The present invention relates to a slab load transfer plate, that is a plate for transferring loads between adjacent cast-in-place slabs and in particular to a plate for transferring loads applied to either slab across a joint to the other slab.

Prior Art

AU forms of activity within buildings require a sound platform on which to operate and concrete floors almost invariable form the base on which such activities are carried out. Manufacturing, retail, distribution, storage and leisure facilities all tend to use concrete floors as a base or wearing surface and the demands upon this system are ever increasing. Such floors are normally formed as a series of individual blocks or slabs of concrete that are individually cast in place separated by joint lines where one slab ends and the next begin. These joint lines are physical breaks in the continuity of construction offering a 'free edge' to the slab and thereby presenting a potential point of weakness and longer-term failure of the flooring system. The UK Concrete Society Technical Report No. 34 Third Edition 'Concrete Industrial Ground Floors - A Guide to Design and Construction' advises that 'the load carrying capacities of a slab at a free edge and at a free corner are approximately 50% and 25% of the capacity at the centre of the slab. '

The increasing demands placed upon concrete flooring are particularly felt at the vulnerable joint lines that lie between slab segments. For example:

• Within the storage, retail and distribution sectors, building and storage racking heights are increasing with a corresponding increase in the loadings imposed on these floors. Where loadings arise close to slab 'free edges', suitable mechanisms effecting load transfer from onβxslab to another are required.

• Increasingly sophisticated materials handling equipment requires that concrete floors have a greater surface regularity than ever before. This is particularly important across joints between slab sections and effective load transfer whilst restraining any vertical movement between these slab blocks is essential to facilitate smooth transit of material handling vehicles travelling across these joint lines. • Wider and longer blocks of concrete are being poured with consequential increases in the width of joint gaps being formed when these slabs shrink under drying. This effect has a direct impact upon the type and frequency of systems employed to transfer loads from one cast-in-place slab to another as an increase in gap width will cause a corresponding increase in bending stresses applied to any load transfer mechanisms employed.

Reference will now be made to certain of the drawings, showing prior art features: Figure 1 is a plan view of a typical concrete floor;

Figure 2 is a plan view of another typical concrete floor; Figure 3 is a plan view of a floor comprising two slabs; Figure 4 is a perspective view of the slab floor having a slab load transfer plate at its joint; Figure 5 is a perspective view of a slab load transfer plate of US patent No

6,354,760 in use;

Figure 6 is a plan view of this plate prior to slab shrinkage; Figure 7 is a plan view of this plate after slab shrinkage; Figure 8 is a diagram showing alternative plate shapes; Figure 9 is a cross-sec tional view of a slab joint in two states, namely under load with a small amount of shrinkage and large amount of shrinkage.

To further explain, a concrete floor 100 is typically made up of a series of individual blocks or slabs of concrete 101 to 109 inclusive as shown in Figure 1 separated by joint lines 110. These individual blocks are cast-in-place separately and provide a number of advantages in that they relieve internal stresses that are likely to arise from drying shrinkage of the cpncrete, they accommodate stresses arising from thermal gain and loss and they can be tailored in size to match the capacity of available plant and workforce. It can be seen from Figure 1 that adjacent blocks meet each other at the joint lines and these joints arise at intervals such that each block has sufficient strength to resist random cracking. However, a number of forces are at play in the construction industry forcing these blocks to get wider and longer: 1. As joints are breaks in the physical continuity of construction they represent a potential point of weakness in the construction of the concrete floor hence fewer joint lines means fewer potential points of failure.

2. The creation of larger blocks increases the speed at which a floor may be laid and this has significant implications for both construction programming and cost to the client.

3. The introduction of fibre reinforcement into concrete slabs has been used to increase the strength of concrete and as a result of this block sizes are increasing.

Figure 2 shows the same area of concrete floor as depicted in Figure 1, however it will be noted that concrete floor 200 is made up of a series of four blocks or slabs 201, 202, 203, 204 separated by joint lines 210, whereas concrete floor 100 is made up of nine smaller blocks or slabs. It can be seen that the combined length of the joint lines in Figure 1 is approximately twice that shown in Figure 2. One of the by-products of increasing slab size is a consequential increase in drying shrinkage and a greater opening of the joint between two abutting slabs. The UK Concrete Society advises that 'for well designed concrete, long term shrinkage strains lie in the range 400 to 600 x 10 ^"6 mm '- this is equivalent to a drying shrinkage of 4 to 6mm for a 10 metre span of concrete slab. These values are, however, mitigated somewhat by restraint and creep and the UK Concrete Society Technical Report No. 34 Third Edition 'Concrete Industrial Ground Floors - A Guide to Design and Construction advises that for a 10 metre span of concrete a shrinkage factor between 2.5 and 3.33mm should be used.

Reference to Figure 3 will snow that a concrete floor 300 where two slabs 301 and 302 abut the drying shrinkage can be the sum of the shrinkage exhibited by each slab. Transferring this into tabular format in Table A below, using a shrinkage factor of- 3.33mm per 10 metres of slab span and giving typical values to Dimension A and Dimension B for the slabs 301 and 302:

Table A

Dimension A Dimension B Total Drying

Dim A Metres Drying Dim B Metres Drying Shrinkage mm

It can therefore be clearly seen that by increasing slab size there is a corresponding increase in drying shrinkage at slab-to-slab joint lines. To accommodate this drying shrinkage, slabs must allowed to move independently of each other in two planes:

1. Perpendicular to the inner surface of the joint line between slabs,

2. Lateral to the inner surface of the joint line between slabs.

However to ensure the regularity and longevity of the wearing surface of the floor, vertical movement between individual blocks or slabs must be restrained and loads (such as those imposed by racking or forklift trucks) applied to one slab must be transferred across the joint line to the adjoining slab. Figure 4 shows these permissible and restrained movements across joint gaps wherein movement perpendicular P and lateral L to the joint line between the two slabs 401 and 402 may be permitted whereas vertical differentials V between the top surfaces of the slabs has to be restrained by some form of mechanism 403.

In summary therefore, whilst the joint gaps between individual blocks or slabs are increasing there is still a requirement for load transfer between adjoining blocks or slabs.

Prior art shows that transferring loads from one block or slab to another is normally achieved by using either steel rods or flat plates:

1. Steel rods (also referred to as dowels) are described in all of US Patent Numbers 5,005,311, 5,216,862 and 5,487,249 all in the Shaw name. They describe dowel bars of circular cross section having a sheath that permits movement perpendicular to the inner surface of the joint line between slabs. US Patent Number 4,733,513 in the Shrader name discloses dowel bars of rectangular cross section with resilient facings attached to the sides of the bars. These systems restrain movement lateral to the inner surface of the joint line between slabs and . vertical movement between slab blocks.

2. Flat plates as described in US Patent Number 6,354,760 in the Boxall name. This patent described the weaknesses inherent in using circular or square steel rods to transfer loads from one slab block to another. The Boxall invention comprises 'α load plate comprising a substantially tapered end having substantially planar upper and lower surfaces adapted to protrude into and engage the first slab, and the load plate being adapted to transfer between the first and second slabs a load directed substantially perpendicular to the intended upper surface of the first slab '. To permit movement the Boxall invention describes a 'block-out sheath ' that could be embedded within the first slab. TAe block-out sheath could have a substantially planar top surface and a substantially planar bottom surface substantially parallel to the upper surface of the first slab '. This sheath which, similar to the Boxall load plate, tapers from one end to the other is larger than the load plate and can be fitted onto the load plate such that the substantially tapered end of the load plate can move within the sheath in a direction parallel to the intersection between the upper surface of the first slab and the joint surface;

Figure 5 shows the essence of the Boxall invention where a concrete slab 501 has been cast encompassing a block-out sheath 502. The load plate 503 is inserted as shown into the block-out sheath 502. Figure 6 shows a plan view of the block- out sheath 602, embedded in the slab 601. The sheath envelops part of the load plate 603 the balance of which is embedded in abutting slab 604. Figure 7 depicts the reaction of the slabs block-out sheath and dowel to drying shrinkage. As this shrinkage occurs the slab 701 begins to withdraw from slab 704 and correspondingly the load plate 703 begins to withdraw from the block-out sleeve 702. This reaction opens gaps denoted by A, B and C in Figure 7 thereby allowing the slabs to move independently of each other excepting in a vertical plane. Figure 8 depicts a number of other shapes of load plate disclosed within the Boxall patent.

The Object of the Invention As noted earlier, the economics of construction allied with fibre reinforcement technology are causing slab or block sizes to increase significantly. As a by-product of this and following drying shrinkage, joint gap widths between slabs are increasing. Similarly, industrial, retail and distribution building heights are increasing and thereby associated loadings arising from racking and material handling systems have increased. In turn the bending forces imposed on dowels and load plates are increasing.

Figure 9 displays two separate slab configurations wherein a joint between slabs 901 and 902 has, under drying shrinkage, opened up by a distance Xi. Slabs 901 and 902 are linked by the dowel or load transfer plate 903 and the bending moment on this dowel or load transfer plate is a product of the load Ll applied to slab 901 and the displacement distance Xi between the two slabs. As the gap between the slabs increases (as shown between slabs 904 and 905) from Xi to X- there is a corresponding reduction in the capacity of the dowel or load plate to resist bending loads such that:

Where: Pbend = the bending capacity of the dowel or load plate the characteristic strength of steel Zp= the plastic section modulus of the dowel or load plate X= the j oint opening

Ys = the partial safety factor for steel.

Hence the capacity of a dowel or load plate to resist bending is directly proportional to the width of the joint opening. The following table is extracted from the UK Concrete Society Technical Report No. 34 Third Edition 'Concrete Industrial Ground Floors - A Guide to Design and Construction' Page 61 Table 9.4 an displays the reduction in bending capacity for various dowel types and sizes.

As can be seen the greater the gap opening the less the capacity of the dowel or load plate to resist bending whilst at the same time the loads imposed on floors are inexorably increasing. Currently the only way to counter these effects is to increase correspondingly the diameter of the dowel (or thickness of the load plate) thereby increasing the plastic section modulus of this dowel or load plate. As both loads and gap openings between slab blocks due to drying shrinkage increase in magnitude, the consequence is that dowel and load plate sizes will have to increase in size and particularly thickness to meet these new requirements. The by-product of this is that dowel and load plate systems will inevitable become heavier possibly giving rise to Health and Safety issues on-site and in addition these systems will become more expensive.

The object of the present invention is to provide an improved system that will provide both the load transfer and movement capacities offered by prior art but with increased resistance to bending under load transfer.

The Invention

According to the invention there is provided a slab load transfer plate having greater depth than its average thickness.

The greater depth enhances its beam stiffness over that which it would have from its average thickness.

In certain embodiments, the greater depth is achieved by forming the plate from plate material. For example it may be curved with its edges transverse of the slab joint in use curved down, or up, in use. Alternatively, it may be bent along one or more lines extending transverse of the slab joint. For instance it may be bent along a median line to give it a Vee - normally shallow Vee - cross-section or it may be bent along two lines to channel shape. Again, it may be cast or otherwise formed to have a central thin plate region and thicker edge regions.

Preferably the plate is tapered in at least one portion, normally a half, of its length corresponding to one half being cast into one slab at the joint, and the other into the other. Normally the tapered portion will be encased in a sheath, to remain in position on shrinkage, with the tapered portion withdrawing from the sheath on shrinkage.

In some embodiments, the non-tapered portion at least is provided with surface formations, to allow it to grip its slab.

Further, in some embodiments compressible strips can be provided along edges of the load plate, to allow lateral movement of the joint.

To help understanding of the invention, various embodiments thereof will now be described by way of example and with reference to the accompanying drawings, in which:

Figure 10 is a diagram showing the application of a load to variously shaped plates.

Figure 11 is a plan view of a curved load plate of the invention with reversed perforations;

Figure 12 is a perspective view of the curved load plate with reversed perforations; Figure 13 is a perspective view of the curved load plate embedded in slab blocks;

Figure 14 is a plan view of another load plate of the invention with a single formed angle and with reversed perforations;

Figure 15 is a perspective view of the load plate of Figure 14; Figure 16 is a perspective view of the load plate of Figure 14 embedded in slab blocks;

Figure 17 is a plan view of a third load plate of the invention with two formed angles and with reversed perforations;

Figure 18 is a perspective view of the third load plate; Figure 19 is a perspective view of the third load plate embedded in slab blocks;

Figure 20 is a plan view of another tapered load plate of the invention with two down-stands; Figure 21 is a top down perspective of the tapered load plate of Figure 20;

Figure 22 is a perspective from beneath the tapered load plate of Figure 20;

Figure 23 is a top down perspective of the tapered load plate of Figure 20;

Figure 24 is a plan view of a tapered sheath the tapered load plate of Figure 20; Figure 25 is a side perspective of the tapered sheath;

Figure 26 is a top perspective of the tapered sheath;

Figure 27 is a perspective from beneath of the tapered sheath;

Figure 28 is a top down perspective of the tapered sheath;

Figure 29 is a perspective view of a tapered load plate 2902 partially embedded in a concrete slab 2901 ;

Figure 30 is a plan view corresponding to Figure 29;

Figure 31 is a view similar to Figure 29, with the sheath being fitted to the tapered load plate;

Figure 32 is a plan view of the tapered load plate 3203 with the load plate sheath 3206 in position;

Figure 33 is a perspective view of the tapered load plate 3303 partially embedded in slab 3301;

Figure 34 is a plan view of the tapered load plate 3403 partially embedded in a concrete slab 3401.

It is known that introducing a curve or an angled downturn (or downturns) into a flat plate increases the section modulus of the plate and significantly increases its resistance to bending under applied loads. Figure 10 shows four configurations wherein: A load Ll is applied to the free edge of a flat plate 1001

A load L2 is applied to the free edge of a curved plate 1002

A load L3 is applied to the free edge of a plate with a single formed angle 1003

A load L4 is applied to the free edge of a plate with two folded angles 1004 In each instance the bending resistance of the curved plate 1002, the plate with a single formed angle 1003 and the plate with two folded angles 1004 exceed that of the bending capacity of the flat plate 1001. Hence the introduction or curves or downturns into load plates will create an improved system that will provide both the load transfer and movement capacities offered by prior art but with increased resistance to bending under load transfer.

One embodiment of a curved load plate 1303 of the invention is shown in Figures 11, 12 and 13. It has with reversed perforations 1304 formed as Vee stampings with their points towards the centre. On casting of blocks 1301 and 1302 with the load plate at the joint between these blocks, the perforations are embedded in the slabs and resist shrinkage of the block away from the joint line 1305.

A second embodiment, of an angle load plate of the invention is shown in

Figures 14, 15 and 16. It has similar reversed perforation 1604 shown embedded in slab blocksl601 and 1602 at the joint 1605 between these blocks.

A third embodiment is shown in Figures 17, 18 and 19 in the form of a plate 1903 having two depending angles 1906,1907 parallel to an axis 1908 transverse to the slab joint 1905 slab blocks 1901and 1902. This plate also has reversed perforations 1904.

Referring now to Figures 20 to 34, the load plate 2001 has a central portion 2002 of basic thickness and edge portions 2003 of at least twice the basic thickness. The plate is basically rectangular, with two tapered corners 2004. In effect it is comprised of two portions .2005, 2006, divided at a line 2007 coincident in use with the slab joint line. The portion 2005 is rectangular and the portion 2006 is regular- trapezoidal. In the rectangular portion, the central portion 2002 extends from the dividing line 2007 to the opposite free edge 2008 and is equal in width to the two thicker portions 2003 on either side. In the trapezoidal portion, the thicker portions taper to the corners 2009 at the end of the free edge 2010 opposite from the dividing line, with the central portion tapering out correspondingly. Normally the thicker portions do not taper to a point, but end with discrete width 2011 at the corners 2009. As shown the central portion extends on one face 2012 of the plate, leaving planar that face, with the other side being formed with a wide, shallow channel 2013 due to the thickness differences. In a non-illustrated alternative, the central portion can be at mid-height in the plate, with channel formations in both the top and bottom faces.

The plate can be machined from solid, but is preferably cast or forged, typically of steel.

As shown in Figure 23, the tapered edges 2021 can be provided with compressible foam strips 2022. These allow block movement parallel to the slab joint, with one or other of the strips becoming compressed against the concrete of the slab cast around it. Such movement is allowed even if the shrinkage on drying has occurred in such a manner as to result in little opening of a gap at the joint line and little freedom of lateral movement as a result of shrinkage away from the tapered edges.

Normally, as shown in Figures 24 to 34, the tapered load plate will be used with a tapered sheath 2031. This is formed complementarily with the load plate, being an injection moulding. It is generally trapezoidal to cover the trapezoidal portion 2006 of the plate and allow it to withdraw smoothly from the concrete of the slab into which it is cast, with the rectangular portion 2005 being set directly into the concrete of its slab, so that it is held firmly in its slab.

The sheath 2031 has a planar top face 2032 corresponding to the planar top face 2012 of the load plate. The bottom face 2033 of the sheath has a channel 2034 corresponding to the channel 2013. Thus the interior of the sheath has two deep, tapered, edge spaces 2035 for the thicker portions 2003 of the plate and a central thinner space 2036 above a central tapered tongue 2037 for the central thinner portion 2002. Thus the majority of the area of the plate is available for load transfer via the sheath, whether the load is so positioned to bear down from the slab to the plate or the plate is bearing down on the slab.

It should be noted that where the internal width of the sheath is such as to accommodate fully the tapered portion 2006 of the plate, including the foam strips 2022, the central tongue 2037 is made narrower than the tapered channel 2013 to allow lateral movement of the load plate in the sheath without withdrawal of the plate from the sheath, as allowed by compression of the foams strips 2022.

The sheath can have ears 2038 and surface formations 2039 to aid in its retention in the concrete of its slab.

Normally, the load plate will be positioned in shuttering (not shown) to be cast into a first slab 2041 via its rectangular portion 2002, as shown in Figures 29 to 31. After removal of the shuttering, the sheath is fitted. Then the second slab 2042 is cast, including around the sheath 2031.

Figures 33 and 34 show the shrinkage of the two slabs away from each other by an exaggerated dimension X. Following drying shrinkage of the slabs, the load plate has partially withdrawn from the sheath. The shrinkage gap X has opened up between the slabs and this distance X is now equal to the distance that the load plate has withdrawn from the sheath. Additionally, gaps A, B develop between the foam strips and the insides 2043 of the sheath. These A, B gaps increase in proportion to the increase in the value of X. However it should be noted that differential lateral shrinkage of the slabs can result in lateral displacement of the load plate in its sheath. Simultaneously with the gaps A, B developing, further gaps C ₅ D develop between the inside edges 2044 of the tapered portions 2003 of the load plate and the central tongue 2035 of the sheath. Despite the freedom for horizontal movement, assuming that the slabs are generally horizontal, the co-operation of the load plate and the sheath, both being set in the their respective slabs, restrict relative, vertical movement of the slabs.