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BACKGROUND OF THE INVENTION 
     The invention relates to suspended ceiling construction and, in particular, to improvements in so-called three-dimensional ceilings. 
     PRIOR ART 
     Suspended three-dimensional ceilings with gentle wave-like configurations have been available for specialty applications where a dramatic or custom look is desired. Such ceilings find application in contemporary office environments, entertainment and gaming complexes, high-bay areas and retail space, for example. 
     The subject ceiling structures include convex (vault) and concave (valley) main grid runners or tees assembled with grid cross members in the form of cross tees or stabilizer bars. Typically, the primary purpose of three-dimensional ceilings is to provide a highly visible decorative structure. Consequently, a precision assembly is especially important so that visually distracting misalignments are avoided. A popular form of three-dimensional ceiling is a one-directional type where the lay-in panels are relatively long and where the joints between panels are not masked by visible cross ties. These one-directional systems are particularly prone to show misalignments of the grid structure and lay-in panels especially where the lay-in panels have a geometric pattern. In prior art constructions, the lay-in panels can take a skewed position on the supporting grid tee flanges. This misalignment is very visible and in severe conditions can even result in a panel falling off of a tee flange. 
     Installation of the main runners of a three-dimensional ceiling is more complex and requires more care than normally expended for conventional planar suspended grid ceilings. For example, considerable care is necessary in placement of suspension hanger wires so that when completed they hang relatively plumb in both directions of the grid. Achieving this condition is made difficult because the spacing between wires is variable depending on the inclination of the area of the grid being suspended. The extra time and effort involved in laying out and achieving a proper spacing for hanger wires longitudinally along the runners can detract from the time and effort spent in properly locating the lateral positions of the wires. These factors are in addition to the physical obstacles or conditions that can exist in the ceiling space which interfere with the proper spacing of the hanger wires. These problems have given rise to the need for a three-dimensional grid system that is more tolerant of imperfect suspension conditions and contributes to efforts at precisely positioning the grid ceiling structure. 
     SUMMARY OF THE INVENTION 
     The invention provides an improved three-dimensional ceiling that has self-aligning features which contribute to increased positional accuracy of both the grid and the panel members. More specifically, the ceiling system has main tees with a cross-sectional configuration that cooperates with specially proportioned lay-in panels to improve the parallelism of the grid tees as well as the parallelism of the panels to the grid tees. In one disclosed system, the main tees have a stem configured with an increased thickness at its lower edge where it joins the panel supporting flanges. Preferably, the thickness of the stem at its lower edge is at least about as large as its thickness adjacent its upper edge where it has a typically enlarged cross-sectional area or bulb for stiffening. This thickened stem geometry allows the components to be dimensioned so as to eliminate excessive lateral clearance between the tees and lay-in panels. The disclosed geometry still allows the panels to be assembled on the tees from a point above the grid without interference with the upper regions of the main tees. 
     The wide stem geometry of the main tees of the invention and correlated width of the lay-in panels is particularly important with one directional three-dimensional style ceilings. This style has no cross-tees at the visible lower face of the grid and, therefore, cannot rely on such structures to gauge and control the spacing between main runners at this face. 
     Stabilizer bars conventionally used to connect adjacent main tees together have a stepped or bridge-like construction to provide clearance for the installation of the lay-in panels. Typically, one-directional panels have their ends bent upwardly to form a flange that is used to couple with a mating end of another panel. The configuration of the stabilizer bars allows end-wise motion of the lay-in panels during installation and must be high enough above the supporting main tee flanges to allow the upwardly extending panel flanges to pass under the stabilizer bars. The somewhat complex geometric stabilizer bar configuration does not lend itself to precise control of the spacing of the lower visible faces of the main tees. 
     Many of the lay-in panel materials are relatively shear because of their translucence and/or perforated design. It is a practice to stagger the locations of the stabilizer bars between successive rows of main tees so that any shadow of a stabilizer bar visible through a lay-in panel is discontinuous and, therefore, less conspicuous. This practice exacerbates the difficulties in precisely positioning the main tees with the stabilizer bars since they do not stack up in a direct line. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view, from above, of portions of a three-dimensional ceiling system embodying the invention, with the majority of the lay-in panels not shown for purposes of clarity; 
     FIG. 2 is an enlarged cross-sectional view of the ceiling system taken in the plane  2 — 2  indicated in FIG. 1; 
     FIG. 3 is a fragmentary perspective view of a stabilizer bar of the illustrated ceiling system; 
     FIG. 4 is an enlarged fragmentary cross-sectional view of the end joint of a pair of abutting lay-in panels and an associated panel splice, taken in the plane  4 — 4  indicated in FIG. 1; 
     FIG. 5 is an enlarged fragmentary perspective view of the ceiling showing an integral hold down tab restraining a lay-in panel against the flange of a supporting tee; 
     FIG. 6 is a cross-sectional view of a modification of a main tee of the invention; 
     FIG. 7 is a cross-sectional view of another modification of a main tee of the invention; and 
     FIG. 8 is a cross-sectional view of still another modification of a main tee of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a specialty three-dimensional suspended ceiling system  10  constructed in accordance with the invention. The system  10  includes parallel rows of main runners or tees  11  interconnected with cross runners  12  to form a grid  13 . Supported on the main runners  11  are decorative lay-in panels  14 . Segments  16 ,  17  of the main runners  11  are curved in vertical planes so as to form vaults  16  or valleys  17 . Typically, an architect or designer can select combinations and patterns of these vaults  16  and valleys  17  or simply all vaults or all valleys as he or she chooses to construct the desired look. The adjacent ends of the segments  16 ,  17  of the main runners  11  are joined together by suitable clips  18  having bendable tabs inserted into appropriate slots provided in the segments adjacent their ends. The main runners  11  are suspended from overhead structure by wires  19  in a generally conventional manner except that the horizontal spacing between wires along a given main runner varies in relation to the inclination of the local part of a runner since the holes for receiving the suspension wires are uniformly spaced along the arcuate length of the runner. This irregular spacing requires extra attention by the installer and can present situations where accurate placement of the suspension points for the wires in both the longitudinal direction of the main runners  11  and in the lateral direction of the cross runners  12  suffers. Inaccurate location of the suspension points causes the wires to be out of plumb and makes it difficult to locate and construct a grid that is “square” so that the cross-runners and joints between panels are perpendicular to the main runners and also makes it difficult to hold the main runners in a straight line lying in an imaginary flat vertical plane. When properly installed, the main tees  11  lie in vertical planes and, from row to row, are in phase with one another so that the local elevation of one main tee is the same as the other tees along a horizontal line perpendicular to all of the tees. A main tee can be manufactured with a radius of curvature, measured at the visible face of its flange  21 , of between 30.5 in. (77.5 cm) to about  229  in. (582 cm) or larger, for example. 
     FIG. 2 illustrates the cross section of a main tee vault segment  16 . The cross section, which is symmetrical about an imaginary vertical central plane has a lower, generally horizontal flange  21  and a generally vertical stem  22 . With reference to FIG. 2, the main tees  11  are of a “narrow face” design such that the flange is relatively narrow, e.g. about {fraction (9/16)} in. (1.43 cm) measured across its edges  23 . The stem  22  includes a narrow, vertical web  24  and an enlarged hollow stiffening bulb  26  adjacent the upper edge of the web  24 . Integrally formed on the stem  22  between opposed portions  27  of the flange  21  adjacent a lower edge of the web  24  is a protrusion or spacer  28  that is preferably continuous with the length of the segment  16 , and is symmetrically disposed about the central imaginary plane of the cross-section. 
     The spacer  28  has generally vertical surfaces  29  that extend above the flange portions  27  a distance that is large in comparison, for example, to the wall thickness of either the flange  21  or web  24 , for example. 
     In the construction illustrated in FIG. 2, the main tee segments  16 ,  17  are made of roll-formed sheet metal such as steel painted or otherwise provided with a protective coating. More specifically, the main tee segments  16 ,  17  are formed of two metal strips, a first strip  31  forming essentially the outline of the tee section and a second strip  32  being a cap that locks the first strip  31  in its rolled configuration when it is rolled over the flange areas of the first strip. The lower or visible face of a tee  16 ,  17  has a hollow, central groove, which is the interior of the protrusion  28 , that is aesthetically desirable for its “reveal” character. Integral “hold-down” tabs  34  are stamped from the web  24  at regularly spaced locations along the segments  16 ,  17 . The valley segments  17  have a cross-section configuration like that of the vault segments except that the area of the bulb  26  is crimped to facilitate forming them into their convex or valley-shape. 
     FIGS. 2 and 3 illustrate details of a typical cross-tie or stabilizer bar  12  that extends between and interconnects with adjacent main runners  11 . The stabilizer bar  12  is preferably formed as a unitary sheet-metal stamping having a main channel body  36 . Each end of the body  36  has a depending leg  37 . The legs  37  are formed with a web mid-section  38  so that the plane of an upper portion  39  of the leg  37  is off-set from the plane of a lower portion  41  of the leg. The offset leg configuration enables the lower portions  41  to abut the web  24  of a main tee segment  16 ,  17  while the upper part  39  extends past the bulb  26  of the main tee segment. 
     The stabilizer bars  12  are assembled on the main tees  11  so that upon completion of the ceiling they are above the planes occupied by the lay-in panels  14 . The stabilizer bars  12  are assembled by positioning integral tabs  42  in slots stamped through the webs  24  of the main tees at regularly spaced locations. Once fully received in the slots, the tabs  42  are bent over against the webs  24  to lock the bars  12  in position. The depending legs  37  of the stabilizer bars  12  hold the channel section  36  well above the main tee flanges  21 . 
     The three-dimensional ceiling system illustrated in FIG. 1 is sometimes referred to in the industry as a “one-directional” style. This style is typically characterized by the absence of visible cross tees and inconspicuous joints between lay-in panels. The lay-in panels  14  are relatively long in comparison to their width being a nominal six feet (1.83 meters) long and a nominal two feet (0.61 meters) wide. The illustrated panels  14  have their ends turned up into flanges  46 . Abutting flanges  46  of adjacent panel ends can be held together with an inverted U-shaped joint splice  47 . The joint splice  47  is advantageously formed of a soft metal capable of being squeezed with pliers or like tools to tighten the abutting flanges  46  together. The lay-in panels  14  are assembled on the grid  13  by sliding them under the stabilizer bars  12 . The vertical height of the main channel body  36  of the bars  12  above the main tee flanges  21  provides ample clearance for the end flanges  46  of the panels  14 . The lay-in panels  14  are typically offered in a variety of materials of different opacity, translucency and/or perforation patterns. Typical lay-in panel materials include smooth or perforated painted aluminum, brass or stainless steel woven mesh, anodized aluminum and translucent fiber-reinforced plastic panels. The thickness of these panels can range from 0.020 in. (0.051 cm) to 0.080 in. (0.203 cm) so that they are relatively flexible. 
     The hold down tabs  42  are bent out of the plane of the web  24  and down against the panels  14  at appropriate locations to make the panels conform to the curvature of the main tees  11 . Typically, the material of the panels  14  is somewhat resilient and tends to maintain a planar configuration when not constrained by the tabs  43 . The lay-in panels  14  have increased lateral stiffness, i.e. compression, between main tees  11  when they assume the curved configuration of the main tees. 
     In accordance with the invention, the main tees  11  and lay-in panels  14  are configured to inter-engage in such a manner that they contribute to their mutual alignment so that the main tees and the panels are urged into precise parallel alignment. By way of example, but not limitation, a panel  14  can be sized with a nominal width of 23.75 in. (60.3 cm) and the stem spacer  28  can have a nominal horizontal thickness of 0.220 in. (0.559 cm). These proportions leave a relatively small nominal clearance of 0.030 in. (0.076 cm) between a panel and the adjacent main runners  11 . This clearance, theoretically, would require adjacent main tees  11  to be parallel to one another and to a panel at the plane of the flange  21  within 0.030 in. (0.076 cm) in six feet. While a nominal clearance of about 0.030 in. (0.076 cm) is most preferred for some applications such as illustrated in FIG. 1, the invention can be practiced by using other clearance dimensions with decreasing precision of positioning. For example, clearances ranging from a nominal clearance dimension of 0.060 in. (0.152 cm) up to as much as about 0.090 in. (0.229 cm), if desired or necessary can be used. 
     It will be appreciated from an understanding of the geometry of the stabilizer bars  12  and their locations remote from the plane of the flanges  21  and their manner of field installation that it is difficult to maintain precise parallel positioning of the main tees  11  at the plane of the flanges  21  simply with the stabilizer bars. The positional accuracy of the flanges  21 , of course, is important because it is these elements that are visible from the space below the ceiling system  10 . Precise control of the position of the main tees  11  with the stabilizer bars  12  is made more difficult by the practice of staggering these stabilizer bars in patterns like that shown in FIG.  1 . The close parallel registration that can be maintained between the tees  11  and panels  14  with the invention results in a high quality finished appearance of the ceiling system  10 . This is especially important with the general type of disclosed three dimensional ceiling since it is under increased visibility by virtue of being a specialty item intended to draw visual attention. Often, the lay-in panels  14  have a regular geometric pattern that accentuates any misalignment between them and the main tees  11 . 
     It is important that the width of the stem of the spacer is at least approximately as large as the maximum width of other portions of the stem—specifically the stiffening bulb  26 —so that the panels  14  can be laid in the grid  13  without undue interference. FIGS. 6-8 illustrate other examples of main tee cross-sectional shapes that can be used in practicing the invention. Typically, the cross-sections are symmetrical about an imaginary vertical central plane. In FIG. 6, a main tee  51  has a cross-section like that of the main tee  11  of FIG. 2 except that the flange portions  52  are proportionately wider. A main tee  53  of FIG. 7 is an extrusion of thermoplastic or thermosetting resin or of aluminum. The tee  53  includes panel supporting flange portions  54 , a stem  56  comprising a web  57 , a solid stiffening bulb  58  and a solid spacer  59 . The spacer  59  includes vertical surfaces  61  for cooperation with the edges of a lay-in panel sized to minimize horizontal clearance between the panels and the main tees  53  as disclosed hereinabove. FIG. 8 shows the cross-section of an extruded main tee  63  formed of suitable plastic or aluminum or other suitable rigid material. The tee  63  includes panel supporting flange portions  64  and a hollow stem  66 . The stem  66  includes vertical spacer surfaces  67  adapted to cooperate with a lay-in panel sized in the manner described above to improve positional accuracy of the grid and panel. 
     It will be understood from the foregoing disclosure that the invention can be employed in various other types of three-dimensional ceiling styles such as those in which the panels are shorter rectangles of nominally 2 ft.×4 ft. (0.610 meters×1.22 meters) or are square, nominally 2 ft.×2 ft. (0.610 meters×0.610 meters). Still further, variants of the invention can utilize conventional cross tees, known in the art, visible from below the panels at selected centers. 
     It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.

Summary:
A suspended three-dimensional ceiling system of improved appearance and performance that includes closely dimensioned main tees and lay-in panels. The main tees have opposed vertical surfaces adapted to abut the edges of the panels to avoid any noticeable non-parallelism between the main tees and/or panels. The vertical surfaces are provided by a protrusion at the juncture between a panel supporting flange and a vertical stem of the main tee. The protrusion allows the panels to be dimensioned to avoid undue interference with a stiffening bulb on the upper part of the stem and provides an attractive reveal on the visible face of the flange.