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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION  
       [0001]     This invention generally relates to a suspended acoustic ceiling assembly with removable tile sections and more particularly to a suspended acoustic ceiling structure wherein sections of the acoustic tiles are selectively removable for enlarged access to a plenum area above the ceiling. The supporting structure for the acoustic tile sections is hidden from view of the persons in the room below.  
       BACKGROUND OF THE INVENTION  
       [0002]     Suspended ceilings have been utilized in building installations for many years. They are typically provided as a grid work of main-runners and cross-runners suspended from the building structure and supporting suitable infill material such as ceiling tiles, light fixtures, air handling devices, etc. In most suspended ceilings the runners have a form of an inverted “T”, and thus, are sometimes called main tees and cross tees. The grid tees are usually formed from a strip of material which has been bent to form a bulb or bead along the top edge of a central web with opposed outwardly extending flanges located at the bottom edge of the web.  
         [0003]     Suspended ceilings are generally formed by hanging main tees along one dimension of a room, the main tees being spaced apart a distance equal to the length of a cross tee which is typically four feet. The main tees are provided with apertures at a regular spacing along the length of the main tee, the apertures receive a strap extending between adjacent cross tees for connecting the cross tees to each other and to the main tee. Typically opposed cross tees are interconnected within the opening, the opposed cross tees extending to either side of the web of the main tee. The connection between the cross tee and the main tee and between interconnected cross tees must, according to many present building code standards, be able to support the ceiling in an emergency situation, such as fire and seismic conditions. Thus the suspended ceiling must provide sufficient integrity such that the connectors will not disengage when exposed to a fire, and also should have sufficient integrity so that in a seismic occurrence, such as an earthquake, the connections will be maintained to enable the infill material of the ceiling to be supported.  
       SUMMARY OF THE INVENTION  
       [0004]     The invention includes a main tee and cross tee suspended from wires attached to a structural ceiling. The main tees have slots for receiving the free end of arms on the upper end of torsion springs. The wound bottom end of the torsion springs engage hooks located at spaced intervals along acoustical panel framing members. These panel framing members are joined at the corners by various means such as pop rivets and bayonet fittings. The framing members are structured with an inwardly extending lip or flange along the length of the member at the bottom thereof. The lip engages a kerf in the acoustic ceiling panels. The kerf extends around the periphery of the panel and provides a mechanism for engaging the lip and holding the panels in place.  
         [0005]     Among the advantages of this invention are those resulting from the fact that it facilitates the ceiling sections that may conveniently be removed from a suspended ceiling structure utilizing the aforementioned structure to provide large clearance access into the plenum space above the ceiling for maintenance personnel and for enabling infill materials such as ceiling tiles and light fixtures to be supported directly from the structural ceiling. The resultant appearance is an aesthetically pleasing ceiling appearance with extremely small reveals between adjacent ceiling tiles.  
         [0006]     It is a further advantage of the invention to provide a hidden supporting structure for a suspended ceiling wherein large ceiling sections are removably supported without sagging.  
         [0007]     Further advantages of the present invention result from the fact that it enables the selectively shaped horizontal panel framing members to be held rigidly parallel in predetermined spaced relationship so that these assemblies can be suspended on wires directly from the building structure above. The main tees, cross tees and panel framing members provide a rigid grid without the necessity of applying heavy structural channels which are conventionally installed in prior art ceiling structures.  
         [0008]     The ceiling produced by this invention is durable, rigid and good looking while hiding the acoustic tile support elements from the viewers located below the ceiling.  
         [0009]     In practicing the invention, the ceiling supporting main tees and cross tees are horizontally aligned and supported on wires directly from the building structure. The tees in turn support framing panels by means of v-shaped torsion springs which engage hooks on the panels and extend upwardly to engage slots in the main tees. Each of the panel frames has at its lower end, an acoustic tile (panel) supporting flange extending transversely, i.e., horizontally inwardly from one side of the framing member. The flanges on the framing members on opposite sides of a ceiling panel are inserted into the kerf (slot) extending along the edge of the tile. Panel framing members on the remaining two sides of the enclosure then engage kerfs on the corresponding sides of the tile and the four framing members are connected to each other at the corners to complete the assembly.  
         [0010]     The invention provides an acoustic panel ceiling system comprising a plurality of ceiling panel framing members with ends interconnected to form a four-sided enclosure, with each panel member having a flange at the bottom thereof extending into the interior of the enclosure. A plurality of notches are formed at spaced intervals along the top edges of each framing member comprising a hook-shaped element.  
         [0011]     A plurality of v-shaped torsion springs having a wound end at the bottom engage the hooks on the framing members and have a pair of arms extending upwardly and outwardly from the wound end, each arm having a hook formed in the free end thereof. A grid suspended from the structure in which the ceiling system is to be mounted is provided comprising main tees and intersecting cross tees. A plurality of pairs of slots are located at spaced intervals along the main tees, the slots being adapted to receive the free ends of the torsion spring arms. A ceiling tile have a kerf formed around the periphery thereof, is engaged with the flanges on the framing members and adapted to abut adjacent tiles with a minimal space between adjacent tiles. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The various features, aspects and advantages of the present invention will be more fully understood from a consideration of the following detailed description in conjunction with the accompanying drawings in which:  
         [0013]      FIG. 1  as a perspective view of a typical portion of the ceiling system showing the ceiling panel, the frame for receiving the panel, the cross tees and main tees for supporting the panel frames and panels and hanger wires for attaching the system to a structure;  
         [0014]      FIG. 2  is a perspective view of a portion of the system according to the present invention showing main tees, cross tees and hanger wires;  
         [0015]      FIG. 3  is a perspective view of a typical perforated wood ceiling panel attached to panel framing members for use in the system according the present invention;  
         [0016]      FIG. 4  is a perspective view of the panel assembly in  FIG. 3  attached to the supporting tee assembly;  
         [0017]      FIG. 5  is a sectional view in elevation showing a kerfed acoustical ceiling panel and framing assembly attached to a main tee and two cross tees;  
         [0018]      FIG. 6  is a sectional view in elevation showing the panel and framing assembly attached to main tees and cross tees with a metal or vinyl ceiling panel attached thereto;  
         [0019]      FIG. 7  is a detailed perspective view of a notch and hook or clip construction on a typical panel framing member with the torsion spring attached thereto.  
         [0020]      FIG. 8A  is a sectional view of the assembly of  FIG. 6  prior to insertion of a pull down tool into the reveal between adjacent tiles.  
         [0021]      FIG. 8B  is a sectional view of the assembly in  FIG. 6  with the pull down tool engaged with a ledge on the panel member; and.  
         [0022]      FIG. 8C  is a sectional view of the assembly of  FIG. 6  with one panel pulled down to enable disengagement of the ceiling tile from the panel member. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     The invention  10  is shown in perspective view in  FIG. 1 . As shown therein, the invention includes a plurality of main tees  12  and cross tees  14 . The main tees  12  are provided with a plurality of torsion spring slots  16  which are located at spaced intervals along the length of the main tees in the flanges extending inwardly from the bottom of the main tees. The main tees and the cross tees provide support for the ceiling panel framing assembly which are attached to the main tees by means of torsion springs  18  which are connected to panel framing members  20  and extend upwardly from torsion spring notches  22  to the torsion spring slots  16 . The main tee and cross tee assembly are supported by hangar wires  23  which are connected to the tee assembly and extend upwardly to be connected to the structure from which the ceiling system is to be hung.  
         [0024]     As shown in  FIG. 1 , a ceiling panel  24  is positioned below the panel framing members  20 . The ceiling panel  24  has a framing slot or kerf  26  extending along the peripheral edge of the ceiling panel. Flange  28  is formed at the bottom of the framing panel members  20  and this flange is adapted and positioned to be engaged in the framing slot  26  of the ceiling panel to hold the ceiling panel in place. In assembly the ceiling panel is mounted to the framing assembly by attaching two panel framing members on opposite sides to the ceiling panel and engaging the same with the kerf extending along the periphery of the ceiling panel. Subsequently, the pair framing panels on the remaining two sides of the ceiling panel are engaged at the corners with the first pair of panel framing members and are secured at the corners by means of pop rivets or a connector tab and connector slot configuration. As can be seen from  FIG. 1  torsion spring notches  22  are located at spaced intervals along each panel framing member to provide flexibility in terms of the way in which panel framing members to which the torsion springs are attached to the tee assembly. Also, as shown in  FIG. 1 , the torsion springs  18  are attached to two sides of the four sided panel assembly.  
         [0025]     The illustration in  FIG. 2  shows the assembly of main tees  12  and cross tees  14  supported by hangar wires  23  in greater detail. As shown therein, the main tees  12  are connected to cross tees  14  by means of straps  32  which extend through vertical slots  34  in the main tees and fastened to the cross tees at the intersections. Torsion spring slots  16  are located at spaced intervals along the main tees. The vertical slots  34  are provided at spaced intervals along the length of the main tees to provide flexibility for mounting cross tees to accommodate different sizes of ceiling tiles.  
         [0026]     Referring now to  FIG. 3 , shown therein is a ceiling panel assembly  35  comprising the panel framing members  20  joined by brackets  36  and pop rivet fasteners  38  which join the adjacent panel framing members at the corners. The ceiling panel member is attached to the panel framing members, either by means of a flange extending around the panel framing members (see  FIG. 1 ) and engaging a kerf in the ceiling panel or, in the alternative, by securing the ceiling panel to the underside of the flange by means of adhesives or the like. As shown therein, the ceiling panel  40  is a typical perforated wood panel  40  and attached thereto is a layer of acoustic insulating material  41  such as Soundtex. The torsion springs  18  are likewise depicted in  FIG. 3  and are attached at the torsion spring notches  22 . The torsion springs  22  comprise arms  43  extending upwardly from a hinge or wound end  42  which engages a folded-down tab or clip  44 .  
         [0027]     The ceiling panel assembly  35  of  FIG. 3  is then attached to the main tee and cross tee subassembly  46  as shown in  FIG. 4 . The torsion spring arms  43  are inserted upwardly through torsion spring slots  16  and the ceiling panel assembly  35  is raised until the panel framing members are flush against the bottom side of the main tees and cross tees. As will be discussed in more detail, the ceiling panel frame assemblies  35  can be easily lowered by the use of a stock tool which is inserted in the reveal between adjacent ceiling panels  40  to engage the edge of a ceiling panel and pull it downwardly causing the torsion springs  18  to slide downwardly through the slots until hooks  48  at the end of the torsion spring arms  43  engage flanges  50  in the main tees.  
         [0028]     A sectional view in  FIG. 5  shows a detail of the assembly of the ceiling panel system supporting a typical acoustical fiberglass panel  52 . As shown therein, the main tee  12  is supported by hangar wire  22 . The main tee is shown in cross section and connected to the main tee  12  are two cross tees  14 . The cross tees are connected to the main tee by strap  32 . Two torsion springs  18  are shown extending upwardly through the flanges of main tee  12 . As shown therein, the ceiling panels are in their raised position and the panel framing members bear directly against the bottom of the main tees and the cross tees. The torsion spring notches  30  on adjacent panel members  52  are shown in opposed position and comprise folded-down tabs or clips  44  to define hooks which are engaged with hinges  42  at the bottom of each of the torsion springs. The flanges  28  of the panel framing members are shown engaged with the kerfs  26  of adjacent ceiling panels  24 . The ceiling panel assembly of the present invention features the close spacing of adjacent ceiling panels with typically a ⅛th of an inch reveal between adjacent acoustical fiberglass panels.  
         [0029]     A sectional view similar to the view shown in  FIG. 5  is shown in  FIG. 6 , with main tee  12  being supported by hangar wire  22  and cross tees  14  joined to the main tee. In this figure the acoustic panels with kerfed edges are replaced by a metal panels  53  which are secured to the flange of the panel framing member by means of adhesives or similar fastening material. The ceiling panel in this instance can be selected from a number of different choices and in certain instances the visible side is covered with a veneer  55  to suit the particular requirements, application and location of the ceiling system being installed. The reveal between metal panels is typically 1/16th of an inch.  
         [0030]      FIG. 7  shows a detail of a torsion spring notch  30  and torsion spring  18 . As shown therein, the notch  30  is formed in the side of a panel framing member and a strap of material in the form of a tab  51  in the center of the notch is bent and folded down to define a hook or clip  54 . The wound end or hinge  42  of the torsion spring is engaged with the hook or clip and secured to the panel member. The arms  43  of the torsion spring extend upwardly and are terminated in hooks  48 . The hooks limit the travel of the torsion spring when the ceiling panel is lowered for purposes of gaining access to the space above the ceiling panel system or for purposes of changing ceiling panels.  
         [0031]     Hook or clip  54  is oversize and provides ample space for engagement with hinge  42 . In assembly hinge  42  is engaged by tab  51  at the notch configuration at the factory as the tab is folded down into its closed position to form the hook or clip. The oversize dimensions of hook  54  provide space for torsion springs  18  to be rotated 90° forward to lie flat with the panel framing assemblies for shipping to a job site. Assembly of the torsion springs and hooks at the place of fabrication eliminates a number of assembly steps at the job site thereby speeding the process of installation and reducing the labor cost at installation.  
         [0032]      FIGS. 8A, 8B  and  8 C illustrate the manner in which a ceiling panel is lowered and then restored to its normal position. As shown therein, a tool  56  having a long blade  57  with a plate  58  affixed thereto, is inserted in the reveal between adjacent panels to engage a ledge  60  formed into the edge of the panel member. The plate  58  engages the ledge as shown in  FIG. 8B  preparatory to pulling the ceiling panel downwardly. As shown in  FIG. 8C , the ceiling panel  24  is lowered and the torsion spring  18  is slid downwardly through the torsion spring slots until the hooks  48  on the torsion springs engage the flanges of the main tee and the ceiling panel is thereby positioned at its lowest limit. After work is done to either replace the panels or to work in the space above the ceiling, the ceiling panel is then manually pressed upwardly until it is flush with the adjacent ceiling panel and the assembly is restored to its normal position.

Summary:
A concealed suspension ceiling assembly with simplified support structure for closely spaced ceiling tiles is described. The ceiling tiles are kerfed and mounted on framing members suspended from a supporting grid. V shaped torsion springs engage hooks on the framing members and extend upwardly through slots in main tees in an intersecting grid of main tees and cross tees. A tool is inserted in the reveal between adjacent tiles to raise and lower a selected tile. Tiles are lowered in predetermined sections to permit access to the space above the suspended ceiling and also to permit repair or replacement of individual tiles.