EP2339082A1

EP2339082A1 - Assembling system between columns and a slab and construction process therefore

Info

Publication number: EP2339082A1
Application number: EP09180602A
Authority: EP
Inventors: Roberto Guidotti; Aurelio Muttoni; Miguel Fernandez Ruiz
Original assignee: Ecole Polytechnique Federale de Lausanne EPFL
Current assignee: Ecole Polytechnique Federale de Lausanne EPFL
Priority date: 2009-12-23
Filing date: 2009-12-23
Publication date: 2011-06-29

Abstract

A concrete slab assembling system comprising:
-a reinforced concrete slab (1);
-a supporting column (2);
-a supported column (3), having a bottom-end with a reference outer diameter thereof;
-an interface adaptor (10) for placement between the supported column bottom-end and the slab top surface;
-said interface adaptor (10) outer diameter at least at the contacting portion with said bottom-end surface, being at least 10% smaller than the reference diameter of the supported column.

The invention also provides the corresponding method for assembling a concrete slab (1) between a supporting column (2) and a supported column (3).

Description

FIELD OF THE INVENTION

The present invention relates to a concrete slab assembling system and a method for assembling a concrete slab between a supporting column and a supported column.

BACKGROUND OF THE INVENTION

Flat slabs are commonly used in reinforced concrete construction for buildings and parking garages. They consist in a reinforced or pre-stressed concrete deck with a flat soffit supported by columns and potentially by walls.
The strength of flat slabs is typically governed by its punching shear strength around columns. In these regions, large shear stresses develop and the column may potentially punch through the slab with development of a conical surface, as shown in Figure 1. Punching of a flat slab is a highly undesirable failure mode since it is brittle and failure occurs without warning signs (limited crack widths and deflections).
Flat slabs are widely used for multi-storey buildings. In these cases, the columns are not continuous, but they are interrupted at flat slabs as shown in Figure 2a. This allows easiness of construction as well as it provides support for the flat slab during construction. In order to increase the speed of construction (and to allow the use of columns with smaller diameters) precast columns are commonly used for such applications. These columns typically present steel plates at their ends, as shown in Figure 2b. They are cast with a concrete whose strength is much higher than that of the flat slabs (two to three times in usual cases). As a consequence, the load carried by the columns is usually significantly larger than the compressive strength offered by the corresponding support region on the flat slab.
Several solutions have been proposed in the past with the aim of increasing the punching shear strength of flat-slabs (stirrups, headed studs, etc).
Some systems have also been developed to transmit the forces carried by the columns through the slab. They however do not take advantage of column loads to increase the punching shear strength.
For instance, DE 201 20 678 U1 deals with precast columns with a special head in the top end of the bottom column. This detail allows supporting the upper column thanks to a thin layer of mortar. Lateral surface of the head is rough in order to provide the necessary shear-carrying capacity. Rebar couplers are also available to provide continuity for the flexural reinforcement of the slab.
EP 1 749 949 A2 differs from the previous one in the way flexural reinforcement passes through the column (with horizontal openings).
EP 1 426 508 A2 deals with joints of precast members. As an application, column-slab joints are considered, with shear forces being carried as previously by rough lateral surfaces.
DE 103 24 291 A1 proposes to link the upper and lower columns by a set of bars crossing through the slab.
WO 2005/098160 A1 also proposes to link the upper and lower columns but discloses a high-performance concrete member instead of a set of bars crossing through the slab.
Thus, there is a need for an improved method and/or arrangement for assembling concrete slabs avoiding the above-mentioned drawbacks.

SUMMARY OF THE INVENTION

Therefore, a general aim of the invention is to provide a system and/or a method for improving the punching shear strength of a concrete slab provided between two columns.
A further aim of the invention is to provide improved interface allowing punching shear strength improvements.
Still another aim of the invention is to provide a concrete slab-assembling configuration allowing achieving punching shear strength improvements.
These aims are achieved thanks to a concrete slab assembling system and a method for assembling a concrete slab between a supporting column and a supported column defined in the claims.
There is accordingly provided a concrete slab assembling system comprising:

a reinforced or pre-stressed concrete slab;
a supporting column;
a supported column, having a bottom-end with a reference outer diameter thereof;
an interface adaptor for placement between the supported column bottom-end and the slab top surface;
said interface adaptor outer diameter at least at the contacting portion with said bottom-end surface, being at least 10% smaller than the reference diameter of the supported column.

The reduced contacting surface between the supported column bottom end and the slab top portion provides an increase of the punching shear strength of the concrete slab. Thus, in order to optimize the punching shear strength, the invention takes advantage of axial forces in the columns. In fact, the applicant came to the very surprising and unexpected conclusion that once the compressive strength of the flat slab has been exceeded in the support region, the punching shear strength and deformation capacity of flat slabs may significantly be increased. Such increase in the punching shear strength is more important as the compressive stress applied by the columns increases. Some limitations to the increase in the strength however apply, as the flexural reinforcement may be governing for the maximum strength of the slab for very high axial loads.
The use of such an interface adaptor provides a load transfer from the supported column to the slab (and thus to the supporting column) concentrated in a load-transfer surface substantially smaller than in cases for which load transfer is materialized with a direct contact between the column and the slab, or for which a footplate of similar diameter than the reference diameter is used.
The reference diameter is preferably the supported column diameter in the contacting zone with the concrete slab.
In a first embodiment, the interface adaptor is substantially disk-shaped.
In a variant, the interface adaptor is provided with a dowel, for cooperation with a blind hole provided in the slab top portion.
In a second embodiment, the interface adaptor is provided with an elongated body having a narrowing portion forming an extremity of said adaptor and being adapted for insertion into a corresponding bore hole provided into the slab top portion. Lateral stress is strongly increased by using an elongated shaped increasing lateral dilatancy as upper column penetrates in the slab. The elongated body preferably further comprises a cylindrical portion and said narrowing portion is co-axial to said cylindrical portion.
The narrowing portion may be substantially conical, or substantially dome-shaped, or with another profile.
The interface adaptor is advantageously provided with a cylindrical portion with a substantially flat end portion for cooperation with supported-column bottom end surface.
The invention also provides a method for assembling a concrete slab between a supporting column and a supported column, comprising:

placing said slab for cooperation on a top head of at least one supporting column;
providing an interface adaptor for placement between the supported column bottom-end and the slab top surface, said interface adaptor having a supported column contacting surface substantially smaller than the supported column bottom end surface.
engaging said supported column on said slab, substantially aligned with said supporting column.

In a preferred embodiment, the supported column diameter of interface adaptor is at least 10 % smaller than the supported column bottom end diameter and most preferably between 10 % to 80 % smaller than the supported column bottom end diameter.
Preferably, before the step of engaging said supported column on said slab, a step consisting in providing a blind hole on the top portion of said slab, substantially aligned with said supporting column central axis, is added.
Preferably, before the step of engaging said supported column on said slab, a step consisting in providing mortar on the slab supporting portion is added.
In another variant, the method further comprises a step consisting in placing a filler layer at the immediate vicinity of said interface adaptor using a filler layer material having a modulus of elasticity substantially lower than the material of said adaptor (modulus of elasticity is at least 50 % lower, and most preferably 95 % or even lower). In a preferred embodiment, the filler layer is crown-shaped, with the interface adaptor substantially centrally placed within said filler layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which:

Figure 1 shows a prior art supporting column and concrete slab arrangement with an illustration of the potential punching cracks;
Figures 2a and 2b present a prior art supporting column, concrete slab and supported column arrangements;
Figure 3 depicts a first embodiment of a concrete slab assembling system using an interface adaptor in accordance with the invention;
Figures 4 to 7 illustrate examples of a second embodiment of the invention using an elongated interface adaptor;
Figures 8a and 8b present a third embodiment of the invention, using a truncated cone interface adaptor;
Figure 9 illustrates a further embodiment of the invention using stirrups in the supported column; and
Figure 10 shows the influence of the different interface adaptor details on the strength of column-slab joints.

DETAILED DESCRIPTION OF THE INVENTION

Figures 2a and 2b illustrate a concrete slab construction of the prior art. The concrete slab 1 is placed between a supporting column 2 and a supported column 3. The supporting column 2 is provided with a top head 5 placed between the slab and the column. The top head 5 may be provided with a dowel 7, adapted for insertion in the bottom portion of the slab. The supported column 3 is provided with a footplate 6 placed between the slab 1 and the column 3. Such footplate 6 may be provided with a dowel 7, adapted for insertion in the upper portion of the slab 1. The top head 5 and the footplate 6 dimensions and profiles are in correspondence with those of their respective columns. Mortar 8 may be provided between the slab and the supported column. As previous mentioned, such an arrangement does not offer an optimized protection against punching shear
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
In a first embodiment, the slab and columns arrangement is configured in order to increase of compressive stress at the interface between the upper column and the slab support region (in order to exceed the uniaxial compressive strength of concrete).
In a second embodiment of the invention, the slab and columns arrangement involves a specifically designed interface adaptor to increase lateral dilatancy as upper column 3 penetrates in the slab 1.
Finally, in a third embodiment of the invention, a combination of these two previous approaches is used.
All these embodiments provide an increase of punching load and deformation capacity of the slab.
Figure 3 illustrates an example of the first embodiment of the invention. In order to increase the compressive stress at the slab-column interface, the contact surface is reduced. This is achieved with the use of an interface adaptor 10 provided between the supported column 3 lower end and the top portion of the slab 1. In the illustrated example, a footplate 6 forms the upper column 3 lower end portion, and the interface adaptor 10 is placed adjacent said footplate 6.
The interface adaptor 10 is preferably made with a material having a substantially high strength and modulus of elasticity, such as for instance steel, ceramic, high or ultra-high performance concrete.
The interface adaptor 10 is configured in order to have a size smaller than the supported column 3 cross-section.
The interface adaptor may be configured with various profile types depending on the applications, the involved loads, the materials, etc. In a preferred embodiment, a disk-shaped interface adaptor is provided. In this embodiment, the load is transferred to the concrete slab 1 via the disk lower flat face.
To fill-up the gap left between the upper column 3 end portion and slab 1, a filler layer 11 may be provided. In the example of Figure 3, the filler layer 11 surrounds the disk-shaped interface adaptor 1. The filler layer material has a substantially lower modulus of elasticity than the interface adaptor. For instance, soft materials such as plaster, neoprene, foams, etc, may be used.
As shown in Figure 3, a dowel 7 may be provided on the interface adaptor 10, on its opposed face with respect to the supported column 3. Also as illustrated, a mortar layer 8 may be added between the concrete slab surface and the interface adaptor, for easier and better placement of the supported column 3.
The dimensions of the interface adaptor are a function of the required increase on the punching shear strength and of the available axial forces in the columns. The thickness of the interface adaptor 10 and of the filler layer 11 is a function of the displacements that the column has to accommodate (typical values of the thickness of the filler layer are between 5 to 30 mm).
Figures 4 to 7 illustrate examples of the second embodiment of the invention. In these examples, the interface adaptor 10 is provided with an elongated body having a substantially flat head for contact with the upper column footplate 6, a cylindrical portion 12a, adjacent said footplate 6, and a narrowing portion 12b, for insertion into a corresponding bore hole 13 provided in the slab top portion.
In the example illustrated in Figure 4, the narrowing portion 12b of the interface adaptor 10 is substantially conical. The resulting cone is preferably cast with a high strength and modulus of elasticity material, as previous mentioned. In this example, the cone is attached to or cooperates with the footplate 6.
Around the cone, a filler layer 11 of soft material can be placed as previously described with the embodiment of Figure 3.
The dimensions and the angle γ of the cone are a function of the required increase on the punching shear strength and of the available axial forces in the columns. The height of the conical adaptor 10 and of the surrounding filler layer 11 of soft material is a function of the displacements that the column has to accommodate (cone height also depends on the thickness of the slab, typically varying between 20% and 90% of it).
Such an embodiment (Figure 4) provides a lateral dilatancy as the column penetrates inside the slab. This dilatancy originates compressive stresses in the concrete and tensile stresses in the flexural reinforcement.
In the examples shown in Figures 5, 6 and 7, the elongated body profile is optimized to maximize lateral dilatancy. The risk of concrete cover spalling near the flexural reinforcement can also be minimized by the use of such devices. More particularly, Figures 5 and 6 show two examples of configuration illustrating the various possibilities given in using different relative dimensions for the cylindrical portion 12a and narrowing portion 12b.
For instance, Figure 6 illustrates a configuration with a conical-shaped narrowing portion 12b extending substantially deep into the slab, near the middle portion thereof, the cylindrical portion 12a substantially corresponding to the thickness of the filler layer 11. The example of Figure 5 involves a much longer cylindrical portion 12a whereas a shorter narrowing conical portion 12b forms the end portion of the elongated body.
Figure 7 discloses an example in which the narrowing portion 12b is dome-shaped.
Figures 8a and 8b show two examples with interfacing adaptors 10 having narrowing portions 12b shaped like a truncated cone. This solution allows exploiting both previous principles simultaneously. The compressive stress at the base of the device is large enough to exceed the uniaxial compressive strength of concrete and, as the column penetrates inside the slab, lateral dilatancy is activated.
All previous concepts are not only restraint to precast columns, but can also be applied to cast-in-place columns. As an example, figure 9 shows a disk-shaped adaptor 10 as described for the example of Figure 3 for a cast-in-place column. In such cases, stirrups 15 have to be provided in the column close to the contact plate in order to ensure sufficient confinement. In precast columns, footplate 6 can provide sufficient load spreading, although additional stirrups can also be added.
A further advantage of the proposed solutions is the fact that interface adaptor 10 forms a joint behaving mostly as a hinge. Thus, the column can accommodate large rotations developing very limited load eccentricities.
The influence of the different interface adaptor details on the strength of column-slab joints is shown in Figure 10. This figure plots the failure envelopes for normal force of the column (N) - shear force carried by the slab (V) for three different adaptor configurations. The dimensions and reinforcement of the slab and column are constant for the three cases. It can be noted that for highly stressed columns, the top flexural reinforcement of the slab (column region) and the bottom (mid-span region) can be strengthened in order to increase the flexural failure load. In Figure 10, reference numeral 16 indicates the development of flexural mechanism with yielding of top flexural reinforcement, reference numeral 17 indicates development of flexural mechanism with yielding of bottom flexural reinforcement, reference numeral 18 shows concrete crushing in radial direction, reference numeral 19 shows failure by punching of the slab, V corresponds to the punching shear force, N corresponds to the force applied by upper column, and C indicates the lateral force of inclined side of special device.
The above detailed description with reference to the drawings illustrates rather than limit the invention. There are numerous alternatives, which fall within the scope of the appended claims. For instance, the shape and dimension of the interface adaptor may vary depending on the application, the involved loads, the materials, the number of columns involved in the construction and the distance between the columns, etc., without departing from the invention.
The word "comprising" does not exclude the presence of other elements or steps than those listed in a claim. The word "a" or "an" preceding an element or step does not exclude the presence of a plurality of such elements or steps. The mere fact that respective dependent claims define respective additional features, does not exclude a combination of additional features, which corresponds to a combination of dependent claims.
The word "diameter" used in the text refers to circular columns. For other column shapes (square, elliptical, rectangular columns, etc) the corresponding diameter refers to the width of the column in the direction considered

Claims

A concrete slab assembling system comprising:
- a reinforced or pre-stressed concrete slab (1);

- a supporting column (2);

- a supported column (3), having a bottom-end with a reference outer diameter thereof;

- an interface adaptor (10) for placement between the supported column bottom-end and the slab top surface;

- said interface adaptor (10) outer diameter at least at the contacting portion with said bottom-end surface, being at least 10% smaller than the reference diameter of the supported column.
A concrete slab assembling system according to claim 1, wherein said interface adaptor (10) is disk-shaped.
A concrete slab assembling system according to claim 1, wherein said interface adaptor (10) comprises an elongated body provided with a cylindrical portion (12a) and a narrowing portion (12b) co-axial to said cylindrical portion.
A concrete slab assembling system according to any one of claims 1 to 3, wherein said interface adaptor (10) is provided with a dowel (7), for cooperation with a blind hole (4) provided into the slab (1) top portion.
A concrete slab assembling system according to any one of claims 1 to 3, wherein said interface adaptor (10) is provided with an elongated body having a narrowing portion (12b) forming an extremity of said adaptor and being adapted for insertion into a corresponding bore hole (13) provided into the slab (1) top portion.
A concrete slab assembling system according to claim 5, wherein said narrowing portion (12b) is substantially conical.
A concrete slab assembling system according to claim 5, wherein said narrowing portion (12b) is substantially dome-shaped.
A concrete slab assembling system according to any one of claims 5 to 7, wherein said interface adaptor (10) is provided with a cylindrical portion (12a) with a substantially flat end portion for cooperation with supported-column bottom end surface.
Method for assembling a concrete slab (1) between a supporting column (2) and a supported column (3), comprising:
a) placing said slab (1) for cooperation on a top head of at least one supporting column (2);

b) providing an interface adaptor (10) for placement between the supported column bottom-end and the slab top surface, said interface adaptor (10) having a supported column contacting surface substantially smaller than the supported column bottom end surface.

c) engaging said supported column (2) on said slab (1), substantially aligned with said supporting column.
A method for assembling a concrete slab according to claim 9, wherein said supported column contacting diameter of interface adaptor is at least 10 % smaller than the supported column bottom end diameter.
A method for assembling a concrete slab according to claim 9, wherein supported column contacting diameter of interfacing adaptor is between 10 % to 80 % smaller than the supported column bottom end diameter.
A method for assembling a concrete slab according to any one of claims 9 to 11, wherein before the step of engaging said supported column (3) on said slab (1), a step consisting in providing a blind hole (4) on the top portion of said slab, substantially aligned with said supporting column (2) central axis, is added.
A method for assembling a concrete slab according to any one of claims 9 to 12, wherein before the step of engaging said supported column (3) on said slab (1), a step consisting in providing mortar (8) on the slab supporting portion is added.
A method for assembling a concrete slab according to any one of claims 9 to 13, further comprising a step consisting in placing a filler layer (11) at the immediate vicinity of said interface adaptor (10) using a filler layer material having a modulus of elasticity substantially lower than the material of said adaptor (10)
A method for assembling a concrete slab according to claim 14, wherein the filler layer is crown-shaped, with the interface adaptor substantially centrally placed within said filler layer.