Patent Grant
8832980
1. Field of the Invention
Embodiments of the present invention relate to structural systems or structures comprising a flexurally deformed panel.
2. Description of Related Art
Structural systems involving more than one panel connected together are commonplace, for example folded plate roofs, boxes, etc. Connecting two originally planar elements together, one of which is substantially deformed, is also known. For example, corrugated paper or card comprises a sheet of plane paper or card which is deformed by means of pressure, heat and water content (but not flexural stress) into a corrugated shape, for example of sinusoidal cross-section, and is then adhered by gluelines to one or two plane sheets of paper or card. However, in the case of corrugated paper or card, the corrugated element is typically deformed in a material state and under conditions such that, were it not attached to the one or more planar sheets, it would still be corrugated in repose. Corrugated plastic constructions, such as Correx® a trademark of Kaysersberg Plastics, a part of D S Smith (UK) Ltd. are made by extrusion, not flexural deformation of the core.
Tied members which are deformed within the elastic range are also known, for example the common bow for projecting arrows, which typically comprises a substantially linear member of wood or a laminate of several materials, which is flexurally deformed and tied at each end by the string of the bow.
Point-of-purchase display devices are also known in which a substantially vertical filmic display is tensioned by one or more bowed linear prop members, typically fixed to and flexed between a heavy base, to which the bottom of the display film is also attached, and a cross-member at the top of the display panel. The bowed prop members are made slightly longer than the display film and are flexurally deformed to induce tension in the display film to keep it flat or plane. A heavy base is required for lateral stability of these systems.
Panels flexed and restrained between two points of a relatively very rigid member are also known, for example, flexed acrylic or other plastic sheets within some light fittings.
British Patent Application No. 8510775 “Constructional Member of Variable Geometry” (Hill and Higgins) discloses substantially linear members comprising interlocked, substantially linear components that can be flexurally deformed and fixed in their deformed geometry by means of discrete mechanical fixings.
In the field of building structures, tied arches and vaults are known, as are flitch beams, slabs, arches and vaults with prestressed ties. Curved and tied building structures are disclosed in U.S. 16,767, U.S. Pat. No. 1,687,850, U.S. Pat. No. 1,762,363, U.S. Pat. No. 1,963,060, U.S. Pat. No. 2,237,226, U.S. Pat. No. 2,287,370, U.S. Pat. No. 2,360,285, U.S. Pat. No. 3,057,119, U.S. Pat. No. 4,536,997 and U.S. Pat. No. 5,595,203.
U.S. Pat. No. 4,865,111 and U.S. Pat. No. 4,979,554 disclose flexed, tied panels forming the ends of a display system.
U.S. Pat. No. 6,311,709 discloses a self-erecting, collapsible fabric dome structure comprising resiliently flexible wire.
U.S. Pat. No. 5,313,666 discloses a flexed panel facial sunshield apparatus.
U.S. Pat. No. 2,160,724 and U.S. Pat. No. 2,862,322 both disclose opaque displays in an assembly comprising an opaque curved card element and a plane element which is “D” shaped on plan, to provide a stable display assembly. The curved and plane components are connected by means of folded card tabs, which will inevitably open up in use and cause reduction of any tension in the plane element. U.S. Pat. No. 5,619,816 discloses a flexed display assembly with string ties. U.S. Pat. No. 6,276,084 and U.S. Pat. No. 6,772,069 disclose means of restraining a display panel in a curved shape.
U.S. Pat. No. 4,749,011 and U.S. Pat. No. 6,065,512 disclose a flexed panel for opening up a flexible bag.
Zips to join two pieces of plastic together are known. U.S. Pat. No. 6,540,085 (Davies) discloses plastic zips comprising teeth attached to side panels and a sliding connector, the side panels typically being heat bonded to a plastic film material being joined.
According to one or more embodiments of the present invention, an assembly comprises a panel, a membrane tie, and a linear connector, the panel being flexurally deformed from an initial geometry and restrained in a flexurally deformed geometry by the membrane tie and the linear connector.
According to one or more embodiments of the present invention, an assembly comprises:
a panel;
a tie; and
a display sign, said display sign being applied to or forming a part of said panel and/or said tie, said display sign facing in one display direction, said panel being flexurally deformed in one direction of curvature from an initial geometry and restrained in a flexurally deformed geometry by the tie, and wherein the one direction of curvature of the panel is reversible to form a reverse flexed panel assembly such that said display sign or another display sign is facing in said one display direction.
According to one or more embodiments of the present invention, an assembly typically comprises:
a panel;
a membrane tie; and
a pull-apart linear connector, the panel being flexurally deformed in one direction of curvature from an initial geometry and restrained in a flexurally deformed geometry by the membrane tie and the pull-apart linear connector, and
wherein the one direction of curvature of the panel is reversible to form a reverse flexed panel assembly.
According to one or more embodiments of the present invention, there is a method of reversing the curvature of a panel within an assembly, said assembly typically comprising a flexurally deformed panel and a tie, said method comprising the steps of:
Embodiments of the invention can have many different geometric forms and many different practical applications. Assemblies may be relatively large, for example demountable and reusable shelters or flat-pack point-of-purchase display assemblies, or may be relatively small, for example a photograph or postcard display system, or extremely small, for example an element of a small spring mechanism.
Components of one or more embodiments of the invention typically are packable and transportable flat, to be assembled remote from the point of manufacture.
A “panel” typically has two plane parallel surfaces and is relatively thin in relation to its overall size. The thickness or minimum dimension of a panel is typically less than one tenth and preferably less than one twentieth and more preferably less than one fiftieth and even more preferably less than one hundredth and even more preferably less than five thousandths of its overall length. Panels are typically semi-rigid in that they may be flexurally deformed through an angle of at least 10° and preferably through 20° and more preferably 90° and even more preferably 180° within the short term, substantially elastic range of the panel parent material or composite material, such that they will substantially regain their original geometry if released immediately after flexure. Panel materials have a stress/strain curve with a substantially elastic range, such as steel, or are materials which ‘creep’ with time under load, such as plastic materials which exhibit nonlinear viscoelastic behavior of creep and/or relaxation upon sustained loading. Panels may be of any shape, for example square, rectangular, triangular, circular, petal shaped (sometimes referred to as petaloid or petalate) or any free-form, irregular shape. A panel is optionally of uniform thickness or tapered or otherwise of varying thickness throughout its area. Panel materials are optionally grossly deformed in the initial geometry, for example by the creation of “plastic hinges” in which a material is locally deformed beyond its elastic range, in some materials referred to as folds or creases, before the initially grossly deformed panel is flexurally deformed within its substantially elastic range according to one or more embodiments of the invention. A panel optionally is of initial single or double (bi-axial) curvature before being flexurally deformed. Such panels are pre-folded or pre-curved in their initial geometry, in order to achieve the desired final, flexurally deformed geometry. Examples of panel materials, typically semi-rigid sheets, for example of plastics materials, are acrylic, polycarbonate, polyester, copolyester, acetate, polyvinyl chloride (PVC) or composite materials, for example glass fibre reinforced or carbon fibre reinforced plastics or resins, or metals, for example steel, stainless steel or aluminum, rubber, rubber compounds, synthetic rubber such as neoprene, or laminates, for example paper or card laminated to a single plastic laminating film or encapsulated by two plastic laminating films, for example of polyethelene, polyester, polypropylene, nylon or pvc, for example either cold-laminated using pressure-sensitive adhesive or hot-laminated using heat-activated adhesive, or so-called “stressed skin” panels comprising two outer layers and an inner cellular or foamic cores, for example aluminum stressed skin panels as used in aircraft construction, or natural materials or processed natural materials, for example timber boards, plywood or chipboard. Optionally, the panel member is of substantially greater flexural stiffness than the membrane tie member. Panels are optionally opaque, translucent or transparent or partially transparent and/or partially translucent, for example see-through graphic panels according to U.S. RE37,186 or U.S. Pat. No. 6,212,805. A panel can typically support its own weight on one edge. A panel according to one or more embodiments of the present invention is capable of being flexurally deformed in opposite directions. Typically, this reversible flexure of the panel is to overcome the effects of viscoelastic creep and/or stress relaxation behavior over time, which reduces the bending and tensile stresses in the panel and tie respectively, thereby reducing the structural performance of an assembly. Upon dismantling an assembly, there is typically residual curvature in one direction and the panel is typically flexed in the opposite direction in a “reverse flexed panel assembly”.
A “reversible panel” is a panel, the direction of curvature of which can be reversed in successive functional assemblies. Many examples of reversed panel functional assemblies are given in the figures and their descriptions herein. A “reversible panel edge stiffener” provides a stiffening restraint to a tied edge to a panel and optionally an increased compression capacity of a tied edge to resist a compressive force applied in line with or parallel to the tied edge and provides a “reverse flexed panel assembly” of the same geometry as the initial assembly geometry.
A “reverse flexed panel assembly” is an assembly in which a panel has been flexed in the opposite direction to its direction of curvature in the immediately preceding construction of an assembly comprising the same or a different tie member.
A “tie” is a tensile member of an assembly which restrains a flexed panel in a flexurally deformed, curved state.
A “linear tie” is a tie that is linear in form, for example a wire, rod, cable, spun twine, string, thread or rope or a monofilament or a bound cluster of rods or monofilaments. A linear tie typically connects two spaced apart points on a panel.
A “membrane tie” is a tie in the form of a membrane, for example a flexible plastic film material, for example of polyester, copolyester, acrylic, polycarbonate, PVC or polyethylene, or a thin sheet of metal, for example of steel, stainless steel or aluminum, or a thin sheet of plywood or paper or card or a fabric, including woven and non-woven fabric, or a laminate, for example paper or card encapsulated by two plastic films, for example of polyester, polypropylene, nylon or pvc, either cold-laminated using pressure-sensitive adhesive or hot-laminated using heat-activated adhesive. Membrane tie members are optionally nets or grids, such as square, triangular, hexagonal or other reticulated nets, or perforated materials, for example perforated steel, aluminum or plastic materials, the perforations being optionally punch-perforated or laser-perforated. A membrane tie typically connects two spaced apart straight or curved lines or loci on a panel.
Membrane ties are optionally of super elastic materials, for example rubber elastic or wound elastic material or elasticated fabric material, for example to create assemblies with large deformation and restitution capabilities. Membrane ties are optionally of hybrid construction, for example filmic ties may have cable or fiber reinforcing elements within them and/or around their perimeter, to add strength where required. Linear elements, for example open rings of cable, are optionally used to distribute the load in membrane ties, for example at discrete connection points to a panel, where there are points of stress concentration. The term “membrane tie” also includes an array of linear elements. A linear element includes a rod, for example of steel or plastic, a cable, such as a steel cable, wire, a rope, string, a monofilament, for example a polyester filament, or a spun natural or artificial fiber, for example thread, twine or a polyester multi-filament fiber. Linear elements of a membrane tie are preferably spaced at less than twenty times the thickness of the panel. Membrane ties are optionally plane, which may be referred to as planar ties, or be curved in one direction, of so-called single curvature, for example as a single curve or, as another example, in a multiple curve, for example in the form of a sinusoidal wave in cross-section, the primary tie function (direction of tensile stress) typically being perpendicular to such curvature or membrane ties are optionally of double or biaxial curvature. Membrane ties are optionally opaque, translucent or transparent, or partially transparent or translucent, for example vision control panels according to U.S. RE37,186 or U.S. Pat. No. 6,212,805.
A “tubular membrane tie” is a type of membrane tie and is typically a flexible membrane in the form of a tube which surrounds the outer surface of the flexurally deformed panel. Typically, the tubular membrane tie is more flexible than the panel.
A “web tie” is typically a membrane tie which is connected to a single continuous curved line or locus in a flexed panel. Optionally, the web tie is more flexible that the panel.
Definitions related to flexibility vary in different arts. Stiffness can be regarded as the inverse of flexibility. For the purpose of this invention, the Flexural Stiffness at one end of an elastic member of uniform cross-section which is pin-jointed at both ends:
Flexural Stiffness=EI/L
where E is the Modulus of Elasticity
I is the second moment of area (Moment of Inertia)
L is the effective length
The Flexural Rigidity of a member cross-section is considered to be:
Flexural Rigidity=EI
where h is the width and t is the thickness of the member.
Typical values for the Modulus of Elasticity (kN/mm2) of some of the materials which may be used for the present invention are:
Preferably the Flexural Rigidity of a membrane tie is less than the Flexural Rigidity of the panel, more preferably less than one hundredth of the Flexural Rigidity of the panel and even more preferably less than one thousandth of the Flexural Rigidity; of the panel.
A “linear connector” typically connects a side or edge of a panel to a side or edge of a membrane tie. The term “linear connector” includes an adhesive layer or “glueline”, a weld or a pre-formed element, for example of plastics or metal, for example an extruded aluminum or plastics “profiled section” or a cold-formed steel section or any novel or known mechanical fixing such as a piano hinge, restraints utilizing friction, or interlocking closure systems, such as VELCRO®, a trademark of Velcro Industries B.V. or Dual Lock™ a trademark of 3M, and zips of any type. In order to connect a semi-rigid sheet of plastic to a plastic film by means of a zip, a transition tape or intermediate tape between the semi-rigid sheet and the side panel of the zip is typically required. The transition tape can be bonded by heat-activated adhesive, pressure-sensitive adhesive or solvent adhesive. Some connection details will be described which have been devised specifically for one or more embodiments of the invention. A linear connector may comprise frictional, magnetic or electrostatic force. A linear connector is optionally discontinuous, for example a plurality of discrete areas of adhesive material, or a layer of adhesive material with a plurality of discrete areas of adhesive material, or a layer of adhesive material with a plurality of areas without adhesive material, a line of discrete spot welds or rivets. The term “linear connector” includes a cable, for example in a ring or loop, which distributes localised stress, for example of the connection of a membrane tie to a corner of a panel. Preferably the linear connector has a direct bond to an elongate area of the panel and/or an elongate area of a membrane tie, the bond for example being provided by a weld or an adhesive layer, a magnetic force or an electrostatic force. Preferably, the direct bond covers an elongate area substantially parallel to an edge of the panel and/or membrane tie, of a width preferably not less than 3 mm and more preferably not less than 10 mm. Optionally, the linear connector is transparent, for example of extruded polycarbonate. A tubular membrane tie can be considered to have a frictional linear connector between a part of its surface and the corresponding adjacent concave surface of the flexed panel, or be considered not to comprise a linear connector but to restrain the panel by means of “wrap-around” tension in the tubular membrane tie.
A “point connector”, sometimes referred to as a “node connector” or “nodal connector” is a connector at a point at which a linear tie is connected to a panel, for example a button or washer, for example of metal, plastic or rubber, or a tied knot or toggle at the end of a linear connector made of string or a screwthread and nut at the end of a rod linear connector. Optionally, a tie connects two points or loci on a panel indirectly, via a “spaced connector” or “spacing element” in an assembly.
A “pull-apart connector” comprises a substantially continuous linear connector that enables a panel and a membrane tie to be separated by only using pulling apart forces in substantially opposing directions. For example, pull-apart linear connectors include the types of linear connectors illustrated in
The ease or degree of reversibility of the direction of flexure of a panel can be classified and sub-classified as, for example:
A “transparent material” in the context of this invention is “water clear” or tinted and allows through vision such that:
The connection of the panel to the tie preferably approximates to what is referred to in the art of structural engineering as a pinned joint or pinned connection, having a bending moment resistance approximating to or tending towards zero.
In a first embodiment of the invention, a rectangular, plane panel, for example a semi-rigid acrylic sheet, is flexurally deformed about one axis and the two opposite edges parallel to this axis are spaced apart and connected by a linear tie member. For example, a semi-rigid acrylic sheet is tied by means of a flexible string with a toggle at each end threaded through a hole adjacent to the centre of each opposing edge of the panel. The resultant structural assembly is dimensionally stable, for example if placed on a horizontal support surface with one of the flexurally curved edges resting on the horizontal support surface, or with the four corners of the panel resting on individual supports or a horizontal support surface. The direction of curvature of the panel is easily reversible and retied by the same string by removing one toggle from one hole, pulling it through to react against the other side of the panel by the other hole, reversing the flexure of the panel and pushing the other toggle through the one hole.
In a second embodiment of the invention, a rectangular, plane panel, for example a semi-rigid acrylic sheet, is flexurally deformed about one axis and the two opposite edges parallel to this axis are spaced apart and connected by a membrane tie member. For example, a semi-rigid acrylic sheet is flexed and tied by a polyester film material, typically of much lower flexural stiffness than the panel. The panel and the membrane tie are typically connected by a linear connector, for example an adhesive layer between the plastic sheet and the plastic film along the two opposite sides. Alternatively, for example, the flexurally deformed or “flexed” panel is a plywood sheet flexed and then tied by another, typically thinner, plywood sheet. In the case of the plywood assembly, for example, a steel angle is connected by screws or gluelines to the plywood panel and the plywood membrane tie. The resultant structural assemblies are dimensionally stable, for example if placed on a horizontal support surface with one of the flexurally curved edges resting on the horizontal support surface, or with the four corners of the panel resting on individual supports or a horizontal support surface. Alternatively, the four corners of such an assembly can be supported on four elevated level supports. For example, the plywood assembly forms a novel form of tied barrel vault roof, an efficient structural roofing system, especially if the open ends of the structure are closed by a “shear diaphragm” stiffening members, for example of further sheets of plywood, which help to maintain the dimensional stability of the structure upon subsequent “dead loading” of any other constructional materials or “live loading”, for example of people on the roof formed by the tied, flexurally deformed panel. The direction of curvature is reversible, the ease of reversibility depending on the nature of the linear connector or connectors.
In a third embodiment of the invention, a rectangular plane panel, for example a semi-rigid acrylic sheet, is flexurally deformed about one axis and a tubular membrane tie surrounds the flexed panel and maintains the panel's flexurally deformed geometry by tension in the tubular membrane tie. The tubular membrane tie preferably extends beyond the edges of the flexed panel and is optionally sealed at one end, for example in the form of a bag, or is optionally sealed at both ends.
In a fourth embodiment of the invention, a rectangular, plane panel, for example a semi-rigid acrylic sheet, is flexurally deformed about one axis and a web tie in the form of a membrane is typically connected along the length of at least one of the curved edges or alternatively is connected along another curved locus of the panel. The web tie is in tension to retain the panel in its desired geometry.
Such structural assemblies may be referred to as “tied, flexurally deformed panel” or “tied, flexed panel” structures. They also may be referred to as “flat-pack, curved structures”. A principal advantage of one or more embodiments of the invention is that the structural assembly is typically fabricated from planar and optionally linear components which can be easily manufactured and subsequently processed, for example printed with a design. The components can be packaged flat or rolled, and can be transported more easily and economically than 3 dimensional structural members that are pre-formed (for example cast concrete structures or conventional steelwork structural members) and can be assembled temporarily, semi-permanently or permanently at sites remote from the component manufacturing site or sites. One or more temporary or semi-permanent embodiments of the invention can be designed to be dismantled easily and re-used or be transported conveniently to recycling or waste disposal centers. Optionally, the assembly is intended to be efficiently stored flat in one location and used occasionally in that one location, for example a podium or display assembly used for occasional public events held in the one location, to be dismantled and stored flat between such events.
The flexed panel or panels and tensioned membrane tie or tie members combine to provide a structural assembly that is typically more stable and has more load-bearing capability than the individual members or the same elements combined in their non-flexed or non-tensioned state. The direction of flexure of a panel is reversible in a reverse flexed panel assembly of typically greater stability and load-bearing capability then the initially constructed assembly.
Panels are typically plane before being flexed for the first time within an assembly and typically have sufficiently high in-plane tensile strength so as not to be conformable to accommodate double curvature. However, a variety of geometric shapes can be achieved by single curvature of plane panels, for example a variety of single curves or repetitive or varied wave shapes can be achieved, as well as a variety of “shell” structures.
Transparent panels and tie membranes are used, for example, to make transparent or partially transparent display assemblies with no independent framing or other such obstruction to through vision. Such assemblies are, in particular, suited to support or comprise one-way vision or other see-through vision control panels, for example as disclosed in U.S. RE37,186 or U.S. Pat. No. 6,212,805. Optionally, linear connector or connectors are also transparent, for example comprising transparent gluelines or transparent profiled sections, for example of clear, extruded polycarbonate.
Assemblies according to one or more embodiments of the present invention are optionally designed to be of variable geometry, typically by enabling the tie member or members to be altered in length, for example by means of tie rods that can be varied in length, for example by means of a turnbuckle, or wound elastic tie members that can be further wound or un-wound. The capability to amend the geometry of an assembly has many potential benefits, for example from minor adjustments to accommodate tolerances or errors in building construction, to substantial changes in geometry, for example to amend the effective area of a tied, flexed panel, for example acting as a sail on a boat or wind-powered electricity generating device.
Assemblies according to one or more embodiments of the present invention are optionally extremely flexible, to allow substantial deflection under load, such deflection being reversible if both the panel and tie elements are not loaded beyond their short-term elastic range. In structural engineering terms, assemblies according to one or more embodiments of the invention typically have a very high Coefficient of Restitution after short-term loading, even those incorporating viscoelastic, plastic materials. A membrane tie member optionally performs a rebound or trampoline function, taking advantage of the stored energy and elastic deformation capability of a suitably designed assembly according to one or more embodiments of the invention. Such properties are useful in the manufacture of many products, from very small spring assemblies to sprung platforms, for example as may be used in “bouncy castles”. One or more embodiments of the invention are optionally used to create energy through changing, repeated flexure of a panel and tensile strain of a membrane tie member, for examples if an embodiment of the invention comprises materials which create an electric current upon flexure, for example buoys at sea are capable of being illuminated by wave action upon an assembly of an embodiment of the invention comprising such flexurally activated material.
Additional elements are optionally used to adapt a tied, flexed panel assembly. For example, further ties or infill material such as flexible foam are used to make a tied, flexed panel assembly into a shock absorbing structure. While most tied, flexed panel structures will be designed to perform within their short-term elastic range, they are optionally designed to ‘fail’, for example by the creation of plastic hinges in a panel, as part of an impact absorption system, for example on a vehicle or as ‘buffers’ or in safety or security barriers.
Assemblies according to one or more embodiments of the present invention are optionally combined “tiled” or otherwise used together, for example a canopy structure can be replicated to produce a building or canopy of a larger size within a required maximum roof profile height.
The ability to use lightweight materials and transport components flat or in roll form means one or more embodiments of the invention can be efficiently packaged and transported by air, sea or land to remote locations and assembled to fulfil needs on a temporary or permanent basis, for example enclosures or other protective structures against sun, wind, sand, precipitation or other natural elements.
Depending primarily on the size of panel member, the flexural deformation of the panel is achieved by purely manual means or requires mechanical means of deforming the panel before being tied to form a stable, tied, flexed panel assembly. For example, temporary clamps can be applied to a panel or holes, slots or recesses may be formed in a panel to enable temporary ties to pull the panel into an “intermediate panel geometry” before attaching the more permanent tie member(s). Optional mechanical assistance in deforming panels includes, for example, scissor mechanisms or a ratchet cable device, typically lever operated, for example a Tirfor™ “grip hoist” by the Tractel Group, USA. Scissor mechanisms, akin to a scissor lift, typically comprise two parallel members which can be moved towards or away from each other but which typically maintain the parallel relationship of the panel sides being drawn together. Flexure is optionally achieved by means of one or more tie straps, which are placed around the panel, initial deflection induced manually or, for example, by a friction buckle or ratchet device, the straps being successively tightened until the required intermediate panel geometry is obtained. After fixing the tie in place and applying any linear or point connector or connectors, the panel is released, transferring the tensile force to the membrane tie, then any temporary restraints are removed, to leave the finished tied, flexurally deformed assembly.
Optionally, clamps enable an eccentric tie force to be applied to the panel, for example by means of a cable, to initiate and then complete flexure. Flexural deformation is optionally assisted by the provision of a temporary framework or jig to restrain the panel in an “intermediate panel geometry”. The final tied, flexurally deformed geometry results from the linear, membrane or web tie member taking up its tension force, typically allowing some “relaxation” from the “intermediate panel geometry” into the “tied, flexurally deformed panel geometry” of the finished assembly.
In some embodiments, some initial and/or intermediate flexural deformation is optionally achieved by differential heating or cooling of the two principal surfaces of the panel.
An assembly optionally comprises a means of edge stiffening, for example the edge of the panel being permanently deformed, for example by an acrylic panel subjected to hot wire bending, or one or more stiffening members being inserted into the assembly.
Assemblies optionally comprise both a linear tie and a membrane tie or optionally comprise a linear tie, a membrane tie and a web tie or optionally comprise a membrane tie and a web tie. For example, a simple enclosure comprises a flexed acrylic sheet tied by a membrane tie, for example of polyethene acting also as a ground sheet, with end panels optionally acting as web ties, for example of canvas or woven polyester, the web ties optionally reinforced by a linear tie, for example of nylon rope sewn into a bottom edge seam of the web tie and connected to corners of the panel.
Temporary enclosures manufactured according to one or more embodiments of the invention have a number of potential advantages over prior art enclosures, for example purely fabric tent enclosures, for example in providing a sheltered observation post with clarity of vision through a transparent flexed panel, for example a clear, transparent polycarbonate sheet. Conversely, vision into the shelter can be a desirable benefit, for example for security reasons, by the human eye or camera. Panel or membrane tie members of the assembly optionally comprise so-called vision control products, for example one-way vision products, for example as disclosed in U.S. RE37,186, for example if a good view out of an enclosure is required in conjunction with obscuration of vision into the enclosure.
Assemblies according to one or more embodiments of the present invention encompass a wide range of size, from large building structures, down to very small scale structures, for example panels of less than 1 mm overall width contained within tubes of less than 1 mm diameter, for example to form a mass of low density, high porosity, sprung elements, for example as an energy absorbing medium.
Additional and/or alternative advantages and salient features of one or more embodiments of the invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, disclose preferred embodiments of the invention.
All the figures are diagrammatic, not to scale and typically not in the correct proportion of thickness of members in relation to their overall dimensions. In numbering the figures, the suffix letter characters I, O, II and OO have been omitted. Referring now to the drawings which form a part of this original disclosure:
FIG. 2AA is a perspective of an assembly comprising a flexed panel with a concave curved edge and a membrane tie display panel.
FIG. 2BB is a perspective of an assembly comprising a flexed panel with a concave curved edge which has been reverse flexed and another membrane tie display panel.
FIG. 2CC is a plan of a panel with two inwardly curved edges.
FIG. 2DD is a perspective of an assembly comprising a panel with two inwardly curved edges.
FIG. 2EE is a perspective of a reverse flexed panel assembly comprising a panel with two inwardly curved edges.
FIG. 2FF is a perspective of an assembly comprising a panel with two outwardly curved edges.
FIG. 2GG is a perspective of a reverse flexed panel assembly comprising a panel with two outwardly curved edges.
FIG. 2HH is a plan of an assembly with a removable display sign and a backing insert.
FIG. 2JJ is a plan of a reverse flexed panel assembly with a removable display sign and a backing insert.
FIG. 2KK is a plan of an assembly with two removable display signs and a backing insert.
FIG. 2LL is a plan of a reverse flexed panel assembly with two removable display signs and a backing insert.
FIG. 2MM is a plan of a panel comprising three legs.
FIG. 2NN is a perspective of an assembly comprising a flexed panel comprising three legs.
FIG. 2PP is a perspective of a reverse flexed assembly comprising a flexed panel comprising three legs.
FIGS. 2QQ and 2RR are perspectives of an assembly with a membrane tie of less width than the connected edges of the panel.
FIGS. 13AA and 13BB are cross-sections through a reversible panel edge stiffener.
FIG. 13CC is a perspective of an assembly comprising a tubular membrane tie.
FIG. 13DD is a cross-section through an assembly with an elastic tape tensioning device.
FIG. 13EE is a perspective of an assembly with an elastic tape tensioning device.
FIG. 13FF is a cross-section through a split tube reversible panel edge stiffener.
Instead of a continuous membrane, the membrane tie may be an array of linear members or a net or a perforated material. In such assemblies the linear connectors 60 may comprise a series of point connectors, discrete elements such as lacing loops attached to the panel edges or holes near the panel edges, reinforced or otherwise, which connect to the tie member or members.
Display messages can be changed in other ways, for example an independent display panel 13, for example a printed piece of paper or card, as illustrated in
Such display units according to one or more embodiments of the invention typically use much less plastic material than prior art plastic display units, for example hot wire formed acrylic display holders typically comprising a continuous piece of acrylic sheet bent to form a base portion and two vertical or sloping portions between which paper or card displays are inserted. The amount of plastic used in various embodiments of the invention can be as little as one quarter or less of that used in hot wire formed prior art units for the same size of display panel, for example as illustrated in prior art
FIGS. 2CC and DD illustrate an assembly comprising panel 10 which has two opposing edges curved inwards, for example to assist access to goods displayed within a retail display embodiment of the assembly, for example jewelry. FIG. 2EE is a reverse flexed panel assembly of FIG. 2DD, as indicated by the reversal of principal surfaces 35 and 36. FIG. 2FF illustrates a panel 10 in an assembly in which two opposing edges of the panel 10 are bowed outwards, for example, in a shelter embodiment to provide better rain protection over the area of the membrane tie 24, for example which also acts as a ground sheet and/or waterproof membrane for the enclosure. FIG. 2GG is the reverse flexed panel assembly of FIG. 2FF, such reversal being undertaken regularly, for example every four months for a polycarbonate panel 10, to overcome the effects of viscoelastic relaxation of the flexural stresses in panel 10 with elapsed time. FIG. 2HH illustrates changeable independent display sign 13 trapped between membrane tie 24 and backing insert 110 having stiffening edges 14 to maintain it in place. FIG. 2JJ illustrates a reverse flexed panel assembly of FIG. 2HH. FIG. 2KK illustrates another variant in which the assembly of FIG. 2HH has another independent display panel 13 inserted behind flexurally deformed panel 10 and FIG. 2LL illustrates the reverse flexed panel assembly of FIG. 2KK. FIG. 2MM illustrates a panel 10 with three feet 51 which, in the tied, flexurally deformed assembly of FIG. 2NN, assist the stability of the assembly on an uneven surface. FIG. 2PP is the reversed flexed panel assembly of FIG. 2NN.
FIG. 2QQ illustrates another example of a display in which membrane tie 24 only extends over part of the length of opposing edges to flexurally deformed panel 10, for example showing a discrete display design 81 on a transparent membrane tie 24 comprising membrane tie display sign 26 enabling a background second display design 82 to be visible through the transparent portions of membrane tie 24, for example to show a subject design 81 in a three-dimensional relationship with background design 82 or a brand logo 81 in front of a brand lifestyle image 82 on principal surface 35 of panel 10. Another brand lifestyle image 82 can be visible from the other side, principal surface 36, panel 10 being reversible as illustrated in FIG. 2RR.
Some practical embodiments of the invention comprise panels and/or membrane ties with transparent plastic laminating film 41 to protect a paper or card display panel, laminated to one or preferably both sides of a paper or card display panel 13, as illustrated in
Another embodiment of the invention does not comprise a separate linear connector but a panel is restrained in its flexurally deformed geometry within a tubular membrane tie. The tubular membrane tie is plane and in tension between two remote edges of the panel. The term tubular membrane tie includes a tube of seamed or seamless flexible material, for example a plastic film or a fabric or a net or a perforated film material. The tubular membrane tie has two ends and preferably the panel is located entirely within the length of the tubular membrane between the two open ends of the tubular membrane. Optionally, one end of the tubular membrane is sealed to form a bag and, optionally, the other end of the tubular membrane is also sealed, for example for packaging a product. The tubular membrane or bag is sealed, for example by adhesive, hot welding or a manual or mechanical sealing device, for example InnoSeal, supplied by InnoSeal Systems, Inc. US.
Some other embodiments of the invention use flexible film bags as a tubular membrane tie. A panel is flexed to an intermediate panel geometry, to enable it to be inserted into the bag, whereupon it is released to press against the inside of the bag in its intended flexurally deformed panel geometry, maintaining the bag in an open condition, prior to any required filling and optional sealing of the bag. Preferably, part of the open end of the bag extends beyond the extremities of the panel to maintain the bag in a substantially fixed geometry and reduce the likelihood of the top of the bag slipping down the panel.
These embodiments having a tubular tie have many practical applications, for example in the display system of
The improved windsock of
It is often advantageous to accurately align one position in any cross-section of a tubular membrane tie, for example a sewn seam in a fabric tube or an adhesive or welded seam in a filmic tube, to a position in the flexed panel, for example one of the opposing edges restrained by the tubular membrane tie.
FIGS. 13AA, 13BB and 13CC illustrate example reversible panel edge stiffeners 112 which provide compressive strength along the tied edges of panel 10. Tubular membrane tie 27, for example of fabric, while tensioned perpendicular to the tied edges, may exhibit a complementary tendency to bunch up in the direction parallel to the tied edges. Optionally the membrane tie is restrained at each end of the tied edges, for example by means of discrete fixing 115, for example of two sided self-adhesive tape or a hook seen as attached to the tubular membrane tie 27 and engaged into the ends of a reversible panel edge stiffener 102, for example a split plastic tube 113, as illustrated in FIG. 13AA, or a profile stiffener 114, for example of extruded plastic or aluminum, as shown in FIG. 13BB. This arrangement stretches the fabric along the length of the tied panel edges, as illustrated in
A linear tie 22 is optionally in the form of a linear tie loop 132, for example in a rectangular configuration of
FIG. 24AA illustrates a rigid or semi-rigid plastic profile section 60 with flexible plastic strip 78 to provide additional frictional resistance to the edge of a panel sliding out of the hook, for example if an assembly suffered impact, and pressure-sensitive adhesive 63 with removable protective liner 65. FIG. 24BB is a similar profile but with a flexible plastic section 78, suitable for sewing to membrane tie 24 with thread 123. FIG. 24CC illustrates a rigid or semi-rigid linear connector 60 with internal sloping flexible “barbs” or “wands” of material 78 to apply lateral pressure and a frictional force to the edge of the panel to be inserted, as illustrated in FIG. 24DD. FIG. 24EE is similar to FIG. 24BB but demonstrates the adaptability of the open linear connector to a different angle of intersection of panel 10 with membrane tie 24.
While some embodiments of the invention are easily assembled manually, others, especially larger embodiments, optionally benefit from the use of jigs and/or mechanical devices to assist assembly. For example, the sequence of assembly shown in
Following assembly, the structural performance of particular embodiments vary depending on their component sizes, their tied, flexurally deformed geometry, their material composition and with time, unless both the panel and the membrane tie are only stressed within their elastic range and continue to be so during the serviceable life of the assembly, for example in the case of suitably stress-limited steel panels and steel membrane ties. Natural materials, such as timber or timber-based products will “creep”, in other words continue to deflect under self weight or “dead loading” and “imposed loading”. Elastic materials have a capacity to store mechanical energy with no dissipation of the energy. A viscous fluid has a capacity for dissipating energy and none for storing it. Viscoelastic materials, such as plastics, are between these two extremes, having a capacity to both store and dissipate mechanical energy. They typically will exhibit viscoelastic behavior of creep under sustained load and/or stress relaxation if restrained in a stressed condition under constant strain. Viscoelastic materials respond in a manner which is dependent on time, upon the magnitude of the initial stress regime and any subsequent amendment of imposed stresses, for example externally applied loading or amended internal stresses, for example by reduction in tensile force in a tie member through creep of a panel within an assembly according to one or more embodiments of the present invention. On the application of subsequent stress regimes, for example the reversal of curvature and thereby flexural stress, the material response is not only determined by the current state of stress but is also determined by past states of stress. The material can be said to have a “memory” of all past states of stress. Similarly, if a deformation is being imposed, for example by a given tie length in a reverse flexed assembly, the resultant stresses depends on the entire past history of deformation. Boltzmann's principle of superposition applies. For example, reversal of flexure from a residual flexed curvature requires greater flexure (change of curvature at all points in the panel), resulting in greater bending stresses in the panel and a greater tensile force in the tie than in the previous construction of the assembly with the same length of tie member. In assemblies which creep and/or relax, the induced bending stresses in the flexurally deformed panel and the tensile force in the membrane tie will decrease. Assemblies according to one or more embodiments of the present invention typically have substantially better structural performance in the resistance of loads, for example in the resistance of vertical or lateral imposed loads, for example from accidental impact, than similar structures without pre-stress. For example, regarding the maintenance of desired geometry, for example membrane tie graphic displays which are required to be maintained in a plane (flat) state, structures according to one or more embodiments of the present invention with their pre-stressed component parts will perform this function far better than similar components pre-formed to the same geometry but not pre-stressed. However, these benefits of a tied, flexed panel assembly reduce with creep or relaxation of any plastic components. The extent of such creep and/or stress relaxation can be measured over time, for example by the use of prior art strain and deflection gauges. Referring to
M=T×H
where M is the bending moment at any point in the panel at height H above the membrane tie and T is the tensile force in the membrane tie, providing there is an effectively pinned connection at the position of the linear connector 60 between the panel 10 and membrane tie 24, as would be provided by many of the linear connectors illustrated in
However, there is great difficulty using the currently available means for structural analysis in pre-determining the tensile force in a membrane tie and therefore the bending moments and the shape of the curve along the length of a panel of an assembly for any given sizes and material properties of a panel and membrane tie. Most theories of structural design and the resultant analysis methods and their computational means rely on assumptions developed for the design of traditional structures, for example for buildings, bridges, etc in which it is desired to restrict the amount of deflection of the overall structure and individual elements for serviceability reasons, for example which typically restrict the maximum deflection of a beam to the span divided by 250. The traditional “beam theory” for the design of conventional structures relies on a number of assumptions which are not satisfied by a typical assembly according to one or more embodiments of the present invention, in which the deflection of the panel is grossly in excess of these assumptions, even the simplest assembly comprising materials which are maintained within their elastic range.
While some methods of analysis can theoretically be applied to any structure, for example finite element analysis, there are assumptions and requirements of such methods that do not ideally lend these methods to such grossly deformed, relatively thin elements. For example, individual elements within a finite element analysis are conventionally not elongated but, for example, comprise a fine triangulated grid with individual triangles having sides of not dissimilar size. In seeking to predict the behavior of a typical panel according to one or more embodiments of the present invention, for example a panel 1 meter long by 1 mm thick, or 10 meters length by 6 mm thickness, hundreds if not thousands of elements along the length of the panel would typically be required if a sufficiently fine grid is provided across the thickness of the panel to enable adequate analysis of resultant stresses.
There is no prior art in the field of structural engineering concerning the flexure of thin panels to induce tension in another structural element, in order to produce a stable, serviceable structural assembly. There is no established means of predicting the performance of such structures, as there has been no prior requirement. One of the reasons such structures have not been devised and used in the past may be because there is no accepted means of reliably predicting their performance by calculation.
These problems of analysis and predicting the performance of assemblies of various embodiments of the invention are even more complicated when plastic materials are incorporated, for example panel sheets of acrylic, polycarbonate or pvc, and/or membrane tie films of polyester or pvc. Creep of one element is interactive with the stresses in the other element or elements of the assembly and the problems of calculation already discussed are greatly worsened by the need for successive or iterative calculations predicting the resultant stresses in any point in time in the life-span of the assembly structure, which are continually changing with time in use. As one example, the opposite edges of a flexed plastic panel which are connected by an elasticated fabric membrane tie will creep inwards, changing the geometry of the assembly and reducing the tensile force in the elasticated fabric membrane tie. For some uses of one or more embodiments of the invention, for example small displays, for example table top displays of postcards or photographs, appropriate member sizes can be relatively easily established by testing, and various embodiments of the invention have been reduced to practice in many such cases, for example as previously described in relation to
Perhaps the nearest practical problem in the art of structural engineering that has been considered from an analytical standpoint is the performance of thin steel plates in compression following buckling, in order to seek to establish the residual strength of a buckled plate with its subsequent gross deformation, for example in considering safety in a resultant collapse mode of a structure. However, the ultimate deflected form of such structures typically involves plastic hinge mechanisms which are not typically achieved in structures according to one or more embodiments of the invention under any anticipated loading condition, and in such prior art analyses, lateral deflection of a failed plate in compression is not important, per se, only its residual strength (for example see: “The Stability of Flat Plates”, P. S Bulson. Pages 406-423). In summary, there is no proven method for reliably predicting the initial stresses within and the subsequent behavior of assemblies according to one or more embodiments of the present invention and any logical approaches to solving the problem are in the realms of very advanced theoretical structural analysis.
Adopting the following nomenclature:
Considering purely elastic behavior, looking at the bending of a rectangular panel that is subjected to a transverse load and assuming that the material stays in the elastic state for large deflections, the deflection of an element of the panel is given by a differential equation that is similar to the deflection of a bent beam. Consider a panel of uniform thickness t and take the xy plane as the middle of the panel and the width of the panel being denoted by h. As in ordinary theory of beams, it can be assumed that the cross-sections of the panel remain plane during bending, so that it undergoes only rotation with respect to the neutral axis.
The curvature of the deflection curve is given in Equation 1, assuming the deflection w is small compared to the length of the beam (which is not the case with typical panels according to one or more embodiments of the present invention).
The lateral strain, εy, must be zero in order to maintain continuity in the panel during bending, from which it follows that the elastic strain, εx, and stress, σx, is given by Equation 2 and Equation 3.
Knowing the applied force P or bending moment M on the panel, the curvature of the bended plate is Equation 4 where EI is the flexural rigidity of the panel.
In the above, it has been assumed that the panel is bent by lateral loads only. If in addition to lateral loads there are forces acting on the middle plane of the panel, these must be considered in deriving the corresponding differential equation of the deflection surface. Timoshenko and Woinowsky proposed the differential equation in
Equation 5 for the deflection of a beam where q is the intensity of a continuous distributed load and Nx, Ny and Nxy are the normal forces per unit length in an element of the panel. X and Y are body forces acting in the middle plane of the panel or are tangential forces distributed over the surfaces of the panel.
Equation 5 is simplified when the boundary conditions are known. Even in the simplest of cases this equation is non-linear and not easily solved. The use of numerical methods such as finite differences has been proposed to solve the non-linear differential equations.
According to “beam theory”, the panel can be assumed to be a cantilever beam of length l, width h and thickness t, as proposed by Timoshenko. Using this assumption, the equations proposed by Bisshop and Drucker (Quarterly of Applied Mathematics, V 3(3), pp 272-275) for the large deflection of cantilever beams can be used to determine the curvature, deflection and horizontal displacement.
The derivation is based on the Bernoulli-Euler theorem, which states that the curvature is proportional to the bending moment (Equation 4). For wide beams, as considered in this case, the flexural rigidity is given by Equation 6.
The curvature of the beam is expressed in terms of the arc length s and slope angle φ in Equation 7. This equation leads to an elliptic integral that can be split up into complete and incomplete elliptic integrals of the first and second kind. In the notation of Jahnke and Emde, the relation for deflection δ and beam length l are given in Equation 8.
With the application of boundary conditions, the horizontal displacement of the loaded end of the beam is calculated with Equation 9 with φ0 the initial slope angle of the beam.
Separately, theoretical curves of an end loaded pillar with pin-jointed ends under progressive axial loading are illustrated in
Considering plastic behavior, in any particular loaded beam, if the load system is increased gradually, yielding would first occur at the extreme fibres of the weakest section in relation to its resultant bending moment. These fibres are then said to be in plastic state and further increase in loading will bring about a considerable increase in strain at this weakest section of the beam, with a redistribution of stress. When the whole cross-section at any point in a structure becomes plastic, no further increase in the moment of resistance is possible without excessive strain and a “plastic hinge” has been developed. So-called “work hardening” can subsequently result in increased moment of resistance.
The main aim is to calculate the bending moment required to form a plastic hinge for any particular cross-section and to determine the distribution of bending moment along the beam at the collapse load. The assumptions made in calculations are:
The fully plastic moment is calculated with Equation 10 and the moment at first yield with Equation 11
The analytical calculations of deflections within the plastic region are uncertain at this stage and the use of numerical computation is suggested to determine the deflection of beams/plates when the material is within the plastic region. Equation 10 and Equation 11 gives an indication at what magnitude of loads plasticity will occur in the material.
In numerical modelling, plasticity theory provides a mathematical relationship that characterizes the elasto-plastic response of materials. There are three ingredients in the rate-independent plasticity theory: the yield criterion, flow rule and the hardening rule.
Numerical modelling is a novel method of applying engineering calculations to almost any engineering problem, be that of a structural, thermal, fluid, electromagnetic, etc. of nature or a combination of these fields. Numerical modelling has proved to be reliable in non-linear problems where the nonlinearities are introduced due to a change of status (contact), geometry (large deflections) and material nonlinearities (stress-strain curves).
The problem of large deflection of beams/plates will include geometrical and material nonlinearities. ANSYS (computer software owned by ANSYS, Inc., a US corporation), employs the “Newton-Raphson” approach to solve nonlinear problems. In this approach, the load is subdivided into a series of load increments. The load increments can be applied over several load steps.
A square panel has been modelled using beam elements. The models looked at the deflection and stress distribution of the panel in the Elastic state and then in the Plastic state. The effect of Creep on the stress relaxation and deformation of the initial curve has also been investigated.
For an Elastic analysis the material is assumed to be pure elastic and does not go into a plastic state no matter the amount of deflection. This type of analysis tends to over-predict the stress and strain calculations when the stresses go above the yield limit of the material. An Elastic analysis is the most basic structural analysis and is good for initial models due to the relatively quick calculations.
In a Plastic analysis the yield stress limit and tangent modulus of the plastic region needs to be specified. For an elastic-perfect plastic material a tangent modulus of 0 is specified and the stress results will not exceed the yield stress. A specified tangent modulus introduces a work hardening effect into the material.
The model consists of a beam with boundary conditions applied to the ends of the beam so that the one end (End 1) is free to move in the vertical direction and the other end (End 2) is free to move in the horizontal direction. End 1 is given a very small vertical displacement to initiate the direction of the desired curvature of the beam. End 2 is then given a large horizontal displacement inwards (towards the beam). This action results in the large deflection of the beam and represents a symmetrical model of a panel that has buckled under axial loads.
Creep is simply the time-dependent deformation of solids under stress. Many equations have been proposed for the calculation of creep strain. It needs to be emphasized that all the many equations proposed for creep can only be given some justification if the right material and test conditions are selected. Creep strain equations can be temperature and stress dependent.
Finite Element Modelling is capable of dealing with creep by using a constitutive law of creep that will be in a form in which the rate of creep strain is defined as some function of stress and total creep strain, β in Equation 12. Various functions for β exist for different material types, stress values and temperature dependence. Different functions also exist for the different stages of the creep: primary and secondary stages.
In conclusion, this brief survey into analytical solutions of beams and plates undergoing large strain deflections indicate that solutions do exist but require a high level of mathematical skills to calculate the deflection and curvature of a panel for given boundary conditions with any degree of accuracy acceptable for commercial use.
Numerical modelling appears to be successful in determining the deflection of the panels. It also has the advantages of calculating stresses, strain, axial forces, bending moments, etc and the application of non-linear material properties such as plasticity, creep and viscoelasticity.
Viscoelasticity is important because in any given assembly in use, although subject to creep, the relationship M=T×H will still apply and substantial deflections within the panel will not typically occur in use, other than to accommodate the reduction in length of the membrane tie owing to the reduction of T. However, plastic materials will continue to suffer substantial reduction in bending stresses with consequent reductions in T by virtue of molecular level restructuring of the plastic material as it “relaxes” under continued flexure without substantial change in overall curvature or shape.
However, one aspect of many embodiments of the present invention is that the effects of creep degradation of the structural performance can be mitigated and even taken advantage of, by reversing the direction of the panel flexure. Referring to
The direction of curvature of a plastic panel 10, shown in its initial geometry in
the Coefficient of Restitution=(H1−H2)/H1
where H1 is the height deformation of the panel in its tied, flexurally deformed panel geometry 6, and H2 is the height deformation following release after creep or viscoelastic “relaxation”. This Coefficient of Restitution will be less the longer the time the assembly remains unreleased. However, a major advantage of one or more embodiments of the present invention is that the viscoelastic creep and relaxation reduction in stresses in the assembly can be countered by reversing the direction of flexure and curvature in the panel, as indicated by the reversal of first panel side 35 and second panel side 36 from the orientation shown in
Aspects of the above review of structural analysis relevant to various embodiments of the invention have been tested in a number of ways, using both elastic panel materials and homogeneous (unlaminated) and laminated viscoelastic panel materials.
The theoretical flexurally deformed geometry of
A range of steel rulers and hacksaw blades representing a panel according to an embodiment of the invention have been tested with vertical, initially axial load applied at the top end, then with progressively imposed vertical and lateral deflection by applied vertical loading (representing tie tension) measured by a spring weighing machine supporting the lower end. It has been found that only a relatively small increase in load is required to achieve substantial deflection of the panel within the elastic range (full restitution being achieved upon release of the panel).
It was concluded that adoption of the Euler Buckling Load of the panel with an appropriate factor of safety for the intended use of the structure would provide a safe and pragmatic approach to design and selection of an appropriate tie member for most of the anticipated embodiments of the invention.
Viscoelastic behavior was assessed by imposing deflections on plastic panels and releasing the panels after elapsed time periods, measuring the residual panel geometry after release, calculating Coefficients of Restitution and finally assessing the implications of reversing panel curvature in reverse flexed panel assemblies. Tests were also undertaken on a laminated paper panel with an array of highly elasticated linear ties, up to a maximum of 20 of such elastic ties. These ties were calibrated so that tie forces could be reasonably accurately assessed with progressive reduction in length of the ties owing to creep deflection of the tied edges of the panel (inwards) with time, over elapsed time periods varying from 24 hours to 3 months. Tests on a laminated paper panel with a length of panel of 280 mm indicated tension forces in the range of 1-2 N (one-two Newton).
While there is potential for much further research into the structural behavior of embodiments of the invention, the following pragmatic guidelines are offered as a result of these theoretical and practical assessments.
1. The deflected form or flexurally deformed geometry of a rectangular, uniform-sectioned panel, with end conditions approximating to theoretical pinned connections, will be substantially the same for all panel sizes and panel materials maintained within their elastic range for any given length of tie member (distance between opposing edges of the panel) and crown (lateral) deflection. The geometry of any infill end panels, bases and other associated elements of actual embodiments can therefore be accurately predicted.
2. The Euler Buckling Load of the panel represents a conservative guide for the tensile load to be designed for in the tie member, while adopting an appropriate factor of safety for the intended application, for example a load factor of 1.5 for display embodiments or 3.0 for roof canopy embodiments.
3. A conservative guide for the tie load in a reverse flexed panel assembly with a viscoelastic panel is that obtained in item 2 amended proportionally to the amount of residual deflection at the time of re-assembly. From
4. Significant, measurable structural performance benefits result in an assembly according to an embodiment of the invention by using a panel with a residual curvature from a previous assembly and flexurally deforming the panel in the opposite direction, if the residual panel geometry exhibits a Coefficient or Restitution of less than 0.9, more preferably less than 0.7, and even more preferably less than 0.5
5. With appropriate analysis and/or practical experience, it is possible to advise owners of assemblies of one or more embodiments of the invention of a recommended elapsed time when the panel should be reversed in flexure to maintain adequate structural performance for the particular embodiment. For example, an assembly could be recommended for reversal of stress after one month or, as another example, after three months from the initial assembly or any previously reverse flexed panel assembly
Embodiments of one or more embodiments of the invention comprising transparent panels and/or transparent membrane ties have many advantages. For example, displays comprising a frameless, clear plastic curved panel supporting a photograph enable the photograph to be illuminated from the rear, for example if located on a window cill, which adds impact and improved perception of the image in the manner of a backlit transparency. Secondly, it is a well-known phenomenon that a conventional, prior art frame surrounding a photograph, a realistic painting or other conventional picture has a negative effect on the perception of the 3-dimensional nature of subject matter in a 2-dimensional image. So-called “cues” to perceiving depth, for example relative size (greater in the foreground), linear perspective (leading to “vanishing points”), color hue (towards the blue end of the spectrum) in the distance) and intensity (stronger in the foreground) are all over-ridden or diminished by a frame which the brain “interprets” as the perimeter of a plane or 2-dimensional image. Prior art transparent framing systems have been developed to overcome this phenomenon, having arrays of dots in two different planes, for example on the front and rear of a frame cut from acrylic sheet, the resulting interference pattern offering the visual perception or illusion of the frame being in a substantially different plane to the framed image, to allow the 3-dimensional cues to be interpreted better by the observer's brain. An observer of a photograph or other image displayed by means of one or more embodiments of the present invention, without a frame and with only transparent means of support behind it, is able to interpret all such 3-dimensional cues without any prior art frame or any opaque means of support visible from any angle detracting from that perceived image. In the case of a postcard or other display with writing or other image on the reverse side, these reverse images are visible through a transparent panel and, in the case of writing or printed text, legible from the other side, which is not the case with conventional, prior art display systems providing an equivalent degree of structural stability.
The same advantages of transparent panels and/or membrane ties and/or linear connectors apply to larger displays, for example floor-mounted displays in a retail environment, as well as one or more embodiments of the invention enabling a cleaner, uncluttered, visual impression than conventional, prior art framing systems. In the case of semi-transparent displays, for example see-through graphics panels according to U.S. RE37,186 or U.S. Pat. No. 6,212,805, there is an added benefit, in that there is little or no visual obstruction to the ambience and security safety aspects of the retail, exhibition or other environment surrounding the display.
However, there is no transparent material that can be flexed to the extent required to create a stable, pre-stressed structure of one or more embodiments of the present invention that does not exhibit viscoelastic creep and/or relaxation behavior. If it is required to design an assembly of reliably predictable performance over an extended lifespan, very advanced methods of structural analysis are required, preferably including for reversible curvature of the panel where appropriate.
The foregoing description is included to illustrate the operation of the preferred embodiments and is not meant to limit the scope of the invention. To the contrary, those skilled in the art should appreciate that varieties may be constructed and employed without departing from the scope of the invention, aspects of which are recited by the claims appended hereto.
This application is the U.S. National Phase of PCT/IB2007/003620, filed Aug. 21, 2007, which in turn claims the benefit of PCT/IB2006/003667, filed Aug. 21, 2006, and U.S. Provisional Patent Application Ser. No. 60/831,306, titled “STRUCTURAL ASSEMBLY WITH A FLEXED, TIED PANEL” filed Feb. 23, 2007, the entire contents of all of which are incorporated by reference herein.
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