Filigran punching shear reinforcement newly assessed

The FDB Filigran punching shear reinforcement consists of spatial welded reinforcement elements with optimized geometry. The punching shear resistance of flat slabs is more than doubled by using FDB, featuring an increase by a factor of 2.1 of a reinforced concrete slab without punching shear reinforcement. According to the manufacturer, this increase factor is higher than that of other punching shear systems. This is the outcome of a comparison with stirrups in accordance with current design standards and double-head studs according to European Technical Assessment (Fig. 1).

The maximum punching shear resistance of slabs with stirrups used as punching shear reinforcement is exceeded significantly. While the use of double-head studs increases the resistance by 40 % compared to stirrups, the use of Filigran punching shear reinforcement improves the resistance by 50 %.

Design program of Filigran free of charge

The simple, parallel installation of the elongated elements of the Filigran punching shear reinforcement in the precast slabs with lattice girders, which are supplemented with in-situ concrete on the construction site, has proven its worth for many years. Additional tests were conducted and evaluated on the basis of a European evaluation document, and the conditions of application were newly assessed and extended.

The FDB Filigran punching shear reinforcement was tested in both partly precast slabs with horizontal shear joint and in monolithic slab sections. An increase factor for the maximum bearing capacity was correspondingly included in the new European Technical Assessment ETA-13/0521 (European Technical Assessment for FDB Filigran punching shear reinforcement, German Institute of Construction Technology, June 14, 2018) for Filigran slabs as well as for cast-in-situ concrete slabs. The same increase factor of k_pu = 2.1 applies to both applications. This enables the structural engineer to design independent of the later type of application. For this purpose, a design program is available for free download at www.filigran.de. Due to the high resistance and the simple installation, the use of FDB Filigran punching shear reinforcement is advantageous both in precast elements and cast-in-situ concrete.

Simplified verification of punching shear reinforcement quantity

The improved design concept with an adapted decay function for the tensions at wall edges and wall corners results in smaller areas which require punching shear reinforcement. The verification of the punching shear reinforcement quantity has been simplified. Maximum distances between the reinforcement elements were determined instead of an increase factor for the determination of the steel cross-section required for high loads. Due to these adaptations, the design result as a whole is more realistic and more efficient.

The punching shear tests carried out on precast floor slabs with horizontal shear joint also confirm the function of the FDB as bond reinforcement. In addition, the test evaluation results in a higher maximum bond resistance for the calculation when using FDB reinforcement compared to the current Eurocode 2. The new ETA-13/0562 allows for an increase of 60 % as against the standard value. This enables higher punching shear loads in precast slabs, up to which no additional mechanical roughening of the bond joint is required. Consequently, the design is simplified and the system safety is increased.