Punching shear reinforcement
Punching shear reinforcement is a special form of reinforcement . It increases the punching shear resistance of a reinforced concrete or prestressed concrete slab and in practice prevents the slab from punching shear failure. This failure is characterized by a frustoconical concrete breakout as a result of a concentrated load application. In the case of flat ceilings, such a point load is introduced by placing them on individual supports. The punching shear reinforcement is built into the slab in order to prevent cracks from forming, which can lead to concrete breaking.
Various punching shear reinforcements are available in construction practice. This article deals with reinforcement elements made of reinforcing steel. They differ in shape and dimension. The respective design determines the efficiency of the reinforcement and thus the respective increase in the punching shear resistance compared to panels without punching shear reinforcement.
Application basis
The use and design of punching shear reinforcement is based on the design standard for reinforced and prestressed concrete structures EN 1992-1-1 (Eurocode 2) in Europe. For special reinforcement, national general building authority approvals or European technical assessments (ETA, abbreviation for "European Technical Assessment") are issued, which are relevant for the application and dimensioning of the respective system.
Installation type
This article describes punching shear reinforcements that are concreted in during the production of the slabs. This is the frequent use. Special systems are available for the subsequent reinforcement of ceilings.
system
Stirrups and bends
The dimensioning standard DIN EN 1992-1-1 (Eurocode 2) regulates the application and dimensioning of stirrups and bends as shear and punching reinforcement. Both reinforcements are formed from reinforcing steel by bending. The shape of the bracket are rectangles with rounded corners. The inner diameter of the fillet usually corresponds to four times the rod diameter used. Brackets are installed perpendicular to the panel level according to the construction rules in Eurocode 2. The stirrups must enclose at least 50% of the longitudinal reinforcement of the slab. The installation is therefore complex.
The punching shear resistance of reinforced concrete slabs with stirrups as punching shear reinforcement was determined in tests on slab sections. Compared to slabs without punching shear reinforcement, the punching shear resistance is increased by a factor of 1.4.
Double head anchor
Double head anchors are reinforcing bars with forged heads at both ends. The heads are used for anchoring in concrete. These reinforcement elements are assessed and dimensioned on the basis of general building inspectorate approvals or European technical assessments (ETA). The individual bolts are arranged perpendicular to the plane of the plate. For easier assembly, the individual bolts are mounted on strips. These serve to secure the position. Manufacturers offer different systems for this. The arrangement of these strips in the floor plan of the ceiling is usually radial to the point load introduction (support).
The assessments (ETA) are issued on a manufacturer-specific basis. According to known ETA, the head diameter of the double-head anchors is three times the rod diameter. When using double head anchors of this size, the punching shear resistance of a plate increases by a maximum of 1.96.
Filigree punching shear reinforcement
The filigree punching shear reinforcement consists of welded spatial reinforcement elements. Their geometry has been optimized to function as punching shear reinforcement. The supporting reinforcing bars are inclined differently. The system is assessed and dimensioned on the basis of a European Technical Assessment. The longitudinal elements of this reinforcement are arranged parallel to each other. This simplifies installation and avoids collision points with other reinforcement.
The use of filigree punching shear reinforcement increases the punching shear resistance of a plate by a maximum of 2.1.
System comparison
The standardized punching shear reinforcements, stirrups and bends, are produced by bending from commercially available reinforcing steel. The manual effort when installing in the plate is high and the increase in resistance is limited. Double head anchors and the filigree punching shear reinforcement are available as prefabricated reinforcement elements and increase the punching shear resistance by 40% and 50% more than stirrups. Double head anchors are often used in reinforced concrete slabs, which are concreted completely in one pour on the construction site (in-situ concrete ceilings). In partially prefabricated filigree ceilings which are reinforced with parallel lattice girders, their installation is difficult. Filigree punching shear reinforcement is usually used here. Due to the high punching shear resistance, this reinforcement is also used in in-situ concrete ceilings.
literature
- F. Fingerloos, J. Hegger, K. Zilch: EUROCODE 2 for Germany, DIN EN 1992-1-1 Dimensioning and construction of reinforced concrete and prestressed concrete structures. Part 1-1: General design rules for building construction with a national annex, annotated version. 1st edition, Beuth Verlag, 2012.
- M. Ricker, F. Häusler: European design rules for double-headed anchors as punching shear reinforcement. In: Concrete and reinforced concrete construction. 109, Issue 1, Ernst & Sohn, 2014, pp. 30–42.
- S. Siburg et al .: Punching shear reinforcement for floor slabs according to Eurocode 2. In: Concrete and reinforced concrete construction. 109, No. 3, Ernst & Sohn, 2014, pp. 170–181.
Individual evidence
- ↑ C. Siburg, F. Häusler, J. Hegger: Punching flat ceilings according to NA (D) to Eurocode 2. In: Civil engineer. Volume 87, May 2012, Table 2.
- ↑ ETA-12/0454, European Technical Assessment from December 17, 2017. German Institute for Building Technology (DIBt), Berlin.
- ↑ ETA-13/0521, European Technical Assessment from June 14, 2018. German Institute for Building Technology (DIBt), Berlin.