Residential basement flooring system and method using pier capitals for supporting pre-cast slabs

Abstract

A residential flooring system is provided for use with expansive soil. The system includes a plurality of pre-cast slabs of hardened material such as concrete with or without structural members such as rebar and/or wire mesh. The system also includes structural members such as drilled piers, helical screws, caissons, or the like that contact the expansive soil and extend upward away from the expansive soil. The structural members have an upper contact surface that extends above the soil. Weight bearing members are attached to the structural members such as to their upper contact surfaces. Each of the weight bearing members includes a bearing surface that is larger (e.g., has a greater area) than the upper contact surface of the pier or other structural member. The pre-cast slabs are positioned on the weight bearing surfaces so that the slabs are supported by the structural members via the weight bearing members.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/679,758 filed May 11, 2005, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates, in general, to the construction of concrete slab floors and flooring systems for residential basements, and, more particularly, to a concrete flooring assembly, and method of fabrication, for use with expansive soils that utilizes pre-cast structural slabs to create void spaces and provides pier capitals to effectively support the slabs a distance above the soil.

2. Relevant Background

Residential buildings or houses are often built on foundations comprising vertical perimeter walls of poured concrete. Since the vertical foundation walls are structural members which support the building, they are usually several feet in depth and function as beams bridging between footers or piers resting on bedrock or stable soil. It is common practice in such buildings to provide a basement, or ground floor, in which at least a portion of the basement walls include the vertical foundation walls and in which the basement floor is a poured concrete slab resting on the soil enclosed by the foundation walls. Typically, the foundation is constructed by first excavating a pit for the basement and for the foundation footers. Then, forms are erected around the periphery of the pit and concrete for the foundation walls is poured into the forms. Next, a form or pan is provided for the flooring slabs and concrete is poured into the floor pans (or, in some cases, the floor is poured directly onto the soil).

A major problem with conventional construction in certain soil and climate conditions is that the location of the basement floor can be unstable due to movement of the underlying soil. Expansive soils are prevalent in many areas of the Unites States and other countries. These expansive soils can expand and contract considerably as a result of cyclical changes in moisture content and/or as a result of freezing and thawing cycles. The soil expansion and contraction problem can be especially severe when the floor is simply a slab of concrete poured onto the surface of the soil that forms the floor of the excavation pit. For example, certain dense clay soils tend to dry out after excavation and then later absorb water and swell. This swelling or expansion causes the slab to move relative to the foundation walls which can generate large forces that are sufficient to crack or break the slab. In general, because the foundation walls must support the building, they are supported by piers or pads on solid ground or bedrock or piers or pads on footings and therefore are very stable. However, when the basement floor is a relatively thin slab of concrete having a large surface area and resting on a large area of soil, it is highly vulnerable to movement due to expansion and contraction of the soil as water is absorbed and released by the soil. The relative motion between the slab and the walls can damage interior walls.

A variety of techniques have been implemented to control the effects of expansive soils on concrete foundations and structural slabs or floors. Generally, each of these techniques attempts to separate the foundation walls and structural slabs or flooring from the heaving soils or to at least absorb some of the expansive forces created by the moving soil. Unfortunately, these techniques have proven to be costly to implement, have increased the complexity of fabricating concrete foundations and flooring, have sometimes caused long-term structural or safety problems, and have reduced spacing between the floor and ceiling.

For example, a common technique of protecting the foundation and slab from the expanding soil is to create a void space under the concrete slab. To create the void, cardboard forms or other degradable material forms are positioned under the form or pan used during pouring of the foundation walls and floor. With time, the material of the void form begins to deteriorate creating a void in which the soil can expand without moving the wall or floor. However, the degradation of the forms typically is accompanied by mold growth and the release of associated toxins, which can result in safety issues within the structure above the concrete foundation. Additionally, jobsite delays and inclement weather during initial construction can result in premature degradation of the cardboard void form and loss of the strength needed to support the curing concrete wall and floor. Each of these techniques involves carefully arranging the void forms and pans onsite in the excavated pit and then pouring concrete that makes up the flooring on site, and typically, the floor is a single, monolithic structure. This onsite work is time consuming and often results in delays due to the time required for finishing and setting of the concrete and due to weather issues.

There remains a need for an improved flooring method and system for creating structural or flooring slabs and protecting the installed slabs from the effects of expansive soils. Preferably, such a method and system would be relatively inexpensive to implement in the cost-sensitive construction industry and lend itself to the field conditions associated with excavating soil and forming structures with concrete. Further, the method and system preferably would result in void spaces being created under structural slabs and control or reduce the use of degradable void forms that may generate mold or need to be removed.

SUMMARY OF THE INVENTION

The present invention addresses the above problems by providing a flooring method and system that includes capitals or plinths on top of piers to provide a wider or extended weight bearing surface. Pre-cast or preformed slabs are placed on the capitals and corresponding piers prior to building the foundation walls of the residential structure. The slabs of the invention are typically pre-cast concrete slabs that include supporting, monolithic ribs or joists of particular width, depth, and spacing for each situation (e.g., for differing load bearing situations). In some embodiments, a poly foam spacer or insulation is inserted between the ribs prior to casting so as to reduce the overall weight, e.g., to form channels between the ribs, and to thermally insulate the floor from nearby soil and air. The pier capitals are included in the flooring system to increase the bearing area of a common drilled pier, caisson, helical screw, or other standard foundational support, and the capitals may be formed in a single pour with a concrete pier or later attached to the foundational support. L-shaped bars, e.g., steel, glass reinforced plastic, or the like may be provided in the slab to provide a mating component and/or lateral support for a wall later poured on or formed on the slabs. Also, the slabs typically will include carrying or edge beams two at opposing ends with the ribs or joists extending and contacting the edge beams.

More particularly, a residential flooring system is provided for use with expansive soil. The system includes a plurality of pre-cast slabs of hardened material such as concrete with or without structural members such as rebar and/or wire mesh. The system also includes structural members such as drilled piers, helical screws, caissons, or the like that contact the expansive soil and extend upward away from the expansive soil. The structural members have an upper contact surface that typically extends above the soil. Weight bearing members or pier extensions are attached to the structural members such as to their upper contact surfaces. Each of the weight bearing members includes a bearing surface that is larger (e.g., has a greater area) than the upper contact surface of the pier or other structural member. The pre-cast slabs are positioned on or abutting the weight bearing surfaces so that the pre-cast slabs are supported in the system by the structural members via the weight bearing members.

In typical embodiments, the pre-cast slabs and weight bearing members are formed of concrete. The weight bearing members generally have sides that extend upward and outward from the upper contact surface of the structural members or piers, e.g., take a frustoconical shape with sides that angle at 40 to 80 degrees (as measured from a horizontal plane passing through the upper contact surface to the sides). The pre-cast slabs may include a planar member, a pair of edge or carrying beams extending from the planar member, and two or more monolithic ribs extending in a spaced-apart manner from the planar member between the edge beams. The edge beams of each of the pre-cast slabs typically abut or contact the weight bearing surfaces such that the slabs are supported a void distance above the expansive soil. In some embodiments, the pre-cast slabs also include insulation positioned between adjacent pairs of the ribs. Further, the system may include a plurality of wall sections positioned upon the pre-cast slabs such that the wall sections are supported first by the slabs and then by the weight bearing members and structural members. In other embodiments, the edge or outer slabs are instead supported by pre-built foundation walls on one end, such as with an angle iron ledge, dowels, keys, or other support members extending outward from the foundation wall, and on pier/capital combinations on the other end.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a residential floor assembly according to an embodiment of the invention including pre-cast slabs positioned on piers with weight bearing extensions or capitals;

FIG. 2 is a sectional view taken along line 2-2 in FIG. 1 showing a cross section of the piers and attached capitals;

FIG. 3 is a sectional view taken along line 3-3 in FIG. 1 showing a cross section of a slab showing carrying or edge beams and a insulating foam-filled channel formed between joists or ribs of the slab;

FIG. 4 is a sectional view taken along line 4-4 of FIG. 1 showing joists or ribs in a slab and the channels formed there between that may optionally be filled with insulating foam; and

FIG. 5 illustrates a perspective view with a cutaway section of a pre-cast slab of one embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawings, like reference numerals indicate like features, and a reference numeral appearing in more than one figure refers to the same element. The drawings and the following detailed descriptions show specific embodiments of the invention with numerous specific details including materials, dimensions, and products being provided to facilitate explanation and understanding of the invention. However, it will be obvious to one skilled in the art that the present invention may be practiced without these specific details and these broader embodiments of the invention are considered within the breadth of the following claims.

In general, the invention is directed to a flooring system that is particularly well suited for use in residential building. The flooring system typically includes a number of structural slabs or assemblies that are adapted for mounting on piers or drilled piers that are modified to include a weight bearing extension or capital. The piers with capitals may be provided in the soil to receive two edge portions of abutting slabs and in some cases four abutting slabs may be supported by a capital and its pier (e.g., when four corners of four adjacent slabs meet). The flooring system may include pre-cast wall sections that are mated with and supported by the slabs or may include a wall that is poured upon an assembled set of flooring slabs. The mounted pre-cast slabs provide void spaces to allow the flooring assemblies to be placed on or in expansive soil. Significantly, these void spaces under the structural slabs and walls are provided without the use of cardboard, wood, and other degradable materials that may rot, mold, or deteriorate in a manner that causes undesirable off-gases or other safety problems or that may increase the complexity and cost of the flooring assembly or significantly reduce the expected life and/or reliability of the finished structure. The flooring assemblies are discussed in detail below with reference to FIGS. 1-5.

One embodiment of a foundation or flooring assembly 100 of the present invention is shown in FIG. 1. Prior to the invention, flooring for building such as individual residences were poured on site, which required significant amounts of concrete finishing and fabrication processes that can be difficult based on weather. In contrast, the flooring assembly 100 makes use of pre-cast slabs 130 that can are manufactured offsite, such as at a warehouse or manufacturing facility, are transported to the site, and then lifted and positioned to form a floor. The slabs 130 are typically formed of concrete and may include structural components such as rebar, mesh or wiring, or the like to increase their strength. Each slab includes a horizontal or planar member (e.g., upper surface) in which may be provided a receiving groove or slot (or other mating components such as dowels when wall pouring is performed) 133 at an end.

With this in mind, the assembly 100 includes wall sections 150 that in this embodiment are pre-cast, too. The wall sections 150 are also typically formed of concrete and are positioned on the floor 130 to mate with the groove 133 in horizontal member or support surface 132. To facilitate mating the wall 150 with the slabs 130, an edge or side 152 of the wall 150 is formed to include a protruding ridge or key that is configured to fit within the ridge or keyway 133 in the slab 130. In other embodiments, the wall 150 is poured after placing the floor slabs 130 and in these cases, dowels may protrude from the end of the slab 130 in place of or in addition to the slot or keyway 133. While not shown, adjacent slabs 130 may also have surfaces adapted for better mating of slabs 130 such as one having an “L” shape while the other has an inverted or opposite “L” shape to receive the next laid slab 130.

As shown, the structural slabs 130 and wall sections 150 are supported by piers 110 above the soil or excavation floor 104 to create a void space between the slabs 130 and the soil 104, which may expand and contract. The piers 110 may be poured concrete, helical screws, or the like, e.g., may be typical drilled piers or caissons as are well-know in the building industry. Significantly, weight bearing extensions or capitals 120 are provided on top of the piers 110 so as to better support the slab and wall weight and to eliminate the need for degradable wood or steel beams and/or void boxes while still providing adequate structural support for the slab during the ongoing use of the slabs 130 in a structure. As shown, the piers 110 typically have at least a portion that extends outward from the soil 104, and the capitals 120 extend from this portion of the pier 110. The capitals or plinths 120 provide a receiving or weight bearing surface 126 formed by the sides 122 that angle outward from the pier 110 to provide a larger diameter, CD. For example, the pier 110 may have a diameter, CD, ranging from 8 to 16 inches, such as 10 inches, while the capital surface 126 may have a diameter that is significantly larger, such as in the range of 18 to 28 inches with diameters in the range of 20 to 24 inches being useful in many applications. A circular surface 126 is shown in FIG. 1 as this is a relatively easy to form and strong shape, but it will be understood that other shapes may be used such as square, rectangular, or another polygonal perimeter.

As discussed, the slabs 130 are typically formed using concrete that is poured into a form such as a metal pan (not shown as it would be used at the offsite manufacturing facility rather than being used on-site and left as part of the floor structure). The slabs 130 may include channels formed between joists or ribs and edge or carrying beams as discussed below, and for additional strength and integrity, the slabs 130 may include steel bars and/or wire mesh, which may also be used to connect the structural slabs 130 to the foundation wall 150 in conventional fashion.

FIG. 2 illustrates a cross sectional view of the floor assembly 100 of FIG. 1. As shown each slab 130 is formed with channels 238 formed by joist/rim members and edge or carrying members as discussed below. In other words, the slabs 130 are not typically solid but instead have channels 238 but in some cases, the channels 238 are filled with insulating material or foam to reduce heat transfer from slab 130 to soil 110 or vice versa. Each slab 130 is supported on top of capitals 120, which in turn are supported by piers 110. As shown, two abutting slabs 130 have end portions 234, 236 that abut or contact each other and are supported by a capital surface 126. For example, each end portion 234, 236 may extend to about the center of the capital weight bearing surface 126. End portions 234, 236 are shown with straight vertical walls but, as discussed above for edges of the slabs 130, the end portions 234, 236 may include surface configurations to facilitate mating (such as paired/inverted “Ls” or the like). Further, finishing may be performed to “fill” cracks or spaces between the slabs 130 and/or walls such as with additional concrete, caulking, or the like.

As shown in FIG. 2, the slab 130 is supported a void distance, VOID, above the soil 104 by the combination of the pier 110, which extends out from the soil a distance or pier height, P_H, and the capital 120, which has a height, C_H, as measured from the top of or mating surface of the pier 110. The pier height, P_H, is in the range of 0 to 12 inches or more with 4 to 8 inches being common but not limiting, and is typically greater than zero to avoid having the soil contacting the sides 122 of the capital 120 upon installation.

The height of the capital, C_H, also may range significantly to practice the invention and will vary with the diameter of the pier 110 and the capital surface 126. This is because the angle, Θm defined by the sides 122 and a plane extending through the top of the pier 110 is selected to not be too steep or it will make the height, C_H, unacceptable large to achieve a desired diameter for the capital surface 126 and not too shallow as then the capital would not have adequate weight bearing strength. Hence, the capital height, C_H, typically will range from about 4 to 16 inches with 6 to 10 inches being preferred (but, again, not limiting). The side angle, Θ, is also typically selected to be steep enough (such as 40 to 80 degrees or the like) to reduce the amount of lift force applied by expanding soil 104. In other words, shallow angles for side walls are undesirable as providing a surface for the soil 104 to contact (as opposed to piers 110 which are typically cylindrical with substantially vertical walls, i.e., the standard use of only cylindrical piers without protruding edges teaches against the use of a weight bearing surface with sides that extend outward and are not vertical).

The pier 110 is often formed as a drilled pier with a hole being drilled followed by a concrete pouring. Rebar or dowel 212 may be provided to facilitate mating of the later formed capital on the top of the pier 110. In other words, the piers 110 are typically formed as an initial step, and then, the capitals 120 are formed in a second or later pouring of concrete, which bonds to the rebar or dowel 212. In some cases, a funnel or frustoconical form is provided on or contacting the pier 110 and concrete is poured into the form. In other embodiments, the capital 120 is formed in a single pouring with the pier 110 by providing the form on top of the hole or pier form. In other embodiments (not shown), the capitals 120 are formed with the piers 110 to provide a monolithic or substantially monolithic structural member. For example, a form may be provided for a single pouring of concrete to produce a pier 110 and the weight bearing extension or capital 120.

FIG. 3 shows another sectional view of flooring assembly 100. In this view, the slab 130 is shown supported on pier weight bearing extensions or capitals 120 to create a desired void between the slab 130 and soil 104. Features of the slab 130 are more fully shown including edge or carrying members 310, 320 on each end of the slab. Typically this is a relatively thick or wide member that is supported on or mates with capitals 120. For example, the slab may be 6 to 10 inches (and often 8 inches) thick, t_SLAB, and the edge beams 310, 320 may be 10 to 16 inches (and, in some cases, about 12 inches) thick or wide as measured from the edge or end of the slab to the channel 238. For added strength, one or more lengths of rebar 312, 322 may be provided extending along the length (or a part of the length) of the carrying members or beams 310, 320. Between the edge beams 310, 320, the channel 238 is shown to be filled with insulation 330, e.g., insulating foam or the like, but this is not required to practice the invention (e.g., see FIG. 5).

In the embodiment 100 of FIG. 3, the horizontal or planar slab member 132 is shown to have a thickness, t_{Hor. Member}, much less than the slab 130, and in some cases, this thickness is selected from the range of 2 to 5 inches, with preferred thicknesses being about 3 inches or somewhat less (such as 2.7 inches). Often wire mesh will be provided in the member 132 for added strength and integrity (see, for example, mesh 420 in FIG. 4). Also, when the wall 150 is poured on site rather than pre-cast, rebar or dowels may be provided as shown at 340 to extend in member 132 and to also extend outward from one of the carrying beams 320 to form a connector or mating dowel 342 with wall 150. A groove 133 may still be provided in the upper surface of member 132 but this is not required in this embodiment.

FIG. 4 shows a cross sectional view of the flooring assembly 100 that shows an end view of two adjacent slabs 130. The slab 130 are shown supported on capitals 120 and piers 110 (i.e., by resting the carrying members (not shown) on the weight bearing surfaces of capitals 120, and in some cases, capitals 120 may be provided to support joists or ribs 430 between the two carrying members or beams). As shown, the slabs 130 include ribs or joists 430 extending downward from the horizontal or planar slab member 132 and extend an elongate manner between the carrying beams or members (not visible in FIG. 4). The ribs or joists 430 define open channels 238 between them, which may optionally be filled with insulation 330. For added strength, the joists 430 may include rebar or other structural components 410 while the horizontal or planar slab member 132 includes mesh or the like 420. The ribs 430 may be rectangular or square in cross section as shown or may take other shapes such as trapezoidal (e.g., with the sides sloping inward toward each other such that the ribs 430 are wider near the slab member 132 than at the surface near the capitals 120).

The channels 238 have a width, W_Channel, defined by the opposing faces of each pair of adjacent joists 430. The joists 430 may have a range of thicknesses or widths to practice the invention such as 3 to 6 inches or the like. Similarly, the number and spacing of the joists 430 may vary and will typically depend on loading requirements (i.e., structural goals) and vary with the panel size, which may be 6 to 12 feet on a side (rectangular or square) with 8 feet being a useful panel length and width. In these embodiments, the joists may be 24 to 30 inches from center to center and this would result in channel widths, W_Channel, of about 18 to 24 inches or the like (which can be left open or filled with insulation 330 as shown).

FIG. 5 illustrates in further detail a slab 130 of the present invention. The slab 130 is formed or pre-cast in off-site or at least not where it is to be positioned/installed in a building. As shown, the form used for the slab 130 is chosen so as to create a number of joists or ribs 330 that extend along the length of the slab 130 between edge or carrying beams such as beam 310. Carrying beam includes rebar 310 as does joists 330 as shown at 410. The slab planar or horizontal member 132 includes a slot or keyway 133 for receiving a matching member of a wall and includes mesh or structural steel/wire 420 for added strength and integrity.

Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the combination and arrangement of parts can be resorted to by those skilled in the art without departing from the spirit and scope of the invention, as hereinafter claimed. For example, some flooring system embodiments of the invention involve the use of the pre-cast slabs with foundation walls that are formed prior to the slabs being positioned. These foundation walls are used to support one end of the slabs (or at least the slabs on the periphery of a floor system). Specifically, it will be understood that a structural floor is often built after the foundation wall is built. The slabs of the invention may then be doweled to the wall, placed on a ledge (such as one formed of steel angle bolted to the foundation wall), or other support assembly or components (such as devices or techniques used to support pans or forms for slabs that are poured on site rather than pre-cast as called for herein). In these embodiments, the foundation wall may be supported by standard piers or other structural members or by the piers and capitals of the present invention. These outer slabs are supported on the opposite end (and, in some cases, at midpoints via their ribs/joists) by the capitals and piers of the present invention as shown in the figures. In these cases, the slabs would not include a groove or keyway for receiving a wall but may include features to facilitate being supported at one end by the foundation wall, e.g., slots or keyways in the edge or carrying beams for receiving or mating with dowels or ledges or the like extending outward from the foundation wall. Figures are not provided showing flooring systems with standard foundation walls as these will be readily understood by those skilled in the art and drawings are not required for an understanding of how these may be used to support a slab of the invention.

Claims

1. A residential flooring system for use with in expansive soil, comprising:

a plurality of pre-cast slabs of hardened material;

structural members contacting the expansive soil and extending upward away from the expansive soil, the structural members having an upper contact surface; and

a plurality weight bearing members attached to the upper contact surface, wherein each of the weight bearing members comprises a weight bearing surface that is larger than the upper contact surface and wherein the pre-cast slabs are supported by the structural members by abutting the weight bearing surfaces of the weight bearing members.

2. The flooring system of claim 1, wherein the pre-cast slabs and weight bearing members are formed of concrete.

3. The flooring system of claim 1, wherein the weight bearing members comprise sides that extend upward and outward from the upper contact surfaces of the structural members.

4. The system of claim 3, wherein the weight bearing members are frustoconical in shape and the sides extend upward and outward from the upper contact surfaces at an angle selected from the range of 40 and 80 degrees.

5. The system of claim 1, wherein each of the pre-cast slabs comprises a planar member, a pair of edge beams extending from the planar member, and two or more monolithic ribs extending in a spaced-apart manner from the planar member between the edge beams.

6. The system of claim 5, wherein the edge beams of each of the pre-cast slabs abuts the weight bearing surfaces such that the pre-cast slabs are supported above the expansive soil.

7. The system of claim 5, wherein the pre-cast slabs further comprise insulation positioned between adjacent pairs of the ribs.

8. The system of claim 1, further comprising a plurality of wall sections positioned upon the pre-cast slabs, whereby the wall sections are supported by the pre-cast slabs, the weight bearing members, and the structural members.

9. A modular flooring system, comprising:

a plurality of concrete slabs comprising a planar member, a pair of edge beams extending from the planar member, and monolithic ribs extending in a spaced-apart manner from the planar member between the edge beams;

piers extending into soil and extending above the soil to expose a circular contact surface having a diameter; and

capitals extending from each of the piers comprising a circular weight bearing surface contacting the edge beams to support the concrete slabs, wherein a diameter of each of the weight bearing surface is greater than the diameter of the circular contact surface.

10. The system of claim 9, wherein the diameter of the weight bearing surface is greater than about 20 inches and less than about 24 inches.

11. The system of claim 9, wherein the capitals and the piers are formed of concrete that is provided in separate pourings.

12. The system of claim 9, wherein the capitals are frustoconical in shape.

13. The system of claim 9, where at least some of the capitals support at least two of the concrete slabs.

14. The system of claim 9, further comprising a wall positioned on the concrete slabs on a surface opposite the capitals.

15. The system of claim 9, wherein the concrete slabs further comprise insulating foam between adjacent pairs of the ribs.

16. A method of assembling a modular residential flooring system for use with expansive soil, comprising:

casting a plurality of concrete slabs at a first location;

transporting the slabs to a second location;

at the second location, providing a plurality of structural members extending into the soil and extending upward out of the soil to expose a support portion;

forming weight bearing members on the support portion of the structural members, wherein each of the weight bearing members comprises a support surface with an area greater than the support portion; and

positioning the concrete slabs adjacent each other and in contact with the support surfaces of the weight bearing members, wherein the concrete slabs form a substantially planar surface and are supported by the structural members via the weight bearing members.

17. The method of claim 16, further comprising forming foundation wall sections at a third location, transporting the wall sections to the second location, and positioning the wall sections in vertical orientation on the concrete slabs.

18. The method of claim 16, wherein the casting of the concrete slabs comprises providing a form and filling the form with concrete, the form having internal surfaces such that each of the concrete slabs comprises a planar member, a pair of edge beams extending from the planar member, and monolithic ribs extending in a spaced-apart manner from the planar member between the edge beams.

19. The method of claim 18, wherein the casting further comprises providing in each of the concrete slabs insulation between adjacent ones of the ribs.

20. The method of claim 16, wherein the providing of structural members comprises spacing the structural members apart a distance such that at each of the concrete slabs is supported by at least four of the structural members.