EP0879328A1

EP0879328A1 - Composite slab, a profile plate thereof and a method for producing a composite slab

Info

Publication number: EP0879328A1
Application number: EP96938228A
Authority: EP
Inventors: Juha VAINIONPÄÄ
Original assignee: Germix Oy
Current assignee: Germix Oy
Priority date: 1995-11-09
Filing date: 1996-11-08
Publication date: 1998-11-25
Also published as: WO1997017509A1; AU7572996A

Abstract

The present invention relates to a composite slab comprising as the lowermost element a shaped bottom plate (1) including longitudinal, upward aligned dovetail folds (4) and a cast part (2) of the slab covering the top area of the slab contiguously and the bottom part of the slab by a cross section with trapezoidal core cavities. The tops of the dovetail folds (4) are provided with anchorages (5, 7, 13) capable of preventing the bottom plate from sliding longitudinally with respect to the cast material and core-forming elements (3) are placed lengthwise immediately above the shaped bottom plate (1), between said dovetail folds (4), whereby the core-forming elements together with folds of the shaped bottom plate (1) form said trapezoidal core cavities into said cast part (2). The invention also concerns an advantageous method for the manufacture of said composite slab.

Description

Composite slab, a profile plate thereof and a method for producing a composite slab

The present invention relates to a composite slab formed by a shaped bottom plate with longitudinal, upward directed dovetail folds and a cast material part which covers in a contiguous manner the upper part of the composite slab and extends downward into a lower part of the slab with a cross section containing trapezoidal lightweighting cavities. The invention also relates to a shaped bottom plate of the above-described kind and a method of producing such a composite slab.

The use of composite slabs aims at more rational building operations with concrete structures. The composite slab forms both the casting form and a major part of the reinforcing steel itself. However, long spans will require a large height of the continuous slab section, whereby technical problems arise. Furthermore, the trapezoidal cross section of the shaped bottom plate necessitates separate facing of the slab bottom side causing extra work and costs. Therefore, instead of composite-slab constructions, prestressed hollow-core slabs have often been used as an alternative choice.

In Finland, floors between storeys are predominantly made using hollow-core slabs. A disadvantage hampering the use of hollow-core slabs is their slow, but due to the grad¬ ual relaxation of the prestressing force, significantly changing degree of sagging, whose uncontrolled progress often necessitates thick top layer castings.

It is an object of the present invention to provide a particularly competitive structure capable of replacing conventional composite-slab structures, and in most cases, hollow-core slabs, too. The goal herein is to achieve a simple but extremely effective slab structure. The characterizing properties of the composite slab ac¬ cording to the invention are presented in claim 1. Furthermore, the characterizing properties of the shaped bottom plate according to the invention are stated in claim 6. A preferred method for producing the composite slab is characterized in claim 7. Since the conventional trapezoidal lightweighting core cavities of the composite slab, on the underside of the cast material, are accord¬ ing to the invention partially formed with the help of moulded pieces of expanded filler material, the shaped bottom plate can be made relatively flat, that is, with shallow corrugations only. The underside of the bottom plate may be made ready-surfaced, whereby separate facing of the composite slab structure underside may be omitted. Sag precompensation can be formed into the composite slab on a simple curved roller conveyor track, wherein the track elevates the center point of the shaped bottom plate slightly above its ends, whereby the finished com¬ posite slab will have a sag precompensation determined by the upward curvature of the track. In a similar manner, the top surface casting of the slab can be trowelled convex or flat on the track as desired.

The dovetail top of the upright fold as such gives good anchorage in the perpendicular direction to the plane of the shaped bottom plate. On the other hand, separate means must be provided to assure anchorage against longi¬ tudinal sliding of the bottom plate. Particularly good anchorage in the longitudinal direction will be achieved by bending the dovetail downward concave at the center of its wide top and simultaneously making creases to the edge corners of the dovetail top at sufficiently close spacings. Most advantageously, the edge corners of the dovetail tops are undulated and are provided with alter- nating indents and outdents. Advantageously, a minimum of free volume is enclosed by the creased dovetail top, whereby the bottom plate is effectively prevented from sliding longitudinally along the upper part of the slab cast from concrete.

The behaviour of the slab structure according to the in- vention is essentially different from that of hollow-core slabs. Owing to the structure of the hollow-core slab, its shear strength over the support area of the slab will be greatly reduced due to the shear stress caused by the compressive force effected in the longitudinal direction of the slab. This effect can be easily managed by virtue of the slab according to the invention using a continuous cast on the end area of the slab.

The invention is next examined with the help of the an- nexed drawings illustrating a number of composite slab embodiments according to the invention, in which diagrams

Figure 1 shows a composite slab structure according to the invention with the cast material partially sectioned;

Figure 2 shows the cross section of the shaped bottom plate at the dovetail fold;

Figure 3 shows a modified embodiment of the composite slab structure with the cast material removed;

Figure 4 shows an improved anchorage shape against longi¬ tudinal sliding of the bottom plate;

Figure 5 shows the cross section of the composite slab elements preassembled for casting; and

Figure 6 shows the longitudinal anchorage arrangement in a composite slab structure of greater height.

Referring to Fig. 1, the composite slab structure shown herein is composed of elongated slab strips, which are designed for being laid adjacently in parallel, after which the joint between the slab ends is sealed by casting. This working method is similar to that used in conjunction with conventional hollow-core slabs. The composite slab is formed by a shaped bottom plate 1, core-forming elements 3 placed thereon and a solid body of castable material 2. The core-forming filler elements 3 may be, e.g., expanded polystyrene pieces glued to the shaped bottom plate. An alternative arrangement uses tubular core-forming elements made from a sheet steel mesh and adhered by spot-welding to the bottom plate. Such filler elements may have a U-shaped cross section, whereby the ends of the filler elements are plugged before casting.

The shaped bottom plate 1 is bent into longitudinally running, conventional dovetail-top folds capable of rendering effective vertical anchorage between the shaped bottom plate and the cast material. To the top of this dovetail fold are made cuts 5, which permit a portion of the segments separated by the cuts 5 to be dented down, thus forming discontinuities on the top of the dovetail fold 4. These steps on the top of the dovetail fold pre¬ vent longitudinal sliding between steel bottom plate and the cast upper part of the slab. Actually, the longi¬ tudinal sliding is inhibited by creases made to the sharp edge corners of the dovetail top which will be described in more detail later in the text. In Fig. 1, the cast material is shown not to extend fully to the slab edge 10. Herein, the purpose is to perform the casting of this edge, which joins the adjacent slab strips to each other, only at the worksite. With this exception, the main body of the slab is cast at the slab manufacturing plant, and the ready-cast slabs are used as building elements similarly to hollow-core slabs by laying them adjacently side-by-side at the construction site. The slab edge may be cast fully to the edge corner. While no additional reinforcing steels are used in the embodiment shown in Fig. 1, in a plurality of cases it is possible to place additional reinforcing steel bars into the concrete prior to casting, particularly to the lower part of the slab, when the overall height of the composite slab is desired- ly made smaller or improved load-bearing capability under fire is required. The reinforcing steel bars may also be prestressed in a conventional manner.

As shown in Fig. 2, the lower gap 11 between the upright sides of the dovetail fold 4 can be made very narrow, whereby the dovetail fold line will become almost indiscernible on the underside of the slab. The cuts shown in Fig. 1 permit a portion of the segments on the dovetail top to be pressed downward concave, whereby these downward-pressed segments 7 form discontinuities with respect to the upper surface of the dovetail fold 4. Furthermore, additional pre-bent reinforcing steel bars 9, which are shown later in Fig. 3, can be inserted into these recesses by sliding one end of the pre-bent bar into one of the alternating guide slots thus formed on top of the dovetail fold 4.

Additionally, adjacently to the dovetail fold 4 are made small ridges 8 which facilitate easy location of the ex- panded-material core-forming filler pieces as the longi¬ tudinal ridges perform correct alignment of the pieces. These ridges also improve the stiffness of the sheet steel bottom plate. Between the ridges, a low-profile pattern may be stamped onto the bottom plate if possible unevenness of the plate resulting from its pressing steps is desired to be concealed.

Referring to Fig. 3, a casting form similar to that of Fig. 1 for the composite slab is shown herein, now com¬ plemented with additional reinforcing steels comprising bars 12 which are inserted in the manner shown in Fig. 2 along the tops of the dovetail fold 4, via the eyelet ties formed by the openings made at the cuts 5. Such reinforcing steels are used as necessary at the ends of the bottom plate, which is the primary place to encounter such critical stresses that may affect the integrity of the composite structure as a whole.

As is evident from Figs. 2 and 3, the core-forming filler pieces will not be extended up to the very ends of the shaped bottom plate, whereby the concrete casting can fill the ends of the composite slab as a solid mass over its entire width.

According to a typical dimensioning scheme, a 1500 mm wide bottom plate blank can be processed into a shaped bottom plate of 1200 mm width. Thus, a 1200 mm wide strip of a composite slab structure such that shown Fig. 1, for instance, may be formed on the shaped bottom plate. The overall strength and load-bearing capability of the structure may be varied by choosing different casting heights for the slab.

The shaped bottom plate according to the invention may be contemplated for use in shallow slab structures without using the core-forming filler pieces, or alternatively, having the filler pieces entirely embedded within the cast material. However, the highest technical benefit of the present structure will be gained from a composite slab structure according to Figs. 1 and 3 , whereby even greater heights of the composite slab structure may be attained using relatively simple means.

As an alternative arrangement, core-forming filler pieces made from sheet steel mesh can be shaped to extend with a low height down so as touch the top of the dovetail fold and then clamped against the dovetail top. Thus, the hollow-core filler piece shaped from sheet steel mesh makes it possible to produce a novel type of hollow-core slab, however, with sound insulating properties superior to those of conventional hollow-core slabs. Firstly, the concrete mix intruding through the mesh falls on the shaped bottom plate so as to form a layer thereon and on the other hand, forms an extremely coarse sound-diffusing structure on the inner surface of the mesh. Actually, the composite slab shown in Fig. 5 may be considered being made in this fashion, whereby the core-forming sheet steel mesh elements actually extend down to the shaped bottom plate 1 which, however, has a thin layer of cast concrete formed thereon due to intrusion of the concrete mix through the mesh. On the inner side of the mesh, the concrete mix forms an uneven layer having a sound- attenuating effect. The modular composite slab structure shown in Fig. 5 and denoted by reference numeral 14 is comprised of essentially the same elements as those described above, namely, a shaped bottom plate 1 of the composite slab, hollow-core filler elements 3 and a body part 2 made from cast material. In the adjacently mounted composite slab strips, the protruding elements 4.1 form a bonding element similar to a dovetail joint, whereby the finishing casting is performed onto these elements.

Instead of concrete, other form-castable materials may also be used such as lightweight concrete made using nodular sintered aggregate.

As a rule, the slab structure according to the invention is delivered to the construction site for use in a single-span installation. If required, the structure may be modified for continuous casting to serve, e.g., dis¬ tributed surface loads, whereby at the slab manufacturing plant the top part of the slab is left uncast by its end areas, the top surface reinforcing steels are placed at the supports of the slab and the top part of the slab is finished over its end areas by casting. The composite slab according to the invention can be fabricated in three alternative manners:

1) The slab is delivered to the construction site entire¬ ly without casting, whereby - the slab skeleton is lightweight to transport, but

- requires support during its installation.

2) The slab is transported to the construction site par¬ tially cast having the dovetail folds covered by casting at the slab manufacturing plant, whereby

- the slab is still somewhat lighter than a ready-cast slab, and

- piping and ducts to be routed in the interior of the slab are easy to mount at the construction site prior to the final casting, whereby, however,

- the slab may require support about it during the installation worksteps depending on the situation. 3) The slab is delivered fully ready-cast.

Referring to Fig. 4, an anchorage arrangement according to the invention against longitudinal sliding of the bottom plate is shown herein suitable for use in the above-described slab structures, said arrangement com¬ prising a dovetail fold 4, however, now having the top of the dovetail fold dented downward concave and having creases made to the edge corners of the dovetail top so that a wavy edge is formed. The creases 5 are accom- plished as small dents made by a suitable type of cold deformation. As far as possible, the goal is to minimize the empty space under the creases, which under heavy load could deform thus permitting the bottom plate to undergo loss of anchorage and sliding relative to the cast concrete. On the other hand, it is possible to achieve such a high anchorage gripping power by making the creases at sufficiently close spacings that a minor empty space under the creases will do no harm. Most advan¬ tageously, the crest-to-valley depth of the crease is from 1- to 15-fold the sheet thickness, in practice most typically having the depth of the crease in the range 2 - 5 mm when the thickness of the sheet blank is from 0.6 mm to 1 mm. The goal herein is to avoid sharp edge corners of the crease that could cut through the con¬ crete. Additionally, the anchorage must perform so that the sheet metal of the bottom plate cannot escape into the empty space under the creases. A sufficient anchorage grip capacity can be attained by an empirical compromise between the empty space under the creases and a practical shape of the creases. As a rule of thumb, it has been found that small creases provide a better anchorage grip than large dents spaced widely apart from each other.

Referring to Fig. 6, a corresponding anchorage arrange¬ ment is shown herein for a composite slab of larger height, wherein the dovetail top can be extended deeper in the body of the slab by providing the dovetail top with an upward extending stem part 14. In this fashion, the dovetail top with its anchorage creases remains fully enclosed in the concrete, well protected under a fire.

In a fire situation, the core cavities formed by the core-forming sheet steel mesh elements act as an escape route for evaporating water. In the interslab seam, the bare mesh makes it possible to achieve an almost conti¬ nuous slab. Herein, the mesh portions at the very ends of the composite slab are left uncast at the slab manufac¬ turing plant. Thus, the final casting of the interslab seams done at the installation site becomes an integral part of such a continuous slab.

Broadly, the anchorage arrangements described herein can be characterized by comprising outdents or indents with noncutting edges and having only a minimal empty space left under such outdents/indents.

Further broadly, the scope of the embodiments of the invention may be extended so that the dovetail anchorage is replaced by any other type of anchorage capable of providing an almost 100 % reliable grip. However, the basic principles of the anchorage concept disclosed herein remain unchanged. Even a vertical fold of the bottom plate with creases made thereto at sufficiently close spacings may fulfill the anchorage needs in the present slab structure. When required, the inferior grip of a modified anchorage arrangement can be compensated for with the help of conventional reinforcing steels. Accordingly, the broader form of the invention includes a shaped bottom plate with suitable anchorages, core- forming elements (advantageously made from sheet steel mesh) and a cast part of the composite slab.

According to a final embodiment of invention, the core- forming elements are replaced by formed core cavities made in conjunction with slip-form casting. Still further, the techniques used in the manufacture of hollow-core slabs can be utilized. A composite slab made with the help of the latter method resembles a hollow- core slab, however, with the difference that the pre- stressed reinforcing steels are replaced by the shaped bottom plate capable of assuring good anchorage to the overlying cast concrete.

Claims

Claims :

1. A composite slab comprising as the lowermost element a shaped bottom plate (1) including longitudinal, upward aligned folds (4) and a cast part (2) of the slab cover¬ ing the top area above the shaped bottom plate as seen in the cross section of the slab and further having the upper part of said fold (4) provided with such anchorages (5, 7) made to the dovetail top of said fold that are capable of preventing the bottom plate from sliding lon¬ gitudinally with respect to the cast material, and said slab further comprising core-forming elements (3) or core cavities situated immediately above said shaped bottom plate (1), between said folds (4), c h a r a c t e r - i z e d in that said anchorages against longitudinal sliding of the bottom plate are comprised of outward projecting creases made to the edge corners of the top of said dovetail fold at constant spacings, said creases forming a continuous, wavy edge of the dovetail top, whereby the projecting creases are placed so densely that the distance between the crease crests is smaller than 5 times the width of the crease crest.

2. A composite slab as defined in claim 1, c h a r - a c t e r i z e d in that the top of the dovetail fold is dented inward and that said creased edge corner forms a wavy dovetail top edge in which the crest-to-valley depth of the creases is from 2 to 15 times the thickness of the sheet metal blank of the bottom plate.

3. A composite slab as defined in claim 1 or 2, c h a r a c t e r i z e d in that the cast part (2) at the ends of the composite slab forms a solid cross section over a short length of the slab.

4. A composite slab as defined in any of claims 1 - 3, c h a r a c t e r i z e d in that said core-forming elements (3) comprise elongated pieces of expanded polystyrene.

5. A composite slab as defined in any of claims 1 - 3, c h a r a c t e r i z e d in that said core-forming elements (3) comprise elongated pieces of shaped sheet steel mesh.

6. A cold-formed, shaped bottom plate (1) essentially planar in the direction of the sheet width dimension for use in a composite slab, said bottom plate incorporating longitudinal, upward directed dovetail folds (4) having anchorages (13) made to the edge corners of the dovetail top for preventing longitudinal sliding of the bottom plate relative to the cast part, c h a r a c t e r ¬ i z e d in that said anchorages against longitudinal sliding of the bottom plate are comprised of outward projecting creases made to the edge corners of the end of said dovetail fold at constant spacings, said creases forming a continuous, wavy edge of the dovetail top, whereby the projecting creases are placed so densely that the distance between the crease crests is smaller than 5 times the width of the crease crest.

7. A method of manufacturing a composite slab, in which slab onto a shaped bottom plate (l) forming an essential part of the slab reinforcing steels is cast a cast part (2) and said shaped bottom plate (1) including anchorages (4, 5, 7) for bonding said bottom plate to said cast part (2) under stress, c h a r a c t e r i z e d in that said shaped bottom plate (1) is handled on an upward curved track and strongly trapezoidal core cavities are formed into said cast part (2) by placing core-forming elements (3) lengthwise onto said shaped bottom plate (1) , after which the casting of said cast part is per¬ formed, whereby the finished composite slab will have a sag precompensation formed by means of said curved casting track.