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CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part patent application of U.S. patent application Ser. No. 12/009,835, filed Jan. 22, 2008, and U.S. patent application Ser. No. 12/830,902, filed Jul. 6, 2010, the disclosure of both applications are incorporated herein by reference. This application also claims the benefit of U.S. Provisional Application No. 61/303,350, filed Feb. 11, 2010, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates in general to impact absorbing underlayment panels. In particular, this invention relates to underlayment panels having deformable elements that compress in a plurality of stages such that a load absorbing gradient is provided in response to an applied force. 
         [0003]    Surfaces such as playgrounds and athletic mats, for example, are scrutinized for their effect on impact forces that cause related injuries to users. Attempts have been made to minimize the force or energy transferred to a user&#39;s body in the event of a fall. Various surface designs that rely on ground materials or layered fabric materials may help reduce the transfer of impact forces. These surface designs, however, are limited by the ability of the materials to spread the impact load over a large area. Thus, it would be desirable to provide a surface having improved impact force absorption and dissipation characteristics. 
       SUMMARY OF THE INVENTION 
       [0004]    This invention relates to an impact absorption panel having a top side and a bottom side. The top side includes a plurality of drainage channels that are in fluid communication with a plurality of drain holes. The plurality of drain holes connect the top side drainage channels with a plurality of bottom side channels. The bottom side channels are defined by sides of adjacent projections that are disposed across the bottom side. 
         [0005]    This invention also relates to an impact absorption panel having a top side and a bottom side where the bottom side has a plurality of projections disposed across at least a portion of the bottom surface. The projections have a first spring rate characteristic and a second spring rate characteristic. The first spring rate characteristic provides for more deflection under load than the second spring rate characteristic. 
         [0006]    In one embodiment, an impact absorption panel comprises a top surface and a bottom surface. The top surface has a three dimensional textured surface and a plurality of intersecting drainage channels. The bottom surface is spaced apart from the top surface and defines a panel section therebetween. A plurality of projections is disposed across at least a portion of the bottom surface. The projections have a first stage that defines a first spring rate characteristic and a second stage defining a second spring rate characteristic. The first spring rate characteristic provides for more deflection under load than the second spring rate characteristic. The plurality of projections also cooperate during deflection under load such that the adjacent projections provide a load absorption gradient over a larger area than the area directly loaded. In another embodiment, the first stage has a smaller volume of material than the second stage. Additionally, the adjacent projections define a bottom surface channel to form a plurality of intersecting bottom surface channels and a plurality of drain holes connect the top surface drainage channels with the plurality of bottom surface channels at the drainage channel intersections. 
         [0007]    In another embodiment, an impact absorption panel includes a top surface and a bottom surface that define a panel section. A plurality of projections are supported from the bottom surface, where the projections include a first stage having a first spring rate and a second stage having a second spring rate. The first stage is configured to collapse initially when subjected to an impact load, the second stage is configured to provide greater resistance to the impact load than the first stage, and the panel section is configured to provide greater resistance to the impact load than the first and second stages. The first stage is also configured to compress and telescopically deflect, at least partially, into the second stage. A portion of the bottom surface is generally coplanar with the truncated ends of adjacent projections such that the coplanar bottom surface portion is configured to provide a substantial resistance to deflection under load compared with the first and second stages. This coplanar configuration of the bottom surface provides a structural panel section having a thickness that is generally equal to the thickness of the panel section plus the length of the projections. 
         [0008]    In yet another embodiment, an impact absorption panel system comprises a first panel and at least a second panel. The first panel has a top surface, a bottom surface, a first edge having a flange that is offset from the top surface and a second edge having a flange that is offset from the bottom surface. A plurality of projections are disposed across the bottom surface. The projections have a first spring rate characteristic and a second spring rate characteristic. The second panel has a top surface, a bottom surface, a first edge having a flange that is offset from the top surface and a second edge having a flange that is offset from the bottom surface. A plurality of projections are disposed across the bottom surface of the second panel and have a first spring rate characteristic and a second spring rate characteristic. One of the second panel first edge flange and the second edge flange engages one of first panel second edge flange and the first panel first edge flange to form a generally continuously flat top surface across both panels. 
         [0009]    In one embodiment, the impact absorption panel is a playground base layer panel. 
         [0010]    Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1A  is an elevational view of a top side of an embodiment of an impact absorption panel suitable as a playground base; 
           [0012]      FIG. 1B  is an enlarged elevational top view of an edge of the impact absorption panel of  FIG. 1A ; 
           [0013]      FIG. 1C  is an enlarged elevational top view of a corner of the impact absorption panel of  FIG. 1A ; 
           [0014]      FIG. 2A  is an elevational view of a bottom side of an embodiment of an impact absorption panel; 
           [0015]      FIG. 2B  is an enlarged elevational bottom view of a corner of the impact absorption panel of  FIG. 2A ; 
           [0016]      FIG. 3  is a perspective view of an embodiment of a panel interlocking feature of an impact absorption panel; 
           [0017]      FIG. 4  is a perspective view of a panel interlocking feature configured to mate with the panel locking feature of  FIG. 3 ; 
           [0018]      FIG. 5  is an elevational view, in cross section, of the assembled panel interlocking features of  FIGS. 3 and 4 . 
           [0019]      FIG. 6  is an enlarged elevational view of an embodiment of a shock absorbing projection of an impact absorption panel; 
           [0020]      FIG. 7  is a perspective view of the bottom side of the impact absorption panel of  FIG. 6 ; 
           [0021]      FIG. 8A  is an enlarged elevational view of an embodiment of a deformed projection reacting to an impact load; and 
           [0022]      FIG. 8B  is an enlarged elevational view of another embodiment of a deformed projection reacting to an impact load. 
           [0023]      FIG. 9  is an enlarged elevational view of another embodiment of a deformed projection reacting to an impact load. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]    Referring now to the drawings, there is illustrated in  FIGS. 1A ,  1 B, and  1 C a load supporting panel having an impact absorbing structure configured to underlie a playground area. The various embodiments of the impact absorbing panel described herein may also be used in indoor and outdoor impact environments other than playgrounds and with other types of equipment such as, for example, wrestling mats, gymnastic floor pads, carpeting, paving elements, loose infill material, and other covering materials. In certain embodiments, the panel is described as a single panel and is also configured to cooperate with other similar panels to form a base or impact absorbing panel system that is structured as an assemblage of panels. The panel, shown generally at  10 , has a top surface  12  that is illustrated having a grid of drainage channels  14 . Though shown as a grid of intersecting drainage channels  14 , the drainage channels may be provided in a non-intersecting orientation, such as generally parallel drainage channels. In the illustrated embodiment, a drain hole  16  is formed through the panel  10  at the intersection points of the drainage channels  14 . However, not every intersection point is required to include a drain hole  16 . The drain holes  16  may extend through all or only a portion of the intersecting drainage channels  14  as may be needed to provide for adequate water dispersion. Though illustrated as a square grid pattern, the grid of drainage channels  14  may be any shape, such as, for example, rectangular, triangular, and hexagon. 
         [0025]    A first edge flange  18  extends along one side of the panel  10  and is offset from the top surface  12  of the panel  10 . A second edge flange  20  extends along an adjacent side of the panel  10  and is also offset from the top surface  12 . A third edge flange  22  and a fourth edge flange  24  are illustrated as being oriented across from the flanges  18  and  20 , respectively. The third and fourth flanges  22  and  24  extend from the top surface  12  and are offset from a bottom surface  26  of the base  12 , as shown in  FIG. 2A . The first and second flanges  18  and  20  are configured to mate with corresponding flanges, similar to third and fourth flanges  22  and  24  that are part of another cooperating panel. Thus, the third and fourth flanges  22  and  24  are configured to overlap flanges similar to first and second flanges  18  and  20  to produce a generally continuous surface of top surfaces  12  of adjoining panels  10 . A panel section  27 , as shown in  FIG. 5 , is defined by the thickness of the panel between the top surface  12  and the bottom surface  26 . 
         [0026]    In an alternative embodiment, the panel  10  may be configured without the first through fourth flanges  18 ,  20 ,  22 , and  24 . In such a configuration, the resulting edges of the panel  10  may be generally flat and straight edges. In another embodiment, the generally straight edge may include projections (not shown) to create a gap between adjoining panels, as will be explained below. In yet another embodiment, the edges may be formed with an interlocking geometric shape similar to a jigsaw puzzle. 
         [0000]    Referring now to  FIGS. 2A and 2B , there is illustrated the bottom surface  26  of the panel  10 . The illustrated bottom surface  26  includes a plurality of projecting shock absorbing structures  28  disposed across the bottom surface  26 . Only some of the projections  28  are shown on the bottom surface  26  so that the drain holes  16  may be clearly visible. Thus, in one embodiment, the projections  28  extend across the entire bottom surface  26 . In another embodiment, the projections  28  may be arranged in a pattern where portions of the bottom surface have no projections  28 . The portion having no projections  28  may have the same overall dimension as the thickness of the panel  10  including the projections  28 . Such a section may be configured to support a structure, such as a table and chairs. This portion of the bottom surface  26  is configured to provide a structural support surface having a substantial resistance to deflection under load compared with the first and second stages  40  and  42 . 
         [0027]    Referring now to  FIGS. 3 ,  4 , and  5 , the flange  24  is shown to include a locking aperture  30  as part of an interlocking connection to secure adjacent panels  10  together. A flange  20 ′ of an adjacent panel  10 ′ includes a locking projection  32 . As shown in  FIG. 5 , the locking projection  32  is disposed within the locking aperture  30 . The diameter of the locking projection is shown as “P”, which is smaller than the diameter of the locking aperture, “A”. This size difference permits slight relative movement between adjoining panels  10  and  10 ′ to allow, for example,  1 ) panel shifting during installation,  2 ) thermal expansion and contraction, and  3 ) manufacturing tolerance allowance. In the illustrated embodiment, flange  18  does not include a locking projection or aperture  30 ,  32 . However, in some embodiments all flanges  18 ,  20 ,  22 , and  24  may include locking apertures and/or projections. In other embodiments, none of the flanges may have locking apertures and projections. 
         [0028]    Some of the flanges include a standout spacer  34 , such as are shown in  FIGS. 4 and 5  as part of flanges  20 , and  20 ′. The standout spacer  34  is positioned along portions of the transition between the flange  20 ′ and at least one of the top surface  12  and the bottom surface  26 . The standout spacer  34  establishes a gap  36  between adjacent panels to permit water to flow from the top surface  12  and exit the panel  10 . The standout spacer  34  and the resulting gap also permit thermal expansion and contraction between adjacent panels while maintaining a consistent top surface plane. Alternatively, any or all flanges may include standout spacers  34  disposed along the adjoining edges of panels  10  and  10 ′, if desired. The flanges may have standout spacers  34  positioned at transition areas along the offset between any of the flanges and the top or bottom surfaces  12  and  26 . 
         [0029]    Referring now to  FIGS. 6 and 7  there is illustrated an enlarged view of the projections  28 , configured as shock absorbing projections. The sides of adjacent projections  28  define a bottom channel  38 . The bottom channels  38  are connected to the top drainage channels  14  by the drain holes  16 . The bottom channels  38  permit water to flow from the top surface  12  through the drain holes  16  and into the ground or other substrate below the panel  10 . In one embodiment, the bottom channels  38  may also store water, such as at least 25 mm of water, for a controlled release into the supporting substrate below. This slower water release prevents erosion and potential sink holes and depressions from an over-saturated support substrate. The channels  38  also provide room for the projections to deflect and absorb impact energy, as will be explained below. Additionally, the bottom channels  38  also provide an insulating effect from the trapped air to inhibit or minimize frost penetration under certain ambient conditions. 
         [0030]    The shock absorbing projections  28  are illustrated as having trapezoidal sides and generally square cross sections. However, any geometric cross sectional shape may be used, such as round, oval, triangular, rectangular, and hexagonal. Additionally, the sides may be tapered in any manner, such as a frusto-conical shape, and to any degree suitable to provide a proper resilient characteristic for impact absorption. The projections  28  are shown having two absorption stages or zones  40  and  42 . A first stage  40  includes a truncated surface  44  that is configured to support the panel  10  on the substrate or ground. The end of the first stage  40  may alternatively be rounded rather than a flat, truncated surface. In another alternative embodiment, the end of the first stage  40  may be pointed in order to be partially embedded in the substrate layer. A second stage or zone  42  is disposed between the bottom side  26  and the first stage  40 . The second stage  42  is larger in cross section and volume than the first stage  40 . Thus, the second stage  42  has a stiffer spring rate and response characteristic than that of the first stage  40 . This is due to the larger area over which the applied load is spread. In another embodiment, the first stage  40  may be formed with an internal void, a dispersed porosity, or a reduced density (not shown) to provide a softer spring rate characteristic. In yet another embodiment, the first stage  40  may be formed from a different material having a different spring rate characteristic by virtue of the different material properties. The first stage  40  may be bonded, integrally molded, or otherwise attached to the second stage  42 . Though the first and second stages  40  and  42  are illustrated as two distinct zones where the first stage  40  is located on a larger area side of the second stage  42 , such is not required. The first and second stages  40  and  42  may be two zones having constant or smooth wall sides where the two zones are defined by a volume difference that establishes the differing spring rates. Alternatively, the projections  28  may have a general spring rate gradient over the entire projection length between the truncated end  44  and the bottom surface  26 . 
         [0031]    Referring to  FIGS. 8A and 8B , the deflection reaction of the projection  28  is illustrated schematically. As shown in  FIG. 8A , a load “f” is applied onto the top surface  12  representing a lightly applied impact load. The first stage  40  is compressed by an amount L 1  under the load f and deflects outwardly into the channel  38 , as shown by a deflected first stage schematic  40 ′. The second stage  42  may deflect somewhat under the load f but such a deflection would be substantially less than the first stage deflection  40 ′. As shown in  FIG. 8B , a larger impact load “F” is applied to the top surface  12 . The first and second stages  40  and  42  are compressed by an amount L 2  under the load F, where the first stage  40  is compressed more than the second stage  42 . The first stage  40  deflects outwardly to a deflected shape  40 ″. The second stage  42  is also deflected outwardly to a deflected shape  42 ″. Thus, the first and second stages  40  and  42  progressively deflect as springs in series that exhibit different relative spring rates. These deflected shapes  40 ′,  40 ″, and  42 ″ are generally the shapes exhibited when an axial compressive load is applied to the top surface. The first and second stages  40  and  42  may also bend by different amounts in response to a glancing blow or shearing force applied at an angle relative to the top surface  12 . 
         [0032]    The projections  28  are also arranged and configured to distribute the impact load over a larger surface area of the panel  10 . As the panel  10  is subjected to an impact load, either from the small load f or the larger load F, the projections deflect in a gradient over a larger area than the area over which the load is applied. For example, as the panel reacts to the large impact load F, the projections immediately under the applied load may behave as shown in  FIG. 8B . As the distance increases away from the applied load F, the projections  28  will exhibit deflections resembling those of  FIG. 8A . Thus, the projections  28  form a deflection gradient over a larger area than the area of the applied load. This larger area includes areas having deflections of both first and second stages  40  and  42  and areas having deflections of substantially only the first stage  40 . Thus, under a severe impact, for example, in addition to the compression of the material in the area of the load, the first stage  40  (i.e., the smaller portions) of the projections compress over a wider area than the are of the point of impact. This load distribution creates an area elastic system capable of distributing energy absorption over a wide area. This produces significant critical fall heights, as explained below. This mechanical behavior of the projections  28  may also occur with tapered projections of other geometries that are wider at the top than at the bottom (i.e., upside down cones). 
         [0033]    Referring now to  FIG. 9  there is illustrated another embodiment of a panel  100  having projections  128  that exhibit a telescopic deflection characteristic. A first stage  140  of the projection  128  is deflected linearly into the second stage  142 . During an initial portion of an impact load, the first stage  140  compresses such that the material density increases from an original state to a compressed state. A dense zone  140   a  may progress from a portion of the first stage  140  to the entire first stage. As the impact load increases, the first stage pushes against and collapses into the second stage  142 . The second stage  142  compresses and permits the first stage to linearly compress into the second stage  142  similarly to the action of a piston within a cylinder. A second stage dense zone  142   a  may likewise progress from a portion of the second stage to the entire second stage. Alternatively, the dense zones  140   a  and  142   a  may compress proportionally across the entire projection  128 . 
         [0034]    The softness for impact absorption of the panel  100  to protect the users, such as children, during falls or other impacts is a design consideration. Impact energy absorption for fall mitigation structures, for example children&#39;s playground surfaces, is measured using HIC (head injury criterion). The head injury criterion (HIC) is used internationally and provides a relatively comparable numerical indicator based on testing. HIC test result scores of 1000 or less are generally considered to be in a safe range. The value of critical fall height, expressed in meters, is a test drop height that generates an HIC value of 1000. For example, to be within the safe zone, playground equipment heights should kept at or lower than the critical fall height of the base surface composition. The requirement for critical fall height based on HIC test values in playground applications may be different from the requirement for critical fall heights in athletic fields and similar facilities. Also, the HIC/critical fall height will vary based on the supporting substrate characteristics. In one embodiment, the panel  10  or the panel  100  may be configured to provide a 2.5 m critical fall height over concrete, when tested as a component of a playground surface, and a 2.7 m critical fall height over concrete in combination with a low pile (22 mm) artificial turf partially filled with sand. In another embodiment, the panel  10  or the panel  100  may provide a 3.0 m critical fall height over a compacted sand base in combination with a low pile (22 mm) artificial turf partially filled with sand. By comparison, conventional athletic field underlayment layers are configured to provide only half of these critical fall height values. 
         [0035]    These HIC/critical fall height characteristic and figures are provided for comparison purposes only. The panel  10  or the panel  100  may be configured to absorb more or less energy depending on the application, such as swings, monkey bars, parallel bars, vertical and horizontal ladders, along with the ages of the intended users. In one embodiment, the projections  28  or  128  may have a first stage height range of 10-15 mm and a second stage height range of 15-25 mm. In another embodiment, the projections  28  or  128  may be configured to be in a range of approximately 12-13 mm in height for the first stage and 19-20 mm in height for the second stage in order to achieve the above referenced HIC figures. The panel  10  or the panel  100  may be made of any suitable material, such as for example, a polymer material. In one embodiment, the panel  10  or  100  is a molded polypropylene panel. However, the panel may be formed from other polyolefin materials. 
         [0036]    The panels  10  or  100  may be assembled and covered with any suitable covering, such as for example, artificial turf, rubber or polymer mats, short pile carpeting, particulate infill, or chips such as wood chips or ground rubber chips. 
         [0037]    The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.

Summary:
An impact absorption panel is adapted for playground use and comprises a panel section and a plurality of projections. The panel section is defined by a top surface and a bottom surface. The plurality of projections extend from the bottom surface of the panel section. The plurality of projections have a first stage and a second stage. The first stage is configured to collapse initially when subjected to an impact load. The second stage is configured to provide greater resistance to the impact load than the first stage. The panel section is configured to provide greater resistance to the impact load than the first and second stages. The first stage can also be distinguished from the second stage by virtue of having a comparatively smaller volume.