A support structure for a rostrum includes a first A-frame; and at least a second A-frame, nestable and retractable into the first A-frame. The support structure may have an open central area and the second A-frame may be substantially the same as, but smaller than, the open central area of the first A-frame. At least one support means is connected to each A-frame for supporting a load, wherein the support is connected to an upper portion of one A-frame. The support extends from the upper portion of the one A-frame and is co-operable with an upper portion of another A-frame. The support structure may be telescopic and configured to be expandable and retractable between a retracted, stored position, and an extended, in use position.

CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a 35 U.S.C. §371 National Phase conversion of PCT/GB2008/001889, filed Jun. 3, 2008, which claims benefit of British Application Nos. 0711106.5, filed Jun. 8, 2007 and 0711330.1, filed Jun. 12, 2007, the disclosures of which are incorporated herein by reference. The PCT International Application was published in the English language. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a support structure for a rostrum and particularly, but not exclusively, to a plurality of collapsible and expandable support structures arranged such that a deck can rest upon the supports to create a rostrum. This invention also relates to a rostrum comprising a deck supported by two or more of said support structures. 
     Support structures are used to provide a secure stand or base on which to locate a raised deck, platform, stage or similar base on which a seating structure can be arranged, thus creating a tiered structure, or auditorium, such as those often found at concerts and outdoor events. Examples of known support structures on which a raised deck may rest can be grouped into different types. 
     DE 217,697 or U.S. Pat. No. 2,724,150 can represent a first group of known support structures used in configuring a rostrum, wherein a number of supports are arranged to support an inclined deck on which a seating arrangement rests. The rostrum disclosed in DE 217,697 is a complex assembly, comprising a number of individual components, which requires labour intensive manual assembly. Although such rostrum arrangements are advantageous because they are suitable for compact storage and are easily transported, they can be unstable during the assembly process. 
     A second group of known rostrum arrangements is represented by U.S. Pat. No. 3,400,502, in which a single unit comprising a number of individual nested units can be expanded and retracted. U.S. Pat. No. 3,400,502 discloses a tiered rostrum, in which each tier is formed of a C-shaped support frame and a deck. Each deck is configured upon a common chassis and cooperates with neighbouring tiers to be expandable and retractable from said common chassis. 
     Rostra created from nested systems, as disclosed in U.S. Pat. No. 3,400,502, overcome the problem of labour intensive assembly by being pre-assembled structures. Being comprised of one single unit, there is an increase in stability during assembly between each of the tiers of the rostrum because each tier is inter-connected to the others within the rostrum structure. 
     Although nested systems overcome the problem of rostra comprising individual components, nested systems often require a permanently fixed deck to maintain the stability of the frame, to prevent the frame flexing or distorting during extension or retraction. Moreover, a single nested unit requires level and stable ground on which to be installed, otherwise the nested unit is prone to instability when resting upon uneven surfaces. Being formed of one single unit, nested systems are often large, heavy and are unsuitable for compact storage and are difficult to transport. 
     SUMMARY OF THE INVENTION 
     It is against this background that the present invention has been made. This invention results from efforts to overcome the problems of known rostrum support structures. Other aims of the invention will be apparent from the following description. 
     Accordingly, the invention resides in a support structure for a rostrum, comprising: a first A-frame; and at least a second A-frame, nestable and retractable into the first A-frame. 
     Each support structure provides a more stable structure than known nested assemblies and is inherently stable, in an extended position, without relying on the decking to provide further support or increased rigidity. The support structure can distribute the load, by being interconnected, across a greater floor surface area and is, therefore, less susceptible to irregularities in the floor surface than conventional nested systems. Further, the distribution of the load across a wider area reduces the likelihood of damage to the floor surface. 
     Each support structure can be completed, assembled and commissioned off-site and can be brought to the location in which the rostrum is to be assembled more easily than conventional nested systems because it is modular. Also, the modular nature of each support structure allows each support structure to be turned at an angle, with respect to other support structures, thus enabling at least two suitably arranged support structures to be arranged to support a curved deck. 
     The first A-frame may have an open central area, and the second A-frame may have a shape substantially the same as, but smaller than, the open central area of the first A-frame. 
     The support structure may further comprise at least one support means connected to each A-frame for supporting a load, wherein the support means is connected to an upper portion of one A-frame, the support means extending from the upper portion of the one A-frame and being co-operable with an upper portion of another A-frame. 
     In the extended position, the support structure may be self-supportingly rigid. The A-frame of each support means may be configured to be vertical, inclined at an angle or adjustable, with respect to the surface on which the support means rests. 
     The support structure may be telescopic and configured to be expandable and retractable between a retracted, stored position, and an extended, in use position. 
     Each A-frame may further comprise a foot arranged at the bottom of each limb of the or each A-frame for supporting each A-frame in an upright position. Each foot of each A-frame may be nestable and retractable between the feet and/or limbs the first A-frame. 
     Each foot of the support structure may comprise wheels or other such means to facilitate the extension and retraction of the support structure. The support structure may optionally comprise manually operated and/or powered-mechanical means for assisting in the extension and retraction of the support structure, which may comprise a system of pulleys, worm gears, hydraulics, pneumatics and/or electric motors. 
     The support structure may comprise stop means configured to control movement of the A-frame, thus preventing one A-frame from being moved beyond the end of the support means of another A-frame. 
     Each support means may be independently adjustable in height and/or angle, with respect to the surface on which the support means rests to increase flexibility in use. 
     In another aspect, the invention resides in a rostrum, comprising at least two of the aforementioned support structures; when extended from their nested and retracted position each support structure is configured to support a deck between the A-frames. The rostrum may comprise at least two or at least three support structures arranged such that the support structures can support a curved deck. 
     In the extended position, a load imposed by a deck may be distributed between the A-frames of the support structure through the support means. 
     Thus there is provided a system of retracting rostra or auditoria, as seen in many sports-halls and theatres. This system is based on nests of A-frames for support, instead of the more usual braced C-frames. Each nest consists of a number of A-frames onto which are bolted the decks to form the floor of each tier. These will then pull out (extend) either manually, or by power, to form a stepped (or tiered) stage or auditoria. Stops fixed to the runners dictate the distance each tier is extended, and then pulls out the following tier. This system can be used with or without seats, built with any number of tiers, with rise heights as desired. 
     Particular advantages of the present invention are as follows:
         Each A-frame support is inherently stable (and not relying on the decking for any support).   The A-frames distribute the load to the floor over a greater area than conventional systems.   The A-frames are less susceptible to any irregularities in the floor surface than conventional systems.   The A-frames conduct the loads imposed by its neighbouring decks much more directly than conventional systems.   It is easy to construct curved auditoria by either extending the A-frames or skewing them on their runners.   The nests are completed, assembled and commissioned off site, and brought to site as finished units onto which the decks are simply bolted. This significantly reduces installation times, and even enables retractable auditoria to be available for hire, due to the ease of assembly.   No other guide mechanism is necessary, as the stops have a second function as guides, ensuring that the frames run out square.       

    
    
     
       In order that the invention is more readily understood, reference will now be made, by way of example, to the drawings in which: 
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an embodiment of the invention comprising a support structure, comprising three A-frames in a retracted and nested condition; 
         FIG. 2  is a perspective view of the support structure shown in  FIG. 1 , in an extended position; 
         FIG. 3  is a perspective view of a rostrum comprising two of the support structures shown in  FIG. 1 , in a retracted and nested condition, with a single deck resting on each of the support structures; and 
         FIG. 4  is a perspective view of the rostrum shown in  FIG. 3 , in an extended position. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIG. 1  of the drawings, a support structure is shown comprising a principal, or first A-frame  10 , and two smaller A-frames  12  nested in the open central area  14  within the principal A-frame  10 . When two A-frames are being described, the larger A-frame  10  is referred to as the first A-frame  10  and the smaller A-frame  12  is referred to as the second A-frame  12 . 
     Each A-frame  10 , 12  comprises two upright limbs  16 , each limb  16  being spaced apart at a lower portion  18  and connected to one another at an upper portion  20 . The upper portion  20  includes an apex  22 , with an open central area  14  beneath said upper portion  20 . From the upper portion  20 , referred to hereinafter as the apex  22 , a support means  24  extends substantially horizontally and transverse to the plane in which the A-frame  10 , 12  lies. The apex  22  is shown as a pointed apex  22  and comprises wheels or rollers, but may be a flat, hipped-type connection. At the lower portion  18  of each A-frame  10 , 12 , a foot  26  extends outwards from the lower portion  18  of each limb  16  in substantially the same direction as the support means. 
     In the nested condition, each of the A-frames  10 , 12  lies in the same vertical plane. In the nested condition, each foot  26  enables the support structure  8  to be self-supporting, and to stand alone without the assistance of any other support. In the embodiment shown, the A-frames  10 , 12  are reinforced by a brace  28  to fix the limbs  16  of each A-frame  10 , 12  a predetermined distance apart. Further, guide means  30  in the form of wheels  30  are included to improve the manoeuvrability of the support structures  8 . Each support structure  8  can be easily manoeuvred and installed in position, before being expanded for use. 
     Each of the upright limbs  16  of the A-frames  10 , 12 , support means  24  and feet  26  are constructed of steel box sectional, although they may alternatively be constructed from any such equivalent structural material such as tubular aluminium sections and the like. Each limb  16  is connected up its apex  22  by welding or may be connected using a hinge, bracket or other joining means (not shown). By using a hinge, each A-frame  10 , 12  could be collapsible as an individual component to enable the support structure  8  to be disassembled from its nested condition. 
     The support means  24 , extending from the upper portion  20  of each A-frame  10 , 12  does not necessarily have to extend horizontally and may alternatively be configured to extend in a downward, or upward, direction from the apex  22  of the A-frame  10 , 12 . Each A-frame  10 , 12  does not necessarily have to be arranged perpendicular to the surface on which it rests, and may alternatively be angled away from the perpendicular, by adjustment of the limb  16  position to enable the support structure  8  to be placed on a sloped surface, while continuing to provide a support structure  8  for a level deck  38  to be placed thereupon. Further, each A-frame  10 , 12  may comprise more than one support means. 
     Although the embodiment shown in  FIG. 1  comprises one foot  26  associated with each limb  16  of each A-frame  10 , 12  to provide support for the A-frame  10 , 12  structure many alternative configurations are possible. Each A-frame  10 , 12 , may comprise only a single foot  26 . Further, each foot  26  may extend in a different direction from the support means. Each A-frame  10 , 12  may, alternatively, comprise a number of feet  26  extending from the lower portion  18  of each limb  16  or may even be arranged at a non-perpendicular angle with respect to the limbs  16  of the A-frame  10 , 12 . 
     In an alternative embodiment (not shown) each A-frame  10 , 12  is a different size such that the height of each support means  24  can be vertically adjusted, while being configured to allow the A-frames  10 , 12  of the support structure  8  to be nested. 
     To reduce material and manufacturing costs, each of the A-frames  10 , 12  may not comprise a foot  26 , thus only becoming stable when at least one of the A-frames  10 , 12  is drawn out from the nested position, such that at least four of the limbs  16  of the support structure  8  are distributed upon a surface on which the support structure  8  is located. 
       FIG. 2  shows the support structure  8  of  FIG. 1  in an extended, in use, condition. To extend the support structure  8 , each A-frame  10 , 12  is drawn out from the open central area  14  beneath the upper A-frame  10  is void. The extent to which each A-frame  10 , 12  slides out is dictated by an interface  32  between the apex  22  of an adjacent A-frame and an end point  34  of each support means. Alternatively, the extent to which each A-frame  10 , 12  slides out is determined by interacting stops  50  arranged on cooperating outer and inner sides of the feet  26 . 
     By way of example, the A-frame  10  is connected, via the support means, to the apex  22  of the A-frame immediately within the upper A-frame  10 . The interface  32  between the support means  24  and the apex  22  of the A-frame  12  is typically a tongue and groove type relationship, known to those skilled in the art. Alternatively, the interface  32  may comprise a wheel or roller assembly on the underside of the support means  24 . Therefore, the A-frame  12  will be drawn out from the open central area  14  within the A-frame  10  until the apex  22  of the A-frame  12  reaches the end of the support means  24  of the A-frame  10 . At the end of the support means  24  of each A-frame  10  a stop means  36  or barrier is provided to prevent the apex  22  of the A-frame  12  from extending beyond the end of the support means. Each of the smaller, dependent A-frames  12 , are drawn out until the support structure  8  is in its fully extended position. 
     In the extended position, the support means  24  of the A-frame  10  is fixed at one end to the upper portion  20  of the A-frame  10 , resting upon and/or connected at the other end to the apex  22  of the A-frame  12 , via the interface  32 . The relationship of the support means  24  between each A-frame applies, similarly, to each A-frame  10 , 12  which has a smaller A-frame located there within. 
     The support means  24  of the smallest A-frame of the support structure  8  is suitably reinforced to bear the weight of a load placed thereupon, or is further supported at its distal end by additional an additional mechanical support (not shown). 
     The relationship between the apex  22  of the A-frame  12  and the support means  24  above created by the interface  32  is such that the support structure  8  is self-supporting before any deck  38  is placed thereupon. At the same time, the interface  32  allows the support structure  8  and each limb  16  in contact with a surface upon which the support structure  8  rests to be adjusted so that the support structure  8  is stable. The interface  32  allows the support structure  8  to flex in a controlled manner, enabling the support structure  8  to rest onto an undulating surface, and to be secured thereafter. By way of example, the interface  32  may comprise a device analogous to a control device often used at the interface  32  between a tripod and a photographic camera to enable the camera to be positioned at any angle and fixed in position. 
     The interface  32 , typically comprising a tongue and groove type relationship, may comprise additional guide means  30  (not shown in detail) for facilitating the movement of the apex  22  beneath the support means  24  of an A-frame  10 . To facilitate the expansion and retraction of each of the A-frames  10 , 12  from the nested position, materials with low-friction or lubricating properties may be used. 
     Stop means  36  configured to prevent the apex  22  of the A-frame  12  from extending beyond the end of the support means  24  may be provided by a latch-type system (not shown), which would comprise a mechanism analogous to a slider mechanism of a seat typically found in an automotive vehicle. Such a latch-type mechanism enables a user to control the amount by which an A-frame  12  can be extracted from the nested support structure  8 , thus increasing the flexibility of the support structure  8 . The stop means  36  may be integrated with the A-frame  10 , 12 , support means  24  and/or the feet  26 . Alternatively, latch type mechanisms may be fitted to the feet  26  and/or the decks  38 . 
     Although not shown, the support structure  8  may comprise mechanical or powered means for facilitating the expansion and retraction of the support structure  8  from its nested condition. The facilitating means may be integrated within the interface  32  between the support means  24  and the apex  22  of the A-frame beneath. Alternatively, the powered means may be integrated with the lower portion  18  of the or each limb  16  of each A-frame  10 , 12 , interfacing with the foot  26  of the A-frame  10  to guide the movement of the A-frame  12  during extension and retraction of the support structure  8 . 
     Referring to  FIG. 3 , two support structures  8  are shown displaced at a predetermined distance apart from one another. A deck  38 , or similar platform, is arranged upon each of the same-height corresponding support means  24  of each of the A-frames  10 , 12  to form a rostrum  40 . In practice, the rostrum  40  would be assembled in location by positioning each support structure  8  in place, locating a deck  38  upon each pair of same-height support means  24  and extending each of the support structures  8  into their extended position. 
     The deck  38 , arranged upon each of the support means  24  may comprise either a single deck  38  or may alternatively comprise a number of individual decks  38  (not shown). Each deck  38  component is typically secured to the support means  24  by means of conventional nut and bolt type arrangement. There are, however, a number of ways, well known to the person skilled in the art, in which the deck  38  can be attached to the support means. 
       FIG. 4  shows the rostrum  40  assembly of  FIG. 3  in an expanded, in use, position. Each of the support structures  8  are shown in an extended position, and a deck  38  is shown resting upon each of the support means  24  to create a tiered rostrum  40 . 
     In an alternative embodiment, the support structure  8  comprises only two upright limbs  16  with a foot  26  extending from the lower portion  18  of each limb  16 . Without the support means, the spatial relationship between each of the A-frames  10 , 12  in the extended position is controlled by the relationship between the lower portion  18  of each limb  16  and the distal end of each foot  26  of the A-frame above. In this embodiment the deck  38  is configured to rest upon the apex  22  of each of the A-frames  10 , 12 , without requiring a support means. 
     In another alternative embodiment, the support structure  8  comprises only two upright limbs  16 , and support means, without any feet  26 . In this embodiment, the support structure  8  becomes self-supporting when at least one of the A-frames  10 , 12  is extended from the nested support structure  8 , and the spatial relationship between each of the A-frames  10 , 12  in the extended position is determined by the relationship between the distal end of the support means  24  and the apex  22  of the A-frame beneath. In use, the interface  32  between the support means  24  and the apex  22  of the A-frame beneath, provide the rigidity of the support structure  8  in its extended position. 
     In a further embodiment, at least two support structures  8  are provided such that a curved deck  38  (not shown) may be placed thereupon. 
     In light of the aforementioned description, the support structure  8  may be configured such that when an A-frame is drawn out from beneath the A-frame immediately above, the extent to which each A-frame slides out is dictated by an interface  32  between the apex  22  of the A-frame  10  and the end point  34  of each support means. Alternatively, interacting stops may be arranged upon the vertical faces of each foot  26 . 
     By way of example, the A-frame  10  is connected comprises a support means, which does not have an interface  32  with another A-frame. Instead, the primary, A-frame  10  connects the apex  22  of the A-frame immediately beneath it via an interface  32  to the support means  24  of the A-frame  12 . Therefore, each of the A-frames  12 , comprising a support means, will be drawn out from the open area beneath the A-frame  10  until the end of the support means  24  of the A-frame  12  reaches the apex  22  of the A-frame immediately above. 
     In view of these and other variants within the inventive concept, reference should be made to the appended claims rather than the foregoing specific description in determining the inventive concept.