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This invention relates to a walkway assembly and in particular, though not exclusively, to a walkway assembly for use externally such as on the roof of a building. 
     The term “walkway” is used herein to relate both to a substantially horizontally extending or slightly inclined surface over which personnel may walk and also to a series of steps which may be in the form of a stairway to assist personnel to move over more steeply inclined surfaces or between two levels. 
     BACKGROUND OF THE INVENTION 
     There is a particular need to protect roof structures from damage by maintenance and other personnel moving thereover, and to assist in ensuring the safety of such personnel. Such needs include also the provision of roof-top fire escape routes. In response to these needs it is well known, particularly for industrial and commercial premises, to construct on a roof top both continuous walkway assemblies, which may be level or slightly inclined, and also series of steps in the form of stairways for movement of personnel over more steeply inclined surfaces or between two levels. 
     The variety of roof pitch angles and the distances over which a walkway is to be provided has meant that in general bespoke on-site construction work is necessary. However the relative difficulties typically encountered when working in a roof top environment, and exposure to weather, makes it particularly advantageous to provide means for minimising or simplifying the on-site construction work. 
     Another requirement that needs to be taken into account is that of ensuring that the tread surface does not of itself present a significant safety hazard. 
     Disadvantages of many known walkway assemblies include difficulty of and time for off-site design work, on-site construction, relative expense, cost of component parts and relatively high weight. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide an improved walkway assembly in which at least some of the aforementioned disadvantages of known types of walkway assemblies and components therefor are mitigated or overcome. 
     In accordance with one aspect of the present invention there is provided a walkway assembly for attachment or attached to a support structure, said assembly comprising:—
         a primary support for on-site attachment to the support structure;   a plurality of tread modules each attached relative to the primary support and moveable between a first position and a second position at which a part of each module is spaced further from the primary support than when at the first position, and   at least one riser unit for supporting the tread modules relative to the primary support in at least the second of the first and second positions.       

     The primary support may comprise two primary support members which lie side by side in spaced apart relationship and extend in parallel with one another in the direction of the length of the walkway assembly. In this case transverse spacers may be provided to extend between the support members to maintain the support members spaced apart, and/or the tread modules may serve to perform a spacing function. However the primary support may be of a different configuration and may, for example, comprise a single elongate member relative to which the tread modules are attached. 
     Preferably each tread module is secured pivotally relative to the primary support. 
     Each tread module may be supported by an auxiliary support that is secured, for example pivotally, relative to said primary support. Said auxiliary support may comprise two auxiliary support members which lie side by side in spaced apart relationship and extend in parallel with one another. As viewed in a direction perpendicular to the primary support, said auxiliary support members may extend substantially parallel with the length of the primary support. 
     A walkway assembly may comprise a single or a plurality of auxiliary supports. 
     For forming a walkway to be used as a stairway the walkway assembly, such as a pre-formed module, may comprise a plurality of auxiliary supports and each auxiliary support may be employed to support one or a plurality of tread modules. Said auxiliary supports in this case may each be pivotally secured to the primary support, each for pivotal movement about respective axes which extend substantially perpendicular to the length of the primary support. 
     If the primary support comprises a pair of spaced apart primary support members, each primary support member may have secured thereto a plurality of auxiliary supports each of a kind comprising a pair of auxiliary support members and with each of the auxiliary support members of a pair being secured to a respective one of the two primary support members. In this case riser units preferably are provided to extend between each of the primary support members and distal regions of each of the auxiliary support members. 
     For a modular walkway assembly to be used as a traverse walkway to extend substantially horizontally over an inclined roof or other support structure a walkway assembly module may, for example, comprise only a single auxiliary support to which all of the tread modules of the walkway assembly module are secured. In this case said auxiliary support may be secured pivotally relative to the primary support for pivotal movement about an axis which extends substantially parallel with the length of the primary support. 
     If the primary support comprises a pair of spaced apart primary support members, one primary support member of a traverse walkway may have an auxiliary support pivotally secured thereto and the other primary support may provide location and support for at least one said riser unit which extends to and supports the auxiliary support at a position remote from the primary support member to which the auxiliary support is pivotally secured. As in the case of a stairway module, the auxiliary support may comprise a pair of auxiliary support members that are inter-connected by and provide support for the plurality of tread modules. One of the auxiliary support members may be pivotally secured relative to one of a pair of primary support members and the other auxiliary support member may be inter-connected with the other of said primary support members by one or more said riser units. 
     From the foregoing description of one aspect of the present invention it will be appreciated that a walkway assembly advantageously, though not necessarily, may be constructed off-site as a pre-formed module and thereby reduce substantially the work needed to be undertaken on-site in conditions which generally are significantly less favourable than off-site. By providing pre-formed modules of different lengths, such as two different lengths (for example 1.5 meter (5 feet) and 3 meter (10 feet) lengths and in view of the fact that on-site tailoring of the length of the walkway often will not be critical to within a few meters, the need to expend time in on-site or off-site cutting to length is avoided or at least significantly minimised. As will also be apparent from the description given below, the primary supports of two modules may be of a kind that readily enables them to be structurally inter-connected thereby to provide a walkway surface devoid of any significant gaps or abrupt changes of deflection characteristics. 
     A further potential advantage arising from the use of pre-formed modules is that there is a significant reduction in the need for detailed on-site survey work and measurement. For many applications the only necessary on-site survey work will be that to establish the length of a walkway run and, in the case of a traverse walkway or stairway, the angle of inclination of the support structure and the type of roof surface to which the primary support of the walkway module is to be secured. 
     The facility for the auxiliary support to be moveable relative to the primary support advantageously allows a module size to be minimised for transportation and, by use of adjustable riser units, for a single module construction to be employed on-site in situations having different angles of inclination to the support structure. However, optionally a module may comprise riser units that can extend to and be lockable at only a single, pre-determined second position. Optionally, alternatively or additionally, a module may comprise riser units that are lockable at the first position, 
     It is not, however, essential that the walkway assembly be of a modular type and the present invention further provides walkway assemblies of a type that are suitable for either modular off-site construction or bespoke on-site construction. 
     In accordance with another aspect of the present invention there is provided a walkway assembly comprising a pair of primary support members which lie side by side in spaced apart relationship and extend in parallel with one another in the direction of the length of the walkway, and a plurality of tread modules which are each supported relative to the support members, each primary support member comprising a side face which comprises a groove formation for location of the primary support member relative to at least one of a support structure and another support member. 
     The tread modules may be secured directly to the support members, for example to upper surfaces of the support members, or they may be secured to an auxiliary support that is secured relative to the primary support members. Said auxiliary support may be moveable relative to the primary support members and may comprise one or more of the other features described herein in respect of an auxiliary support. 
     Preferably each said support member comprises two side faces each having a groove formation. 
     Preferably the or each said groove formation is provided in a side face which extends substantially perpendicular relative to an upper support surface of the support member. 
     It is further preferred that the support member has a cross-sectional shape which is symmetrical about two mutually perpendicular axes, for example one parallel with a side face or upper support surface and the other perpendicular relative thereto. 
     Each groove formation may define a channel which is generally of a C shape in cross section whereby within the groove there are two abutment surfaces each of which lies at a respective side of the groove opening. 
     Suitable materials for the support members include metals such as aluminium and steel (typically galvanised) and also plastics such as a polyamide (e.g. nylon) or a composite such as glass reinforced polyamide. 
     The groove formations preferably are open ended to allow for ease of insertion of components such as the heads of securing means, such as bolt heads or nuts employed in use for securing the support member to a connector for connection of two lengths of support members or to a retainer (e.g. a clamp or retention bracket) by means of which the support member may be retained relative to a support structure such as a roof structure. 
     The support members may be tubular in cross-section. End caps may be provided to close the ends of the tubular passages of the support members and also to serve to provide a protective cover over any sharp edges which may be present at a cut end of a support member. End caps may extend over ends of the groove formations or may leave the groove formations open-ended. The end caps may be of moulded plastics and may be secured in position as a friction fit. 
     The walkway assembly, whether or not in the form of a module, may comprise straight connectors such as metal strips for joining two lengths of support members end to end, each strip being provided with apertures for securing means. A connector may be secured to a side face of a support member by a bolt the head of which lies within the groove whereby a nut secured to the outer end of the bolt enables a part of the connector to be clamped to one of the support members. A connector may be positioned internally within a groove and may be similarly secured relative to the support member by a securing means that extends outwards through an aperture in the connector, or the connector may be provided with a threaded aperture for engagement by a bolt that extends inwards into the groove. Other shapes of connectors may be used, e.g. right angled connectors or connectors having two limb portions inclined relative to one another at an angle other than a right angle. 
     Thus it will be appreciated that the provision of a primary support having a said groove formation allows two primary supports readily to be joined end to end or, for example, at right angles relative to one another. 
     The retainers for attachment to the support structure may be substantially similar to those already well known, but adapted as may be necessary to enable them to be secured readily relative to the grooved side face of a support member. 
     The tread modules may be of a kind as described in our co-pending UK patent application GB 0921366.1 and entitled Tread Module. 
     The tread modules preferably are of a kind having a width (as considered perpendicular to the length of the walkway) which is greater than the depth of the module and preferably the width is between four times the depth and twice the depth, more preferably between three times and twice the depth. 
     Each step of a stairway may comprise only one or a plurality of tread modules. The tread modules may be of a kind severable to form a sub-module as described in our aforementioned patent application. One or more sub-modules, with one or more whole tread modules, may be provided at a step position so as to provide an appropriate depth of tread surface for the particular angle of inclination of the stairway. Similarly, a horizontal walkway section may comprise a plurality of whole tread modules and one or more sub-modules. 
     For a walkway which is to extend over a roof or other support structure which lies at a significant angle of inclination the present invention teaches that primary support members may be positioned substantially parallel with and be supported by the inclined surface and that riser units are employed to enable tread modules to be supported in the substantially horizontal orientation, for example to be supported on one or more auxiliary supports. Similarly riser units may be employed to form a stairway that connects between two levels without the presence of an inclined surface for support of the primary support members. For forming a walkway that traverses horizontally over an inclined roof structure riser units may be employed to locate one of a pair of auxiliary support members at a position elevated above the roof surface. 
     The riser units may be of adjustable effective length and may be employed in the formation of a stairway assembly that does not necessarily incorporate support members having a grooved side face. 
     Thus in accordance with another aspect of the present invention there is provided a walkway assembly for movement of personnel over an inclined surface or between two levels, said walkway assembly comprising a pair of primary support members which lie side by side in spaced apart relationship and extend in parallel with one another in the direction of the length of the walkway, and a plurality of tread modules which each extend between and are supported relative to the primary support members, one edge of each tread module lying closer to one or each of the primary support members than an opposite edge of the tread module and said plurality of tread modules being supported by riser units each of which is of an articulated type comprising two leg sections the relative angle of which is selectively adjustable 
     The end of one leg section of each riser unit preferably is pivotally securable to a support member lying on and or supported by the support structure and the end of the other leg section is pivotally securable relative to the tread module, e.g. an auxiliary support member to which the tread module is secured. The riser unit preferably possesses the further feature that the other ends of the two leg sections comprise pivot positions whereby the leg sections may pivot relative to one another, and locking means may be provided to enable the two leg sections to be locked either parallel with one another or at a chosen angle of relative inclination whereby the effective length of the riser unit may be adjusted to that required to enable the or each tread module to be supported in a substantially horizontal orientation. 
     Optionally one or each of the leg sections may be provided with a plurality of pivot positions, such as the positions of pivot pin apertures, whereby for a given angle of relative inclination of the leg sections the effective length of the riser unit may be adjusted. 
     The locking means may comprise for example a retention pin or nut and bolt which extends through an aperture provided in one leg section at a position off-set from the pivot position in said other end of the one leg section and one of a series of apertures provided in said other end of the other of the two leg sections, said series of apertures comprising apertures each off-set from the pivot position of that leg section and said apertures of the series being circumferentially spaced from one another about said pivot position. In consequence by appropriate selection of that aperture of the series through which the locking means extends the selected effective length of the riser unit may be secured against further relative movement of the leg sections under the action of loads carried by the tread module(s). 
     The riser units may be of a type that limit the maximum angle of movement of tread modules away from the primary support member. 
     In the case of a stairway assembly module the riser units may be of a type which can adopt a first position in which they allow the outer, leading edge regions of one tread module to lie in an overlapped manner over the inner edge of a neighbouring tread module. 
     A tread module positioned on an inclined surface may be supported by an auxiliary support member of a kind having a side face with a grooved formation substantially as described herein for the aforementioned support members whereby the end of one leg section of a riser unit may be secured to that auxiliary support member. 
     For a walkway assembly in the form of a stairway the tread modules (and including any sub-modules) may each be supported by a pair of auxiliary support members one end of each of which is secured, e.g. pivotally, to a primary support member which lies substantially parallel with the surface of the inclined support structure (or extends between two levels) and the distal end region of which is secured to an end of said riser unit. 
     For a walkway assembly which is in the form of a substantially horizontally extending walkway that extends over an inclined support structure the riser units may be employed to support an outer auxiliary support member that extends parallel with the length of the walkway to support outer regions of tread modules. The inner ends of the tread modules may be supported by another auxiliary support member that serves as an inner support member to support regions of the tread modules nearest the inclined support structure. That inner auxiliary support member may be pivotally secured to support members which lie substantially parallel with the surface of the inclined support structure. 
     An auxiliary support may be secured to a primary support by pivot means that is attached to a surface of the primary or auxiliary support which is in a plane substantially parallel with a tread module surface or surface of a support structure. However, and especially if the primary and/or auxiliary support comprises a support member having a side face which comprises a groove formation, the pivot means may be secured to said side face. 
     For forming an installation comprising a substantially horizontally extending walkway and a stairway, and irrespective of whether the horizontal walkway and stairway are aligned in a common direction or, for example, extend at right angles relative to one another, a tread module of the stairway may be arranged to form a part of the walkway or a tread module of the walkway may be positioned to serve as an upper or lower tread of the stairway. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which:— 
         FIG. 1  is a perspective view of part of a walkway installation in accordance with the present invention; 
         FIG. 2  is a cross-sectional view of a support beam of the assembly of  FIG. 1 ; 
         FIGS. 3 a    &amp;  4   a  each show means for securing a support beam to a roof structure, and  FIGS. 3 b  and 4 b    show exploded views of features of  FIGS. 3 a  and 4 a   , respectively; 
         FIGS. 5 a  and 5 b    show means for connecting two support beams end to end,  FIG. 5 b    showing an exploded view of a feature of  FIG. 5   a;    
         FIGS. 6 a  and 6 b    show means for securing two support beams at right angles relative to one another,  FIG. 6 b    showing an exploded view of a feature of  FIG. 6   a;    
         FIG. 7  shows part of the stairway of  FIG. 1 ; 
         FIG. 8  shows in exploded form components of the stairway of  FIG. 7 ; 
         FIG. 9  is a perspective of a traverse walkway, and 
         FIG. 10  is an end view of another transverse walkway. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A walkway  10  (see  FIG. 1 ) for a support structure in the form of roof comprising a flat section and an inclined section comprises a horizontal walkway assembly module  11  and a stairway assembly module  12 . 
     The walkway and stairway assemblies  11 , 12  each comprise a pair of mutually parallel primary support members each in the form of a beam  13 , the two support beams lying side by side in spaced apart relationship. 
     Each beam  13  (see  FIG. 2 ) is an aluminium extrusion comprising an upper support surface  14  and a pair of side face walls  15  each of which is grooved such that in each side wall there is an opening  16  to a substantially C shaped retention channel  17  as viewed in transverse cross-section. 
     The channels  17  in each side face are of the same cross-sectional shape and comprise a pair of abutment surfaces  18  disposed respectively above and below the opening  16 . The cross-section of each channel departs from an exact C shape in that the channel has upper and lower recess regions  19  whereby, for a purpose described below, a rectangular-section plate may locate and be supported in upright manner in the channel. Additionally the base region  20  of each channel defines a recess that may, for example, accommodate the head of a bolt that extends through an inserted plate and outwards of the opening  16 , and may act to prevent rotation of the bolt head. 
     In the case of a roof or other support structure that comprises a rib formation, such as a standing seam, the beams may each be secured to the roof by clamps  25  (see  FIG. 3 ) comprising in cross-section a pair of limbs  26  which can be clamped together by bolts  27  to embrace the rib. One of the limbs has an integral flange formation  28  to which is secured a clip  29  having a hook shaped end  30  that fits in the beam opening  16  thereby to secure the beam against the flange formation when a nut  31  is tightened to draw the clip downwards towards the flange formation. 
     In the case of a roof structure having a surface to which a retainer bracket may be bolted, the channel of a beam may be used non-rotatably to locate a bolt  32   a  that extends outwards and through one limb  33   a  of an L shaped bracket  34  (see  FIG. 4 ) to enable the bracket to be bolted firmly to the beam. The other limb  33   b  of the bracket is formed with an opening to enable the bracket to be secured by a bolt or self-tapping screw  32   b  to the roof structure. 
     In addition to facilitating ease of attachment of a primary support beam to a roof structure the grooved side walls also facilitate ease of connection of two lengths of the beam in an end to end manner or to form a right angled interconnection, such as a T junction, between two lengths as shown respectively in  FIGS. 5 and 6 . In each of  FIGS. 5 and 6  a first beam section  35  has secured thereto a connector  36  or  37  by means of a bolt the head of which is non-rotatably secured in the channel recess  20  with the outer end of the bolt being accessible for location of a nut to enable the connector to be tightened against the beam side wall. In the case of  FIG. 5  the connector is in the form of a flat plate having two apertures whereby it may serve to interconnect between two bolts provided respectively in ends of the two lengths of beam. In the case of  FIG. 6  each connector is an L shaped right-angled bracket to enable a T section junction to be created. 
     Prior to or, or optionally, after securing the beam sections to a roof or other support structure tread modules  41  (see  FIG. 1 ) are secured to the upper surfaces  14  of the beams by means of self tapping screws which locate in recesses  38  in the tread modules. These tread modules may, for example, be of the kind described in our aforementioned co-pending UK patent application GB 0921366.1. 
     In this embodiment the angle of inclination of the stairway  12  is such that a single module  41  would not provide a suitable depth of tread. Thus at each step position there is provided a whole module and a one third sized sub-module section  40  which has been formed by cutting of a whole module. 
     The stairway assembly  12  comprises a pair of spaced apart beams  13  secured to the inclined surface of the roof structure in a manner as aforedescribed for the horizontal walkway assembly  11 . 
     For each tread position (see  FIG. 7 ) each beam has an auxiliary support beam  45  secured thereto. Each auxiliary beam is secured pivotally relative to a support beam  13  by means of a hinge  46  (see  FIGS. 7 and 8 ) the arms of which are secured by self-tapping screws respectively to an upper surface  14  of a beam and underside surface  47  of auxiliary beam  45 . Exposed ends  58  of each auxiliary beam  45  are covered by protective end caps  39  which are each an interference fit in the end of a beam. 
     The distal end  58  of each auxiliary beam  45  is supported by an articulated riser unit  48  comprising two leg sections in the form of pivotally interconnected arms  49 , 50  one,  49 , of which is pivotally connected and secured to the beam  13  and the other arm  50  being pivotally connected and secured to the distal end  58  of the auxiliary beam  45 . 
     The ends of the leg sections are secured by bolts  51 , 52  respectively to the support beams  13  and distal end regions  58  of the auxiliary beams  45 . 
     The leg sections are each pivotally interconnected by a nut and bolt assembly  53  that extends through pivot apertures  54 , 55 . 
     When the arms are at a chosen angle of relative inclination they are locked relative to one another by a lock pin  59  inserted through a second aperture  56  in one leg section and which is aligned with one of a series of apertures  57  provided in the other leg section and circumferentially spaced relative to the pivot aperture  54 . 
     Accordingly, the facility for varying the relative inclination of the leg sections of each riser unit, the position at which an end is secured to a support beam  13 , and, optionally, also to the auxiliary beam  45  provides for ease of on-site adjustment to ensure that the tread modules  41  are acceptably level. 
     The riser units may be employed also for the construction of a traverse type walkway for extending horizontally over an inclined roof surface as shown in  FIG. 9 . Tread modules  60  are secured to a pair of auxiliary support beams  61 , 62  one  61  of which is pivotally secured by hinges  63  relative to a roof mounted primary support beam  64  secured to a roof structure. The other auxiliary support beam  62  is maintained elevated relative to another roof mounted primary support beam  65  by riser units that correspond substantially with the aforedescribed riser units  48 . In this configuration the ends of each riser unit are not secured directly to the groove in a beam side wall. Instead end brackets  66  are employed each to pivotally connect with an end of a leg section and secure it to either the underside of beam  62  or upper face of beam  65 . 
     In an alternative construction for a traverse walkway module, see  FIG. 10 , U shaped brackets  70  are attached to the respective ends of each riser unit  48  and allow attachment to the grooved side walls of the beams  62 ,  65 . Also, instead of the aforedescribed hinge  63 , use is made of hinges  72  which are secured to the grooved side walls of the beams  61 ,  64 . In this and the aforedescribed embodiment the primary support beams  64 , 65  are each secured, in use, directly to a roof surface and additionally are maintained spaced apart by cross beams  73  having grooved side walls. That is, the cross beams form part of a prefabricated traverse walkway module.

Summary:
A walkway assembly ( 10 ) for attachment to a support structure such as a roof of a building includes a primary support ( 13 ) for on-site attachment to the support structure, a plurality of tread modules ( 41 ) each attached relative to the primary support and moveable between a first position and a second position at which a part of each tread module is spaced further from the primary support than when at the first position, and at least one riser unit ( 48 ) for supporting the tread modules relative to the primary support in at least the second of the first and second positions.