FIELD OF THE INVENTION The present invention relates to structural steel decking and in particular to structural steel decking constructed from profiled steel on which concrete is poured to form a composite slab.

BACKGROUND OF THE INVENTION Light-gauge steel decking is used as permanent formwork in the construction of composite slabs. During all stages of construction it must support the construction live loads and stacked materials applied or placed on its top face. Its profile or cross-sectional shape has a significant bearing on the behaviour of the steel decking prior to the concrete it ultimately supports hardening.

Trapezoidal steel decking is characterised by ribs having webs inclined to the horizontal at less than or equal to 90 degrees as shown in Fig. l(a). In contrast, re-entrant ribs have webs that are inclined at an angle greater than 90 degrees (see Fig. l(b)). Re-entrant ribs interlock with the hardened concrete.

Trapezoidal decking is nestable, which is a significant advantage for minimising the volume of a pack of sheets for transportation from the factory to a building site. However, under the pressure from sheets above, lower sheets spread laterally apart and can have significantly reduced overall depth and increased overall width when removed from the pack. This causes problems with installation on site, and may result in reduced strength and safety.

Before the sheets can be fixed down they must be laid out on the supports. Certain types of supports, e.g. hollow masonry or light-gauge steel walls, or

even solid concrete walls, are difficult to fix to and this may even not be possible. Spreading may cause particular problems for construction workers, who can also damage and distort the decking simply by walking on it. Transporting pieces of construction equipment across the decking poses even greater problems. Sheets can also become misaligned if the spreading is uneven between the ends of the decking (as it normally is). They may also cause problems fitting between fixed boundaries of the building if individual panels are wider than allowed for.

As a result of these problems, fixing down the ends of the sheeting at every rib is common practice when using trapezoidal decks in steel-frame buildings - see Fig. 2 in which the arrows 25 indicate fixing points. However, this slows down construction and may otherwise be avoidable, e.g. in a protected location where wind uplift is not a problem and the sheets cannot otherwise be dislodged, fixing may otherwise be unnecessary.

Re-entrant decks do not experience significant problems with spreading of the steel ribs, and therefore having to fasten down the sheeting ends to permanent or temporary supports is often averted.

The load-carrying capacity of trapezoidal decks can be significantly reduced if they are not fixed down. Again, this is not normally the case for decks with reentrant steel ribs. In long-spanning applications using deep trapezoidal decking, end diaphragms must be pre-fitted to the steel beams to prevent spreading of the decking and premature collapse. These diaphragms must also resist the lateral pressure of the full depth of the concrete over the steel beams, and therefore are substantial and costly.

A common problem experienced using decks with open steel ribs, whether the profile is trapezoidal or re-entrant, is for concrete to leak through them while being poured. Various means have been devised to limit the amount this occurs. For economics, end diaphragms need to be multi-functional and serve this purpose too.

Several other conventional means for sealing the ends of the steel ribs are known, for instance, tapes and foams may be used manually on site to seal the ends. However, this and other solutions do not strengthen the ends of the decking structurally. They also tend to be labour intensive and costly too.

Various means by which the ends of a trapezoidal deck can be both sealed and strengthened by preventing spreading are also known. For instance, plywood can be cut to the profile of the steel decking and can be supported off the formwork used to construct the soffit of a bandbeam.

When trapezoidal decking is laid out on supports and not fixed down, under load it will spread laterally. Moreover, in the complete absence of lateral restraint (slight friction excepted) the decking can be completely flattened under relatively light loading compared to what is expected during construction, particularly when the concrete is poured. Such flattening is obviously highly undesirable, and the gross deformations lead to an almost complete loss of bending and shear capacity when taken to the extreme.

Lateral restraint must be provided to the decking panels to avoid such flattening occurring at least at the supports. When multiple panels are laid down alongside each other, they laterally restrain each other to some degree, but there is always still the issue of edge panels. Fixing of every rib of every panel, as shown in Fig. 2, becomes essential.

Once the very weak lateral spreading failure mode is suppressed by providing lateral restraint, web crippling failure at ends comes into play and can take various forms. It occurs in decks with trapezoidal or re-entrant steel ribs.

It is an object of the invention to provide an improved trapezoidal steel decking for supporting construction loads and materials including wet concrete and subsequently longitudinally reinforcing the concrete after it has hardened.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a structural steel decking panel for supporting construction loads and materials including wet concrete and subsequently longitudinally reinforcing the concrete after it has hardened, the decking panel including: a first longitudinally extending rib; a second longitudinally extending rib spaced apart from and parallel to the first rib; an elongate pan joining the first and second ribs and forming a base, each rib having a top portion and a pair of web portions, the web portions diverging as they extend down from the top portion to the base; and first and second end zones, each end zone extending from and formed from a corresponding said longitudinally extending rib, each end zone comprising two opposed web end portions joined by an inclined portion sloping downwards towards the base.

Preferably each web end portion and its corresponding said web portion lies substantially within a single web plane.

Preferably each inclined portion has a pair of divergent longitudinal edges, the longitudinal edges formed by a Z-fold.

Preferably each longitudinal edge lies substantially within the web plane, whereby the panel is nestable with like panels.

Preferably the ribs have a terminal squash zone adjoining the end zone, whereby the top and web portions of the rib are squashed to all lie adjacent and substantially within, or parallel to a squash plane parallel to the base.

According to a second aspect of the invention there is provided a structural steel decking panel for supporting construction loads and materials including wet concrete and subsequently longitudinally reinforcing the concrete after it has hardened, the decking panel including: a first longitudinally extending rib; a second longitudinally extending rib spaced apart from and parallel to the first rib; and an elongate pan joining the first and second ribs and forming a base, each rib having a top portion and a pair of web portions, the web portions diverging as they extend down from the top portion to the base, characterised in that the ribs have a fold zone at an end, whereby the top portion slopes downwards towards the base and the web portions are folded such that in part they form part of a Z-fold that lies adjacent and substantially within or parallel to a transverse inclined fold plane.

Preferably each rib has a terminal squash zone adjoining the fold zone, whereby the top and web portions of the rib are squashed to all lie adjacent and substantially within, or parallel to a squash plane parallel to the base.

According to a third aspect of the invention there is provided a formwork assembly for a composite concrete deck, the assembly including: (i) a decking panel having: a first longitudinally extending rib; a second longitudinally extending rib spaced apart from and parallel to the first rib; an elongate pan joining the first and second ribs and forming a base, each rib having a top portion and a pair of web portions, the web portions diverging as they extend down from the top portion to the base; first and second end zones, each end zone extending from and formed from a corresponding said longitudinally extending rib, each end zone comprising two opposed web end portions joined by an inclined portion sloping downwards towards the base; and first and second terminal squash zones, each squash zone adjoining a corresponding said fold zone, whereby the top and web portions of the rib are squashed to all lie adjacent and substantially within, or parallel to a squash plane parallel to the base; and

(ii) a beam edge form lying across and underneath the decking panel, the edge form and decking panel mutually positioned such that the squash zones abut the edge form so as to substantially prevent concrete leaking over the edge form into a void formed under the ribs.

Preferably the edge form and decking panel are mutually positioned such that the squash zones extend over and beyond the edge form to provide for

penetration of a finished concrete beam after a concrete pour.

Specific embodiments of the invention will now be described in some further detail with reference to and as illustrated in the accompanying figures. These embodiments are illustrative and are not meant to be restrictive of the scope of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Preferred embodiments of the invention are illustrated in the accompanying representations in which:

Fig Ia shows a background art trapezoidal steel decking panel.

Fig Ib shows a further background art deck having re-entrant ribs.

Fig 2 is a cross-sectional view showing typical fixing of background art decking to steelwork.

Fig 3a is a top perspective view of a structural steel decking panel according to a first embodiment of the invention.

Fig 3b is an underside view of the panel of Figure 3a.

Fig 3c is a top perspective view of an alternative structural steel decking panel according to the invention.

Fig 4a is a top perspective view of a decking panel according to a second embodiment of the invention.

Fig 4b is an underside perspective view of the panel of Fig 4a.

Fig 5a is a diagrammatic view of the decking sheet of Figs 4a and 4b. Fig 5b is a part sectional view of a part of the decking sheet shown in

Fig 5a.

Fig 6 is a diagrammatic side view of the decking shown in Fig 5a.

Fig 7 shows a typical section through section line X-X in Fig 6.

Fig 8 is a similar view to Fig 7 showing the fold incompletely formed.

Figs 9a and 9b show a variant of embodiments showing Figs 3a and 3b in which outward folding has occurred (as opposed to inward folding).

Fig 10 shows the decking of Fig 4a and 4b in use.

Fig 11 shows a non-optimal close-to-vertical fold zone. Figs 12a, 12b and 12c show alternative fold zone geometries.

Fig 13 shows alternative configurations of the decking panels of Fig 3a and 3b or 4a and 4b placed over multiple spans.

A first embodiment of the invention is illustrated in Figs 3a and 3b. Referring first to Fig 3a, a structural steel decking panel 10 for supporting construction loads and materials including wet concrete and subsequently longitudinally reinforcing the concrete after it has hardened is shown. The decking panel 10 includes a first longitudinally extending rib 20, a second longitudinally extending rib 40 spaced apart from and parallel to the first rib 20, and an elongate pan 30 joining the first and second ribs 20 and 40 and forming a base. Each rib 20 and 40 has a top portion (flange) 45 and a pair of web portions 42 and 44, the web portions diverging as they extend down from the top portion 45 to the base. The decking panel 10 is characterised in that the ribs have a folded zone 48 at an end, whereby the top portion 47 slopes downwards towards the base and the web portions 42 and 44 are folded inwards such that they lie adjacent and substantially within or parallel to a transverse inclined fold plane. This is clearly illustrated in Fig 3b in an underside view.

An alternative embodiment of the invention is shown in Fig 3c. With this embodiment, a single rib 20 is provided. Rib 20 has a strengthening section 21 on its top flange 45. A plurality of strengthening dimples 77 axe also provided in the webs 42 and 44 of the decking panel 10.

An alternative embodiment of the invention is shown in Figs 4a and 4b. This is the same embodiment shown in Figs 3a and 3b except that a terminal squash zone 50 adjoining the fold zone 48 is formed whereby the top and web portions 42, 44 and 47 of the rib 40 are squashed to all lie adjacent and substantially within, or parallel to a squash plane level with (or through) the base.

The special geometry of the press-fold is described initially with reference to the drawings showing the fold lines 64, 65 and 66 in Figs. 7 and 8, noting that an actual example of the inwardly-folded press-fold in Fig. 7 is shown in Figs 3a & 3b. The press-fold geometry is such that:

(i) the portions of webs 44 of the deck below the folded portion of decking (i.e. below the lowest fold line 66 shown in Figs. 5a, 5b and 6 are not distorted at all by the folding operation, and therefore have to remain in their original shape; (ii) the folded portion of decking (everything above the lowest fold line 66) effectively forms a flat, single plane of decking (comprising an inclined flange 47) that strongly and rigidly restrains the tops of the webs from moving laterally, which would otherwise lead to spreading of the steel ribs or premature failure of the webs in crippling; and (iii) the nestability of the decking is not affected.

The mechanism by which an inward press-fold is formed is shown in the top right-hand rib in Fig. 5a. It is important that the dimension B measured at the cross-section at the boundary between the "squash zone" 62 and "fold zone" 60 is at least as wide as the base width A. This principle applies along the full length of the fold zone where the base width is measured at the level of the

lowest fold line. This ensures that the top of the web remains in its original position, thus satisfying requirement (i) immediately above. The position of the fold line 65 along which point D is located must then be chosen to satisfy requirement (ii).

The importance of requirement (ii) is explained with reference to Figs. 7 and 8, which shows a typical section X-X in Fig. 6. In Fig. 7 requirement (ii) is met, and the structural action is sound, while in Fig. 8 the fold has been incompletely formed, considerably weakening the web due to gross imperfections where the vertical reaction from the underlying support is transmitted up the webs in compression, and premature buckling and spreading are possible. It is worth noting that in practice the steel used to form the press-fold may be high-tensile and low in ductility with a tensile-to- yield-stress ratio as low as unity. The steel sheeting used to make the press- folded ends shown in Figs. 3a and 3b had these properties, and proved to be suitable without fracturing of the steel decking occurring along the fold lines, provided the fold radius was not less than about lmm.

Concerning requirement (iii), the decking remains nestable provided that in the fold zone the outer edges 49 of the inclined top flanges 47 are pressed flush with the top edges of the webs portions 44 as illustrated in Figs 3c and 5b.

An outward fold is shown in Figs. 9a and 9b, which may similarly be formed, provided the two requirements (i) and (ii) above are satisfied. It is possible that forming the fold this way the anchorage of the decking in hardened concrete may be enhanced because the edges of the fold protrude past the web on the top concrete face. However, it is visually less attractive, so may not be the preferred option, and particularly given that with modern decks it

may not be necessary to enhance their mechanical interlock in this manner. Moreover _/ the decking will not be nestable if an outward fold of the webs is used.

Two other aspects of the geometry of the press-fold are (numbering them consecutively from above):

(iv) that a flat squash zone may be created at the end of the deck (see Figs. 4a and 4b), so that in some forms of construction it can protrude past the outer edge of the (temporary) supporting element into the side of the monolithic concrete construction without concrete leakage occurring, also preventing the open ribs of the steel decking encroaching into the side of the monolithic concrete member; and (v) the lap joints of the steel deck (seen for example at the edges of the trapezoidal deck drawn in Fig. 5a) are not damaged by the press-fold process, so the deck may be joined together on site normally.

An example of a press-folded end with the squash zone is shown in Figs. 4a and 4b. One of its purposes is explained in Fig. 10 by way of an example where the ends of the decking (the decking forming the main bottom-face longitudinal reinforcement in a slab spanning between monolithic bandbeams) are required to penetrate the side of the bandbeam. Otherwise, under ultimate load conditions, the decking could delaminate from the soffit of the slab due to "peeling" effects. Therefore, this is a normal requirement for decking, even with an interlocking shape, i.e. peeling must be avoided since it leads to premature brittle failure of the longitudinal shear connection between the steel decking and the concrete in the slab.

It is clear from Fig. 10 that if all of the squash zone 50 extends beyond the inner-most edge 72 of the temporary support 70 on the beam side, then concrete will leak up from underneath the decking end, and a fundamental requirement of the press-fold that it seals the decking ends will not be met. The minimum length of the squash zone 50 will depend on various factors such as the compressive strength of the concrete, i.e. needs to be longer if weaker concrete is used, to avoid premature spalling under the squash zone at ultimate load. An absolute minimum value would be 15mm, so allowing for the squash zone to sit at least 10mm on the edge form, the squash zone should normally be at least 25mm long. Its maximum length may be limited by the cover to the reinforcement in the side concrete element, i.e. the decking should not conflict with the vertical reinforcing stirrups in the reinforcing assembly 17 as shown in Fig. 10.

It is important to note that as a general principle, the press-folded end may be formed without the squash zone and it will still function satisfactorily by preventing spreading of the ribs and providing adequate end shear strength.

It is also generally important that the end reaction is not concentrated solely through the squash zone. The length of the fold zone must also be considered in relation to the overall depth of the steel deck. Both of these factors are discussed next.

The press-folding of the ends of trapezoidal steel decking overcomes the problems associated with the structural behaviour at the ends of the deck as described above in the "Background of the Invention". In particular, the specially-designed press-folded ends:

(a) allow the ends of the decking to be sealed against concrete leakage irrespective of the form of construction, and includes all types of composite steel-frame buildings and concrete buildings with their respective special issues; (b) prevent the open steel decking ribs from encroaching into the side of a monolithic concrete supporting member, which could otherwise reduce the member's strength, stiffness, or create a durability issue;

(c) prevent the ends deforming laterally without any fixing, i.e. prevent spreading during transport, handling and when under load in the structure while acting as f ormwork;

(d) prevent the trapezoidal steel ribs failing prematurely by web crippling, leading to flexurally stiffer panels with reduced vertical deflection under load (particularly important when the wet concrete is poured), and very high end shear capacity making flexural failure within the span much more likely which leads to increased utilisation of the steel and possible economic advantages by using thinner sheeting, as well as providing aesthetic advantages due to reduced local deformation of the sheeting in support regions making it visually more regular and acceptable when the soffit is left exposed in the final structure, e.g. in a carpark;

(e) can be used throughout the building construction process without requiring the decking to be fastened to supports at its ends, making construction faster and more economical;

(f) can be used to laterally support the central regions of adjoining panels by placing the panels in staggered arrangements, thus also avoiding having to fasten the panels to internal supports

where they are continuous over the supports as illustrated in Fig.13; (g) provide greater rigidity and strength to provide stiffer and stronger lateral restraint to supporting steel beams when this is required, e.g. in unpropped construction of composite steel- framed buildings with long-spanning steel beams, which does require the sheeting to be fixed to the steelwork by fasteners or plug welds; and (h) can maintain nestability.

Structural testing has confirmed that it is possible to design the press-folded end to not fail under severe end shear conditions, such that under normal conditions experienced in practice, a flexural failure of the decking will occur away from the end zone and possibly govern the design of the decking (i.e. determine the minimum base metal thickness required), resulting in optimal designs.

In practice it would be possible to fold the end of a steel deck into a wide range of geometries, each of which can critically affect the structural performance of the end regions subjected to high shear force.

The geometry of the fold zone and its position over a support is shown in Figs. 11, 12a, 12b and 12c. Unlike normal end diaphragms or end-capping, it would not be optimal to form a vertical press-fold, i.e. close to zero overall length. This is because web crippling would remain an issue in the end region immediately adjacent to the press-fold, i.e. the effective height of the webs is not reduced where the concentrated reaction is applied near the ends as shown in Fig. 11, and therefore the panel strength would not be a maximum.

(Note: when the panel end rotates as the panel is loaded, all of the contact pressure is applied at the inner edge 74, if support 70 can not rotate)

As can be seen in Figs. 12a, 12b and 12c, the optimal inclination of the top flange 47 of the fold zone is when the contact point at the inner edge 74, of the rigid, rotationally-fixed support 70, lies within the fold zone (i.e. alternatives 12a and 12b, but not 12c). However, there must be sufficient depth of web above this point (alternative 12b) for the part of the fold zone resting on the support 70 to not fail in bending (as is more likely for alternative 12a). Bearing in mind that the minimum end bearing dimension (dimension Y in Fig. 12a) to avoid accidental dislodgement of the sheeting during construction is typically about 30mm, while the normal bearing end length is about 50mm, it follows that for normal steel decking, which is in the vicinity of 40 to 80mm deep, that a fold length of between 50mm (minimum) and 100mm (maximum) is likely to be satisfactory (noting that the squash zone may not always be present).

The stabilising effects of the press-folded ends may be taken advantage of in multi-span configurations with the decking panels extending over multiple spans by using the detail in Fig. 13. With the arrangement shown in Fig. 13, 4 lateral support beams 90 are shown. Multi-span sheets 80 with press-folded ends 50 span over all four lateral beams 90. Adjacent to the multi-span sheets 80 are single span sheets 10 also with press folded ends 50. With the arrangement shown in Fig. 13, single span sheets 10 are used in every third deck laterally placed over the beams. In other applications, more or less single span sheets 10 may be required to stabilise multi-span sheets. An important benefit is that the steel sheets do not necessarily have to be fixed to the supports. Construction efficiencies and cost savings result, and

there are no restrictions on the material used to form the supporting members or walls with regard to fixing.

While the present invention has been described in terms of preferred embodiments in order to facilitate better understanding of the invention, it should be appreciated that various modifications can be made without departing from the principles of the invention. Therefore, the invention should be understood to include all such modifications within its scope.