required.  
         [0004]     Two by ten or two by twelve joists made of solid wood are also very common. A major problems associated with these are that they require old growth timber to provide large enough lumber. In addition, when oriented edgewise, they provide an inferior nailing surface compared with that provided by truss chords oriented flatwise. This is particularly important when adding subfloors and ceilings to the joists since only an approximate location of the supporting member can be found. In addition, span lengths are greatly diminished by the load carrying properties of these timbers.  
         [0005]     A variety of open web truss designs are known. However, design of most open web trusses requires the use of metal due to structural requirements of the truss. The use of a large number of metal and wood components increases the complexity of truss component alignment and assembly as well as increases costs. Additionally, typical open web truss designs require that the manufactured length of the truss closely match the as-built span of the building because the truss configuration (and metal truss components in particular) makes field length adjustment or trimming nearly impossible.  
         [0006]     There are a number of hybrids that combine the features of open web truss center configuration with closed web trimmable end configuration. The products typically exhibit a degree of structural deficiency in completing the transition between solid web joist end and open web truss center. The transitioning problem is caused because the joist end behaves structurally like a beam and resists imposed loads with bending member stresses, whereas the truss center behaves structurally like a truss and resists imposed loads with axial member stresses. The most common limitations of this difficult transition is a requirement that the trusses be installed with a certain designated side up, or trim is limited to a small amount and perhaps requiring that both ends be trimmed equally.  
       SUMMARY OF THE INVENTION  
       [0007]     The described structure provides a lightweight, yet strong open web trimmable truss formed from a wood containing material. The truss assembly includes a pair of chord members. The chord members each include a mortise and a trimmable end block groove. A web element extends between the chord members. Tenons disposed on opposite ends of the web element engage with said mortise. The outer most web elements in the truss assembly also include a web element groove. Additionally, the trimmable truss includes a trimmable end block. The trimmable end block has opposed trimmable end block chord edges that engage the chord members via trimmable end block groove. The trimmable end block also has a web element edge that engages the web element groove. Generally, the truss assembly is constructed from a wood containing material.  
         [0008]     The trimmable truss assembly also includes a self locking joint. The self locking joint is formed by the interaction of a mortise and a pair of “mated” web element tenons. A first web element includes tenons formed by a first tenon seat cut and a first tenon square cut. Further, a second web element includes a second tenon defined by a second tenon seat cut and a second tenon square cut. When mated, the first tenon is adjacent the second tenon and both are within the mortise. The mortise may be either a single taper mortise or a double taper mortise. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0010]      FIG. 1  is an exploded isometric view of an open web trimmable truss according to the present invention;  
         [0011]      FIG. 2  is a side view of the open web trimmable truss illustrated in  FIG. 1 ;  
         [0012]      FIG. 3  is a end view of the open web trimmable truss illustrated in  FIG. 2 ;  
         [0013]      FIG. 4  is a side, partial sectional view of a chord member of the truss assembly of the present invention;  
         [0014]      FIG. 5  is a side view of a trimmable end block according to the present invention;  
         [0015]      FIG. 6  is a side view of a web member according to certain aspects of the present invention;  
         [0016]      FIG. 7  is an isometric view of the outermost web member according to certain aspects of the present invention;  
         [0017]      FIG. 8  an exploded isometric view of and aspect of the open web trimmable truss according to the present invention;;  
         [0018]      FIG. 9  is a top view of a section of chord member with a double taper mortise according to certain aspects of the present invention;  
         [0019]      FIG. 10  is a bottom view of a pair of web members according to certain aspects of the present invention;  
         [0020]      FIG. 11  is a isometric view of a mated pair of mated web members according to certain aspects of the present invention;  
         [0021]      FIG. 12  is an isometric of a web member according to certain aspects of the present invention;  
         [0022]      FIG. 13  is an isometric view of a mated pair of mated web members according to certain aspects of the present invention;  
         [0023]      FIG. 14  is a top view of a pair of web members and chord member according to certain aspects of the present invention;  
         [0024]      FIG. 15  is a top view of a section of chord member with a single taper mortise according to certain aspects of the present invention;  
         [0025]      FIG. 16  is a bottom view of a pair of web members according to certain aspects of the present invention;  
         [0026]      FIG. 17  is an isometric view of a mated pair of mated web members according to certain aspects of the present invention; and  
         [0027]      FIG. 18  is a side elevation view showing a variety of sized trimmable trusses made in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     The present invention provides an open web trimmable truss that utilizes a novel, self-centering and locking joint. By way of overview and with reference to  FIGS. 1-4  and  6 , an embodiment of the present invention includes chord members  22  and a web  26  integrally combined to form a trimmable truss  20 . The web  26  includes two distinct regions, a solid region on both ends of the trimmable truss  20  and an open region positioned between the solid regions. The solid region is generally indicated by trimmable end block  30  and the open region indicated by the region of the trimmable truss  20  incorporating web elements  28 . The chord members  22 , trimmable end blocks  30  and web elements  28  are all integrally connected to form a lightweight and strong trimmable truss  20 . The trimmable truss  20  typically has an I-beam cross-sectional shape. Specific details of the trimmable truss  20  are described with more particularity below.  
         [0029]     The trimmable truss  20  is constructed from various wood containing materials. A suitable example of the type of wood containing material the chord members  22  and web elements  28  are constructed from laminated strand lumber such as TimberStrand® sold by Weyerhaeuser Company. The trimmable end block  30  may be any known engineered panel, such as, without limitation, plywood or oriented strand board. It will be appreciated however that other materials may be used without parting from the spirit and scope of the present invention.  
         [0030]     Chord members  22  include two types of cutouts. A mortise  36  is configured to receive an interlocking pair of tenons  40 , described in more detail below. The mortise  36  may or may not include a bore  58  extending through the chord member  22 . Additionally, the chord member  22  includes a trimmable end block groove  32  configured to receive a trimmable end block chord edge  48  of the trimmable end block  30 . Those skilled in the art will appreciate that the trimmable end block groove  32  may be a tapered groove if the trimmable end block edge  48  is beveled. Likewise, if the trimmable end block edge  48  is not beveled, the trimmable end block groove  32  may be untapered. With regards to the trimmable truss  20 , both chord members  22  contain the same cutouts. As formed, the spacing of one chord member  22  is laterally offset from the other to facilitate formation of the trimmable truss  22 .  
         [0031]     As best seen in  FIGS. 2, 5  and  7 , the tenons  40  of the web elements  28  mate and engage the mortise  36 . Likewise, the trimmable end block edge  48  of the trimmable end block  30  engages the trimmable end block groove  32 . Additionally, a trimmable end block web element edge  50  of the trimmable end block  30  is formed at an angle to match the angle of the outer most web element  28   a . The outer most web element  28   a  includes a web element groove  60  matched to receive the trimmable end block web element edge  50 .  
         [0032]     As best seen in  FIGS. 2 and 5 , the trimmable end block  30  includes an end block notch  52 . The end block notch  52  is positioned a distance from an intersection point between a main axis of the trimmable end block chord edge  48  and the trimmable end block web element edge  50 . The end block notch  52  is located a sufficient distance from the intersection to allow positive engagement of the chord member  22 , outer most web element  28   a  and trimmable end block  30 . As the end block notch  52  makes the length of the trimmable end block web element edge  50  shorter than the length of the outer most web element  28   a , it is not necessary for the web element groove  60  to extend the entire length of the web element  28   a.    
         [0033]      FIG. 8  depicts one aspect of the behavior of truss  20  under load. Specifically, The FIGURE depicts the reactions to loading within the web element  28 , outer most web element  28   a , trimmable end block  30 , and chord members  22 . The truss  20  is subjected to applied loads along the length of its span which produce an end reaction. Those skilled in the art will appreciate that the given loading illustrated will place the respective chord members  22  in tension or compression. The effect of the end reaction on the trimmable end block  30  is one of causing it to behave like a beam and rotate away from the reaction force. This “beam action” yields a glue line shear force between the trimmable web member chord edge  48  and the trimmable member groove  32 .  
         [0034]     A transition zone  49  is also present when the truss  20  is under loading. The transition zone  49  is that part of the truss  20  where the truss  20  switches from behaving like a beam to behaving like a truss  20 . The transition zone  49 , in general terms, refers to interaction between the trimmable end block web element edge  50  and the web element groove  60  of the outermost web element  28   a.    
         [0035]     Under loading illustrated, the truss  20  places the outer most web element  28   a  in tension as illustrated by resultant tension load R 2  and web member  28  under compression as illustrated by, opposite but equal, resultant tension load R 1 . This resultant tension load R 2  gets transferred to the trimmable end block  30  via a tension shear between the trimmable end block web element edge  50  and the web element groove  60  of the outermost web element  28   a.    
         [0036]     It will be appreciated that truss  20  may be used with either chord member  22  on top. As such, the outer most web element  28   a  may be either in tension or compression, depending upon how the truss  20  is utilized. In either configuration, the tension or compression within the outer most web element  28   a  is transferred to the trimmable end block  30  as described above. Those skilled in the art, will appreciate that without this transfer zone  49 , the loading in the outer most web element  28   a  would be carried into the chord member  22 . The result of such an arrangement would lead to either the web element wanting to pull out of engagement with the chord member  22  or wanting to drive further into the chord member  22 . Both of which are undesirable conditions in a truss  20 .  
         [0037]      FIGS. 9-11  illustrate various features of an embodiment of the present invention. Specifically,  FIG. 9  illustrates a double taper mortise  36   a  in chord member  22   a . The term “double taper” is meant to indicate a mortise  36   a  tapered in two directions. The first taper direction is a centering taper  39 . The direction of this taper is parallel to the main axis of the chord member  22   a , or along the X axis as indicated in the FIGURE. The centering taper  39  is typically cut at about a 45 degree angle relative to a plane passing vertically into the plane of the paper along the Y-axis. However, a mortise  36   a  having a centering mortise  39  cut at an angle above or below 45 degrees is also considered within the scope of this invention.  
         [0038]     In this embodiment, mortise  36   a  also includes a locking taper  37 . The direction of this taper is perpendicular to the main axis of the chord member  22   a , or along the Y-axis as indicated in the FIGURE. The locking taper  39  is typically cut at about a 5 degree angle relative to a plane passing vertically into the plane of the paper along the X-axis. However, a mortise  36   a  having a locking taper  37  cut at an angle above or below 5 degrees is also considered within the scope of this invention.  
         [0039]     The locking taper  37  is configured to taper inwardly. As best seen in  FIG. 9 , the locking taper  37  has its widest spacing represented as M 1  and its narrowest spacing as M 2 . The width of M 1  is selected to be some width greater than the thickness of the mated tenons  41   a  as indicated as T 1  in  FIG. 11 . However, the width of M 2  is selected as being some width less than the thickness of the mated tenons  41   a . In this manner, the locking taper  37  at M 1  permits the mated tenons  41   a  to be easily inserted into the mortise  36   a , while applying an increasing compression force to the mated tenons  41   a  as they approach M 2 . This increasing compression force is the self-locking feature for this embodiment. The relative sizing of M 1 , M 2  and T 1  are generally selected to create an interference fit of about 0.005 to about 0.035.  
         [0040]     With respect to all embodiments and as discussed above, the web element  28  includes a tenon  40  at each end. For simplicity reasons only we will limit our description to a single tenon  40  with the understanding that discussion of one tenon  40  applies equally to all tenons  40  for a given embodiment. For all embodiments, one tenon&#39;s orientation on the web element  28  relative to the other tenon&#39;s orientation is best described as the tenon profile on opposite ends of a web member  28  are identical in profile but with opposing orientation. This orientation can be visualized as rotated 180 degrees in the plane of the trimmable truss  20  about the centroid of the finished web element  28 .  
         [0041]      FIGS. 10-14  illustrate various features of web element  28  with respect the double taper mortise  36   a  embodiment. Tenon  40  is formed by two cuts, a seat cut  42  and a square cut  44 . The seat cut  42  is formed by a pair of angled cuts, one cut on each side of the web element  28  per tenon  40 . The angle is typically at about 45 degrees relative to the longitudinal axis of the web element  28 . However, it will be appreciated that other geometries may be used when forming the seat cut  42 . With the double taper mortise embodiment, the seat cut  42  is made such that the outer surface  40   b  of tenon  40  is beveled.  
         [0042]     The square cut  44  is made at an angle of about 90 degrees relative to the longitudinal axis of the web element  28 . The square cut  44  is only formed on a single side of the web element  28  per tenon  40 . The square cut  44  may be made to form a substantially right angle tenon edge profile  56   a . Conversely, the tenon edge profile  56   b  may be a rounded, or fillet type cut as depicted best in  FIG. 12 . Likewise, the back edge  58   a,b  may be left square or rounded over. Additionally, other edges may be rounded or the cuts configured to form fillet type corner edges. Those skilled in the art will appreciate that rounding or forming fillet type cuts helps to limit or relieve stress concentrations in the region of the cut or fillet. Thus, the overall strength of the tenon is maximized.  
         [0043]      FIG. 11  illustrates the web elements  28  wherein the respective tenons  40  are mated, or otherwise configured as they would be when inserted into the double taper mortise  36   a . As formed, the mated tenons  40  position the longitudinal axis of one web element  28  at about a 90 degree angle relative the longitudinal axis of the other web element. A nip  46  is present at the outer most point of the mated tenons  40 . The nip  46  is generally arranged to prevent any section of the tenon from extending through bore  58  when the mated tenons  40  are inserted into the mortise  36   a .  FIG. 13  illustrates how the mated tenons would are configured when the tenons  40  include the rounding features discussed above and shown in  FIG. 12 .  
         [0044]      FIGS. 15-17  illustrate an additional embodiment of the present invention. Specifically, this embodiment uses a single taper mortise  36   b . The single taper mortise  36   b  embodiment includes a chord member  22   b  wherein the single taper mortise  36   b  includes only centering taper  39   b . The centering taper  39   b  of this embodiment is formed in a similar fashion as the double taper mortise  36   a . The centering taper  39   b  is generally formed with a taper about 45 degrees relative to a plane vertical with the page. Those skilled in the art will appreciate that other geometries are available for the centering taper  39   b  without departing from the spirit and scope of the invention.  
         [0045]     The width of the single taper mortise  36   b  is indicated as M 3 . With reference to  FIGS. 15 and 17 , M 3  is only slightly less than the thickness of mated tenons  40  as indicated by T 3 . The relative sizing of M 3  and T 3  are generally selected to create a interference fit of about 0.005 to about 0.035. To increase the ease at which the mated tenons  40   b  are inserted into the single taper mortise  36   b , the tenons include a nip bevel  47 .  
         [0046]     With specific reference to  FIG. 16 , the square cut  44  is typically made such that the mating surface  49  is beveled, or otherwise not parallel to the main axis of the web  28 . However, the seat cut  42  is made such that the outer surface of tenon  40  is parallel to the main axis of the web  28 .  
         [0047]      FIG. 18  depicts a variety of trimmable trusses  20  of varying size. It will be appreciated that the trimmable nature of this truss design allows for a single trimmable truss  20  to be used in a variety of locations. Additionally, the adjustable length of the trimmable truss  20  compensates for inconsistencies in structural span during construction of a building.  
         [0048]     When formed, the web elements  28 , the trimmable end blocks  30  and the chord members  22  are additionally secured together by the application of an adhesive, such as a resin. The adhesive is typically an alkaline phenolic resin. However, other adhesives may be used, such as, without limitation, water-soluble and non-water-soluble alkaline and acidic phenolic resins, resorcinol-formaldehyde resins, urea-formaldehyde resins, isocyanate resins, melamine, and epoxy resin.  
         [0049]     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.

Summary:
A wood containing, open web trimmable truss assembly composed of opposed chord members defining an open web central structure that is bounded at each ends by a closed, trimmable end block. The open web central structure is formed by a plurality of web elements attached to the chord members via a mortise and tenon coupling. The trimmable end block is attached to both chord members and the outer most web element.