Stepped structure

A stepped structure (100) comprising a plurality of separate run portions (1), wherein at least one of said plurality of separate run portions (1) comprises upper and lower sheets (20) each sheet having a forward longitudinal end portion (14, 24) bent downwards and a rear longitudinal end portion (12, 22) bent upwards, and a core material (30) between said upper and lower sheets (10, 20).

This application is the U.S. national phase of International Application No. PCT/GB2009/002138, filed 7 Sep. 2009, which designated the U.S. and claims priority to GB Application No. 0816774.4, filed 12 Sep. 2008, the entire contents of each of which are hereby incorporated by reference.

The present invention relates to a stepped structure such as a stepped riser, perhaps a seating riser e.g. for a sports stadium or other entertainment venue.

To increase the revenue from sporting and other events, it is desirable to maximize the number of spectators that can be accommodated in a sports stadium or other venue. To do this it is necessary to provide additional tiers of seats, often resulting in structures in which a significant portion of the upper bowl seating cantilevers over other parts of the structure. Accordingly, the weight of risers supporting such seating should be minimized to reduce the size and cost of the supporting structure. To reduce transient and resonating vibrations associated with sporting and entertainment events the risers must be stiff, have sufficient mass, or be constructed with materials having good damping characteristics. Existing designs of seating risers are made of prestressed or precast concrete or steel. Known riser sections are generally constructed from concrete as it allows for long clear spans between rakers (typically 12,200 mm) with reasonable vibration control since concrete has a damping coefficient of 0.2, good fire resistance and relatively low maintenance cost. The major disadvantage of concrete construction is that the riser section is heavy, e.g. about 10 T for a two tier riser, with self weight (deadload) equal to the design superimposed live load due to use and occupancy. It is therefore necessary to provide heavier, stronger, stiffer and more costly superstructure and foundations to support the riser sections, especially for large cantilever seating sections.

To minimise self weight, and hence reduce the cost of the superstructure and foundations, the riser sections may be constructed with folded steel plates that are supported by intermediate rakers and a secondary steel framework. Typically the maximum span for this type of construction is approximately 6100 mm and the self weight about 40% of an equivalent concrete structure. However, steel risers are more susceptible to sound and vibration problems, having a damping coefficient of 0.1, and have additional costs associated with the fabrication and erection of the intermediate rakers and secondary steel framework.

Structural sandwich plate members are described in U.S. Pat. No. 5,778,813 and U.S. Pat. No. 6,050,208, which documents are hereby incorporated by reference, and comprise outer metal, e.g. steel, plates bonded together with an intermediate elastomer core, e.g. of unfoamed polyurethane. These sandwich plate systems (often referred to as SPS structures) may be used in many forms of construction to replace stiffened steel plates, formed steel plates, reinforced concrete or composite steel-concrete structures and greatly simplify the resultant structures, improving strength and structural performance (e.g. stiffness, damping characteristics) while saving weight. Further developments of these structural sandwich plate members are described in WO 01/32414, also incorporated hereby by reference. As described therein, foam forms or inserts may be incorporated in the core layer to reduce cost and/or weight and transverse metal shear plates may be added to improve stiffness.

According to the teachings of WO 01/32414 the foam forms can be either hollow or solid. Hollow forms generate a greater weight reduction and are therefore advantageous. The forms described in that document are not confined to being made of light weight foam material and can also be make of other materials such as wood or steel boxes, plastic extruded shapes and hollow plastic spheres.

GB 2,368,041 discloses a stepped riser comprising a sandwich structure having upper and lower metal plates and an intermediate layer of plastics or polymer materials bonded to the metal plates so as to transfer shear forces therebetween i.e. a SPS structure. The plates are pre bent into the desired stepped riser shape and welded together and then the intermediate layer is injected into the stepped riser shaped cavity between the two plates. The sandwich structure plates used in forming the stepped riser have increased stiffness as compared to steel plates of comparable thickness and avoid or reduce the need to provide stiffening elements. This results in a considerably simpler structure with fewer welds leading to both simplified manufacture and a reduction in the area vulnerable to fatigue or corrosion. However, the structure into which the elastomer is injected is bulky and complicated to assemble.

One aim of the present invention is to provide an improved structural member.

The present invention provides a stepped structure comprising a plurality of separate run portions, wherein at least one of said plurality of separate run portions comprises upper and lower sheets each sheet having a forward longitudinal end portion bent downwards and a rear longitudinal end portion bent upwards, and a core between said upper and lower sheets.

This significantly simplifies production of a stepped riser and assembly. Furthermore, the stepped structure can be made with only bends of approximately 90° (e.g. 90.6°) thereby allowing the stepped structure to be made without specialised bending equipment. The upper and lower sheets may be identical in profile so that a single sheet bending line may be used to fabricate both sheets. Furthermore, the number of welds needed to manufacture a stepped structure (optionally with the sandwich plate system (SPS)) is kept low. This not only reduces the cost of welding but also eliminates a potentially fatigue prone detail. Also, the present design thereby avoids greater potential for welding distortion. Furthermore, the individual elements from which the stepped structure is made relatively are easily transportable and a plurality of separate run portions can be stacked. Fixing together of the separate run portions and fixing to a frame work is also simplified. The separate run portions can be fabricated at a manufacturing plant and transported to the site for assembly.

The materials, dimensions and general properties of the sheets of metal and core of the invention may be chosen as desired for the particular use to which the stepped riser is to be put. In general they may be as described in U.S. Pat. No. 5,778,813 and U.S. Pat. No. 6,050,208 for the case that the core is of a polymer or plastics material. Steel or stainless steel is commonly used in thicknesses of 0.5 to 20 mm (preferably 3-5 mm) and aluminium may be used where light weight is desirable. Similarly, the core may be a plastics or polymer material which is preferably compact (i.e. not foamed) and may be any suitable material, for example an elastomer such as polyurethane, as described in U.S. Pat. No. 5,778,813 and U.S. Pat. No. 6,050,208. Lightweight forms or inserts may also be included as described in WO 01/32414. The first sheet of metal may be painted or have a different surface treatment applied to improve traction.

A stepped structure according to the present invention can be designed to meet relevant serviceability criteria and construction constraints related to vibration and deflection control, and plate handling. The resulting structure is light, stiff and, with the plastics or polymer material's inherent dampening characteristics, provides improved structural and vibration response performance over risers built with stiffened steel plates and rolled sections (secondary steel work) or those built with prestressed concrete.

FIRST EMBODIMENT

FIG. 1shows a cross-section in the transverse direction through a separate run portion1according to the present invention. The separate run portion1can be used for forming a stepped structure100(seeFIG. 2), for example a seating riser for use in a theatre or small stadium etc.

Typically a section of seating has a width of between 5 and 15 metres and is supported at each end by raker beams which can cantilever over other parts of the stadium. Seats are then placed on run portions1of the stepped structure. The run portions1are generally horizontal and steps between the run portions1are termed rise portions2which are generally vertical. The stepped structure can be assembled on site or can be pre-assembled partially or completely.

As can be seen fromFIG. 1in the structural element the separate run portion1(which is elongate in the longitudinal direction) is made of an upper sheet10and a lower sheet20. The upper and lower sheets10,20are comprised of first and second metal plates, preferably steel plates though other materials may be suitable. For example, the sheets10,20may be made of a fibre reinforced plastic or be made of a metal other than steel, for example aluminium.

The thickness of the upper and lower sheets10,20may be, for example, in the range of from 0.5 to 20 mm. Parts of the structure expected to experience wear in use may be formed with thick metal layers and/or surface profiling, e.g. to improve grip. Alternatively coatings may be used.

Between the upper and lower sheets10,20is a core30. The core30is preferably of plastics or polymer material, preferably a compact thermosetting material such as polyurethane elastomer, so as to form a structural plate member (SPS) which acts as the run portion or the tread of the structural member. The core30may be a concrete layer. The concrete layer may be normal concrete which typically weighs about 2400 kg/m3(e.g. between 2100 and 2700 kg/m3), but preferably light weight concrete which typically weighs about 1900 kg/m3(e.g. between 1200 and 2200 kg/m3), more preferably ultra light weight concrete that typically weighs about 1200 kg/m3or less (e.g. between 500 and 1200 kg/m3). The concrete may be of any type of cementitious material (e.g. cements such as Portland cement, fly ash, ground granulated blast furnace slags, limestone fines and silica fume). The core30is formed of a material which transfers shear forces between the upper and lower sheets10,20. The core3may have a thickness in the range of from 15 to 300 mm (preferably 15-30 mm, e.g. 20 mm) and is bonded to the upper and lower sheets10,20with sufficient strength and has sufficient mechanical properties to transfer shear forces expected in use between those sheets10,20. The bond strength between the core30and the sheets10,20should be greater than 3 MPa, preferably 6 MPa, and the modulus of elasticity of the core material should be greater than 200 MPa, preferably greater than 250 MPa, especially if expected to be exposed to high temperatures in use.

For low load applications, such as staircase risers, where the typical use and occupancy loads are of the order of 1.4 kPa to 7.2 kPa, the bond strength may be lower, e.g. approximately 0.5 MPa. By virtue of the core layer30, the structural sandwich plate member has a strength and load bearing capacity of a stiffened steel plate having a substantially greater plate thickness and significant additional stiffening.

To manufacture the structural member, the inner surfaces of sheets10,20are prepared, e.g. by acid etching and cleaning and/or grit blasting or any other suitable method, so that the surfaces are sufficiently clean to form a good bond to the core material.

The core material is preferably injected or vacuum filled into a cavity and then allowed to cure in the cavity. In order to manufacture the separate run portion1in this way, a cavity is formed between the sheets10,20by sealing longitudinal ends of the structural plate member (as is described below) and transverse edges of the structural plate member (for example by welding a face plate between the upper and lower sheets10,20or by placing or welding an edge bar60(seeFIG. 6) between the upper and lower sheets10,20at their transverse edges). Thus, a core cavity is formed between the upper and lower sheets10,20and core material can be injected into the core cavity by injection ports (not shown) either in the plates or the member attached at the transverse ends. Vent holes can be provided in any convenient position. Both vent holes and injection ports are preferably filled and ground flush after injection is completed. During injection and curing of the core material, the sheets10,20may need to be restrained to prevent buckling due to thermal expansion of the core caused by the heat of curing. Alternatively, especially for relatively small risers, the structural member may be put into a mould for injection of the core material. In fact, due to the geometry of the rise portion(s)2of the present invention and which are described below, buckling of the upper and lower sheets10,20during injection and curing of the core material is unlikely and this is a further advantage of the present invention.

Although not shown, spacers, light weight forms, shear plates and other inserts may be positioned in the core cavity before the upper and lower sheets10,20are fixed in place. Spacers are advantageous because they ensure that the spacing of the sections, and hence the core thickness, is uniform across the riser. Furthermore, other low density bulking materials may be used in the core material such as micro spheres and these help in keeping the weight of the structural member low and cost down. Detailing, such as seat and safety rail mounts may be welded or otherwise fixed onto the structural member as desired before injection or after curing of the core. In the latter case however, care must be taken to avoid damage to the core.

FIG. 1illustrates that the upper sheet10and lower sheet20of the separate run portion are bent at their longitudinal ends. That is, the upper and lower sheets10,20are formed of three portions. These are a rear longitudinal end portion12,22, a forward longitudinal end portion14,24and a central portion16,26. The central portion16,26is positioned between the rear longitudinal end portion12,22and the forward longitudinal end portion14,24.

The core30is generally only present between the upper and lower sheets10, adjacent to the central portion16,26. That is, the core30does not extend all the way along the transverse direction of the sheets10,20(though there may be some plastics or polymer material between the rear longitudinal end portions12,22and/or the forward longitudinal end portions14,24due to imperfect sealing between those two portions, as described below). The core30does not extend from one run portion1to another. That is, there is a break in the core30between adjacent separate run portions1, e.g. the core30is not continuous throughout the structure. Put another way, the core30is not continuous through the stepped structure. At least part of a or each rise portion2of the stepped structure does not comprise a core (of plastics or polymer (load bearing) material). The rise portion2is substantially core free and is substantially comprised of only plates, for example metal plates. The plates may be the rear longitudinal end portions12,22and forward longitudinal end portions14,24. The rear and forward longitudinal end portions12,22,14,24may have no core between them. In particular no core exists between rear longitudinal end portions12,22and forward longitudinal end portions14,24of adjacent run portions. A central portion of the rise portion2is core free.

As can be seen inFIG. 1, the rear longitudinal end portions12,22are generally perpendicular to the central portions16,26. Similarly, the forward longitudinal end portions14,24are generally perpendicular to the central portions16,26. The angles may not be exactly 90°, for example to allow the run portion1aslope of 1:100 downwards so that it can drain. The forward longitudinal end portions14,24are bent downwards from the central portions16,26. The rear longitudinal end portions12,22are bent upwards from the central portions16,26. The term “bent” does not necessarily mean that the sheet is formed into that shape by bending (though this may be the case, particularly if the sheets are made of metal), but it is used to indicate that the sheets are unitary (i.e. not formed by welding three plates together, for example). Therefore, if the sheets10,20are made from fibre reinforced plastic, for example, the sheets may originally be formed in the shape illustrated inFIG. 1and no actual physical bending may take place even though the end portions are bent upwards and downwards.

FIG. 1illustrates a separate run portion1. What is meant by the term “separate” is that the run portion is detached from other run portions and other components of the stepped structure. In particular, neither the upper sheet10nor the lower sheet20is used to form parts of a further run portion.

The rear longitudinal end portion12of the upper sheet10is substantially parallel to the rear longitudinal end portion22of the lower sheet20. Both rear longitudinal end portions12,22overlap. That is, a line which is perpendicular to the plane of both rear longitudinal end portions12,22will pass through both rear longitudinal end portions12,22. The same is true for the forward longitudinal end portions14,24.

The forward longitudinal end portions14,24and rear longitudinal end portions12,22are present for two main reasons. First those parts of the sheets10, are used to seal a cavity between the central portions16,26of the upper and lower sheets10,20which is then filled with core material30. In that case the core30may be injected into the cavity. However, this is not necessarily the case and a pre-cast slab of core could be adhered to the inner surfaces of the central portions16,26of the upper and lower sheets10,20. Second, the rear longitudinal end portions12,22and forward longitudinal end portions14,24can be used for fastening the separate run portion1to an adjacent separate run portion1. This can be done by using fasteners, for example screw fasteners or rivets. Alternatively this could be done by welding.

Two embodiments are illustrated inFIGS. 2 and 3as to how adjacent separate run portions1could be attached, though there are other ways in which this can be achieved. In this way the forward longitudinal end portions14,24and rear longitudinal end portions12,22form at least part of the rise portion2between adjacent separate run portions1.

As indicated above, a cavity is formed between the upper sheet10and lower sheet20which is substantially sealed from outside. At the longitudinal ends this is done by sealing between the rear longitudinal end portions12,22and by sealing between the forward longitudinal end portions14,24.FIG. 1illustrates one way in which this sealing is accomplished. Other ways in which the sealing may be accomplished are illustrated inFIGS. 4,5and9.

In the embodiment ofFIGS. 1 and 2(as well as that ofFIG. 3) the sealing is achieved by contact between the longitudinal end portions12,22,14,24. That is, the inner surfaces of the rear longitudinal end portions12,22touch and the inner surfaces of the forward longitudinal end portions14,24touch. By clamping the rear longitudinal end portions together and by clamping the forward longitudinal end portions14,24together, the sealing can be achieved. In the case where the core material30is injected on site, the clamping can be achieved by first assembling the stepped structure as illustrated inFIG. 2, prior to injecting. Optionally a weld may be made between longitudinal end portions12,22and14,24, in particular in the embodiments ofFIGS. 1,2,3and5. The welds may be made prior to injecting the core or after injecting the core. It is easiest if the two longitudinal end portions12,22and14,24are made different lengths so that a fillet weld may be used.

As can be seen, inFIG. 1, both the upper sheet10and lower sheet20have the same shape. That is, the lower sheet20is simply an upper sheet10turned the other way around. This has advantages in manufacture because then a single plate bending line may be used to fabricate both upper and lower sheets10,20. Furthermore, the fact that only bends of approximately 90° are necessary also means that manufacture is likely to be much simpler. Also, the sheets10,20can be stacked and easily transported to the site for assembly.

FIG. 2illustrates how a plurality of separate run portions1can be assembled to form a stepped structure100. Adjacent separate run portions1are fastened together. The adjacent separate run portions are fastened directly together (contrary to the embodiments ofFIGS. 3-5). Although inFIG. 2the fastening together is illustrated by way of bolts52,54, other ways of fastening could be used. For example, fastening could be way of rivets or by way of at least one weld. However, it is preferred to avoid the use of welding where possible in order to reduce production costs and time, as well as eliminating associated distortions. All welds are not necessarily eliminated however, as the cavities between the upper and lower sheets10,20need to be sealed at their transverse end portions. This is usually accompanied, as described above, by welding a face plate or an edge bar60between the upper and lower sheets10,20.

As can be seen inFIG. 2, the separate run portions1are fastened together via their longitudinal end portions12,14,22,24. That is, an upper or first separate run portion1ais attached to a lower or second separate run portion1bby connecting together at least one forward longitudinal end portion14,24of the upper separate run portion1ato at least one of the rear longitudinal end portions12,22of the lower separate run portion1b. In fact, the rear and forward longitudinal end portions are of different lengths so that an overlapping stepped joint can be formed between the upper separate run portion1aand lower separate run portion1b. In fact, at least one upper fastener52passes through both forward longitudinal end portions14,24of the upper separate run portion1aand only one rear longitudinal end portion22of the lower sheet20the lower separate run portion1b. At least one lower fastener54passes through both rear longitudinal end portions12,22of the lower separate run portion1band only one forward longitudinal end portion14of the upper sheet10of the upper separate run position1a. However, the opposite could also work. In the approach illustrated inFIG. 2however, the visible joint to the outside of the stepped structure is positioned close to the lower run portion1band this is preferred. Other attachment systems could be used.

The lower fastener54can conveniently be used to connect the stepped structure to a supporting beam50. All holes for fasteners may be punched on the production line.

An intumescent material may be positioned between the inner surfaces of the rear longitudinal end portions12,22and between the inner surfaces of the forward longitudinal end portions14,24. The use of an intumescent material can help seal off the cavity for the core30and can also help in fire prevention and in particular opening up or de-gassing of the cavity in fire situations.

The intumescent material may be on either side of the fasteners52,54. However, the intumescent material is preferably on the side of the fastener52,54nearer to the core30.

FIG. 3shows a second embodiment which is the same as the first embodiment except as described below. InFIG. 3the forward longitudinal end portions14,24are the same length. Similarly, the rear longitudinal end portions12,22also extend away from the central portions16,26by the same amount as each other. However, instead of being joined to the longitudinal end portions of the adjacent separate run portions, the forward longitudinal end portions14,24of the upper separate run portion1aare attached to the top of a plate40and the rear longitudinal end portions12,22of the lower separate run portion1bare attached to the lower side of the same plate40. Therefore, the plate can be seen as a rise plate40. Therefore, the rise portion2is made up of the forward longitudinal end portions14,24, the plate40and the rear longitudinal end portions12,22.

The fastening arrangement is the same as in the first embodiment, namely by an upper fastener52passing through both the forward longitudinal end portions14,24of the upper separate run portion1aand the plate40and a separate lower fastener54passing through both of the rear longitudinal end portions12,22of the lower separate run portion1band the plate40.

The sealing between the forward longitudinal end portions and between the rear longitudinal end portions is the same as in the first embodiment.

FIG. 4illustrates a third embodiment which is the same as the second embodiment except as described below. InFIG. 4a single sheet makes up both the upper sheet10and the lower sheet20. The sheet is bent over by 180° at a bent portion35which is at the end of the forward longitudinal end portions14,24or the rear longitudinal end portions12,22(both illustrated). In this embodiment only one of the longitudinal end portions can be sealed by the bent portion35. The other of the longitudinal end portions will need to be sealed by a different method. It is expected that the use of an intumescent material between the longitudinal end portions will not be as effective for a downwardly facing joint as for an upwardly facing joint. Therefore it is preferred that the bent portion35is at the ends of the forward longitudinal end portions14,24rather than at the ends of the rear longitudinal end portions12,22.

FIG. 5shows a fourth embodiment. TheFIG. 5embodiment is the same as theFIG. 3embodiment except as described below. However, in theFIG. 5embodiment, the rear longitudinal end portions12,22and the forward longitudinal end portions14,24are crimped together using a crimp37. This provides a better seal than simple bolting. The crimp may be on either side of the fasteners32.

Any of the ways of sealing of the above embodiments can be used with any other way. For example the forward longitudinal and portions may be sealed by a bent portion35and the rear longitudinal and portions may be sealed by a crimp37.

FIGS. 6-8illustrate how two stepped structures100may be joined together at their transverse ends (i.e. two stepped structures which are positioned next to each) other so that a run portion1of one stepped structure100is then continued by a run portion1of the other stepped structure.

FIG. 6illustrates how two adjacent run portions may be joined andFIGS. 7 and 8illustrate how two adjacent risers of different designs may be joined. In all cases, it is desirable to have a joint which is translatable thereby to be able to take up thermal expansions and contractions.

As is illustrated inFIG. 6, an edge bar60has been welded in place (using welds62) between the upper and lower sheets10,20along the transverse ends. The forward upper end edge and rear lower end edge of the edge bar may be machined in order that they fit into the curve of the bend of the upper and lower sheets10,20. The machining may be a simple 45° cut. The lower sheet20of the left hand run portion1ais shorter in the transverse direction than the upper sheet10. The outer end of the edge bar60is substantially level with the outer end of the upper sheet10(albeit leaving enough space for the weld62). Therefore the edge bar60at its bottom outer edge provides a landing surface for the bottom sheet20of the adjacent run portion. In the adjacent run portion1bthe lower sheet20protrudes further than the upper sheet10and the edge bar60has its outer end substantially co-plannar with the outer edge of the upper sheet10(albeit allowing for the weld62). Using this configuration the inner surface of the lower sheet20of the adjacent run portion16engages with the outer bottom surface of the edge bar60of the left hand run portion1a. Sealing between the two run portions1a,1bcan be done by the provision of a tube or rod64positioned between the two edge bars60. The tube or rod can be of silicone or other non-absorbent material. Placed on top of the rod or tube64is a fire resistant barrier66. The fire resistant barrier66can be applied in the form of a gel, for example, which then sets. This arrangement can provide both sealing properties as well as fire resistant properties.

FIG. 6also illustrates how the assembly may be attached to a frame100. Through holes80are machined through the edge bar60and upper and lower sheets10,20. A recess85for the head of a bolt90is also present in the top of the edge bar60. After a bolt90has been placed in the through hole80and attached to a frame100with a nut92, a cover plate110may be welded in place over the recess85so that the upper sheet10and cover plate110provide a continuous flat top surface.

For the sealing of adjacent rise portions, an overlap also needs to be engineered. In the case ofFIG. 7, each run portion2is comprised of a single plate. Therefore a backing bar70is welded to one of the plates so that an overlap between the two rise portions is present. The gap between the two rise plates2can then be filled with fire resistant sealant66as in theFIG. 6embodiment. However, in order to avoid adherence of the material66to the backing plate70, a bond breaker68is adhered to the backing plate70prior to filling in of the material66.

In theFIG. 8embodiment each riser is made of two plates. By making the edges of the plates different lengths, adjacent risers can be arranged to overlap. The same filling procedure with bond breaker can then be applied as in theFIG. 7embodiment.

FIG. 9illustrates a further embodiment as to how adjacent separate run portions1could be attached. The embodiments ofFIG. 9is the same as the embodiment ofFIG. 2except as described below.

InFIG. 9, the forward longitudinal end portion14of the upper sheet10is brought into contact with the rear longitudinal end portion22of the lower sheet20of the adjacent separate run portion1. Therefore bolts52can pass through only two plates and the run portion2is only two plates thick.

Additionally illustrated inFIG. 9is a further way in which the cavity is substantially sealed from outside. A gasket140may be positioned between rear longitudinal end portions12,22and/or forward longitudinal end portions14,24of a single separate run portion1. After positioning of the gaskets140in place, the core3may be injected into the cavity formed between the upper and lower plates10,20. In some circumstances it may be acceptable to have no further sealing (for example in indoor arenas or where the stepped structure is in a low stress situation). Alternatively after injection of the core3the upper and lower sheets10,20may be welded together at their longitudinal end portions12,22,14,24. Such a welded structure has improved stress performance and water tightness.

Additionally illustrated inFIG. 9are two possible positions of seat180which may be mounted on the stepped structure. As can be seen seats180may be fastened to the stepped structure through one or more brackets190. The brackets190may be attached to either a run portion1or a rise portion2. In this way a stepped structure according to the present invention can be used to provide tiered seating, for example in a sports stadium, a stadium of another kind, an arena, a theatre etc.

Materials

If the sheets10,20are made of metal and other metal parts of the structural member described above, are preferably made of structural steel, as mentioned above, though these may also be made with aluminium, stainless steel, galvanised steel or other structural alloys in applications where lightness, corrosion resistance or other specific properties are essential. The metal should preferably have a minimum yield strength of 240 MPa and an elongation of at least 10%.

The core material should have, once cured, a modulus of elasticity, E, of at least 200 MPa, preferably 275 MPa, at the maximum expected temperature in the environment in which the member is to be used. In civil applications this may be as high as 60° C.

The ductility of the core material at the lowest operating temperature must be greater than that of the metal layers, which is about 20%. A preferred value for the ductility of the core material at lowest operating temperature is 50%. The thermal coefficient of the core material must also be sufficiently close to that of the steel so that temperature variation across the expected operating range, and during welding, does not cause delamination. The extent by which the thermal coefficients of the two materials can differ will depend in part on the elasticity of the core material but it is believed that the thermal expansion coefficient of the core material may be about 10 times that of the metal layers. The coefficient of thermal expansion may be controlled by the addition of fillers.

The bond strength between the core and sheets must be at least 0.5, preferably 6, MPa over the entire operating range. This is preferably achieved by the inherent adhesiveness of the core material to metal but additional bond agents may be provided.

The core material is preferably a polymer or plastics material such as a polyurethane elastomer and may essentially comprise a polyol (e.g. polyester or polyether) together with an isocyanate or a di-isocyanate, a chain extender and a filler. The filler is provided, as necessary, to reduce the thermal coefficient of the intermediate layer, reduce its cost and otherwise control the physical properties of the elastomer. Further additives, e.g. to alter mechanical properties or other characteristics (e.g. adhesion and water or oil resistance), and fire retardants may also be included.

Whilst an embodiment of the invention has been described above, it should be appreciated that this is illustrative and not intended to be limitative of the scope of the invention, as defined in the appended claims. In particular, the dimensions given are intended as guides and not to be prescriptive. Also, the present invention has been exemplified by description of a seating riser but it will be appreciated that the present invention is applicable to other forms of stepped structure.