Abstract:
An apparatus for opening a mechanical bridge. The apparatus includes at least two adjacent bridge spans where the first of the bridge spans is removable from its initial position and the second of the bridge spans is at least partially movable into the space originally occupied by the first span so that at least a portion of the second bridge span can be separated from yet a third adjacent span. This forms an opening between the second and third spans. Alternatively, a span can be moved laterally and then longitudinally to open a section of the bridge.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The field of the invention is mechanical bridges and more specifically bridges including spans that are openable. 
     Bridges are required to facilitate convenient rail and vehicular traffic over rivers, streams, dams and the like (hereinafter collectively referred to as rivers). While bridges are necessary, unfortunately bridges can impede passage of vessels along rivers there below. In order to accommodate both rail and vehicular traffic over rivers and travel along the rivers by ships, barges, etc, bridge designers have developed several different mechanical type bridges including one or more bridge spans that can open and close. 
     One mechanical bridge type is generally referred to as a vertical-lift bridge. A vertical-lift bridge typically includes vertical towers at either end of a bridge span. When positioned for vehicular traffic, the span is in a low position where a top surface is aligned with top surfaces of adjacent bridge spans. To accommodate travel below the bridge span the span can be raised between the towers. 
     While vertical-lift bridges can accommodate both river and vehicular travel, these bridges have several shortcomings. First, vertical-lift bridges, while accommodating some river travel, still restrict travel as the lifted span remains above the area through which travel occurs. Second, the motors and other mechanical equipment required to lift the bridge span are relatively large and therefore expensive. In addition, because of the mechanics involved with vertical-lift bridges, maintenance costs for vertical lift bridges are relatively high. 
     Another mechanical bridge type is a swing span bridge. A typical swing span bridge includes a moveable span that pivots about a vertical axis to provide required opening clearance for navigation traffic. Swing spans are typically symmetrical with equal length cantilevers to each side of the vertical axis. Some swing span bridges, however, are configured with unsymmetrical cantilevers that are counterweighted to balance the bridge. Swing span bridges are advantageous as they provide unlimited vertical clearance for river bound traffic when the span is open. 
     Unfortunately swing spans also have several shortcomings. First, when a swing span is horizontally pivoted into the open position the span ends are generally considered to be navigational hazards. The span ends are directed against movement of water bound traffic and therefore are prone to vessel collision. Thus, often substantial fender systems are required to protect the span and vessels in the area. Second, swing spans typically require twice as much moveable length span as other mechanical span designs to provide the same opening width. This is because, as indicated above, most swing span bridges require equal length span segments cantilevered about the vertical pivot point. Third, the mechanical components required to manipulate the large span sections are generally relatively large and therefore relatively expensive. 
     Yet one other mechanical bridge type is referred to generally as a bascule type bridge. A typical bascule bridge includes a leaf that pivots about a horizontal axis to provide a required opening and clearance for river bound traffic. Counterweights are usually provided to balance the weight of the span and minimize the operating requirements on the drive machinery. The bascule span bridges provide unlimited vertical clearance when open. 
     Bascule bridges, like the other bridge types described above, have several shortcomings. First, the counterweight required to balance the bascule span is typically rather large. As most mechanical bridges are relatively low to the water, the counterweights are typically positioned above a span adjacent the moveable span. To support the counterweight these bridge types often require large and expensive overhead framing systems and massive foundations below the spans to handle the overturning moments that occur. Second, the mechanics required to control a bascule bridge are extremely complex and therefore expensive. Third, bascule bridges requiring massive counterweights are relatively unsafe in certain geographic areas that are subject to seismic tremors. 
     Thus, there is a need for a mechanical bridge that is simple, relatively inexpensive, provides unlimited vertical clearance and that does not require massive overhead or counterweight components. 
     BRIEF SUMMARY OF THE INVENTION 
     It has been recognized that a relatively simple bridge design can overcome many of the shortcomings of the prior art bridges described above. To this end, by moving a bridge span essentially within a single vertical plane from a supporting position into a storage position, system mechanics can be greatly simplified without sacrificing safety. To this end, in one embodiment, an openable span is moved laterally from a supporting position and then longitudinally along the side of an adjacent span to open a space for water bound traffic. In another embodiment an adjacent span is removed from its position adjacent an openable span and then the openable span is moved at least in part into the adjacent position to open a space for water bound traffic. 
     Thus, one object of the invention is to provide a simplified openable bridge design. This object is accomplished by minimizing required vertical span movement. In some embodiments there is no vertical span movement while in other embodiments vertical movement is limited in several ways. First, the vertical distance of movement is minimized. Second the size of the span that has to be moved is limited. To this end, when a first span is vertically moved and then a second span is horizontally moved into the space originally occupied by the first span, the first span is only half as large as the second span and hence a minimally sized span is vertically moved. 
     Another object is to provide a relatively safe mechanical bridge. To this end, because vertical span movement is limited, above deck structure is minimized. Because above deck structure is minimized bridges constructed according to the present teachings are relatively safe in various environments including those that may be subject to periodic earth quakes and other disruptive natural phenomenon. 
     Yet one other object is to provide a relatively inexpensive bridge system. Because most span movement is horizontal relatively small motors can be used to move spans on rollers as opposed to lifting the spans. 
     One other object is to provide a bridge where, when a span is open, the open space can accommodate passage of any vessel there below. To this end the present design has no components that remain above the open space after a span is moved. 
     Consistent with the above objects and advantages, the present invention includes a bridge assembly comprising first, second and third adjacent piers, the first and second piers defining a first space there between and the second and third piers defining a second space there between, third and fourth spaces above the first and second spaces, respectively, a first bridge span positionable so as to traverse the distance between the first and second piers within the third space, a second bridge span positionable so as to traverse the distance between the second and third piers within the fourth space, a first motivator linked to the first bridge span for moving the first span into and out of the third space and a second motivator linked to the second bridge span for moving at least a portion of the second bridge span from the fourth space to the third space so that at least a portion of the fourth space is unobstructed. 
     In some embodiments the invention further includes at least one intermediate pier between the second and third piers, the space between the intermediate and third piers being a fifth space, the space above the fifth space being an openable space, the openable space being the portion of the fourth space that is unobstructed when the portion of the second bridge span is moved to the third space. In some cases the first, second, third and intermediate piers are essentially equi-spaced. 
     In some embodiments roller members are provided between the tops of the piers and the spans thereabove. The rollers may be mounted to the tops of the piers. 
     In several embodiments the first span has a span width, each of the first and second piers has a pier width that is substantially twice as wide as the span width, first and second in-line sections of the first and second piers, respectively, aligned with the third pier and defining an in-line space, a supporting space above the in-line space, first and second lateral sections of the first and second piers laterally adjacent the first and second in-line sections, respectively, the lateral sections defining a lateral space there between, a receiving space above the lateral space, the first motivator for moving the first span between the supporting space and the receiving space. 
     In some embodiments the first and second in-line sections and first and second lateral sections comprise one lateral load bearing element and the first span comprises another lateral load bearing element and the assembly further includes a first track mounted to a first one of the lateral load bearing elements and rollers mounted to the second of the lateral load bearing elements, the rollers supportable on the track to facilitate rolling of the first span between the supporting and receiving spaces. Here, the track may be secured to the first span and the rollers may be secured to the tops of the piers. 
     The assembly may further include an intermediate pier between the second and third piers wherein the second and intermediate piers comprise a longitudinal load bearing element and the second span comprises another longitudinal load bearing element and, wherein, the assembly further includes a second track mounted to a first one of the longitudinal load bearing elements and rollers mounted to the second of the longitudinal load bearing elements, the rollers supportable on the track to facilitate rolling of the second span such that the at least one section moves between the fourth space and the supporting space. The longitudinal load bearing element that includes the second and fourth piers may also includes the first pier. 
     In some embodiments the first motivator moves the first span between the third space and a space above the third space. In other embodiments the first motivator moves the first span between the third space and the first space. 
     The second span may include first and second ends, a top and a bottom, the second end adjacent the third pier when the second span is in the fourth space, the assembly further including first and second aligning apparatus at the second end and the top of the third pier, respectively, the second aligning apparatus receiving the first aligning apparatus when the second span is moved into the fourth space so as to align the second span with the third pier. The first aligning apparatus may include a first inclined surface. Similarly, the second aligning apparatus may include a second inclined surface. In addition, the second aligning apparatus may include a guiding roller. 
     Another embodiment of the invention includes a method for opening a section of a bridge where the bridge includes several spans that are longitudinally arranged along the length of the bridge including at least first and second adjacent spans that, when the bridge is closed, occupy first and second spaces, respectively, the method comprising the steps of moving the first bridge span from the first space, moving at least a segment of the second bridge span from the second space into the first space so that at least a portion of the second space is unobstructed. 
     According to one embodiment, when the first span is in the first space and the second span is in the second space the first and second spans are aligned longitudinally and, the step of moving the first bridge span includes moving the first span from the first space laterally and wherein the step of moving the second span includes moving the second span longitudinally. In another embodiment, the step of moving the first bridge span includes moving the first span upward and out of the first space. 
     In yet another embodiment the invention includes a bridge assembly comprising first, second and third adjacent piers, each of the second and third piers including an in-line section and an adjacent lateral section, the in-line sections aligned along a longitudinal axis and the lateral sections aligned along a lateral axis that is essentially parallel to the longitudinal axis, the first pier and second pier in-line section defining a first in-line space there between, the second and third pier in-line sections defining a second in-line space there between, a space adjacent the first in-line space and the second lateral section defining a first lateral space, the second and third lateral sections defining a second lateral space there between, third and fourth in-line spaces above the first and second in-line spaces, respectively, and third and fourth lateral spaces above the first and second lateral spaces, respectively, a first bridge span positioned so as to traverse the distance between the first and second piers within the third in-line space, a second bridge span positionable so as to traverse the distance between the second and third piers within the fourth in-line space, a first motivator linkable to the second bridge span for moving the second span between the fourth in-line space and the fourth lateral space and a second motivator linkable to the second bridge span for moving at least a portion of the second bridge span from the fourth lateral space to the third lateral space so that at least a portion of the fourth lateral space is unobstructed. 
     Here the assembly may further include at least one intermediate pier between the second and third lateral pier sections, the space between the intermediate and third lateral section being a fifth space, the space above the fifth space being an openable space, the openable space being the portion of the fourth lateral space that is unobstructed when the portion of the second bridge span is moved to the third lateral space. The first lateral section, second lateral section, third lateral section and intermediate pier may be essentially equi-spaced. 
     The invention further includes a method for opening a section of a bridge where the bridge includes at least first and second adjacent spans that are longitudinally alignable along the length of the bridge and are supported by at least first, second and third piers, each pier including an in-line section and a lateral section laterally positioned with respect to the in-line section, the space between the first and second in-line pier sections being a first in-line space, the space between the second and third in-line pier sections being a second in-line space, the space above the first and second in-line spaces being a third in-line space and the space above the second in-line space being a fourth in-line space, the space between the first and second lateral pier sections being a first lateral space, the space between the second and third lateral pier sections being a second lateral space, the space above the first lateral space being a third lateral space and the space above the second lateral space being a fourth lateral space, when the bridge is closed, the first and second spans occupying the third and fourth in-line spaces, respectively, the method comprising the steps of: moving the second bridge span laterally from the fourth in-line space to the fourth lateral space and moving at least a segment of the second bridge span from the fourth lateral space into the third lateral space so that at least a portion of the fourth space and a portion of the fourth lateral space are unobstructed. 
     These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefor, to the claims herein for interpreting the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIGS. 1 a  through  1   d  are schematic diagrams illustrating a first embodiment to the present invention; 
     FIG. 2 is a cross-sectional view taken along the line  2 — 2  of FIG. 1 c;    
     FIG. 3 is a cross-sectional view taken along the line  3 — 3  of FIG. 1 b;    
     FIG. 4 is a schematic view taken along the line  4 — 4  of FIG. 3; 
     FIGS. 5 a  through  5   c  are schematic views of a second embodiment of the present invention; 
     FIG. 6 is a schematic cross-sectional view taken along the line  6 — 6  of FIG. 5 a;    
     FIG. 7 is a schematic cross-sectional view taken along the line  7 — 7  of FIG. 5 a;    
     FIGS. 8 a  through  8   c  are schematic diagrams illustrating a third embodiment of the present invention; 
     FIG. 9 is a cross-sectional view taken along the line  9 — 9  of FIG. 8 a;    
     FIG. 10 is similar to FIG. 9, albeit illustrating an extended shaft and a raised span; 
     FIG. 11 is a plan view of the assembly of FIG. 5 b  taken along the line  11 — 11 ; 
     FIG. 12 is a plan view of the assembly of FIG. 5 b  taken along the line  12 — 12 ; 
     FIG. 13 is a view similar to FIG. 11, albeit with a bridge span in a different position and retracted lifts; and 
     FIG. 14 is a view similar to FIG. 12, albeit with a bridge span in a different position and retracted lifts. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings wherein like reference numerals represent similar elements throughout the several views and, more specifically, referring to FIGS. 1 a  through  1   d,  a first embodiment of the present invention will be described in the context of a mechanical bridge  10  including a plurality of piers  9 ,  12 ,  14 ,  16  and  18  (only five illustrated) that begins at a first shore  20  and traverses over a river  22  to a shore (not illustrated) opposite shore  20 . Piers  9 ,  12 ,  14 ,  16  and  18  are equi-spaced so as to equally accept load from traffic passing across the bridge thereabove. In addition to piers  9 ,  12 ,  14 ,  16  and  18 , bridge  10  also includes a plurality of spans or bridge sections  24 ,  26 ,  28  and  32  that traverse the distance between the tops of piers  9 ,  12 ,  14 ,  16  and  18  and provide a deck  34  above river  22  for vehicular travel. Spans  24 ,  26  and  32  have essentially identical lengths and traverse the distance between the tops of two adjacent piers. For example, span  26  traverses the distance between the tops of piers  12  and  14 . One special and relatively long span  28  traverses the distance between the tops of three adjacent piers including piers  14 ,  16  and  18 . Thus, span  28  is twice as long as any of the other spans in bridge configuration  10 . 
     Referring specifically to FIG. 1 b,  piers  12  and  14  are approximately twice as wide as the other piers that make up bridge  10 . To this end, pier  12  includes an in-line section  38  that is, as the name implies, in-line with other piers (e.g.,  16 ,  18 , etc.) that form the bridge and a lateral section  40  that is laterally positioned with respect to in-line section  38 . Similarly, pier  14  includes an in-line section  42  and a lateral section  44 . Lateral sections  40  and  44  are aligned to the same side of the in-line sections  38  and  42  and are capable of supporting a bridge span thereabove. 
     For the purpose of explaining this first embodiment of the invention it is advantageous to define the illustrated piers in a specific manner and also to define various spaces with respect to those piers. To this end, referring still to FIGS. 1 a  through  1   d  and also to FIG. 1 e,  piers  12 ,  14  and  18  will generally be referred to as first, second and third piers, pier  16  will be referred to as an intermediate pier, the space  48  between piers  12  and  14  will be referred to as a first space, the space  50  between piers  14  and  18  will be referred to as a second space, the space above first space  48  and, in FIG. 1 a  occupied by span  26 , will be referred to as a third space  52 , the space above second space  50  and, in FIG. 1 a,  occupied by span  28 , will be referred to as a fourth space  54 , the space between in-line pier sections  38  and  42  will be referred to as an in-line space  41 , the space above in-line space  41  will be referred to as a supporting space  43 , the space between lateral pier sections  40  and  44  will be referred to as a lateral space  45  and the space above lateral space  45  will be referred to as a storage space  47 . In addition, end  64  of span  28  will be referred to as a leading end  64 . In addition, the space between piers  16  and  18  will be referred to as a fifth space  56  and the space thereabove which in FIG. 1 a  is occupied by a portion of span  28  will be referred to as an opening space  58 . 
     With the spaces and piers as defined above and referring to FIGS. 1 a  through  1   e,  according to this first embodiment of the invention, a section of bridge  10  can be opened to allow river bound traffic to pass through the open section. To this end, according to a first step in the process of clearing a passage through bridge  10  for river bound traffic, first span  26  is removed from supporting space  43 . This is accomplished by moving first span  26  laterally from supporting space  43  to storage space  47  so that lateral sections  40  and  44  of piers  12  and  14 , respectively, support span  26 . This condition is illustrated in FIG. 1 c.    
     Next, second span  28  is moved longitudinally along the tops of the in-line piers and pier sections so that at least a portion of span  28  is positioned within supporting space  43 . When span  28  is moved in this manner, an opening is created between spans  28  and  32 . By moving span  28  as far as possible into supporting space  43  so that approximately half of span  28  is within space  52 , the entire opening space  58  is rendered unobstructed so that river bound traffic can pass therethrough. 
     To close the bridge  10 , the above described process is simply reversed. To this end, a first step in closing the open space  58  is to drive span  28  toward span  32  until leading end  64  of span  28  is received and supported on the top of pier  18 . Next, span  26  can be moved form it&#39;s lateral position illustrated in FIG. 1 d  to its in-line position as illustrated in FIG. 1 b  where span  26  traverses the distance between and is support by adjacent in-line pier sections  38  and  42 . 
     Referring now to FIGS. 1 b  and  2 , the mechanism used to slide or move span  26  from the supporting space  43  to the storage space  47  will be described in more detail. To this end, the tops of piers  12  and  14  comprise a first lateral load bearing element while the underside of span  26  comprises a second lateral load bearing element. Rollers are provided on one of the lateral load bearing elements while one or more tracks are provided on the other of the lateral load bearing elements. The rollers and tracks cooperate to facilitate lateral movement of span  26 . Similarly, the tops of piers  14  and  16  comprise a first longitudinal load bearing element and the underside of span  28  comprises a second longitudinal load bearing element. Rollers are provided on one of the longitudinal load bearing elements while one or more tracks are provided on the other of the longitudinal load bearing elements. The rollers and tracks on the longitudinal elements cooperate to facilitate longitudinal movement of span  28 . Specifically, at the tops of each pier  12  and  14 , rollers are provided that facilitate easy movement of span  26  between the supporting space  43  and the storage space  47 . The rollers in this embodiment are identical at the tops of piers  12  and  14  and therefore, only the rollers corresponding to the top of pier  12  will be described in detail. The configuration at the top of pier  12  includes a timber  70 , a pier cap  72 , a first motivator  74  and the first span  26 . Timber  70  includes a lower end (not illustrated) that extends down through the river (see  14  in FIG. 1 a ) and is embedded in the bottom of the river and a top end  76 . Pier cap  72  is a concrete member and is formed about the top end  76  of timber  70 . Although not illustrated in FIG. 2, a plurality of timbers adjacent timber  70  are provided that support cap  72  and the other bridge components thereabove. 
     Cap  72  forms three roller housings  78 ,  80  and  82  that generally face upward. A central roller housing  80  includes a plurality of rollers  84  that form an upward facing roller surface  86  for supporting span  26  thereabove. Lateral roller housings  78  and  82  each support a plurality of rollers  88 ,  90 , respectively, that form support surfaces  92  and  94  for guiding and supporting span  26  thereabove. 
     Surfaces  92  and  94  are tilted in a direction toward central roller housing  80  and therefore restrict movement of span  26  in other than the direction between supporting space  43  and storage space  47 . 
     Motivator  74  is a motor and is securely mounted to a side  98  of cap  72  (see also FIGS. 1 b  through  1   d  in this regard). Motor  74  includes a shaft  96  that extends up above cap  72 . At the distal end of shaft  96  a large gear having vertically aligned teeth is mounted. 
     Span  26  includes a bottom support  104  and various components that form a top support  106  that will be described in more detail below. Bottom support  104  is preferably formed of concrete and has a top surface  102  and a bottom surface  105 . Top surface  102  is essentially flat and provides a support deck for components  106  thereabove. Bottom surface  106  forms three separate roller recesses  108 ,  110  and  112  that form roller surfaces  114 ,  116  and  118 , respectively. A central roller surface  116  faces downward and is sized so as to receive surface  86  of rollers  84  thereon. Similarly, roller surfaces  114  and  118  are sized and configured so as to receive rollers  88  and  90 , respectively, corresponding to the lateral rollers as illustrated. 
     A lateral edge  120  of span  26  forms a gear receiving surface having teeth sized to receive the teeth of gear  100 . Edge  120  extends so that the teeth of gear  100  are received within the teeth of edge  120 . While structure  104  is illustrated and described above as being formed of concrete, it should be appreciated that certain of the features may be formed of other more suitable materials used for specialized purpose. For instance, a steel member may be mounted to member  104  that forms the teeth  120  that cooperate with motor  74  to move span  26 . Similarly, flat steel plates may be provided on the surfaces of each of roller surfaces  114 ,  116  and  118  that may be greased to facilitate easy movement of rollers there along. 
     While only a single roller system is illustrated in FIG. 2, it should be appreciated that several roller systems like the one illustrated in FIG. 2 may be provided at the top each of the first and second piers  12 ,  14 , respectively. For instance, in one embodiment at least four roller assemblies would be equi-spaced along the top of each of piers  12  and  14 . It should also be appreciated that, because efficient roller systems reduce the amount of power required to move large objects, a relatively small motor  74  should be able to move a span  26  back and forth between the supporting space  43  and storage space  47 . To this end, to move span  26 , motor  74  is driven and applies a force to span  26  that drives the span  26  either into or out of the figure illustrated in FIG.  2  and therefore either toward or away from supporting space  43  (see also FIGS. 1 c  and  1   e ). 
     While first span  26  remains fully supported during movement between supporting space  43  and storage space  47 , as illustrated in the FIG. 1 sequence of drawings, second span  28  is not fully supported during movement between spans  12  and  18 . In other words, span  28 , at certain times during movement, is cantilevered about one or more piers so that at least segments of span  28  are out and over open spaces therebelow. For this reason, a relatively more complex roller system is contemplated to maintain span  28  in a stable configuration during movement. Referring now to FIGS. 1 a  through  1   d  and also to FIG. 3, the components that are used to construct the top of pier  16  are illustrated. The components in FIG. 3 include two timbers  130 ,  132 , a pier cap  134 , a span lower structure  136 , a span upper structure  138  and a second motivator  140 . Timbers  130  and  132  both extend down to the bottom of the river bed to provide support. The top ends  142  and  144  of timbers  130  and  132 , respectively, extend into a lower surface of cap  134 . Cap  134  forms a plurality of roller housings that together cooperate to provide support span  16  and also to provide guidance to span  16  as span  16  is moved. Four roller housings are illustrated including housings  146 ,  148 ,  150  and  152 . A plurality of rollers  154  are mounted in housing  150  and form a support surface  156  that faces upward. Similarly, a plurality of rollers  158  are mounted in housing  152  and form a support surface  160  that faces upward. A plurality of rollers  162  are mounted in housing  148  and form a vertical guiding surface  164 . A plurality of rollers  166  are mounted in housing  146  and provide a downward facing restraining surface  168 . Other roller assemblies may be provided along the length of cap  134  to facilitate easy movement of span  16  thereabove. Motivator  140  is similar to the motivator  74  described with respect to FIG.  2  and therefore will not be described here in detail. Suffice it to say that a gear  170  extends from the motivator  140  and includes vertically aligned teeth that open, at least to one side, facing an edge  172  of lower structure  136 . 
     Lower structure  136  includes a top surface  173  for supporting upper structure  138  and a bottom surface  175 . Bottom surface  175  forms a plurality of recesses (e.g.,  174 ,  176 ) that are sized and positioned so as to receive upward facing rollers that are mounted within cap  134 . Thus, recess  174  forms a load bearing surface  180  that receives support surface  160  while recess  176  forms a load bearing surface  182  that receives support surface  156 . An upper portion of edge  172  contacts guidance surface  164  to restrain lateral movement of span  16 . Upper surface  173  forms an upward facing restraining surface  188  that contacts downward facing restraining surface  168 . 
     Upper structure  138  includes a plurality of I beams  190  that support a concrete road surface  192  thereabove. A guide rail  194  is provided along a lateral edge of member  192 . Referring also to FIG. 2, configuration of upper structures  106  and  138  is relatively unimportant with respect to what is believed to be novel and therefore are not explained here in detail. Suffice it to say structures  106  and  138  must be rigid and must be securely mounted to the top surfaces of lower structures  104  and  136 , respectively. 
     As in the case of the roller system illustrated in FIG. 2, the system illustrated in FIG. 3 is only exemplary and a plurality of roller systems like the one illustrated in FIG. 3 would likely be provided at various locations along the tops of piers  14 ,  16  and  18 . 
     Referring now to FIGS. 1 a  through  1   d  and also to FIG. 4, while span  28  is to be constructed of concrete and steel and other types of rigid materials and therefore should be extremely rigid, where the open space  58  is relatively large (e.g., 60-100 feet), while span  28  is being moved from its open position to the position where space  58  is closed, leading edge  64  may bow downward a small distance when span  28  is at its most extended point and just prior to support by pier  18 . For this reason, in an advantageous embodiment, a guiding mechanism is provided at the receiving edge of pier  18  for “lifting” the leading edge  64 . To this end, the underside  200  of leading edge  64  is sloped so that underside  200  can be used to guide span  28  upward when edge  64  reaches pier  18 . In addition, a guiding component  201  is attached to the bottom of cap  134 . Guiding component  201  extends longitudinally from the under surface of cap  134  and includes a sloped surface  202  that is effectively a mirror image of sloped surface  200 . In addition, a plurality of rollers  204  are provided on sloped surface  202  to reduce friction between surfaces  200  and  202  during reception of span  28 . 
     Referring now to FIGS. 5 a  through  5   c,  a second embodiment of the invention is illustrated. This second embodiment, like the first embodiment, includes a plurality of piers  9 ,  12 ,  14 ,  16  and  18  and a plurality of spans  24 ,  26 ,  28  and  32  that traverse the distance between the piers. Each of piers  9  and  12  have a width that is generally the same width as each of the spans (e.g.,  24 ). Each of piers  14 ,  16  and  18 , however, has a width that is approximately twice as wide as the width of any of the spans (e.g.,  28 ). To this end, pier  14  includes an in-line section  250  that is in-line with piers  9  and  12  and a lateral section  252  that is laterally positioned with respect to in-line section  250 . Similarly, pier  16  includes an in-line section  254  and a lateral section  256  while pier  18  includes an in-line section  258  and a lateral section  260 . 
     Referring now to FIGS. 1 a  and  5   a  through  5   c,  as above, in order to understand the second embodiment, it is advantageous to define specific piers by specific names and specific spaces with respect to those piers by specific names. To this end, piers  12 ,  14  and  18  are referred to generally as first, second and third piers, while pier  16  is referred to as an intermediate pier. In FIG. 1 a,  the space between piers  12  and  14  is referred to as a first in-line space, the space  50  between piers  14  and  18  is referred to as a second in-line space, the space above first in-line space  48  is referred to as a third in-line space  52  and the space  54  above second in-line space  50  is referred to as a fourth in-line space  54 . The in-line spaces are aligned along a longitudinal axis  27 . 
     In addition, referring to FIGS. 1 a,    5   a  and  6 , the space that is adjacent each of first in-line space  48  and lateral pier section  252  is referred to as a first lateral space  270  and the space above first lateral space  270  is referred to as a third lateral space  272 . Referring to FIGS. 1 a,    5   a  and  7 , the space between lateral pier sections  252  and  260  is referred to as second lateral space  274 , the space thereabove is referred to as fourth lateral space  276 , the space between intermediate pier  256  and lateral section  260  is referred to as a fifth space and the space above the fifth space is referred to as an openable space. The lateral spaces are aligned along a lateral axis  290 . 
     With the piers and spaces defined above, operation of the bridge illustrated in FIGS. 5 a  through  5   c  can be easily understood. Referring still to FIGS. 5 a  through  5   c  and also to FIGS. 6 and 7, initially, to facilitate vehicular traffic over the bridge, second span  28  is in the fourth in-line space  54  (see FIG.  7 ). To open the bridge and allow water bound traffic to pass therethrough, first, span  28  is moved from the fourth in-line space  54  laterally to the fourth lateral space  276  so that span  28  is supported on the tops of lateral sections  252 ,  256  and  260  of piers  14 ,  16  and  18  as illustrated in FIG. 5 b.  Next, span  28  is moved longitudinally along the lateral axis  290  to the left as illustrated in FIG. 5 b  until a segment (i.e., the lefthand half of span  28  referred to as the “openable space” above) is positioned within third lateral space  272  as illustrated in FIG. 5 c.  After this second move, an open space  300  is formed between piers  16  and  18  to allow water bound traffic to pass therethrough unobstructed. 
     To reclose the bridge, the process as described above is reversed. To this end, span  28  is first moved to the right as illustrated in FIG. 5 c  until the entire span  28  is within fourth lateral space  276 . Then span  28  is moved laterally back into space  54  above the in-line sections of piers  14 ,  16  and  18 . 
     The movement systems used in the second embodiment would be similar to those used in the first embodiment including motivators, roller assemblies and tracks, and should be configurable by one of ordinary still in the art. Nevertheless, it should be appreciated that while this embodiment is contemplated, in some ways, this embodiment is less preferred than the first embodiment because the movement system mechanics would be more complex. This is because the movement mechanics have to facilitate movement of span  28  in two separate directions (i.e., laterally and then longitudinally). In addition to the motivators for span movement laterally and longitudinally, this design would also likely require some other moveable components. 
     Referring now to FIGS. 5 a,    11  and  12 , exemplary movement assemblies at the tops of piers  14  and  18  are illustrated. Specifically, the assemblies illustrated are located at the tops of lateral pier sections  252  and  260 . Pier  14  includes timbers  450  and  452 , a pier cap  454 , several lateral roller assemblies  456  (only one illustrated), a hydraulic lift  451  for each lateral roller assembly, a span assembly  28  and at least two longitudinal roller assemblies  458  and  460 . From the first embodiment description above the functions, configurations and operation of most of the components of FIGS. 11 and 12 should be understood and therefore will not be explained again here in detail. 
     Hydraulic lift  451  is mounted on a top surface of cap  454  and includes an upwardly extending shaft  462 . Roller assembly  456  is mounted at the top end of shaft  462 . Lift  451  is capable of changing the vertical elevation of roller assembly  456  and other span components (e.g.,  28 ) thereabove. 
     Referring to FIGS. 5 a,    11 ,  12  and  2 , the lateral roller assemblies at the tops of in-line pier sections  250 ,  254  and  258  need not include hydraulic lifts (e.g.,  451 ) and therefore are more akin to the assemblies illustrated in FIG.  2 . In their highest position (i.e., with corresponding hydraulic lifts  451  extended to a maximum point), the rollers of assemblies  451  would be at the same vertical height as, and would be aligned with, the stationary roller assemblies on the in-line pier sections  250 ,  254  and  258 . Thus, at their lowest position (i.e., with lifts  451  retracted), the rollers of assemblies  451  would be below the stationary rollers at a lowest level (see also FIGS.  13  and  14 ). 
     Referring again to FIGS. 11 and 12, longitudinal roller assembly  458  extends up from cap  454  at a lateral end of cap  454  and forms a receiving bay  470  designed to receive a lateral edge  472  of span  28 . To this end upper and lower roller banks  474  and  476 , respectively, are provided in bay  470  for supporting edge  472 . The space D 1  between roller banks  474  and  476  is slightly greater than the width D 2  of end  472 . The second longitudinal roller assembly  460  includes a single upward facing roller bank  461  on a side of hydraulic lift  451  opposite assembly  458 . Importantly, as best seen in FIG. 12, cap  454  extends longitudinally past an adjacent end  449  of span  28  on a side of lift  451  opposite pier  18 . 
     Referring to FIGS. 12 and 14, upward facing roller banks  461  and  476  are at the same vertical height which is slightly higher than the top of roller assemblies  451  when those assemblies  451  are in their lower positions. 
     Referring to FIGS. 5 a  and  12 , the components at the top of lateral pier section  260  are similar to the components at the top of section  252  with a few exceptions. Similarities include a supporting timber  490 , a pier cap  482 , a hydraulic lift  492  and several (only one shown) lateral roller assemblies  494 . A first distinction is that only a single roller assembly  480  is provided at the top of pier cap  482  on the same side of lift  492  as pier  14 . Assembly  480  includes a roller bank  421  that defines an upward facing support surface  423 . The height of surface  423  is identical to the heights of surfaces  460  and  467  and is slightly higher than the tops of assemblies  494  when assemblies  494  are in their lowest positions. On the side of lift  492  opposite roller assembly  480 , span  32  rests on, and is securely mounted to, the top of pier  18 . 
     Referring still to FIGS. 11 and 12, span  26  forms an under surface  498  that defines downwardly extending track members  500  and  502 . Each track member  500 ,  502  is shaped so as to be received and supported by roller assemblies  451  and  494  there below and thus extend laterally across span  28 . Track members  500  and  502  do not extend completely across span  28 , but rather stop short of end  472 . This is so that end  472  can be received within bay  470 . 
     In operation, to open the bridge, referring to FIGS. 11 through 14, with span  28  in the in-line position (see FIG. 5 a ) and lifts  451  and  492  extended, a motivator (not illustrated), drives span  28  laterally in direction  508  on roller assemblies  456  and  494  into a position supported above lateral pier sections  252 ,  256  and  268 . An intermediate span position is illustrated in FIG.  11 . The motivator continues to drive span  28  until end  472  is aligned with but longitudinally displaced from bay  470  as illustrated in FIGS. 5 b  and  12 . Next, lifts  451  and  492  are lowered. When lifts  451  and  492  are lowered, span  28  comes to rest and be supported on the upward facing roller bank surfaces  467  and  423 . Span  28  supported by roller assemblies  461  and  421  is illustrated in FIG.  14 . Note that lifts  451  and  492  need only be lowered a very small amount and therefore span  28  is only lowered very slightly. 
     Continuing, referring to FIG. 14, the second motivator (not illustrated) drives span  28  longitudinally in the direction indicated by arrow  510 . As span  28  is driven longitudinally, end  449  is received within bay  470  and is supported and restrained by roller assemblies  474  and  476 . Referring specifically to FIG. 14, roller assembly  458  should be positioned relative to assembly  480  such that span end  449  is fully received between banks  474  and  476  prior to opposite span end  447  coming off roller bank  421 . This ensures that end  447  will be supported in a cantilevered manner upon becoming unsupported. 
     The second motivator continues to drive span  28  in the direction of arrow  510  until span  28  is in the position illustrated in FIG. 5 c  where space  300  is completely open for river bound travel. 
     To close space  300  and facilitate vehicular travel, span  28  is driven from its location in FIG. 5 c  to the location in FIG. 5 b  while being supported on longitudinal roller assemblies  458 ,  460 ,  480 , etc. Next, lifts  451  and  492  are extended to lift span  28  up and above longitudinal assemblies  460  and  480  and so that span  28  is supported by lateral assemblies  456  and  494 . Continuing span  28  is driven from the position in FIG. 5 b  to the position in FIG. 5 a.    
     Importantly, when span  28  is in the in-line position (see FIG. 5 a ), span  28  is fully supported by rigid mechanical rollers as opposed to hydraulic lifts. This makes for a more resilient bridge system. 
     Referring now to FIGS. 8 a  through  8   c,  a third embodiment of the present invention will be described in the context of a bridge  330  that includes a plurality of piers  9 , 12 ,  14 ,  16  and  18 . In addition, bridge  330  includes spans  24 ,  26 ,  28  and  32  that traversing a distances between the piers, each of spans  24 ,  26  and  32  traversing a distance between adjacent piers and span  28  being approximately twice as long as the other spans, traversing a distance between piers  14 ,  16  and  18 . 
     To understand this third embodiment, as with the embodiments described above, it is helpful to specifically label several of the piers and the spaces relative thereto. To this end, piers  12 ,  14  and  18  are referred to as first, second and third piers, respectively, pier  16  is referred to as an intermediate pier, the space between piers  12  and  14  is referred to as a first space  333 , the space between piers  14  and  18  is referred to as a second space, the space above the first space is referred to as a third space  336 , and the space above second space  334  is referred to as a fourth space  338 . Third space  336  is approximately the same size as span  26  and the space thereabove is referred to as a fifth space  340 . 
     In operation, to open a section of bridge  330 , first, with span  26  supported between piers  12  and  14  and within third space  336 , first span  26  is raised up and into fifth space  340  thereabove. After this move, the bridge is in the configuration illustrated in FIG. 8 b.  Next, span  28  is moved from fourth space  338  toward third space  336  such that a segment [e.g., approximately the left half of span  28  as illustrated] of span  28  moves into third space  336 . After this move, bridge  330  is configured as illustrated in FIG. 8 c  with a leading end  350  of second span  28  supported on the top of pier  16 . In this configuration, the space between and above piers  16  and  18  is completely unobstructed and water bound traffic can pass there through. 
     To reclose bridge  300 , the method described above is simply reversed. To this end, span  28  is moved toward span  32  until leading end  350  contacts and is supported by the top of span  18 . This configuration is illustrated FIG. 8 b.  Next, span  26  is lowered until that span is supported on the tops of piers  12  and  14  as illustrated in FIG. 8 a.    
     Referring now to FIGS. 8 a  and  9 , in order to raise and lower span  26 , the components illustrated in FIG. 9 are provided at each end at each of piers  12  and  14 . Because the components at each end of each of piers  12  and  14  are generally the same, only the components provided at one end of pier  12  are illustrated. The components at pier  12  include a timber  370 , a lower construct  374 , an upper construct  376  and a motivator  378 . Timber  370  has a lower end (not illustrated) that extends down to the bottom of the river and an upper end  380  that is received by and supports cap  372 . As above, other timbers would also be provided to support cap  372 . Cap  372  forms an upper surface  382  that is essentially flat. Motivator  378  is mounted to cap  372  in any manner known in the art. Motivator  378  is simply a lifting mechanism including a hydraulic motor of some type and a shaft  390  that extends upwardly therefrom. A distall end  392  of shaft  390  can be raised and lowered in a manner explained in more detail below. 
     Referring still to FIG. 9, lower structure  374  includes a concrete base member  394  and a stopper member  396  that extends downward therefrom. One or more other stopper members  396  (not illustrated) would be provided along the length of member  394  to support that member above surface  382 . 
     Upper structure  376  includes a plurality of eye beams  398  that form a lattice and support a deck  400  thereabove. Deck  400  forms top and bottom surfaces  402  and  404 , respectively. The lattice formed by beams  398  contact under surface  404 . In addition, distall end  392  of shaft  390  contacts under surface  404 . Surface  402  provides a driving deck for vehicular traffic. 
     Referring now to FIGS. 9 and 10, the components of FIG. 9 are shown in FIG. 10 in a raised position where shaft  390  has been extended to raise both the upper and lower structures  376  and  374 , respectively. Once raised, a space  410  is provided between span  26  and surface  382 . Referring also to FIG. 8 b  and FIG. 8 c,  once span  26  is raised as illustrated in FIG. 10, span  28  is rolled into space  410 . The support rolling structure used to roll span  28  is similar to the structure illustrated in FIG.  3 . 
     It should be understood that the methods and apparatuses described above are only exemplary and do not limit the scope of the invention, and that various modifications could be made by those skilled in the art that would fall under the scope of the invention. For example, while the embodiments above include roller assemblies mounted to the tops of piers, other embodiments may include roller assemblies mounted to the undersides of spans. In addition, referring to FIGS. 8 a - 8   c,  instead of moving span  26  upward, span  26  may be lowered to provide a space for span  28 . Moreover, referring to FIGS. 5 a - 5   c  and  11  through  14 , while that embodiment shows span  28  being vertically repositioned between lateral and longitudinal moves, in other embodiments rollers may be raised and lowered so that the vertical span position remains essentially constant. Furthermore, while two motivators are described above, it should be appreciated that some embodiments may require only a single motivator. In addition, embodiments with additional vertical restraints are contemplated. 
     To apprise the public of the scope of this invention, the following claims are made: