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PRIORITY CLAIM 
       [0001]    This application is based upon and claims the benefit of U.S. provisional application Ser. No. 62/312,260, filed Mar. 23, 2016, which is incorporated fully herein by reference in its entirety for all purposes. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to modular platform systems, such as are used to provide crossover platforms and access platforms in workplace environments. 
       BACKGROUND OF THE INVENTION 
       [0003]    Platform systems are utilized in a variety of industrial and commercial facilities to provide access from one location to another. Generally speaking, such platform systems often fall into one of two categories: crossover platforms and access platforms. Crossover platforms function as bridges, providing a path over some obstruction, such as pipes or industrial machinery. Access platforms, on the other hand, typically allow a user to reach or access a desired area or object from another location. 
         [0004]    Usually, platform systems are specially designed and constructed for a particular location and application. While this provides an acceptable platform for one situation, it is not adaptable or reconfigurable as the situation changes (or for another situation). To address this limitation, the art has provided modular platform systems which are configurable for use in a variety of different situations. These systems utilize a number of components, which can be selected and assembled according to the needs of a particular situation. In this way, a wide variety of different crossover and access platforms can be provided using different combinations and configurations of the available components. One such modular platform system is shown and described in U.S. Pub. No. 2013/0015016 (“the &#39;016 publication”), filed on Jul. 16, 2011 and accorded application Ser. No. 13/184,499, which is incorporated herein by reference in its entirety for all purposes. 
         [0005]    The present invention recognizes the foregoing considerations, and others, of the prior art. 
       SUMMARY OF THE INVENTION 
       [0006]    In accordance with one aspect, the present invention provides a platform system comprising a platform structure having at least one modular platform unit with platform connection features on side surfaces thereof to which other components can be connected, the other components including at least two of another modular platform unit, a stair unit having a plurality of steps, a tower unit, a handrail unit, and a ladder unit. The platform system according to this aspect further includes a stair assembly connected to the platform structure via the platform connection features. The stair assembly comprises a plurality of stair units connected together to provide a number of steps equal to the sum of steps of the stair units. For example, the plurality of stair units may comprises at least three stair units each having no more than six steps. 
         [0007]    According to some exemplary embodiments, the left and right stringers of each stair unit may comprise a side plate having upper and lower stair connection features for attachment of upper termination elements and base elements, respectively. Stair units of the stair assembly may be interconnected together at an interconnection location via web plates that are attached to the side plates using the stair connection features in lieu of upper termination elements and base elements at the interconnection location. 
         [0008]    Handrail sections of the stair units may preferably be connected to form a continuous handrail along a length of the stair assembly. For example, the handrail sections of the stair units may be connected together using transition handrail sections. Internal couplings (e.g., comprising expansible coupling elements engaging an inner surface of the handrails for connecting the handrails together) may be used to connect the handrail sections together. 
         [0009]    The platform system may further comprise a tower assembly connected to the platform structure via the platform connection features, the tower assembly comprising a plurality of tower units connected together to provide a total height corresponding to a height of the stair assembly. In such embodiments, the tower units may be interconnected using generally U-shaped brackets. Moreover, the handrail units and handrails of the stair assembly may be attached using a nut plate locking assembly inserted into a tubular end of a handrail support post. 
         [0010]    According to another aspect, the present invention provides a stair assembly for use in a platform system. The stair assembly comprises a plurality of stair units each comprising a pair of left and right side plates having a plurality of spaced apart steps extending therebetween. The plurality of stair units are arranged such that left side plates of the plurality of stair units are axially aligned with each other and right side plates of the plurality of stair units are axially aligned with each other. Left side plates of adjacent stair units of the plurality of stair units are rigidly interconnected and right side plates of adjacent stair units of the plurality of stair units are rigidly interconnected. 
         [0011]    For example, the adjacent left side plates and the adjacent right side plates may be in end-to-end abutment and interconnected via web plates. According to some preferred embodiments, the left side plates, the right side plates, and the web plates may be L-shaped. Furthermore, the left and right side plates of a lowermost stair unit may have respective removable base elements attached thereto. Likewise, the left and right side plates of an uppermost stair unit may have respective removable upper termination elements attached thereto. 
         [0012]    A further aspect of the present invention provides a tower assembly for use in a platform system. The tower assembly comprises a plurality of tower units (e.g., two, three, or more) stacked one on top of the other, each of the tower units having an inverted generally U-shaped configuration with a top portion and pair of depending leg portions. Adjacent tower units are interconnected via a pair of removable side brackets. 
         [0013]    According to some embodiments, each of the tower units comprises at least one cross-support extending between the depending leg portions. First and second removable base fittings may be attached to respective of the depending leg portions of a lowermost tower unit of the plurality of tower units. A separate top piece may be located above the top portion of an uppermost tower unit of the plurality of tower units. The top portion and the depending leg portions of the tower units may be formed of rectangular tubing. 
         [0014]    A still further aspect of the present invention provides a method of assembling a stair assembly for use in a platform system. One step of the method involves obtaining a plurality of stair units each comprising a pair of left and right side plates having a plurality of spaced apart steps extending therebetween. The plurality of stair units are arranged such that left side plates of the plurality of stair units are axially aligned with each other and right side plates of the plurality of stair units are axially aligned with each other. Another step involves rigidly interconnecting the left side plates of adjacent stair units of the plurality of stair units and right side plates of adjacent stair units of the plurality of stair units using a plurality of web plates. 
         [0015]    According to another aspect, the present invention provides a combination comprising a plate structure defining at least one fastener through-hole. A tubular member defining a fastener receipt hole is also included. A nut plate locking device having a nut plate and an associated spring plate is received in the tubular member, the spring plate carrying at least one spring urging it away from the nut plate. The nut plate has a threaded hole aligned with the fastener through-hole of the plate structure and the fastener receipt hole of the tubular member. A threaded fastener extends through the fastener through-hole of the plate structure and the fastener receipt hole of the tubular member into threaded engagement with the threaded hole of the nut plate such that the tubular member is connected to the plate structure. 
         [0016]    A washer piece having a flat side and a curved side may be situated between the plate structure and the tubular member. Moreover, the nut plate locking device may have a flange located at one end thereof to limit insertion into the tubular member. According to some embodiments, the threaded hole may be defined by a nut attached to the nut plate. 
         [0017]    Other objects, features and aspects of the present invention are provided by various combinations and subcombinations of the disclosed elements, as well as methods of practicing same, which are discussed in greater detail below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    A full and enabling disclosure of the present invention, including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying drawings, in which: 
           [0019]      FIG. 1  shows types of components that may be used in a platform system in accordance with an embodiment of the present invention. 
           [0020]      FIGS. 2 and 3  are perspective views illustrating an access platform ( FIG. 2 ) and a modification thereof ( FIG. 3 ) that can be configured using different combinations of the components. 
           [0021]      FIG. 4  is a perspective view of a stair unit (in this case a 6-step unit) in accordance with an embodiment of the present invention. 
           [0022]      FIG. 5  illustrates a stair unit (in this case a 5-step unit), along with certain ancillary components. 
           [0023]      FIG. 6  is an exploded perspective view of multiple stair units that can be combined to create a longer stair assembly. 
           [0024]      FIG. 7  is an assembled perspective view of the multiple stair units of  FIG. 5 . 
           [0025]      FIGS. 8A and 8B  illustrate the manner in which handrail sections can be connected to each other in the stair assembly of  FIG. 6 . 
           [0026]      FIG. 9  is an enlarged perspective view of the manner in which handrails may be attached to a stair stringer (or “stair support”) in accordance with an embodiment of the present invention. 
           [0027]      FIG. 10  is an enlarged view of the handrail attachment illustrated in  FIG. 9 , partially cut away to reveal a nut plate locking device in accordance with an embodiment of the present invention. 
           [0028]      FIG. 11  is a perspective view of the nut plate locking device of  FIG. 10 . 
           [0029]      FIG. 12  illustrates an exemplary tower (or “stand”) unit in accordance with an embodiment of the present invention. 
           [0030]      FIG. 13  is an exploded view of the tower unit of  FIG. 12  to better show certain components thereof. 
           [0031]      FIG. 14  is an exploded perspective view of multiple tower units that can be combined to create a taller tower assembly. 
           [0032]      FIG. 15  is an assembled perspective view of multiple tower units. 
           [0033]      FIGS. 16A and 16B  illustrate an exemplary ladder unit in assembled and disassembled states, respectively. 
           [0034]      FIGS. 17A through 17C  show different stages of connection between ladder assembly components in accordance with an aspect of the present invention. 
           [0035]      FIG. 18  shows the manner in which a longer ladder may be created according to an aspect of the present invention. 
       
    
    
       [0036]    Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0037]    It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary constructions. 
         [0038]      FIG. 1  illustrates primary components of an exemplary platform system in accordance with an embodiment of the present invention. Such components include modular platform units  100 , stair units  200 , tower (or “stand”) units  300 , platform handrails  400 , and ladders  500 . The components may be assembled in different ways, and in different combinations, to provide a platform needed in a particular situation. In this case, the illustrated components are compatible and interchangeable with components of the platform system described in the &#39;016 publication. 
         [0039]      FIG. 2  illustrates a platform assembly  10  formed of combinations of components as shown in  FIG. 1 . In this regard, assembly  10  includes a stairway  12  leading to one end of an elevated platform  14 . The other end of platform  14  is supported at the desired elevation by a support tower  16 . Handrails are located along the sides of stairway  10 , as well as sides of platform  14 . If necessary for access purposes, or unnecessary for safety (such as if the platform assembly is beside a wall), some of these handrails might not need to be provided. 
         [0040]    In this example, each of stairway  12 , platform  14 , and tower  16  is formed of a plurality of smaller units that are preferably assembled on-site. For example, stairway  12  has fifteen stairs (“steps”) along its rise, and may be formed of three 5-step units (or a 6-step unit, a 5-step unit, and a 4-step unit, etc.). Similarly, the tower  16  may be formed of multiple tower units assembled on site to achieve the desired height. Different heights of tower units may be sold so that the tower heights correspond to the different heights of stairways (or ladders) that can be formed using components of the system. As can be seen, platform  14  is formed in this example of three platform units  100  connected to form a longer rectangular platform. One skilled in the art will recognize, however, that more or fewer platform units  100  can be provided, and they can be connected to form various shapes of platforms, such as rectangular platforms, L-shaped platforms, T-shaped platforms, etc. In a preferred embodiment, platform units  100  may be identical to the modular platforms in the &#39;016 publication, having standard connection features (e.g., a pattern of holes for fasteners) on all four sides thereof. 
         [0041]      FIG. 3  shows a modification of platform assembly  10 , denoted  10 ′, in which a second stairway  18  has been attached to the distal end of platform  14 . In this case, stairway  18  is a single 5-step unit, although different size units or combinations thereof may be used at this location also. In this variation, platform  14  serves as a landing en route to a higher platform  20 , which is supported on its other end by a tower  22 .  FIG. 3  thus illustrates one manner in which the overall platform system allows modifications and adaptations depending on the particular situation in which it will be used. 
         [0042]    Because the stairs, towers, and ladders can be made to a desired height by combining a relatively small number of different units, a wide variety of platform heights can be achieved. In fact, a greater number of different platform heights can be achieved using a smaller number of dedicated component sizes than would otherwise be the case. For example, assuming the largest stair size available in a prior art platform system is an 11-step unit, it would not be possible to have a rise higher than eleven steps without an intervening landing. According to the present invention, a stair assembly of, for example, fifteen steps may be made by combining multiple units, such as three 5-step units. Thus, for example, the longest individual stair unit may be a 6-step unit, but longer stairways (i.e., greater heights) can still be achieved in comparison with the prior art by various combinations of the stair units (the same is true for the tower units and ladder units.) As a result, the manufacturer (or distributor) may stock a fewer number of different unit sizes while offering more platform system heights and combinations. This allows a reduction in inventory for the manufacturer and greater flexibility to the user. In addition, it is often cheaper to ship several smaller units than one larger unit, resulting in a reduction in shipping costs as well. 
         [0043]    Referring now to  FIG. 4 , stair unit  200  can be described in more detail. As can be seen, stair unit  200  in this example has a total of six steps (such as step  202 ) that are attached to and extend between a pair of lateral stringers  204   a  and  204   b . Each of the stringers comprises an L-shaped side plate, such as plate  206 . For example, plate  206  may be a cut section of extruded steel stock. Various holes may be defined in plate  206  at regular and/or predetermined locations for attachment of the steps, handrails, and other features. In this regard, terminal elements may be removably attached to the ends of plate  206  for situations where stair unit  200  is at the bottom or top of the stairs (or is itself used as an entire set of stairs). 
         [0044]    For example, the bottom of stair unit  200  in this case has a pair of base elements  208   a  and  208   b  attached to the respective side plate. As shown, base elements  208   a  and  208   b  define a flat bottom for resting against a support surface such as a floor. The top of stair unit  200  in this case has a pair of upper termination elements  210   a  and  210   b  also attached to the respective side plates. Base elements  208   a - b  and upper termination elements  210   a - b  may be formed in any suitable manner, such as casting. A pair of gussets  212   a  and  212   b  may be attached to the respective upper termination elements  210   a - b  for connection to the underside of a modular platform  100  (similar to the gussets described in the &#39;016 publication). Cross supports (such as cross support  213 ) may extend between the stringers under the steps to provide additional structural integrity. 
         [0045]    Stair unit  200  further has a pair of sloped handrails  214   a  and  214   b  also attached to the stringers  204   a  and  204   b . Each of the handrails is preferably formed of three separable sections, namely lower section  216 , middle section  218 , and upper section  220 . For example, lower section  216  can be removed (along with base elements  208   a  and  208   b ), leaving at least middle section  218  in place, if another stair unit is to be located below this stair unit. Similarly, upper section  220  can be removed (along with upper termination elements  210   a  and  210   b ), leaving at least middle section  218  in place, if another stair unit is to be located above this stair unit. The sections of handrails  214   a  and  214   b  may be connected together using suitable internal couplings, such as those described below. Otherwise, the overall configuration and external appearance of handrails  214   a  and  214   b  is intended to resemble the handrails shown in the &#39;016 publication. 
         [0046]      FIG. 5  illustrates an exemplary stair unit  200 ′ similar to the one shown in  FIG. 5 , but in this case having five steps. Ancillary components that may be provided with stair unit  200 ′ to facilitate its use in a longer stair assembly are also shown. In this regard, a pair of transition handrail sections  222   a  and  222   b  can be used to span the gap otherwise left by the removal of handrail sections  216  or  220 . A plurality of internal couplings, collectively indicated at  223 , may also be provided to facilitate the interconnection of handrail sections. Web plates  224   a  and  224   b  are used instead of base elements  208   a - b  or upper termination elements  210   a - b  depending on whether the other stair unit is above or below this stair unit. In this regard, web plates  224   a  and  224   b  attach to a respective side plate  206  using the same holes as had been used for base elements  208   a - b  or upper termination elements  210   a - b , but extend farther so as to also attach to the side plate of the next stair unit. 
         [0047]    In this regard,  FIG. 6  shows three stair units  200   a ,  200   b , and  200   c , each a 5-step stair unit in this example, about to be combined in order to produce a 15-step stair assembly. In stair unit  200   a , upper termination elements  210   a - b  have been removed. Additionally, transition handrail sections  222   a - b  have replaced handrail upper sections  220 . In stair unit  200   b , base elements  208   a - b  and upper termination elements  210   a - b  have been removed. In addition, handrail lower sections  216  and handrail upper sections  220  have been removed. Transition handrail sections  222   a - b  are used in place of handrail upper sections  220 . In stair unit  200   c , base elements  208   a - b  and handrail lower sections  216  have been removed. 
         [0048]    Turning now to  FIG. 7 , the combined 15-step stair assembly  226  is illustrated. As can be seen, the stair units&#39; side plates  206  have been interconnected on both sides using web plates  224 . In other words, a pair of web plates  224  interconnects stair units  200   a  and  200   b  using the same attachment holes in side plates  206  that had been used by the upper termination elements  210  of stair unit  200   a  and the base elements  208  of stair unit  200   b . Similarly, a pair of web plates  224  interconnects stair units  200   b  and  200   c  using the same attachment holes in side plates  206  that had been used by the upper termination elements  210  of stair unit  200   b  and the base elements  208  of stair unit  200   c . The handrails of stair units  200   a ,  200   b , and  200   c  are secured together using internal couplings to produce a continuous handrail structure on both sides of the steps. While three stair units of 5-steps each have been used in this example, one skilled in the art will appreciate the wide variety of different combinations that are allowed using embodiments of the present invention. 
         [0049]    Referring now to  FIGS. 8A and 8B , an exemplary internal coupling  228  is illustrated. Coupling  228  in this example has a pair of coupling elements  230   a - b  that are loosely connected together. At least one threaded hole  232  is defined in coupling  228 . Hole  232  aligns with a set-screw hole  234  defined in an end of a tubular handrail  236 . The set screw itself is indicated at  238 , along with a typical hex wrench  240  used to tighten it in this case. 
         [0050]    In order to connect handrails of different stair units together, internal coupling  228  is first positioned in the open end of one tubular handrail with set screw  238  in place. The tubular handrail of the other stair unit is abutted with the tubular handrail having coupling  228 . Set screw  238  is then tightened so that coupling elements  230   a - b  expand into firm engagement with the inner surfaces of both tubular handrails. Preferably, hole  234  will be located on the underside of the tubular handrail so it will not normally be seen. One skilled in the will appreciate that a similar coupling arrangement will preferably be used in every set of tubular handrails that will abut in the overall stair assembly. 
         [0051]    Referring now to  FIGS. 9-11 , a preferred manner for attaching the handrails to the stair stringers  204  will be described. In this regard, a pair of vertically-aligned holes are defined in stair stringer  204  at each location where a vertical support post for the handrail is to be attached. In  FIG. 9 , for example, support posts  242   a  and  242   b  are shown attached to stringer  204 . 
         [0052]    As can be seen most clearly in  FIGS. 10 and 11 , support post  242  is attached in this embodiment using a nut plate locking device (or “assembly”)  244 . Nut plate assembly  244  includes a rigid nut plate  246 , typically formed of steel or other suitable metal, carrying a pair of threaded nuts  248   a - b . Nuts  248   a - b  are in this case separate pieces that have been permanently and securely attached to nut plate  246 , such as by welding. One skilled in art will appreciate that nut plate  246  and nuts  248   a - b  could alternatively be formed as a unitary part. 
         [0053]    As shown, a spring plate  250 , typically formed of a suitable polymeric material such as nylon, is loosely coupled to the back of nut plate  246 . In this regard, spring plate  250  includes a pair of projections removably received in corresponding upper and lower slots defined in nut plate  246 . Flexible arms  254   a - b  slightly urge nut plate  246  away from spring plate  250 . This allows relative movement between nut plate  246  and spring plate  250  to facilitate insertion of nut plate assembly  244 . Also, the spring action of arms  254   a - b  tends to hold nut plate assembly  244  in the correct position until tightening can occur. Spring plate  250  preferably includes a flange  256  at its bottom end that limits insertion of nut plate assembly  244  such that the nuts  248   a - b  will aligned with corresponding holes in the support post. 
         [0054]    Support post  242  is then attached to stringer  204  using bolts  258   a - b  that extend through holes in the stringer into threaded engagement with respective nuts  248   a - b . As bolts  258   a - b  are tightened, nut plate  246  is drawn into secure engagement with the inner surface of post  242  to eliminate looseness in the handrail. In this embodiment, post  242  has a circular cross-section all the way to its bottom end. Thus, a handrail washer  260  with a flat side and a curved side is preferably positioned between stringer  204  and post  242 , as shown. In other embodiments, the bottom of support post  242  can be formed into a “D” shape, thus eliminating the need for handrail washer  260 . One skilled in the art will appreciate, however, that forming such a D-shaped portion of support post  242  involves additional processing steps that are avoided if washer  260  is used. Also, while nut plate assembly  244  has been described in relation to attachment of a support post of a stair handrail, one skilled in the art will appreciate that it can also be used to attach handrail units  400  to modular platform units  100 . 
         [0055]    Referring now to  FIGS. 12 and 13 , an exemplary tower unit  300  is illustrated. In this exemplary case, tower unit  300  corresponds to the rise of a 5-step stair unit, although a variety of tower units of different heights will typically be available corresponding to the different heights of available stair units. As shown, tower unit  300  has a main tower component  302  generally having the shape of an inverted “U” and formed of rectangular tubing in this example. Cross-supports  304  may extend between the legs of tower component  302  to facilitate structural integrity. 
         [0056]    Similar to stair unit  200 , tower unit  300  includes removable components that may or may not be used, or are used differently, depending on the situation. In this regard, a pair of removable base fittings  306   a - b  are provided at the bottom of the respective legs of tower component  302 . For example, base fittings  306   a - b  may be attached using a pair of vertically aligned attachment holes extending through the legs of tower component  302 . If this particular tower unit  300  is not the lowest tower unit in a taller tower assembly, these same holes are used to connect it to another tower unit below. 
         [0057]    Removable side brackets  308   a - b  and top piece  310  are also provided. Side brackets  308   a - b , which are generally U-shaped as shown, are shiftable to allow interconnection to another tower unit above. Top piece  310  is used at the top tower unit of the overall tower assembly (or at the top of tower unit  300  if it is used alone). In this example, top piece  310  is formed of rectangular tubing, and sits atop the upper beam of main tower component  302 . Top piece  310  is held in place by fasteners (e.g., bolts) extending through aligned holes defined in it and brackets  308 . 
         [0058]    Referring now to  FIG. 14 , a pair of tower units  300   a  and  300   b  are being combined to form a taller tower assembly. As tower unit  300   b  will be the top unit in the tower assembly, it carries top piece  310 . On tower unit  300   a , brackets  308   a  and  308   b  are shifted up to accommodate the legs of tower unit  300   b . After the legs of tower unit  300   b  are correctly positioned, fasteners (e.g., bolts) are inserted through aligned holes in the brackets and legs to secure everything together. (While a shiftable bracket is shown in this example, one skilled in the art will appreciate that different brackets could be used for attachment of top piece  310  and interconnection of tower units, if desired.) Advantageously, the tower assembly can be easily produced lying flat on the ground and then moved into an upright position. 
         [0059]    In  FIG. 15 , a tower assembly is shown that was formed of three tower units  300   a ,  300   b , and  300   c . While the three tower units each have the same height in this example, tower units of different heights may be mixed and matched to achieve a desired overall height, as noted above. 
         [0060]    A ladder unit  500  is illustrated in  FIGS. 16A and 16B . As shown, ladder unit  500  includes side rails  502   a - b  having rungs (such as rung  504 ) extending therebetween. In this embodiment, rails  502   a - b  extend into handrails  506   a - b  at their upper ends. Referring particularly to  FIG. 16B , ladder unit  500  has a lower section  508  and an upper section  510  that can be separated to allow expansion of ladder height. As shown in  FIGS. 17A through 17C , the interface between lower section  508  and upper section  510  may be connected together using coupling elements  230  as described above. 
         [0061]      FIG. 18  illustrates how a taller ladder assembly may be formed from a plurality of ladder sections. In this case, an additional lower section  508 ′ is added below an existing ladder unit  500 . A spacer section  512  may be provided to ensure that attachment features for the overall ladder assembly are in the correct locations. In the illustrated embodiment, the ladder assembly is used in conjunction with a tower assembly of the type described above. Upper mounting brackets  514  attach to the side of a platform, whereas offset brackets  516  attach along legs of the tower assembly. 
         [0062]    It can thus be seen that the present invention provides a modular platform system that is easily adaptable into a wide variety of configurations. One skilled in the art would be able to determine the most appropriate materials from which to form the various components described above in order to meet anticipated strength and rigidity requirements. In many cases, however, the various structural components may be formed of steel or another suitable metal. 
         [0063]    While preferred embodiments of the invention have been shown and described, modifications and variations may be made thereto by those of ordinary skill in the art without departing from the spirit and scope of the present invention. In addition, it should be understood that aspects of the various embodiments may be interchanged both in whole or in part to yield still further embodiments. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to be limitative of the invention as further described in the appended claims.

Summary:
A platform system comprising a platform structure having at least one modular platform unit with platform connection features on a side surface thereof to which other components can be connected, the other components including at least two of the following: another modular platform unit, a stair unit, a tower unit, a handrail unit, and a ladder unit. A stair assembly is connected to the platform structure via said connection features, the stair assembly comprising a plurality of stair units connected together to provide a number of steps equal to the sum of steps of the stair units.