GB2181171A

GB2181171A - Concrete slab for a false or platform floor having strengthening ribs on the underside

Info

Publication number: GB2181171A
Application number: GB08622989A
Authority: GB
Inventors: Christopher R Smith; Colin H Holmes
Original assignee: INTEK FLOORS Ltd
Current assignee: INTEK FLOORS Ltd
Priority date: 1985-09-27
Filing date: 1986-09-24
Publication date: 1987-04-15
Also published as: EP0219985A3; GB8523810D0; EP0219985A2; GB8622989D0

Abstract

A concrete slab for false or platform floors has a downwardly extending peripheral reinforcing rib 2, stiffening ribs 4, 5 extending across the slab in two orthogonal directions, a weight not exceeding 20 kgs for an area of 600 x 600 mm and fulfils the medium-grade requirements of Property Services Agency Specification 08.801. It may have further diagonal reinforcing ribs (16 Figure 2) at the corners or a further reinforcing rib 7 inwardly spaced from the peripheral reinforcing rib. <IMAGE>

Description

1 GB 2 181 171 A 1

SPECIFICATION

Concrete slab for a platform f loor 4 The invention relates to concrete sla bs su itable for use in raised f loors or access f 1 oors known as "false f loors 5 or " platform floors" in which the slabs may be supported at their corners and laid side-by-side to form a floor surface.

Such slabs or panels are used for providing false floors at a variety of locations, including buildings accom modating corn puters and other electrical equ i pment which are connected to cables. A false floor made u p of slabs which may be lifted and removed for access as required allows cables and other items of equ ipment to be accom modated below the fa Ise f loor su rface, in a space formed between the lower surface of the slabs and the f loor surface of the building on which they rest. The slabs are commonly square and supported at their corners on pedestals which stand on the floor surface of the building. The pedestals may be adjustable in height so that the slabs are supported to form a perfectly stable, level surface when laid side-by-side. The upper su rf aces of the slabs are f lat a nd a re genera 1 ly covered by carpeti ng or othersu rface 15 finish.

Slabs for use in false f loors have to meet a number of requirements. They must have a high strength to withstand both and point loads and distributed loads. The slabs must have a low deflection u nder load in order to provide a stable support surface and they should have a high degree of dimensional accuracy in orderto be transferable and interchangeable. The slabs should be structurally stable, resistant to fire unaffected in performance by moisture and have sound insulating properties; the latter is important as the space between the sla bs a nd the f loor of the bu ilding f orm a resonant cavity. It is also desirable that the sla bs should have the lowest possible weight to facilitate handling, removal and replacement of the slabs and to minimise the load imposed on the supporting structure.

Performance Specification MOB 08.801 for platform floors, published in August 1985 bythe Property 25

Services Agency of the United Kingdom Government Department of the Environment, lays downtest standardsto be observed forfalse floorslabs.

False floor slabs have been made of a variety of materials including metals such as aluminium,wood, metal/wood composites and light-weight concrete/metal composites. European Patent Application 0135240 of CBR Beton discloses a floor slab of concrete containing constituents such as expanded glass, expanded clayor 30 polystyrene balls to reduce the weight of the slab. However, hitherto there has not been produced a concrete false floor slab of acceptable weightwhich meets all the standards laid down in Specification MOB 08.801 mentioned above, notably regarding failure load and resistance to impact by soft and hard bodies falling onto the flat upper surface of the slab.

According to one aspect of the invention, there is provided a rectangular reinforced concrete slab fora 35 platform floor having a flat upper surface and having on its underside a downwardly projecting peripheral reinforcing rib extending around the slab periphery and a plurality of downwardly projecting stiffening ribs extending across the underside of the slab in two orthogonal directions, the stiffening ribs extending in at least one direction extending to a lesser depth belowthe upper surface of the slab than the peripheral rib, the slab having a weight not exceeding 20 Kg when having a square upper surface area of 600 x 600 mm and fulfilling 40 the medium-grade structural requirements of Property Services Agency Performance Specification 08.801.

Rectangular slabs for platform floors, including square slabs, maybe made in various dimensions but a square slab of surface dimensions 600 mm x 600 mm, which is intended to be supported by a pedestal at each of its four corners without intermediate pedestals between the corners, is generally regarded as a standard size of floor slab. The upper weight limit of the slabs according to the invention mentioned herein is quoted as 45 referring to slabs of this standard size. Itwill be understood howeverthatthere maybe made slabs of other dimensions which are within the scope of the invention and of which the weight does not exceed 20 Kg per036 m 2 of area (i.e. the area of the standard square slab).

In one embodiment of the invention, the underside of the slab is provided with an inner reinforcing rib extending continuously around the slab adjacent and within the peripheral reinforcing rib, the inner reinforcing rib extending below the slab upper surface to substantiallythe same depth as the peripheral rib. The reinforcing ribs maybe trapezoidal in cross-section and extend to a depth of about 40 m m below the upper slab surface. The peripheral rib may have a lower surface width of about 25 mm, the inner reinforcing rib a lower surface width of about 18 mm, and the inner reinforcing rib maybe about 75 mm from the edge of the slab (measured up to the mid-point, that is the longitudinal axis of symmetry, of the rib).

The stiffening ribs running in one direction may extend to a depth of 34 mm, and the stiffening ribs in the other direction to a depth of 28 mm, from the slab surface. The ribs maybe separated byflat-bottomed recesses, the bottoms of the recesses forming the underside of the main body of the slab itself forming the floorsurface.

It has been found that such a slab, of area 600 mm x 600 mm and weighing not morethan 20 Kg,when supported by suitable pedestals at the four corners, can fulfill the medium-grade structural requirements given in the above-mentioned Performance Specification 08.801, paragraphs P4.01 to P4.05. These require ments may be summarized as follows.

1. The slab shall support a load of 4.5 KN distributed over an area of 300 mm square, orof 3.0 KN over 25 mm square, orof 8.0 KN/M2 uniformly distributed, at any point without deflection exceeding 0.4% of the shortest 65 2 GB 2 181 171 A 2 span and without residual deflection exceeding 0.50 mm.

2. The slab shall supportthree times these loads for 5 minutes withoutfailure.

3. Theslab shall withstand impact by a 40 Kg soft mass dropped from a height of 1 metre, and by a 4.5 Kg hard mass having a 50 mm hemispherical end and dropped from a height of 600 mm.

4. The slab shall not move more than 1.5 mm horizontally, or 1.00 mm vertically, when subjected to 250,000 cycles of the pedestrian dynamic load test quoted in Paragraphs T 14 and T34 of the Performance Specification.

In another embodiment of the invention, the portions of the underside of the slab adjacentthe angles ofthe slab are provided with reinforcing ribs extending diagonally acrossthe slab. The diagonal reinforcing ribs may be of the same dimensions asthe peripheral ribs, for example a depth of about40 mm, a trapezoidal cross-section and a lowersurface width of about 25 mm. In this embodimentthe stiffening ribs running in one direction may have a depth of about40 mm and those in thetransverse direction a depth of about34 mm.

In yet another embodiment, the underside of the slab is provided with diagonal reinforcing ribs extending acrossthe whole of the slab between opposite corners. In this embodimentthe peripheral reinforcing rib may again have a depth of 40 mm butthe diagonal ribs may have a lesser depth, for example 34 mm.

In the two last-mentioned embodiments the recesses between the reinforcing ribs maybe provided with auxiliary ribs of triangular cross- section extending parallel to the slab edges.

Slabs according to the invention will now be described byway of example with reference to the accompanying drawings in which:

Figure 1 shows the underside of a concrete slab, Figure 1A and 18 are cross-sections of the slab of Figure 1 along A-A and B-B respectively, Figure2 shows another slab, Figures2A and 28 are cross-sections along A-A and B-B respectively of the slab of Figure 2, Figure 3 shows yet another slab, Figures 3A and38 are cross-sections along A-A and B-B respectively of the slab of Figure 3, Figure 4 is a partial cross-section of the slab of Figure 1 on an enlarged scale.

The slab of Figure 1 comprises a square body of concrete having a f lat uppersurface of dimensions 600 mm x 600 mm to serve as a floor surface. In use, the slab is supported by pedestals at its four corners to form a platform floor. The underside of the slab is provided with a peripheral trapezoidal strengthening rib 2 extending around the whole periphery of the slab having a lower edge 40 mm below the upper surface of the 30 slab and reinforced by steel reinforcing bars 3. The underside is also provided with orthogonal trapezoidal stiffening ribs 4 and 5 extending across the slab between the opposite sides of the peripheral rib and parallel to respective slides of the slab. The lower surfaces of ribs 4 are 34 mm belowthe slab upper surface, those of the ribs 5 are 28 mm belowthe upper surface. Ribs 4 and 5 are reinforced by steel bars 6. Ribs 2 have a width attheir lower end of 25 mm, and ribs 4 and 5 a width of 18 mm at their lower end. The ribs are separated by flat-bottomed recesses and the thickness of the slab in the recesses is 14 mm.

In orderto f urther reinforce the edges of the slab an innertrapezoidal strengthening rib 7 is provided. Rib 7 also extends 40 mm below the upper surface and is reinforced by steel bar 8. The lower surface of rib 7 has a width of 18 mm and the mid-point of rib 7 is 75 mm horizontally f rom the edge of the slab. The horizontal separation between the parallel ribs 4 and 5, and between ribs 4 and 5 and the inner peripheral rib 7, is 90 mm. 40 The slab of Figure 2 comprises a square body of concrete having a f lat upper surface 11 of dimensions 600 mm x 600 mm. The underside of the slab is provided with a strengthening rib 12 of trapezoidal cross-section projecting downwardly and extending around the whole periphery of the slab. The depth of rib 12 is 40 mm and its lower surface width 25 mm. The underside is also provided with stiffening ribs 13 extending acrossthe underside parallel to one pairof sides of the slab. Ribs 13 are also of trapezoidal cross-section and extend tothe 45 same depth below the slab as ribs 12. The slab is also provided with stiffening ribs 14 extending across the slab orthogonally to and between ribs 13. Ribs 14 are of trapezoidal cross-section and extend downwardly to a lesser depth, 34 mm, than ribs 12 and 13. Ribs 12,13 and 14 are ail provided with longitudinal steel reinforcing bars embedded in the concrete. 50 Ribs 12,13 and 14togetherform a pattern of squares on the underside of the slab and within these squares, 50 exceptforthe squares atthe corners of the slab, there are provided auxiliary ribs 15 of triangularcross-section and extending downwards by a lesser depth than ribs 14. These auxiliary ribs are arranged parallel to both pairs of sides of the slab and extend between the opposite ribs 12,13,14 defining the squares. The squares atthe corners of the slab are not provided with auxiliary ribs 15 but instead comprise diagonal 55 reinforcing ribs 16 of depth and cross-section similarto ribs 12 and provided with similar reinforcing bars. The 55 lowersurfaces of ribs 16 are flush with those of ribs 12. The purpose of diagonal ribs 16 is to give the slab increased strength adjacent the corners, where the maximum stress is concentrated when the slab is supported at its corners and a load is imposed on the upper surface of the slab. The slab shown in Figure 3 is also square, having a flat upper surface 21 of dimensions 600 mm x 600 mm and a reinforcing rib 22 of trapezoidal cross-section extending downwardly around the whole periphery of the 60 slab. The depth of rib 22 is 40 mm and its lower surface width 25 mm. The slab has one reinforcing rib 23 similar to ribs 22 extending across the slab parallel to one pair of sides of the slab and also has stiffening ribs 25 of triangular cross-section extending in both orthogonal directions between the reinforcing ribs.

The slab is also provided with a further reinforcing rib 24 extending across the slab orthogonal to rib 23 and with diagonal reinforcing ribs 26 extending between opposite corners of the squares defined by ribs 22,23 and65 4 3 GB 2 181 171 A 3 24. Ribs 24 and 26 are of lesser depth (34 m m) than ribs 22 and are reinforced by embedded steel bars in the same manner as ribs 22. As in the slab of Fig ure2, these diagonal ribs increase the strength of the slab at its corners which are subject to stress when the sl a b is supported at the corners and bears a load.

The slabs of a I I the Figures have a maxim u m depth of 40 m m and, as shown in the drawings, the outside surfaces of the peripheral ribs are inwardly inclined in the downward direction so that the maxim u m horizontal 5 dimensions of the slab are those of its upper surface (600 m m x 600 m m). The slab upper surfaces maybe accurately dimensioned to provide a close fit when the slabs are laid side-by-side to form a false floor. The edges of the slabs maybe provided with a protective I ipping of a plastics materia I such as PVC. Alternatively, as shown in Figure 4 the edges of the slabs maybe provided with a meta I edging 32 which is preferably cast into the edge of the slab when the slab is cast in a mould. Such metal edging maybe provided on all the slabs of 10 Figures 1-3.

As shown in the drawings,the lowersurfaces of the peripheral reinforcing rib atthe corners of the slabare provided with recesses 31, of depth of about 2 mm. These recesses assistcorrect location of the slab onthe pedestals used to supportthe corners of the slabwhentheslab is installed in a building. The pedestals maybe provided with bosses upstanding from the pedestal surfaceto engagethe recesses. The recesses mayalso 15 assistcorrect location of the metal reinforcing barswithin the concrete during casting.

A hole may be provided in the panel to accommodate a service outlet boxto allow passage of cables, pipes and the likethrough the slab and additional reinforcing ribs may be provided surrounding the hole. Theslab mayalso beformed with one or more apertures to facilitate lifting and handling of theslab.

The preferred composition of the concrete usedforcasting theslab is asfollows:

Cement Inertfiller of density from 800 to 1000 Kg /M3 and 25 particle size upto 3 mm Pulverized fuel ash Micro silica slurry Steel fibres from 650 to 750 Kg from 750 to 850 Kg from 120 to 160 Kg from 70to 90litres from 45to 60K9 The slabs of all the Figures are formed by casting a concrete having the following compositions percubic 30 metre:

Cement 700 Kgs Inertfiller (up to 3 mm diameter) 800 Kgs Pulverised fuel ash 140 Kgs 35 Micro silica slurry (Emsac F 100, a slurry containing 50% silica in 50% water) 80 litres Steel fibres (0.4 mm x 30 orOA mm X 25 mm 52 Kgs 40 The slabs are formed by conventional casting methods and the reinforcing bars are incorporated in the reinforcing ribs in known manner.

All the above-described slabs have a weight not exceeding 20 Kg and meetthe requirements of Performance Specification MOB 08.801.

Claims

1. A rectangular reinforced concrete slab fora platform floor having a flat upper surface and having on its underside a downwardly projecting peripheral reinforcing rib extending around the slab periphery and a plurality of downwardly projecting stiffening ribs extending across the underside of the slab in two orthogonal 50 directions, the stiffening ribs extending in at least one direction extending to a lesser depth belowthe upper surface of the slab than the peripheral rib, the slab having a weight not exceeding 20 Kg when having a square upper surface area of 600 x 600 mm and fulfilling the medium-grade structural requirements of Property Services Agency Performance Specification 08.801.

2. A slab according to Claim 1, in which the underside of the slab has an inner reinforcing rib extending 55 continuously around the slab adjacent and within the peripheral reinforcing rib, the inner reinforcing rib extending below the slab upper surface to substantially the same depth as the peripheral rib.

3. A slab according to claim 2, in which the peripheral and inner reinforcing ribs are of substantially trapezoidal cross-section and extend to a depth of about 40 mm below the slab upper surface, the peripheral rib has a lower surface width of about 25 mm and the inner reinforcing rib has a lower surface width of about 18 60 mm and the centre of the inner reinforcing rib is separated from the slab edge by about 75 mm.

4. A slab according to claim 2 or 3, in which the stiffening ribs extending in one direction are of lesser depth below the slab surface than the peripheral rib and the stiffening ribs extending in the other direction are of lesser depth below said surface than the ribs extending in said one direction, the strengthening and stiffening ribs being separated byflat-bottomed recesses.

4 GB 2 181 171 A 4

5. A slab according to claim 4, in which the stiffening ribs extending in one direction have a depth belowthe upper slab surface of about 34 mm and the stiffening ribs extending in the other direction have a depth of about 28mm.

6. A slab according to claim 4 or 5, in which the stiffening ribs are of substantially trapezoidal cross-section, have a lower surface width of about 18 mm and are separated from each other by a centre-to-centre distance Of 5 about 90 mm.

7. A slab according to claim 1, in which the portions of the underside of the slab adjacent the angles of the slab are provided with reinforcing ribs extending diagonally across the slab.

8. A slab according to claim 7, in which the peripheral reinforcing rib and the diagonal reinforcing ribs are of substantialiytrapezoidal cross-section, extend belowthe slab upper surface by about 40 mm and have a 10 lower surface width of about 25 mm.

9. A slab according to claim 8, in which the stiffening ribs extending in one direction have a depth belowthe slab upper surface of about 40 mm and the stiffening ribs extending in the other direction have a depth of about 34mm.

10. A slab according to claim 1, in which the underside of the slab is provided with diagonal reinforcing ribs 15 extending across the slab between opposite corners of the slab.

11. A slab according to claim 10, in which the peripheral reinforcing rib extends downwardly to a depth of about 40 mm below the slab upper surface and the diagonal reinforcing ribs extend to a depth of about 34 mm, the reinforcing ribs being of substantially trapezoidal cross-section and having a lower surface width of about 25 mm.

12. A slab according to anyone of claims 7 to 11, in which the recesses formed between the reinforcing ribs are provided with auxiliary ribs of triangular cross-section extending parallel to the slab edges.

13. A slab according to any preceding claim, in which the outside surfaces of the peripheral ribs are inwardly inclined in the downward direction.

14. A slab according to any preceding claim, in which the slab edges are provided with a protective lipping 25 of plastics material or with a metal edging.

15. A slab according to any preceding claim, in which the lower surfaces of the peripheral reinforcing rib at the corners of the slab are provided with recesses.

16. A slab according to any preceding claim, formed by casting a concrete containing the following constituents per cubic metre:

Cement inert filler of density from 800 to 1000 K 91M3 and particle size up to 3 mm Pulverized fuel ash Micro silica slurry Steel fibres from 650 to 750 Kg from 750 to 850 Kg from 120 to 160 Kg from 70to 90litres from 45to 60Kg.

17. A concrete slab fora platform floor, substantially as hereinbefore described with reference to Figures 1, 40 2 or3 or4of the accompanying drawings.

Printed for Her Majesty's Stationery Office by Croydon Printing Company (L) K) Ltd,2187, D8991685.

Published by The Patent Office, 25 Southampton Buildings, London WC2A 'I AY, from which copies maybe obtained.

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