Patent Publication Number: US-9422714-B2

Title: Wooden frame truss with enhanced fire resistance

Description:
BACKGROUND 
     The present invention relates generally to wooden frame construction materials and techniques, and more specifically to an open-web wooden truss with enhanced fire resistance. 
     Since their introduction in about 1960, light timber, open-web wood trusses have become one of the most widely used engineered wood building products employed in commercial construction. According to the Wood Truss Council of America (WTCA), such trusses are lightweight, easy to install, and have nailable chords for easy attachment of roof decking and ceiling materials. Open-webbing provides great benefits to plumbers and electricians, without the need to spend time cutting holes in floor members. Less cutting reduces jobsite labor and reduces potentially critical errors that could result in compromising the structural integrity of the components. Open-web wood trusses are lighter, less expensive and can be stronger than large, single “closed web” support members. 
     When subject to fire damage, the weak link or cause of failure of such open-web trusses is the detachment of the metal gusset plates used to connect the framing members together. Under load, as the wood chars and the metal gusset plates heat up under fire, the teeth of the metal gusset plates lose strength and holding power. The loss of the gusset plate on the bottom chord of a truss can lead to tensile forces pulling the truss apart. The loss of a gusset plate on the top chord will cause any web members attached to the top chord to pull away. Both situations will significantly reduce the load-carrying capacity of the installed truss and may even lead to a truss collapse. 
     Thus, engineered building components provide adequate strength under normal loading, but under fire conditions, these truss systems can fail, leading to the collapse of roof, floors, and possibly the entire structure. Truss systems are usually hidden, and fires within truss systems can go undetected for long periods of time, resulting in loss of structural integrity prior to discovery of the fire. Structural design codes often do not factor in the decreased system integrity as the fire degrades the structural members. Accordingly, there is a need for open-web truss systems having enhanced fire resistance. 
     SUMMARY 
     The above-identified need is met by the present open-web wooden truss assembly, in which approximately six-inch wide gypsum wallboard batten strips are attached to a lower surface of the lower chord of the truss. The width of the strip compared to the approximate 4-inch width of the chord, means that the wallboard batten strips extend laterally beyond edges of the chord. In this manner, a fire resistant barrier is created that protects both the chord from charring, and the metal gusset plates from degrading due to the intense heat generated by the fire. In addition, the batten strips also provide a ledge of approximately 1 inch extending away from, and along each edge of the chord, creating a support platform for insulation installed prior to the fabrication of the wallboard ceiling. It has been found that a ceiling made from trusses equipped with the present batten strips, and having a ceiling of two layers of fire rated ⅝ inch gypsum wallboard fastened to the strips, successfully resisted charring by fire for at least two hours. 
     A preferred construction system using the present truss includes the truss with the batten strip along the lower chord, insulation strips inserted between trusses and supported by the ledges defined by the batten strips, then RC- 1  sound attenuation channels secured transversely to the batten strips, so that the batten strips are acoustically decoupled from the wallboard ceiling boards attached to the RC- 1  channels. The gypsum wallboard ceiling panels or boards are then secured to the RC- 1  channels, as is well known in the art. Preferably two thicknesses of ceiling boards are attached to the RC- 1  channels. 
     More specifically, a wooden frame truss with enhanced fire resistance is provided and includes an upper chord extending along a longitudinal axis, a lower chord disposed below the upper chord and extending along a vertically displaced, parallel axis, a plurality of supports attached between the upper and lower chords, and a plurality of metal gusset plates securing the supports to the chords. At least one wallboard batten strip is attached to an underside of the lower chord, the strip being constructed and arranged so that the strip defines a ledge extending from each side of the lower chord. 
     In another embodiment, a ceiling system is provided, and includes a plurality of wooden frame trusses with enhanced fire resistance, each truss including an upper chord extending along a longitudinal axis, a lower chord disposed below the upper chord and extending along a vertically displaced, parallel axis, a plurality of supports being attached between the upper and lower chords. A plurality of metal gusset plates secures the supports to the chords. At least one wallboard batten strip is attached to an underside of the lower chord, each strip being constructed and arranged so that the strip defines a ledge extending from each side of the lower chord. At least one length of insulation is inserted between adjacent beams and supported by the ledges, and at least one RC- 1  strip is secured to an underside of each batten. In addition, at least one wallboard panel is secured to the at least one RC- 1  strip. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a side elevation of a conventional open-web wooden truss; 
         FIG. 1B  is a side elevation of the present ceiling system having enhanced fire resistance and including the present truss; 
         FIG. 1C  is a fragmentary end view of the present ceiling system shown in  FIG. 1B ; 
         FIG. 2  is a bottom perspective view of the ceiling of  FIG. 1B  under construction using the present open-web wooden trusses; 
         FIG. 3  is a bottom perspective view of the present trusses assembled in a ceiling, and strips of insulation installed between the trusses; 
         FIG. 4  is a bottom perspective view of the ceiling of  FIG. 1B  under construction, with RC- 1  channels secured to the battens; 
         FIG. 5  is a bottom perspective view of the present ceiling with a first layer of wallboard ceiling panels secured to the RC- 1  channels; 
         FIG. 6  is a bottom perspective view of the present ceiling with a second layer of wallboard ceiling panels secured to the first layer and to the RC- 1  channels; and 
         FIG. 7  is a bottom perspective view of the present ceiling showing the wallboard taped and ready for painting. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1A , a conventional open-web wooden truss is generally designated  10 . Such trusses are typically used for supporting a floor of a building, such as, but not limited to commercial buildings or residences, especially multi-story, multi-family residences. The truss  10  includes an upper chord  12  extending along a longitudinal axis “L.” The chord  12  is typically made of a 2×4 board commonly used in residential and commercial construction. A lower chord  14  is disposed along an axis “M”, extending in a vertically spaced, generally parallel orientation to the chord  12 . A plurality of structural supports, also referred to as supports  16  are located between the upper and lower chords  12 ,  14  and are secured to the chords. As is the case with the upper chord  12 , the lower chord  14  and the supports  16  are preferably made of 2×4 lumber, however other sizes for all of these components are contemplated depending on the situation. 
     As is known in the art, metal gusset plates  18  are used for securing the supports  16  to the associated chords  12 ,  14 . The plates  18  are provided with a plurality of pointed teeth (not shown) for gripping the wood, and are hammered or pressed in place during construction of the truss  10 . As described above, during fire conditions, conventional trusses  10  have been known to fail, in part due to charring of the chords  12  and  14 , and also to the deterioration of the gusset plates  18 , which results in the supports  16  detaching from the chords, and the structural failure of the truss. 
     Referring now to  FIGS. 1B, 1C and 2 , an open-web wooden truss with enhanced fire resistance according to the present invention is generally designated  20 , and is depicted as part of a ceiling system, generally designated  22 . Components shared with the truss  10  are depicted in the truss  20  using identical reference numbers. 
     A main distinguishing feature of the truss  20  is the attachment of at least one wallboard batten strip  24  to an underside  26  of the lower chord,  14 . Each batten strip  24  is preferably cut from a conventional gypsum wallboard panel, having a thickness of either ½ or ⅝ inch depending on the application. In width, each batten strip  24  is approximately 6-8 inches, however the width may vary to suit the application. Each batten strip  24  is constructed and arranged so that the strip defines a ledge  28  extending from each side  30  of the lower chord  14 . It is preferred that the chord  14  is generally centered upon the batten strip  24 , so that the ledges  28  extending from each side  30  are relatively equal to each other. In an embodiment, the ledges  28  extend one inch from the corresponding sides  30 , however it is contemplated that the distance extending from the side may vary to suit the application. Further, it is preferred that the batten strips  24  extend coextensively along a length of the lower chord  14 . 
     Referring now to  FIGS. 1C and 3 , an advantage of the ledges  28  is that they provide a support location for lengths or bats or strips of insulation  32 , typically fiberglass, however other types of conventional ceiling insulation are contemplated. Using the present trusses  20 , the strips of insulation  32  are easily installed and retained without supplemental fastening between adjacent ledges  28 , facilitating the creation of a heat and sound insulating barrier. 
     Referring now to  FIGS. 1B, 1C and 4 , beneath the batten strips  24  are attached a plurality of spaced, parallel RC- 1  acoustical decoupling channels  34 . The RC- 1  channels  34  are fastened to an underside  36  of the batten strips  24  using threaded fasteners or the like, as is known in the art, and extend in a direction that is normal to the longitudinal axes “L” and “M”. 
     Referring now to  FIGS. 1B, 1C and 5 , at least one, and preferably a plurality of wallboard ceiling panels  38  are secured to the at least one RC- 1  channels  34 . Since the panels  38  are fastened to the RC- 1  channels and not to the trusses  20 , they are acoustically decoupled from the trusses, and provide a quieter environment for the living space below the ceiling system  22 . 
     Referring now to  FIGS. 1C and 6 , in a preferred embodiment, a second layer of wallboard ceiling panels  40  is fastened both to the RC- 1  channels  34  and to the first layer of panels  38 . As is well known in the art, the panels  40  are installed to be oriented normally to the panels  38 . Upon installation of the panels  40 , a resulting double thick layer of ceiling panels creates a further fire resistant barrier for the ceiling system  22 . Also, as is known in the art, seams  42  between adjacent panels  40  are taped at  44 , and fastener holes  46  are covered with joint compound  48  as seen in  FIG. 7 . 
     Referring again to  FIG. 1B , the truss  20  supports a subfloor  50 , including a layer of plywood and optionally poured underlayment, as is known in the art. 
     Tests of the present system  22  including the truss  20  have shown that the system withstands fire for at least two hours. The test procedure was performed at the Western Fire Center, Inc., Kelso, Wash. The test procedure was pursuant to ASTM E119 and employed a modified construction described in Underwriters Laboratory design assembly L577. The testing was performed using a horizontal fire resistance test furnace employing the fire endurance conditions and standard time-temperature curve described in ASTM E119. Temperature measurements were taken inside the natural gas furnace using 9 thermocouples (TC f ) connected to a computerized data acquisition. TC f  locations were symmetrically disposed and distributed to show the temperature near the exposed face of the test assembly. 
     According to the test criteria the system/assembly will have sustained the applied load for the indicated two hour time without passage of flame or gases hot enough to ignite cotton waste; and transmission of heat through the assembly will not have resulted in a temperature rise of more than 139° C./250° F. above its initial temperature, or if a temperature higher than 30% (181° C./325° F. of the specified limit occurs at any one point on the unexposed side of the assembly. 
     In the test, on the underside of the lower chord  14  bottom of each truss  20  was applied a 6 in. (2¾ inch on exterior rim support) gypsum batten strip (same material that was used for the ceiling), initially fastened with #6 2¼ in. screws at 12″ on center. Full strips (10′) and short strips (3′) were alternately placed over each truss. Unfaced fiberglass insulation (Owens Corning ProPink® R-11, M44) strips were loosely laid on top of the batten strips, each strip of insulation being 3⅝×20 in. The density of the insulation was 0.49 lb./ft. 3 . Steel (25GA) resilient channels (RC) were fastened with #6 screws perpendicular to the underside of the batten strip at 12 in. on center, alternating between screws already fastened into the batten strip. The RC channel was located so that the perpendicular gypsum (butt) joint for the exposed face could be fastened 2 in. in both directions (4 in. total) with a 4 in. overlap splice. The ceiling (exposed side below trusses) was lined with two layers of USG ⅝ in. SHEETROCK® Brand Gypsum Panels, FIRECODE® fire rated C Core, which was supplied in 4′×10′ lengths before being cut to size. The measured mass and average thickness of the butt and tapered edge of the gypsum were 2.49±0.01 lb./ft. 2 , 0.63±0.01 in., and 0.58±0.01 in., respectively. 
     Gypsum panels were installed perpendicular to the underside of the RC and fastened with Type S steel screws (1 in. and 1⅝ in.) spaced 8 in. on center and located 1 in. from the tapered edge and ½ in. and 2 in. from the butt joint edge for the base and face layers, respectively. All exposed gypsum joints were taped (2 in.) and covered with two layers of dry mix joint compound. The exposed nail heads were also covered with two layers of compound. 
     A superimposed load was applied to the assembly provide a distributed load of 51.56 lb. f /ft., or a combined live (40 lb. f /ft. 2 ) and dead (23.5 lb. f /ft. 2 ) load on the top chord of 63.5 lb. f /ft. 2 . The loading of the assembly was accomplished by 20 weighted barrels (581.6 lb./barrel or 11,632 lb. total), each placed upon two 50 in., nominal 4×4 wood bunks, evenly spanned over three trusses (5 barrels spanning 4′×14.10′). This load applied was the calculated maximum theoretical load permitted by nationally recognized design standards. The deflection of the floor assembly was measured with one linear voltage displacement transducer (LVDT) located at the geometric center of the assembly. 
     After the furnace exposure and test termination, the fire was extinguished with water. Some of the gypsum wallboard had fallen away due to the extinguishment, but the RC and batten strips remained in place, with some bowing of the RC mid-cavity. Much of the fiberglass insulation was still in place with deterioration on the bottom, but largely whole on the top. A few gusset plates were observed to be pulling from the trusses, and the lower chord was charred, although no charring was observed on the bottom of the subfloor. The 2¼ inch screws and batten strips were sufficient to hold the RC and wallboard in place for the duration of the test. 
     The floor/ceiling assembly passed the requirements for the 2-hr fire endurance test, according to ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials. The fire resistance assembly had a finish rating of 100 min., rounding to the nearest integral minute. The floor/ceiling assembly did not allow flames to pass through the assembly for the 125 min. based from when the test was terminated. There was no unexposed temperature failure for average or single-point thresholds (139° C. +ambient, 181° C. +ambient) during the 125 min. test. Therefore, this assembly was considered certifiable for a 2 hr. 5 min resistance time. 
     While a particular embodiment of the present wooden frame truss with enhanced fire resistance has been shown and described, it will be appreciated by those skilled in the art that changes and modifications may be made thereto without departing from the invention in its broader aspects and as set forth in the following claims.