CA2121452A1 - Concrete raft foundation - Google Patents

Abstract

CONCRETE RAFT FOUNDATION
ABSTRACT
A concrete raft foundation is cast integrally to form an upper slab and a plurality of downwardly depending joists in the form of a grid pattern on the underside of the slab. The joists are defined in the casting process by wooden box forms which are laid on the ground prior to the casting process. The wooden box forms are defined by planar sheet material and reinforcing posts. The rebar for the concrete slab and the joists is supported in position by support elements carried on the box forms.
Each box form includes a base plate which projects outwardly to define a shoulder onto which the concrete is cast. A duct can be cast into the concrete at the periphery.

Description

~ 2121~52 CONCRETE RAFT FOUNDATION
FIELD OF INVENTlt)N
This invention relates to a concrete raft foundation for resting upon a ground surface and for supporting a buildin~ thereon and to a collapsed box forrn for use in manufacturing the foundation.
BACKGROUND (;)F INVENTION
Concrete raft foundations are designed as plates strong enough to exert a continuous uniform pressure on the soil below over the entire raft area, regardless of the distribution of loads superimposed on them. Rafts rnay be used to s,~read the weight of buildings uniformly over an area of soil havin~ a low load bearing capacity or a layer of relatively soft rigid insulation placed over permanently frozen ground (permafrost) to prevent thawing. Rafts may also be used to increase the bearing pressure on heaving soils (clay) to exseed their swelling pressure and thereby prevent uplift, unevenness and cracking. The simplest raft is a solid concrete slab with two way ~rids of steel reinforcing bars near its top and bottom. This type of raft is commonly used in heavy industrial construction but is uneconomical for light residential and commercial buildings. In these cases the raft is constructed with a thin base slab including a system of ups~anding narrow concrete ribs and a thin top slab, leaving the spaces between the ribs void. This method saves on concrete and reinforcing steel but construction labour is exceedingly high and it is therefore hardly ever used.
This invention permits construction of reinforced concrete raft foundations for light industrial, residential and commercial buildings eco~nomically by rninimizing the amount of labour and materials required.

2 2121~2 An essential part nf this invention are prefabrieated, self-aligning, self spacin~, expendable wooden void forms. These permit simultaneous concreting of the raft top slab including a two way rib system below it. Each typical void form consists of four equal sides and a square top cover. All the pieces are manufactured off site, shipped flat and assembled at the construction site into boxes with 4 sides and the top closed and open at the bottom. The materials used in the manufacture of the void forms are preferably the least expensive rough cut lumber available (for example popl?~r) and preferably the most inexpensive sheet material that can withstand the pressure of the wet concre~e ~for example 7/16"
aspenite).
The base rails along the bottom of the sides are the only parts of the void forms that are in contact with the soil and are therefore made of preservative treated wood preferably pine to prevent rotting. These box bases, which project one half of the widths of the ribs past the face of the boxes all around, aet as void form spacers and prevent the boxes from floating up In the wet concrete.
The dimensions of the void forms are governed by engineering and economical considerations and thus can vary in accordance with requirements. In one example i~ appears that 4" (100mm) wide by 12"
i300mm) deep ribs spaced at 4'0" ~1.20 m) on centers both ways with 4"
(100mm~ thick top and bottom slabs or 6" (152mm) wide and 16" (400mm) deep ribs at 4'0" on centers with 4" thick top and bottom slabs will satisfy most requirements. The rafts with top and bottom slabs would then become 20" and 24" thick rafts. Without the bottom slab the rafts would become 16" and 20" deep. By modifying the dimensions of the void forms 3 2~21~2 the width of the ribs may be adjusted as required for distribution of point loads ~columns etc.).
Example No. 1: A RAFT WITH TOP AND BQTTOM SLAB
This type of raft is especially suited for areas with soils havin~
low bearin~ capacities. The construction procedure is as follows:
Excavate the area to be covered by the raft plus one foot all around to the desired depth of the raft plus three inches l75mm). Install the forming for the edge of the raft all around, its top bein~ the top of t~ e raft.Level the bottor~ of the excavation by screedin~ a layer of wet sand over it with the top of the sand bein~ 1 1/2" below the bottom of the raft. Place a 10 rnil. thick polyethylene film over the sand and turn its edges up to the top of the side forms. Pour a 4" thick concrete slab, reinforced with a two way grid of reinforcing bars in its cent~r and screed its top to level. When the concrete has su~ficiently hardened to walk on it place the prefabricated self spacin~ wooden void forms on it. Place reinforcing bars into the bottom of the 2-way joist spaces usin~ donut shaped bar supports. Install joist spacers at the top edges of the void forms which are also reinforcing bar supports for the top slab reinforcing.
Install the two way grid of top slab reinforcing, to be iocated in the center of ~he 4" thick top slab. Concrete th~ joists into the spaces between the void forms and the slab over their tops, preferably by pumpin~
the concrete. Screed and steel trovei the top of the rafts to produce a smooth and level floor.
Exarnple No. 2: A RAFT WITH T()P SLAB ONLY ~ `-This type of ra~ is especially suitecl for areas with heavin~ soils (clay). The construction procedure is the same as in example no. 1, except ~ 4 .
that the 4" bottom slab is omitted and the depth of the excavation is less by the thickness of this slab. The idea is that the width of the concrete joists is selected to exert the maximum allowable bearing pressure for the particular subsoil and if swelling occurs the swellin~ pressure is relieved sideways into the void spaces without harming the integrity of the raft.
The thickness of the top and/or bottom slabs including the width and depth of the joists can be selected as required by standard engineering principles for any raft application. The relatively thick raft slabspermit the incorporation of air ducts for heating andlor ventilation purposes, thus eliminating the need for overhead ductwork. A layer of rigid thermal insulation around the periphery of the raft may also be incorporated in the raft to reduce heat losses.
One embodiment of the invention will now be described in conjunction with the accompanying drawinys in which BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a vertical cross-sectional view through a portion of a concrete raft according to the present invention, the cross-section being taken along the lines 1-1 of Figure 6.
Figure 2 is a cross-sectional view along the lines 2-2 of Figure 1.
Figure 3 i5 a cross-sectional view alon~ the lines 3-3 of Figure 1.
Figure 4 is an isometric view showing the structure of one of the box forms of the concrete raft foundation of Figure 1.
Figure 5 is a verticai cross-sectional view through a support s~ructure for ~he peripheral wall of the concrete raft.

... j .- . . . . . ..

5 %3~14~2 Figure 6 is a top plan view of one portion of the concrete raft of Figure 1.
DETAILED DESCRIPT!QN
A concrete raft foundation is shown particularly in Figures 1 and 6 and comprises an upper slab 10 together with a plurality of depending joists 11. The slab 10 has a predetermined substantially constant thickness across the full extent of the slab and the joists arranged, as shown in Figure 6, in parallel row.s and columns extending downwardly from the underside of the slab and integrally formed therewith.
Within the slab is provided a grid 12 of reinforcing bars 13 and 14 arranged in a grid pattern at right angles of a spacing sufficient to provide the necessary strength within the slab in accordance with good engineering practice.
At the base of each joist is provided also a reinforcing bar 15 so again the reinforcing bars 15, 16 are arranged again at right angles with each running along a respective one of the joists. The thickness of the joist, the hei~ht of the joist and the dimensions of the reinforcing bars are again selected in accordance with good en~ineering practice and these details are not essential to the present invention.
The slab and the jois~s are formed by casting in situ with the forms provided by box forms 17 of the construction shown particularly in Fi~ures 1 and 4. The box forms remain in place after casting and consti~ute an integral part of the concrete raft construction.
Each box form comprises four side panels 18, a top panel 19 and base pla~es 20.

2121~52 --~ 6 Each base plate 20 comprises a wooden plate having a depth of for example 2 inches and a transverse width of for example 6 inches and a len~th substantially equal to the len~th of one side of the box.
Each side panel 18 comprises a sheet 21 of a water resistant structural material such as aspenite which has sufficient strength to receive and support the wet concrete when cast and is resistant to ro~ting and deterioration. Attached to the aspenite sheet is a bottom strip 22 of rough lumber of for example 2" x 2" dimension and runnin~ along substantially the full len~th of the sheet. On top of the bottom strip 22 is provided a plurality of posts 23 which extend vertically upwardly therefrom. Each sheet has three such posts attached thereto one at an end of the sheet and the other two positioned at 1/3 and 2/3 of the way along the sheet leaving the far end free from a post for receiving the end post of the next adjacent sheet.
Thus each of the side panels 18 are identical and when attached together form the structure shown in Fi~ure 4 with the sheets upstanding and at right angles so as to form an open box having an open bottom and an open top. Each of the posts 23 is spaced from the top edge 24 of the sheet 21 by a distance equal to the width of the rough cut lumber material together with the thickness of the aspenite sheet for receiving the top panel 19 thereon. The bottom strip 22 is nailed to the underside of the post 23 which in turn are nailed to the sheet. The bottom strip 22 is then nailed vertically downwardly onto the base plate 20 so as to form the side sheet and the base plate into an inte~ral structure as best shown in Figure 1. The base plate 20 extends outwardly beyond the outside surface of the sheet 21 and also may extend inwardly beyond the inside surface of the bottom strip 22.

7 2121~.~2 The top plate 19 comprises a square sheet of the same sheet material on the underside of which is attached four strips 26 of the rough cut lumber. These act as joists Iying across the top of the posts. The two end strips adjacent the edges of the sheet 25 lie across the top of the posts of one side panel 18 and the two central strips span the width of the box from a post on one side to a post on the other side. Thus the top sheet is structurally supported by the joists or strips 26 forming a rigid box form.
The box forms can be shipped in partially assembled flat construction including the top panel 19 and, separately, the base plates 20 with the side panels 18 attached thereto. The box forms can then be rapidiy assembled simply by nailing together the four side panels and the top panel onto the top of the posts of the side panels. When assembled the box form takes up the shape shown in Figure 4 which defines a closed top box with the base plate extending around the base of the box and presenting a shoulder projecting outwardly therefrom.
The construction further includes a plurality of rebar support elements generally indicated at 30 for supporting the top reinforcing bars 13 and 14 within the slab and rebar support elements 31 for supporting the reinforcing bars 15 and 16 at the bottom of the joist.
Each of the rebar support elements 30 comprises a small sheet portion 32 of the sheet material to3ether with a transverse supporting wood strip 33 to which the sheet is nailed. The width of the sheet as best shown in Fi~ure 1 is such that it just fits between adjacent upright sheets 21 of adjacent box forms. The spacin~ between the upriyht sheets 21 is defined by the width of the shoulder portion of the base plate 20 which projects outwardly beyond the upright sheet. Thus as shown in Fi~ure 1 the plates ~_~ 8 2121d~2 are arranged to abut thus definin0 a specific spacin~ between the upright plates and thus the width of the cast concrete joist. The wooden strip 33 spans the space between the upright strips and thus rests upon the top edge of the strips and upon the upper surface of the sheet 25. The sheet portion 32 hangs downwardly between the upright sheets 21 and also projects slightly upwardly as indicated at 34 so that the position of the top edge of the sheet portion 32 defines that acquired spacing between the underside of the reinforcing bar 14 and the top surface of the sheet 25.
The reinforcing bar support elements 30 are positioned between each box form and the next box form along the parallel rows therebetween and thus define a common horizontal plane for receiving the reinforcing bar 14 thereupon. The reinforcing bars 13 which are arranged at right angles to the bars 14 can simply lie over the top of the bars 14. As is conventional practice, the bars are then tied together by tie wires to hold the structure integral.
The rebar support elements 31 comprise a plurality of donuts in the form of a thin disc 35 of square outside shape and with a central opening 36 for receivin~ the reinforcing bar 15 threaded there through. The discs or donuts can be cut simply from a block of wood which is drilled longitudinally with the hole 36 and is then cut into the separate pieces as shown in Figurc 3. After threading of the disc onto the reinforcing bar 15, they can be properly spaced along the length of the reinforcing bar and the reinforcing bar lowered into the location for receiving the cast concrete forming the joist so that the disc sits upon the upper surface of the base plates 20. The reinforcing bar 16 are then simply laid across the top surfaces of the bars 15 again as shown in Figures 1 and 3.

9 2121~52 The outside peripheral edge of the raft foundation is defined by a layer of insulation material 40 and an upright sheet 41, the latter being exposed at the ed~e of the raft foundation to defin~ a facing plate visible at the edge.
Around the periphery of the box forms, an outer most joist 42 is wider ~han the joists between the box forms. During formation of the raft foundation, the outer plate 41 and the insulation layer 40 are connected together and attached ~o a nailing strip 43. A support structure for the outer piate comprises a wedge 44 holding the lower most edge in position to prevent toppling inwardly. The plate 41 is then supported by a temporary strut arrangement including a vertical wedge 45 and an inclined beam 46. The wedge 45 is fastened to the plate 41 by a temporary nailing strip 47. The beam 46 is supported its outer end by a second wedge 48.
The temporary structure thus holds the plate 41 in vertical orientation for the subsequent poring of the concrete.
The thicker outer joist 42 allows the mounting in that joist of a duct 50 of circular cross-section which is embedded within the concrete of the joist at a position spaced from the top surface at a depth so that it is accessible for connection of an air outlet 51 duct. Thus the duct 50 is available for connection to a source of ventilation, heating or cooling air without the necessity for internal duct work within the building.
In manufacture of the raft foundation, initially the site is prepared by the steps of firstly excavatin~ the area to the desired level removing all boulders and other obstacles, secondly installing the necessary services to ~he site including plumbing, electrical and gas; and thirdly placing a layer of 1-2" of wet sand across the site for leveling purposes.

Io 21~1~52 Where build-up is acquired due to low areas of the site, it is necessary to place more wet sand and to vibrate the sand to form a level and firm base.
The sand extends beyond the boundaries of the forms or vertical plates 41 for at least one foot. A sand fill layer 55 is applied partly up the outer plate41 to provide structural support for the plate.
The outer sheet 41 and the insulation layer 40 are generally supplied as separate elements. In one example the outer sheet 41 can comprise the p,uralitv of such sheets of dimensions 2' x 8' of 1/2" plywood.
The insulation is supplied in dimensions 18" x 8' x 2" thick of rigid Styrofoam board and these are connected using the 2 x 4 nailer 43. 3 1/2"
nails are driven through the outside of the sheet 41 and the insulation into the nailer to hold the nailer strip 43 in place. The nailer strip 43 carries theplurality of 3 1/2" nails which are directed inwardly into the interior to engage into the cast concrete and to anchor the nailing strip to the concrete when cast. The insulation is sandwiched between the plywood outer sheet and the nailer in showing that the bottom of the insulation and the plywood are flush and the top of the nailer is 1 1/2" below the top of the insulation.
This leaves a spacing of approximately 6" between the top of the insulation and the top of the outer sheet 41. The nailer 43 extends beyond the end of the plywood and insulation to enable the forms to be fastened end to end.
The forms are installed in showing that ~he insulation ship lap joints but together following which the extending nailer is nailed to the next form thus tying the forms together. The inclined brace 46 and wedge forms are located at ends of the sheets and also at 4 foot spacing to provide sufficient structural stren~th to receive the concrete when poured. The ., . .. . , ~.......... :

. , , ll 21~ 2 backfill 55 is formed from a layer of wet sand which has a clay layer over the sand for a water shedding arrangement.
The box forms are supplied in flat arrangement and are fabricated and laid throughout the interior of the form as shown in Fi~ure 6.
It may be necessary to cut to length a row of the box forms so that they are no longer square but are reduced width so as to ensure that the box forms arranged edge to edge across substantially the full width of the form for the foundation.
Prior to laying the box forms in position, a heavy gauge moisture barrier is laid over the entire foundation and up the sides of the foundation forms to the level of the nailer. The boxes are then laid in position edge to edge as shown in Figure 6. Preferably the boxes are installed with the cover panel o~ the box omitted thus enabling ~he boxes to be lifted and carried into position more easily. The cover panels are then attached and nailed into position to complete the rigid structure of the box.
The donuts are then threaded onto the lower rebar 15 and the bars lowered into piace between the box forms. The transverse rebars 16 are then lowered on to the parallel bars 15 to form the required srid pattern at the base of the joist following which the bars are tied together to properly locate the bars 16 along the joist.
In an arrangement where the duct 50 is used, aspenite bases nailed to a 2 x 4 spacer are placed in the perimeter duct space prior to the rebar being placed. The duct work is then laid according to the plan and tied down to the aspenite base with tie down straps. This prevents the duct work from floatin~ up when the concrete is poured.

12 212~4~2 ,~

The upper sp~ces 30 are then placed in position at ths mid-point between each side of each box and nailed to the boxes. This spacer holds the spacing between the boxes and also serves as the support for the top rebars previously described. The rebar is then placed in position on top of the supports, the transverse rebars are applied and the tying is completed.
When the forms and the box forms are complete and assembled and the rebar applied, the concrete is poured over the area to form simultaneously the concrete slab and the joists both around the periphery and between the box forms.
The structure of the present invention provides a raft foundation of si~nificant strength in view of the depth of the joists while reducin~ the wei3ht of the raft foundation itself due to the voids defined by the box forms. The base plates of the box forms receive the cast concrete thereon so as to hold the box forms down in position and to prevent any floating .
The structural stren~th of the raft foundation while avoiding the very hi~h weight of a constant thickness of foundation allows the foundation to lie upon compressible soils without the necessity for prior vigorous compaction conventionally required. The strength of the raft foundation allows it to aceommodate some compaction of the soils or the formation of voids in certain areas under the raft foundation where the danger of collapse.
In areas where heaving of ~he soil can be expected by moisture absorption or by frostl this heavin~ is accommodated b~ the soil being pressed into the void under the box form without heaving the joists. Thus _~ ~3 21 21 4 ~2 the void under the box form relieves the pressure in the soil without heaving of the slab itself or danger of cracking of the slab at the joists.
Since various modifications can be made in my invention as herein above described, and many apparently widely different embodiments of same made within the spirit and scope of the claims without departing from such spirit and scope, it is intended that all matter contained in the accompanying s,r,ecification shall be interpreted as illustrative only and not in a limiting sense.

Claims (20)

1. A collapsed box form for use in constructing a concrete raft foundation comprising four separate rectangular side panels for connection to form box walls with an open top, and a cover panel for positioning over the open top, each of the side panels and the cover panel being formed from a plurality of wooden reinforcing strips and a rigid sheet material attached thereto, the sheet material being resistant to moisture from the cast concrete and of sufficient strength to support the weight of concrete cast thereon, and four base plates each formed from preserved wood having a length substantially equal to the length of the respective side and arranged such that, on assembly, each side wall has a respective one of the rigid base plates thereon projecting outwardly to at least the outer side thereof for resting on the ground surface and receiving the cast concrete thereon.

2. The box form according to Claim 1 wherein each of the side panels includes a plurality of vertical wooden reinforcing strips and wherein the top panel includes a plurality of horizontal wooden reinforcing strips, the reinforcing strips being arranged such that the horizontal reinforcing strips rest upon upper edges of the vertical reinforcing strips.

3. The box form according to Claim 1 wherein each of the side panels includes a bottom wooden reinforcing strip extending horizontally therealond, the bottom reinforcing strip being arranged for attachment to the rigid base plate.

4. The box form according to Claim 1 wherein each of the side panels includes a plurality of vertical wooden reinforcing strips, a first of the strips being arranged at one end of the panel and further ones of the strips being arranged at spaced positions along the length of the panel with an opposed end of the panel being free from a reinforcing strip so as to allow engagement with the reinforcing strip at one end of an adjacent one of the side panels.

5. The box form according to Claim 1 wherein the sheet material is formed from aspenite.

6. A concrete raft foundation for resting on a ground surface and for supporting a building thereon, the foundation comprising an upper cast concreted, integral slab member defining an area of the foundation, a plurality of spaced depending cast concrete joist elements formed integrally with the slab member on an underside thereof, the joist elements including two sets arranged mutually at right angles with the elements of each set being parallel and substantially equi-distantly spaced across the slab member, the elements of the two sets having intersections at right angles and being integrally connected at the intersections so as define a plurality of rectangular cells between adjacent joist elements, a plurality of reinforcing bars arranged in a grid pattern in the slab member, a reinforcing bar extending longitudinally of each joist element, and a plurality of box forms each arranged in a respective one of the cells, each box form comprising four rectangular side panels interconnected to form box walls with an open top, a cover panel over the open top, each of the side panels and the cover panel being formed from a plurality of wooden reinforcing strips and a rigid sheet material attached thereto, the sheet material being resistant to moisture from the cast concrete and of sufficient strength to support the weight of concrete cast thereon, each side wall having a rigid base plate thereon projecting outwardly to at least the outer side thereof resting on the ground surface and receiving the adjacent joist element thereon.

7. The foundation according to Claim 6 wherein each of the side panels includes a plurality of vertical wooden reinforcing strips and wherein the top panel includes a plurality of horizontal wooden reinforcing strips, the reinforcing strips being arranged such that the horizontal reinforcing strips rest upon upper edges of the vertical reinforcing strips.

8. The foundation according to Claim 6 wherein each of the side panels includes a bottom wooden reinforcing strip extending horizontally therealong, the bottom reinforcing strip being attached to the rigid base plate.

9. The foundation according to Claim 6 wherein each of the side panels includes a plurality of vertical wooden reinforcing strips, a first of the strips being arranged at one end of the panel and further ones of the strips being arranged at spaced positions along the length of the panel with an opposed end of the panel being free from a reinforcing strip so as to allow engagement with the reinforcing strip at one end of an adjacent one of the side panels.

10. The foundation according to Claim 6 wherein the sheet material is formed from aspenite.

11. The foundation according to Claim 6 wherein the base plate of one side wall of the box form has a side edge thereof in a butting engagement with a side edge of the base plate of a parallel side wall of an adjacent box form so as to hold the adjacent side walls of the box form and the next adjacent box form at a required spaced position.

12. The foundation according to Claim 11 wherein each of the base plates comprises a wooden beam.

13. The foundation according to Claim 12 wherein the wooden beam is arranged beneath a lower edge of the respective side wall and projects outwardly to both sides thereof.

14. The foundation according to Claim 6 including a spacer member mounted on an upper edge of a side wall of a box form and upon an upper edge of a parallel side wall of an adjacent box form for supporting the reinforcing bars of the grid pattern in the slab member.

15. The foundation according to Claim 14 wherein each spacer member comprises a first portion bridging the space between the adjacent side walls and resting upon the respective top panel of each of the box forms and the second portion in engagement with outside surfaces of the respective side walls to hold the outside surfaces at the required spaced position, the second portion including an element thereof projecting upwardly from the top panels for receiving the reinforcing bars resting thereon.

16. The foundation according to Claim 15 wherein each box form includes four such spacer members each arranged on a respective side wall of the box form and extending therefrom to the next adjacent box form.

17. The foundation according to Claim 6 wherein the reinforcing bar extending longitudinally of each joist element includes a plurality of ring members engaged there around at spaced positions therealong, each of the ring members including a portion thereof extending downwardly from the reinforcing bar into engagement with the base plate

18 for supporting the reinforcing bar at a position spaced upwardly from a lowermost surface of the joist element.
18. The foundation according to Claim 6 including a plurality of outer support wall members extending substantially vertically at outermost edges of the slab member and extending vertically from an upper surface of the slab member downwardly to a position at a bottom of an outermost one of the joist elements, the outer wall being separate from the box forms and spaced outwardly therefrom so as to define a space therebetween receiving said outermost joist element.

19. The foundation according to Claim 18 wherein the outer wall includes a layer of an insulation material thereon.

20. The foundation according to Claim 19 including a duct member laid in the slab member adjacent the outer wall for communication of heated air around the periphery of the slab member.