EP2851479B1 - System for reinforcement against crushing with transverse rods having anchoring heads - Google Patents

Description

INTRODUCTION

The support zones of the reinforced and / or prestressed concrete slabs are subjected to very high bending and shearing forces. They are thus often determining for the resistance and the capacity of deformation of the structure [1].

1. 1. Modifier les dimensions de la surface d'appui et/ou les armatures flexionnelles de la dalle.
2. 2. Disposer une tête métallique permettant d'élargir la surface d'appui de la dalle (solution souvent adoptée pour de sollicitations très importantes).
3. 3. Plier les barres d'armature de flexion pour leur permettre de reprendre l'effort tranchant. Cette solution a été pratiquement abandonnée depuis les années 60's mais elle est encore présente dans quelques systèmes spéciales d'armature, plutôt pour l'amélioration de la capacité de déformation des dalles en béton.
4. 4. Ajouter des armatures transversales (de dimensions et propriétés comparables à l'armature à la flexion de la dalle) façonnées spécifiquement afin d'empêcher à la fissure conique de poinçonnement de s'ouvrir sans contrôle.
5. 5. Disposer de fibres dans le béton (éléments linéaires en acier ou plastique de faible diamètre et longueur et orientées de façon aléatoire).

Failures in the bearing areas are normally caused by punching the concrete [1], ie by the formation of a conical crack causing the bearing surface to penetrate into the slab (see figure 1 ). In order to increase the resistance and deformation capacity of these regions, different solutions have been applied in the past:

1. 1. Modify the dimensions of the support surface and / or the flexural reinforcement of the slab.
2. 2. Arrange a metal head to widen the support surface of the slab (solution often adopted for very important solicitations).
3. 3. Bend rebar flexion to allow them to regain shear strength. This solution has been practically abandoned since the 1960's, but it is still present in some special reinforcement systems, rather for improving the deformation capacity of concrete slabs.
4. 4. Add transverse reinforcement (of similar dimensions and properties to the slab reinforcement) specifically shaped to prevent the conical punching crack from opening unchecked.
5. 5. Place fibers in the concrete (linear steel or plastic elements of small diameter and length and oriented randomly).

When the modification of the slab dimensions is not possible for economic or functional reasons, the use of transverse reinforcement is the solution most often used. It allows the efficient use of the material with the arrangement of the elements (steel) with high strength and ductility.

STATE OF THE ART IN RELATION TO THIS INVENTION

• Des étriers (barres d'armatures ordinaires) façonnés de façon a faciliter la pose de l'armature de flexion (figure 2a)
• Paniers d'armatures, avec plusieurs brins d'armature ancrées par adhérence (figure 2b)
• Barres d'armatures droites avec tête d'ancrage, disposées en vertical ou inclinées (une ou plusieurs barres de montage pouvant être présent dans un côté afin de faciliter la pose des éléments, figure 2c,d)
• Barres d'armatures droites avec tête d'ancrage d'un côté et soudés sur un rail de l'autre (figure 2e)

Compared to reinforcement systems used for reinforcing concrete slabs, several systems are currently used:

• Stirrups (ordinary reinforcing bars) shaped to facilitate the installation of the bending reinforcement ( figure 2a )
• Reinforced baskets, with several reinforcement strands anchored by adhesion ( figure 2b )
• Straight rebar with anchoring head, arranged vertically or inclined (one or more mounting bars may be present in one side to facilitate installation of the elements, Figure 2c, d )
• Straight rebars with anchor heads on one side and welded to one rail on the other ( figure 2e )

Other solutions or combinations of the previous solutions are always possible [2]. In addition, systems composed of sealed or through rods anchored to the ends by means of plates or mechanical elements are also used, but their use is restricted to reinforcement of existing slabs ( figure 2f ).

The performance of the different systems depends strongly on the quality of the anchorage (forged or welded heads, anchoring by adhesion) and the structural rules applied to the system (spacing of the transverse reinforcement, quantity of reinforcement available ( figure 3 [2,3]).

DESCRIPTION OF THE INVENTION

In order to increase the resistance to shear and punching, it has been observed experimentally that inclined bars show a better performance than vertical parts ( figure 3 ). However, excessive inclination of the rods can lead to anchoring problems in the compressed area of the slab (intrados). This solution has also revealed difficulties for the laying of transverse reinforcement, particularly when they are not parallel to the directions of the flexural reinforcement.

EP 1 630 315 A1 discloses a reinforcing system against the punching of a concrete slab according to the preamble of claim 1.

The invention provides a reinforcing system against the punching of a concrete slab according to claim 1. In one embodiment according to the invention, a combination of vertical rods and curves with anchor head is proposed, see figure 4 .

The rods are made of standard or high strength steel bars with two anchor heads (one at each end of the bar, figure 4 ). The bars can be straight or curved. For curved pieces, the curvature can be obtained by a succession of point bends or by a constant bending radius, and straight zones can also be envisaged according to the static behavior of the system and the method of manufacturing the parts.

The curved rods, which are more statically efficient, are arranged close to the bearing region (where the forces are maximal) whereas the straight pieces are arranged in more distant regions subject to lower forces, see figure 4 .

The stems are arranged radially from the edge of the column ( figure 5 ) with variable number depending on the degree of stress. In parallel rays flexural frames, the combination of curved and straight pieces is envisaged. On the other hand, for other radii, vertical straight pieces are envisaged in order to facilitate the laying of the reinforcement (see figure 5 ).

• D'accroitre les performances statiques par rapport à des armatures avec têtes d'ancrage verticales (voir figure 3). Ceci est justifié par la présence des éléments courbes inclinés dans la zone où ils sont interceptés par la fissure de poinçonnement
• De profiter des avantages de pose de tiges verticales pour les zones avec armature flexionelle non parallèle aux tiges ou loin de la zone de rupture (voir figure 5)

This arrangement of rods allows:

• To increase the static performances compared to reinforcements with vertical anchoring heads (see figure 3 ). This is justified by the presence of inclined curved elements in the area where they are intercepted by the punching crack
• Take advantage of the advantages of vertical rods for zones with non-parallel flexionelle reinforcement to the stems or far from the rupture zone (see figure 5 )

This system also targets an optimization of the cost of the product with special parts (curves) in the region where the solicitations are more important and straight parts (more economical and easy to dispose) in less busy areas or with increased installation difficulties. See as well Figures 6A to 6B .

APPLICATIONS

The object of the invention is to enable the reinforcement against punching of concrete slabs subjected to very high concentrated loads. The goal in terms of strength is to exceed those of similar products (punching rebar systems) currently in the market.

This product could be used as a replacement for more efficient (and more expensive) products such as metal heads against punching.

Another objective of this product is to improve the deformation capacity of the slabs, similar to the raised bars. This will allow the use of this system as an integrity armature (reinforcement to prevent slab collapse after accidental punching).

BIBLOGRAPHIE

1. [1] Muttoni, A. Punching Shear Strength of Reinforced Concrete Slabs Without Transverse Reinforcement, ACI Structural Journal, Farmington Hills, Mich., Vol. 105, No. 4, 2008, pp. 440-450
2. [2] Fernández Ruiz, M., Muttoni, A. Punching Shear Strength of Reinforced Concrete Slabs with Transverse Reinforcement, ACI Structural Journal, Farmington Hills, Mich., Vol. 106, No. 4, 2009, pp. 485-494
3. [3] Fernández Ruiz, M., Muttoni, A., Performance and Design of Punching Shear Reinforcing Systems, 3rd International Fib Congress, Washington, USA, 2010, 15 p .

DESCRIPTION OF THE FIGURES

AT

STATE OF THE ART

Figure 1:

Punching crack developing in a bearing zone of a slab without transverse reinforcement.

Figure 2:

Examples of existing punching armature systems: (a) stirrups; (b) stirrup cages; (c) vertical studs; (d) inclined studs; (e) studs welded to a rail; and (f) glued reinforcement.

Figure 3:

Performance of reinforcement systems against punching according to their anchorage and type of reinforcement (adapted from [3]).

B

INVENTION

Figure 4:

Cross section of a reinforced slab with the proposed invention: several curved studs are arranged near the column (maximum stress zone) while straight studs are arranged in less stressed regions.

Figure 5:

Plan view of the layout of the pieces and the bending reinforcement of the slab.

Figure 6A:

Anchor with 2 folds in two opposite directions in the shape of "S".

Figure 6B:

Anchor with a succession of punctual folds and a straight part between the folds or anchors with two or more punctual folds and a straight part between the bends.

Figure 7A:

Anchor with feet enlarged by an increase in volume.

Figure 7B:

Anchor with at least one head not perpendicular to the stem.

Figure 7C:

Anchor with an enlarged head to improve the introduction of efforts into the concrete.

Systems of Figures 7A to 7C do not include a curved bar. These systems are not part of the invention.

Claims (12)

1. System for reinforcing a concrete slab against crushing, comprising studs that extend transversely with respect to the slab when the system is installed in the slab, said studs consisting of a transverse rod with an anchoring head at each end of said rod, said anchoring heads having a wider diameter than the diameter of the rod, said studs having an anchoring head positioned close to the upper surface of the slab and an anchoring head positioned close to the lower surface of the slab when the system is installed in the slab, characterised in that the rod of said studs is curved between said anchoring heads.
2. System according to claim 1, characterised in that the curved rods have at least one straight section.
3. System according to claim 1 or 2, characterised in that the curved rods have at least one vertical section relative to the concrete slab.
4. System according to any one of claims 1 to 3, characterised in that the curved rods have at least two angular offsets in the same or different directions.
5. System according to any one of claims 1 to 4, characterised in that in combination the curved rods form an arched structure.
6. System according to any one of claims 1 to 5, characterised in that at least one of the anchoring heads is positioned at an inclined angle relative to the corresponding ends of the curved rods.
7. System according to any one of claims 1 to 6, characterised in that at least one of the anchoring heads is positioned at a right angle relative to the corresponding ends of the curved rods.
8. System according to any one of claims 1 to 7, characterised in that the curved rods have a foot that is enlarged relative to the anchoring shank as the lower anchoring head.
9. System according to any one of claims 1 to 8, characterised in that the curved rods have a region that is enlarged relative to the rest of the curved or straight rods as the lower end thereof.
10. System according to any one of claims 1 to 9, characterised in that it includes a combination of studs with curved rods and studs with straight rods.
11. System according to any claim 10, characterised in that the studs with curved rods are arranged close to the bearing region while the studs with straight rods are arranged in the more peripheral regions.
12. System according to any one of claims 1 to 11, characterised in that the curved or straight rods are inclined towards the zone of a concrete slab where they are intersected by any potential shearing cracks.

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