Abstract:
An auto-centering structural bearing transfers vertical loads to a supporting structure while preventing transfer of lateral loads. In one embodiment, the structural bearing includes: parallel lower, middle, and upper bearing plates; a lower roller bed sandwiched between the lower and middle bearing plates and laterally displaceable in a first direction; an upper roller bed sandwiched between the middle and upper bearing plates and laterally displaceable in a second direction; lower centering means including springs compressible by lateral displacement of the middle bearing plate and the lower roller bed in the first direction; and upper centering means comprising springs compressible by lateral displacement of the upper bearing plate and the upper roller bed in the second direction. Upon removal of loads causing such lateral displacements, the springs will rebound to their unstressed states, thereby returning the displaced bearing plates and roller beds to their centered positions.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to structural bearings for transferring large vertical loads to a supporting structure without physical connection between the bearing feet and the supporting structure. More particularly, the invention relates to a structural bearing that transfers little or no lateral load to the support structure in response to external lateral loads exerted upon the supported load. 
       BACKGROUND OF THE INVENTION 
       [0002]    In various industrial contexts, it is commonly required to provide structural bearings for supporting vertical loads while preventing transfer of significant lateral forces to the supporting structure. Examples include structural bearings in bridges and larger buildings that must be able to carry large vertical loads without allowing transfer of lateral loads to the supporting structure due to wind loads, seismic loads, or expansion or contraction induced by temperature changes. As well, it is commonly desirable to prevent the development of lateral reactions against supporting structures and foundations that can otherwise develop in some structures due to inherent structural characteristics. For example, ‘rigid frame’ building structures can in some cases exert lateral forces against supporting structures or foundations, even under vertical loading alone. 
         [0003]    In such situations, prevention of lateral load transfer to the supporting structure, or prevention of lateral reactions in rigid frame structures, is commonly achieved by allowing the bearings to move laterally relative to the supporting structure, with such lateral movement being facilitated by rollers of some type, or the bearings may be slide bearings using a low-friction material such as PTFE (polytetrafluoroethylene). 
         [0004]    In other scenarios, it is necessary to temporarily support large vertical loads on a supporting structure without transferring lateral loads, such as in conjunction with cantilevered mobile drilling rigs used to drill closely-spaced oil wells, particularly in extremely cold conditions. In such drilling operations, multiple wells are drilled at linear spacings of 10 or 12 feet, with the wellheads being disposed within a heated enclosure. The roof of the wellhead enclosure has hatches spaced to match the well spacing. Well drilling is carried out using a wheel-mounted or track-mounted mobile drilling rig having a cantilevered superstructure that carries a typically sliding rig floor. The mobile rig is positioned adjacent to the wellhead enclosure with the cantilevered superstructure extending over and beyond the wellhead enclosure. The mobile rig is movable parallel to the line of wells, such that it can be longitudinally aligned with each well location as required. 
         [0005]    When the mobile rig is longitudinally aligned with a selected well, the free end of the cantilevered superstructure must be supported before the rig floor and mast section can be laterally positioned over the well and drilling operations commenced. For this purpose, a heavy girder is installed adjacent to the wellhead enclosure on the side opposite the mobile rig. The cantilevered superstructure is provided with two or more telescoping support legs that can be extended to bear upon the girder, without any mechanical connection or anchorage to the girder. When it is desired to move to a new well location, the support legs are retracted vertically away from the girder, and the mobile rig can then be relocated as required. 
         [0006]    The girder is typically supported by spaced columns such that the top of the girder is at an elevation well above the roof of the wellhead enclosure, which may put the girder 25 or 30 feet above the ground. The vertical load exerted on the girder by each support leg during well-drilling operations can be in the range of 500,000 to 600,000 pounds. These large vertical loads create high frictional resistance across the contact interface between the support legs and the girder, such that large lateral loads exerted on the drilling rig structure by wind or seismic forces will react in part against the girder. This is undesirable not only because of the resultant torsional stresses induced in the girder, but also because of the resultant large bending moments induced in the structural columns supporting the girder (not to mention lateral loads and bending moments induced in the piles or other foundation systems supporting the columns). 
         [0007]    For the foregoing reasons, there is a need for improved structural support bearings that will transfer large vertical loads to a supporting structure without allowing the transfer of significant lateral loads the supporting structure, but also without requiring lateral displacement at the contact interfaces between the support bearings and the supporting structure. The present invention is directed to this need. 
       BRIEF SUMMARY 
       [0008]    The present disclosure addresses the foregoing need by providing a structural bearing apparatus for supporting vertical loads while preventing the transfer of significant lateral loading to the supporting structure without relative movement at the contact interface between the bearing and the supporting structure, and which will automatically return to a neutral or centered position upon removal of lateral loads acting on the supported vertical load. In one embodiment, the structural bearing incorporates:
       a lower bearing plate adapted to bear upon a supporting structure, with or without physical connection thereto;   an upper bearing plate positioned above and parallel to the lower bearing plate, and incorporating means for mounting or connecting a supported vertical load (such as a component of a building structure);   a first (or lower) load-bearing lateral displacement means disposed between the lower and upper bearing plates, whereby when the supported vertical load is subjected to lateral loading in a first direction:
           the upper bearing plate will be laterally displaced, in the first direction, relative to the lower lateral displacement means;   the lower lateral displacement means will be laterally displaced, in the first direction, relative to the lower bearing plate; and   vertical loads applied to the upper bearing plate will be transferred through the lower lateral displacement means to the lower bearing plate and then to the supporting structure; and   
           centering means, for returning the upper bearing plate and the lower lateral displacement means to their neutral positions upon removal of the applied lateral loads.       
 
         [0016]    As used in this patent document, the term “neutral position” (or, alternatively, “centered position”) means a position assumed by the described component when the structural bearing is not subjected to lateral loads. 
         [0017]    In a preferred embodiment, the structural bearing is adapted to similarly respond to lateral loading in a second direction (typically but not necessarily perpendicular to the first direction). In this preferred embodiment, the structural bearing further includes a middle bearing plate and a second (or upper) load-bearing lateral displacement means, positioned between the first (or lower) load-bearing lateral displacement means and the upper bearing plate, such that when the supported vertical load is subjected to lateral loading in both the first and second directions:
       the upper bearing plate will be laterally displaced, in the second direction, relative to the upper lateral displacement means;   the upper lateral displacement means will be laterally displaced, in the second direction relative to the middle bearing plate;   the middle bearing plate will be laterally displace, in the first direction, relative to the lower lateral displacement means;   the lower lateral displacement means will be laterally displaced, in the first direction, relative to the lower bearing plate; and   vertical loads applied to the upper bearing plate will be transferred in turn through the upper lateral displacement means, the middle bearing plate, the lower lateral displacement means, and the lower bearing plate, and then to the supporting structure.       
 
         [0023]    In this embodiment, the structural bearing includes additional centering means, for returning the middle bearing plate and the upper lateral displacement means to their neutral positions upon removal of the lateral loads causing lateral displacement thereof. 
         [0024]    In one embodiment, the lower and upper load-bearing lateral displacement means are provided in the form of roller beds each comprising roller means in the form of a plurality of elongate rollers mounted in a retaining frame or cage, with the elongate rollers in each roller bed having parallel and coplanar rotational axes. 
         [0025]    However, the present invention is not limited to lateral displacement means using elongate rollers. By way of non-limiting example, lateral displacement means in alternative embodiments could comprise different roller means, which (by way of non-limiting example) could be provided in the form of multiple sets of wheel-like rollers mounted on parallel axles, or in the form of a ball bearing bed comprising ball bearings disposed within a suitable retaining frame or cage. 
         [0026]    Other alternative embodiments may use the lateral displacement means may comprise a lubricated steel slide plate, with the slide plate being slidable relative to the bearing plates above and below it. Lubrication of the slide plate could be provided by a suitable grease or oil, or alternatively by applying a low-friction material or coating to the surfaces of the slide plate (and/or the surfaces of the associated bearing plates). Lateral displacement means in accordance with such alternative embodiments may be best suited (but not necessarily restricted) to applications requiring support of comparatively light vertical loads, with lateral displacement means using heavy-duty rollers or ball bearings being a preferred choice for (but not restricted to) applications requiring support of heavier vertical loads. 
         [0027]    In one embodiment, each centering means comprises helical springs arranged so as to be compressed in response to lateral loading and corresponding lateral displacement of the associated lateral displacement means or bearing plate, such that upon removal of the lateral load, the compressed springs will automatically restore the associated lateral displacement means or bearing plate to its initial position as the springs rebound to their unstressed states. However, the present invention is not limited to centering means using helical compression springs. By way of non-limiting example, centering means in alternative embodiments could comprise springs that are put into tension rather than compression in response to lateral displacement of associated lateral displacement means or bearing plates. Other embodiments could use springs of a type different from helical springs. 
         [0028]    Moreover, centering means for use with structural bearings in accordance with the present invention do not necessarily have to use springs of any type. By way of non-limiting example, further alternative embodiments may be devised using hydraulic cylinders, screw jacks, or other known devices that can be shortened or elongated in response to a force exerted by lateral displacement of an associated bearing plate or lateral displacement means, and which will naturally rebound or will be otherwise restored to a neutral or unloaded state upon removal or relaxation of the applied lateral force, thereby moving the displaced bearing plate or lateral displacement means back to a neutral or centered position (by means of suitable mechanical linkages). Centering means in accordance with still further embodiments may be adapted to mobilize gravity forces to re-center the bearing plates and lateral displacement means upon removal of lateral loads. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    Embodiments of the invention will now be described with reference to the accompanying figures, in which numerical references denote like parts, and in which: 
           [0030]      FIG. 1  is a conceptual view of a prior art mobile drilling rig having a cantilevered drill floor, shown positioned over a wellhead enclosure. 
           [0031]      FIG. 2  is an isometric view of an auto-centering structural bearing in accordance with one embodiment of the present invention. 
           [0032]      FIG. 2A  is an isometric view of the structural bearing in  FIG. 2 , with protective covers installed. 
           [0033]      FIG. 3  is a first side view of the structural bearing in  FIG. 2 , in a neutral or centered configuration. 
           [0034]      FIG. 3A  is a first cross-section through the structural bearing in  FIG. 2 , in a centered configuration with protective covers installed. 
           [0035]      FIG. 3B  is a cross-section similar to  FIG. 3A , but with the structural bearing in a laterally offset configuration. 
           [0036]      FIG. 3C  is a side view similar to  FIG. 3 , but with the structural bearing laterally offset as in  FIG. 3B . 
           [0037]      FIG. 4  is a second side view of the structural bearing in  FIG. 2 , in a neutral or centered configuration. 
           [0038]      FIG. 4A  is a second cross-section through the structural bearing in  FIG. 2 , in a centered configuration with protective covers installed. 
           [0039]      FIG. 4B  is a cross-section similar to  FIG. 4A , but with the structural bearing in a laterally offset configuration. 
           [0040]      FIG. 4C  is a side view similar to  FIG. 4 , but with the structural bearing laterally offset as in  FIG. 4B . 
           [0041]      FIG. 5A  is an exploded isometric view of the auto-centering structural bearing in  FIG. 2 , illustrating the assembly of the lower roller bed, the lower bearing plate, and the first (or inner lower) centering means. 
           [0042]      FIG. 5B  is an exploded isometric view illustrating the assembly of the middle bearing plate with the subassembly in  FIG. 5A . 
           [0043]      FIG. 5C  is an exploded isometric view illustrating the assembly of the middle bearing plate with the subassembly in  FIG. 5B , in conjunction with the second (or outer lower) centering means. 
           [0044]      FIG. 5D  is an exploded isometric view illustrating the assembly of the upper roller bed and the middle bearing plate, in conjunction with the third (or inner upper) centering means. 
           [0045]      FIG. 5E  is an exploded isometric view illustrating the assembly of the upper bearing plate with the subassembly in  FIG. 5D . 
           [0046]      FIG. 5F  is an exploded isometric view illustrating the assembly of the fourth (or outer upper) centering means with the subassembly in  FIG. 5E . 
           [0047]      FIGS. 5G and 5H  are exploded isometric views illustrating the installation of optional protective covers over the subassembly in  FIG. 5F . 
       
    
    
     DETAILED DESCRIPTION 
       [0048]      FIG. 1  illustrates a prior art mobile drilling rig  200  having tracks  205  for a sliding drill floor (not shown), plus two cantilevered superstructure sections  210  each having a telescoping support leg  215 . Mobile rig  200  is shown positioned with cantilevered sections  210  extending over a wellhead enclosure  220  such that support legs  215  are in position for downward extension to bear upon a girder  245  supported by columns  240  and running along one side of enclosure  220 . Enclosure  220  has a roof  230  with a plurality of access hatches  235  positioned along the length of enclosure  220  according to the spacing of wellheads (not shown) enclosed within enclosure  220 . To drill a well at a designated wellhead location, mobile rig  200  is moved as required in a direction parallel to girder  245  (as indicated by the dual-headed arrows in  FIG. 1 ) so as to be aligned in a first (or longitudinal) direction with the selected well. The corresponding access hatch  235  in roof  230  is then opened, and sliding rig floor (not shown) is moved in a lateral direction along tracks  205  until the rig mast (not shown) is centered over the wellhead location. 
         [0049]      FIGS. 2 through 4C  illustrate a first embodiment of a structural bearing  10  in accordance with the teachings of the present invention.  FIGS. 5A-5H  illustrate the detailed assembly of the structural bearing  10  in  FIGS. 2 through 4C . 
         [0050]    In the illustrated embodiment, structural bearing  10  comprises a lower bearing plate  20  having an upper surface  21  and a lower surface  25 , with lower surface  25  being intended for resting on a structural support (such as girder  245  shown in  FIG. 1 ). The size and thickness of lower bearing plate  20  will be selected to suit material properties and design criteria applicable to specific intended uses. By way of non-limiting example, in one embodiment intended for use to support large vertical loads from a telescoping support leg under a cantilevered section of a mobile drill rig, lower bearing plate  20  is 48 inches by 60 inches in plan dimensions, and 2.50 inches in thickness. 
         [0051]    Preferably but not necessarily, lower bearing plate  20  comprises an upper plate  20 U overlying a lower plate  20 L, as shown in  FIGS. 3 and 4 . In this preferred embodiment, upper plate  20 U is preferably made from a wear-resistant material such as (but not restricted to) QT100, which is a quenched and tempered, high-strength weldable steel. Upper plate  20 U thus defines upper surface  21  of lower bearing plate  20 . This construction allows lower plate  20 L to be fabricated from a structurally sufficient mild steel rather than the alternative of making upper plate  20  entirely from a more expensive material such as QT100 to provide optimal wear resistance when structural bearing  10  is in service under heavy vertical and horizontal loads. Upper plate  20 U and lower plate  20 L may be joined to form an integral lower bearing plate  20  by any suitable means. By way of non-limiting example, and as illustrated in  FIG. 5A , one way of doing this is to provide upper plate  20 U with a number of preferably elongate slots  23 , so that upper plate  20 U can be welded to lower plate  20 L. 
         [0052]    Also as best seen in  FIG. 5A , a pair of spaced and parallel lower fence members  22  are mounted to and project upward from upper surface  21  of lower bearing plate  20 . In the illustrated embodiment, and for reasons explained later herein, one lower fence member is longer than the other; for clarity, reference numbers  22 A and  22 B will be used to denote the longer and shorter fence members, respectively. A lower roller bed  30 , comprising a plurality of heavy-duty cylindrical rollers  34  rotatably mounted in parallel within a lower roller frame  31 , is positioned upon upper surface  21  of lower bearing plate  20  so as to be rollingly movable thereupon, in either direction transverse to lower fence members  22  (which in turn define and limit the range of movement of lower roller bed  30  relative to lower bearing plate  20 ). The length and diameter of rollers  34  will be selected to suit case-specific design criteria. By way of non-limiting example, in one embodiment intended for use to support large vertical loads, rollers  34  are 3.00 inches in diameter and about 36 inches in length. 
         [0053]    In the illustrated embodiment, lower roller frame  31  comprises parallel side members  32  extending between parallel end members  33 , with side members  32  being adapted (e.g., with suitable bearing means) to support rollers  34  in rotatable fashion. Persons skilled in the art will appreciate that end members  33  are not essential to the invention, and also that lower roller frame  31  in alternative embodiments may have side members  32  that are not parallel. 
         [0054]    Lower bearing plate  20  is also provided with a first centering means for biasing lower roller bed  30  toward a neutral or centered position relative to lower bearing plate  20 . Persons skilled in the art will readily appreciate that the first centering means can be provided in a variety of forms using known concepts and technologies, and the present invention is not limited by or restricted to the use of any particular type of centering means. However, as shown in  FIGS. 5A and 5B , the first centering means in the illustrated embodiment, comprises two pairs of helical compression springs  40 , with each pair of springs  40  being disposed around an elongate spring rod  42  extending between and mounted (using suitable mounting hardware such as rod mounting brackets  43 ) to a pair of spaced abutments  24  which in turn are mounted on upper surface  21  of lower bearing plate  20 , adjacent to and clear of the travel path of lower roller bed  30 , such that spring rod  42  is parallel to the direction of travel of lower roller bed  30 . In this embodiment, abutments  24  effectively serve as lateral guide means for lower roller bed  30  as it moves between fence members  22 A and  22 B, preventing or limiting lateral displacement of lower roller bed  30  relative to lower bearing plate  20  in a direction parallel to the axes of rollers  34 . 
         [0055]    Each spring rod  42  passes through an opening  38 A in a lug member  38  projecting laterally outward from a medial region of a corresponding side member  32  of lower roller frame  31 , such that for each pair of helical springs  40 , one spring  40  is disposed around the corresponding spring rod  42  on each side of the corresponding lug member  38  on lower roller frame  31 . As illustrated, each spring rod  42  will preferably carry a washer  41  on either side of and adjacent to the corresponding lug member  38  to facilitate uniform application of compressive force into springs  40 . 
         [0056]    Accordingly, when lower roller bed  30  is moved in either direction between lower fence members  22 , one helical spring  40  on each side of lower roller bed  30  will be compressed between a corresponding lug member  38  and a corresponding abutment  24 . Removal of the external force causing the movement of lower roller bed  30  will in turn relieve the compressive load in the compressed springs  40 , which as a result will urge lug members  38 , and lower roller bed  30  with them, back toward the neutral or centered position relative to lower bearing plate  20 . 
         [0057]    As will be described later in this specification, the illustrated embodiment of structural bearing  10  comprises second, third, and fourth centering means using helical compression springs similar to the first centering means described above. For enhanced clarity and to distinguish between the various centering means and related components, the first centering means may be alternatively referred to as the inner lower centering means, and helical springs  40  may be alternatively referred to as inner lower springs  40 . Similar alternative terminology will also be used for the other centering means described later herein. 
         [0058]    Referring to  FIGS. 3 ,  4 , and  5 B, structural bearing  10  also comprises a middle bearing plate  60  having an upper surface  61  and a lower surface  65 . In the illustrated embodiment, middle bearing plate  60  comprises a middle plate  60 M (which may be made from mild steel), and upper and lower plates  60 U and  60 L, which like upper plate  20 U of lower bearing plate  20  are preferably made from a wear-resistant material such as QT100. Accordingly, in this embodiment, upper and lower plates  60 U and  60 L thus define upper and lower surfaces  61  and  65 , respectively, of middle bearing plate  60 . Also similar to upper plate  20 U of lower bearing plate  20 , upper and lower plates  60 U and  60 L of middle bearing plate  60  may be secured to middle plate  60 M by welding, facilitated by slots  63  formed in upper and lower plates  60 U and  60 L. 
         [0059]    As illustrated in  FIGS. 5B and 5C , with lower roller bed  30  positioned on upper surface  21  of lower bearing plate  20 , middle bearing plate  60  is positioned over lower roller bed  30  such that lower surface  65  of middle bearing plate  60  contacts rollers  34  of lower roller bed  30 . Accordingly, when a lateral force F 1  is applied to structural bearing  10  in a direction transverse to the axes of rollers  34 , as shown in  FIGS. 3B and 3C , and while the overall assembly is under vertical compressive load as well, lower roller bed  30  will roll over lower bearing plate  20  in the same direction. As a result, middle bearing plate  60  will roll over rollers  34  a corresponding amount in the same direction. 
         [0060]    To facilitate centering of middle bearing plate  60  relative to lower bearing plate  20 , structural bearing  10  preferably incorporates a second (or outer lower) centering means generally similar to the first (or inner lower) centering means described previously, and as best understood with reference to  FIGS. 5C and 5D . In the illustrated embodiment, the second (or outer lower) centering means comprises two pairs of helical compression springs  50  (or outer lower springs  50 ), with each pair of outer lower springs  50  being disposed around an elongate spring rod  52  extending between and mounted (using rod mounting brackets  53 ) to a pair of spaced abutments  68  which in turn are mounted to and project downward from lower surface  65  of middle bearing plate  60 , externally adjacent and parallel to a corresponding pair of inner lower springs  40  of the first centering means. Each outer lower spring rod  52  passes through an opening  28 A in a lug member  28  mounted to and projecting upward from a medial side region of lower bearing plate  20 , such that for each pair of outer lower springs  50 , one spring  50  is disposed around the corresponding outer lower spring rod  52 . Accordingly, when middle bearing plate  60  is laterally displaced in either direction relative to lower roller bed  30  and lower bearing plate  20  as previously described, one outer lower spring  50  on each side of lower roller bed  30  will be compressed between a corresponding lug member  28  and a corresponding abutment  68 . This can be seen in  FIG. 3C , in which the compressed outer lower spring is indicated by reference number  50 A. Preferably, each outer lower spring rod  52  carries a washer  51  on either side of the corresponding lug member  28  to facilitate uniform application of compressive force into outer lower springs  50 . 
         [0061]    Removal of external force F 1  will relieve the compressive load in compressed outer lower springs  50 A, which as a result will urge middle bearing plate  60  back toward a neutral or centered position relative to lower bearing plate  20 . 
         [0062]    Middle bearing plate  60  may be adapted to accommodate lateral displacement relative to lower bearing plate  20  without vertical separation when structural bearing  10  is in a suspended condition (such as, for example, when incorporated into a vertically extendable support leg  215  as in the mobile cantilever drill rig  200  shown in  FIG. 1 ). It will be readily apparent to persons skilled in the art that this preferred feature can be provided in a variety of ways by non-inventive adaptation of known concepts and technologies. 
         [0063]    In the illustrated embodiment, however, this is accomplished by forming abutments  24  on lower bearing plate  20  with outwardly-extending elongate flanges  24 A as shown in  FIGS. 5A and 5B , and providing each abutment  68  mounted to the underside of middle bearing plate  60  with one or more inwardly-projecting lugs  66  configured and located to slide under flanges  24 A of abutments  24 , as best seen in  FIG. 4A . When structural bearing  10  is in a suspended condition, lower bearing plate  20  will be effectively suspended from middle bearing plate  60  due to flanges  24 A (which are connected to lower bearing plate  20 ) supported on lugs  66  (which are connected to middle bearing plate  60 ). 
         [0064]    In alternative embodiments, intended for use in service conditions in which structural bearing  10  will at all times rest on a supporting structure and therefore will not be suspended, there will be no necessity for means for preventing vertical separation between lower and middle bearing plates  20  and  60 . In such service conditions, rollers  34  will at all times maintain compressive contact with upper surface  21  of lower bearing plate  20  and with lower surface  65  of middle bearing plate  60 . In such alternative embodiments, the second (or outer lower) centering means may be unnecessary, depending on the magnitude of the vertical load applied to structural bearing  10 . Provided that it has sufficient strength, the first (or inner lower) centering means by itself may be effective to center middle bearing plate  60  as well as lower roller bed  30  upon removal of loads or conditions causing lateral displacement thereof in the first direction. As mentioned previously, when lower roller bed  30  is laterally displaced relative to lower bearing plate  20  in the first direction, middle bearing plate  60  will be resultantly displaced a corresponding amount in the same direction relative to lower roller bed  30 , due to the fact that rollers  34  roll equally relative to both upper surface  21  of lower bearing plate  20  and lower surface  65  of middle bearing plate  60 . Therefore, if middle bearing plate  60  is in compressive contact with rollers  34 , the action of the first centering means to urge lower roller bed  30  back toward its neutral or centered position will have a corresponding effect on middle bearing plate  60 , barring slippage between rollers  34  and lower surface  65  of middle bearing plate  60 . 
         [0065]    As shown in  FIGS. 5A-5C , the longer fence member  22 A extends across the ends of abutments  24  while shorter fence member  22 B extends between abutments  24  so as to leave clearance to allow middle bearing plate  60  to slide into position over lower roller bed  30 , with lugs  66  of abutments  68  sliding under flanges  24 A of abutments  24 . Suitable end plates  27  are then mounted to the ends of abutments  24  adjacent to the ends of shorter fence member  22 B, thus providing a second limit for lateral displacement of middle bearing plate  60  relative to lower bearing plate  20 . 
         [0066]    As illustrated in  FIGS. 5C and 5D , middle bearing plate  60  includes a pair of spaced and parallel upper fence members  62  (more specifically, longer fence member  62 A and shorter fence member  62 B) projecting upward from upper surface  61  of middle bearing plate  60 . An upper roller bed  70 , comprising a plurality of parallel cylindrical rollers  74  rotatably mounted within an upper roller frame  71 , is positioned upon upper surface  61  of middle bearing plate  60  so as to be rollingly movable thereupon, in either direction transverse to upper fence members  62  (which in turn define and limit the range of movement of upper roller bed  70  relative to middle bearing plate  60 ). In the illustrated embodiment, upper roller frame  71  comprises parallel side members  72  extending between parallel end members  73 , with side members  72  being adapted to rotatably support rollers  74 . 
         [0067]    As shown in  FIGS. 5D and 5E , middle bearing plate  60  is also provided with a third (or inner upper) centering means for biasing upper roller bed  70  toward a neutral or centered position relative to middle bearing plate  60 . In the illustrated embodiment, the third centering means comprises two pairs of helical compression springs  80  (or inner upper springs  80 ), with each pair of springs  80  being disposed around an elongate inner upper spring rod  82  extending between and mounted (using rod mounting brackets  83 ) to a pair of spaced abutments  64  which in turn are mounted on upper surface  61  of middle bearing plate  60 , adjacent to and clear of the travel path of upper roller bed  70 , such that inner upper spring rod  82  is parallel to the direction of travel of upper roller bed  70 . In this embodiment, abutments  64  effectively serve as lateral guide means for upper roller bed  70  as it moves between fence members  62 A and  62 B, preventing or limiting lateral displacement of upper roller bed  70  relative to middle bearing plate  60  in a direction parallel to the axes of rollers  74 . 
         [0068]    Each inner upper spring rod  82  passes through an opening  78 A in a lug member  78  projecting laterally outward from a medial region of a corresponding side member  72  of upper roller frame  71 , such that for each pair of helical springs  80 , one spring  80  is disposed around the corresponding spring rod  82  on each side of the corresponding lug member  78  on upper roller frame  71  (preferably with washers  81  on each side of lug member  78 ). 
         [0069]    Accordingly, when upper roller bed  70  is moved in either direction between upper fence members  62 , one helical spring  80  on each side of upper roller bed  70  will be compressed between a corresponding lug member  78  and a corresponding abutment  64 . Removal of the external force causing the movement of upper roller bed  70  will in turn relieve the compressive load in the compressed springs  80 , which as a result will urge lug members  78 , and upper roller bed  70  with them, back toward the neutral or centered position relative to middle bearing plate  60 . 
         [0070]    It will be immediately apparent that upper roller bed  70 , the third centering means, fence members  62 , and abutments  64  in the illustrated embodiment are similar in configuration and construction to lower roller bed  30 , the first centering means, fence members  32 , and abutments  34 , respectively. However, the direction of travel of upper roller bed  70  is transverse to the direction of travel of lower roller bed  30 . Accordingly, the illustrated embodiment of structural bearing  10  accommodates lateral loading in two directions, and is auto-centering in both directions upon removal of the lateral loads. 
         [0071]    Referring to  FIGS. 3 ,  4 , and  5 E, structural bearing  10  also comprises an upper bearing plate  100  having a lower surface  105 . In the illustrated embodiment, upper bearing plate  100  comprises an upper plate  100 U (which may be made from mild steel), and a lower plate  100 L, preferably made from a wear-resistant material. Accordingly, in this embodiment, lower plate  100 L thus defines lower surface  105  of upper bearing plate  100 . 
         [0072]    As illustrated in  FIGS. 5E and 5F , with upper roller bed  70  positioned on upper surface  61  of middle bearing plate  60 , upper bearing plate  100  is positioned over upper roller bed  70  such that lower surface  105  of upper bearing plate  100  contacts rollers  74  of upper roller bed  70 . Accordingly, when a lateral force F 2  is applied to structural bearing  10  in a direction transverse to the axes of rollers  74 , as shown in  FIGS. 4B and 4C , and while the overall assembly is under vertical compressive load as well, upper roller bed  70  will roll over upper bearing plate  100  in the same direction. As a result, upper bearing plate  100  will roll over rollers  74  a corresponding amount in the same direction. 
         [0073]    To facilitate centering of upper bearing plate  100  relative to middle bearing plate  60 , structural bearing  10  preferably incorporates a fourth (or outer upper) centering means, which as shown in  FIG. 5F  may comprise two pairs of helical compression springs  90  (or outer upper springs  90 ), with each pair of outer upper springs  90  being disposed around an outer upper spring rod  92  extending between and mounted (using rod mounting brackets  93 ) to a pair of spaced abutments  102  which in turn are mounted to and project downward from lower surface  105  of upper bearing plate  100 , externally adjacent and parallel to a corresponding pair of inner upper springs  80  of the third centering means. Each outer upper spring rod  92  passes through an opening  69 A in a lug member  69  mounted to and projecting upward from a medial side region of middle bearing plate  60 , such that for each pair of outer upper springs  90 , one spring  90  is disposed around the corresponding outer upper spring rod  92 . Accordingly, when upper bearing plate  100  is laterally displaced in either direction relative to upper roller bed  70  and middle bearing plate  60  as previously described, one outer lower spring  90 A on each side of upper roller bed  70  will be compressed between a corresponding lug member  69  and a corresponding abutment  102 . This can be seen in  FIG. 4C , in which the compressed outer upper spring is indicated by reference number  90 A. Preferably, each outer upper spring rod  92  carries a washer  91  on either side of the corresponding lug member  69  to facilitate uniform application of compressive force into outer upper springs  90 . 
         [0074]    Removal of external force F 2  will relieve the compressive load in compressed springs  90 A, which as a result will urge upper bearing plate  100  back toward a neutral or centered position relative to middle bearing plate  60 . 
         [0075]    Upper bearing plate  100  may be adapted to accommodate lateral displacement relative to middle bearing plate  60  without vertical separation when structural bearing  10  is in a suspended condition. In the illustrated embodiment, this is accomplished by forming abutments  64  on middle bearing plate  60  with outwardly-extending elongate flanges  64 A as shown in  FIGS. 5B through 5E , and providing each abutment  102  mounted to the underside of upper bearing plate  100  with one or more inwardly-projecting lugs  104  configured and located to slide under flanges  64 A of abutments  64 , as best seen in  FIG. 3A . When structural bearing  10  is in a suspended condition, middle bearing plate  60  will be effectively suspended from upper bearing plate  100  due to flanges  64 A (which are connected to middle bearing plate  60 ) supported on lugs  104  (which are connected to upper bearing plate  100 ). 
         [0076]    In alternative embodiments, intended for use in service conditions in which structural bearing  10  will at all times rest on a supporting structure and therefore will not be suspended, there will be no necessity for means for preventing vertical separation between middle and upper bearing plates  60  and  100 . In such alternative embodiments, the fourth (or outer upper) centering means may be unnecessary, for reasons essentially as set out previously with respect to alternative embodiments not requiring the second (or outer lower) centering means. 
         [0077]    As shown in  FIGS. 5A-5C , the longer fence member  62 A extends across the ends of abutments  64  while shorter fence member  62 B extends between abutments  64  so as to leave clearance to allow upper bearing plate  100  to slide into position over upper roller bed  70 , with lugs  104  of abutments  102  sliding under flanges  64 A of abutments  64 . Suitable end plates  67  are then mounted to the ends of abutments  64  adjacent to the ends of shorter fence member  62 B, thus providing a second limit for lateral displacement of upper bearing plate  100  relative to middle bearing plate  60 . 
         [0078]    Structural bearing  10  is provided with mounting means (generally indicated by reference number  110 ) for mounting structural bearing  10  to a supported structural element, such as (by way of non-limiting example) to the lower end of a support leg  215  as in the mobile cantilever drill rig  200  in  FIG. 1 . In the illustrated embodiment, mounting means  110  is provided in the form of one or more mounting brackets  114  extending upward from upper bearing plate  100  as shown in  FIG. 2  and other drawings, with holes  115  to receive a shear pin (not shown) inserted through one or more mating brackets (not shown) on the supported structural element. The shear pin is preferably round in cross-section to allow swivelling between the supported structural element and structural bearing  10  about a swivel axis X- 1  as shown in  FIG. 3A  and other drawings. Persons of ordinary skill will readily understand how structural bearing  10  may be thus mounted to a supported structural element notwithstanding that the above-described mounting arrangement is not illustrated in the drawings. Furthermore, persons skilled in the art will appreciate that alternative forms of mounting means  110  may be readily devised in accordance with known concepts and methods. 
         [0079]    As illustrated in  FIGS. 2A ,  5 G,  5 H and other drawings, structural bearing  10  is preferably provided with covers to protect against entry of contaminants such as rain, snow, and dust, while at the same time accommodating lateral displacement in response to lateral loading in any direction. In the illustrated embodiment, a fixed cover  120  is provided in the form of a rectilinear box with side walls  121 , a top member  122  with an opening  123 , and an open bottom. Fixed cover  120  and opening  123  are sized and configured such that when fixed cover  120  is mounted with side walls  121  supported upon and fastened to lower bearing plate  20 , the entire movable subassembly (i.e., lower roller bed  30  plus all components supported thereby) can move through full ranges of lateral displacement in both directions, without physical interference with fixed cover  120 . 
         [0080]    To accommodate this movement, mounting means  110  projects upward through opening  123  in top member  122  of fixed cover  120 . In order to protect against entry of contaminants through opening  123  regardless of the lateral position of the movable subassembly, a travelling cover  125  with an opening  126  is mounted to mounting means  110  in any suitable fashion, such that travelling cover  125  extends over top member  122  of fixed cover  120 , and such that the perimeter edge  127  of travelling cover  125  will always overlap top member  122  of fixed cover  120  regardless of the lateral position of the movable subassembly. In the illustrated embodiment, travelling cover  125  is mounted to mounting means  110  by interposing a base plate  112  between upper bearing plate  100  and mounting brackets  114 , such that travelling cover  125  can be fastened to base plate  112  along the periphery of opening  126  in travelling cover  125 . Optionally, and as best seen in  FIGS. 3A and 4A , top member  122  of fixed cover  120  may be formed with an upturned lip  122 A around the periphery of opening  123 , and travelling cover  125  may be formed with a downturned lip  127 A around perimeter edge  127 , for further protection against entry of contaminants into the inner workings of structural bearing  10 . 
         [0081]    Persons skilled in the art will appreciate that the protective cover means described and illustrated herein are by way of example only, and that alternative suitable cover means can be readily devised without departing from the principles and concepts of the present invention. Moreover, it is to be understood that protective cover means are not essential to the present invention, and do not form part of the broadest embodiments of the invention. 
         [0082]    It will be readily appreciated by those skilled in the art that various modifications of the present invention may be devised without departing from the scope and teaching of the present invention, including modifications which may use equivalent structures or materials hereafter conceived or developed. To provide one particular and non-limiting example, and as previously suggested herein, alternative embodiments can be devised to accommodate lateral displacement either way from a centered or neutral position but only in two opposite directions (e.g., lateral displacement to the north or south, but not to the east or west). Such alternative embodiments would require only one roller bed, disposed between a lower bearing plate and an upper bearing plate. Accordingly, such embodiments would substantially correspond to the illustrated embodiment, but without the middle bearing plate, the upper roller bed, and the third and fourth centering means. For variant assemblies that will not be suspended, and for which no means for preventing vertical separation between the lower and upper bearing plates will be necessary, it may be sufficient to provide only a single centering means. 
         [0083]    Another alternative embodiment would accommodate operational conditions where anticipated lateral displacement of one bearing plate relative to another (e.g., lateral displacement of the middle bearing plate relative to the lower bearing plate) would be in one direction only, relative to a centered or neutral position (e.g., lateral displacement to the north but not to the south). In this embodiment, each associated centering means would need only a single compression spring on each side of the assembly. 
         [0084]    In a variant combining the two alternative embodiments described immediately above, the principles of the present invention could be applied to accommodate lateral displacement from the neutral position in two opposite directions (e.g., north and south) and only one transverse direction (e.g., east). A further variant would accommodate lateral displacement from the neutral position in only a single direction (e.g., to the north, but not to the east, south, or west); in such embodiments, only a single roller bed would be required, with upper and lower bearing plates. 
         [0085]    In yet further alternative embodiments, intended for service conditions in which the auto-centering structural bearing will always be supported from below and will not be suspended, there will be no need for means for preventing vertical separation between adjacent bearing plates, such as lugs  66  on abutments  68  or lugs  104  on abutments  102 . 
         [0086]    It is to be especially understood that the invention is not intended to be limited to any described or illustrated embodiment, and that the substitution of a variant of a claimed element or feature, without any substantial resultant change in the working of the invention, will not constitute a departure from the scope of the invention. It is also to be appreciated that the different teachings of the embodiments described and discussed herein may be employed separately or in any suitable combination to produce desired results. 
         [0087]    In this patent document, any form of the word “comprise” is to be understood in its non-limiting sense to mean that any item following such word is included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one such element. Any use of any form of the terms “connect”, “engage”, “couple”, “mount”, “attach”, or any other term describing an interaction between elements is not meant to limit the interaction to direct interaction between the subject elements, and may also include indirect interaction between the elements such as through secondary or intermediary structure. Relational terms such as “parallel”, “perpendicular”, “coincident”, “intersecting”, and “equidistant” are not intended to denote or require absolute mathematical or geometrical precision. Accordingly, such terms are to be understood as denoting or requiring substantial precision only (e.g., “substantially parallel”) unless the context clearly requires otherwise.