Flat slabs with lattice girders as semi-precast construction system feature horizontal composite joints between the semi-precast concrete slabs and the in-situ concrete topping. Both the punching shear resistance and the load-bearing capacity of the composite joint have to be ensured for these floor slabs. Using the Filigran punching shear reinforcement not only increases the punching shear resistance, but also the maximum resistance of the composite joint. A faultless composite joint in the highly loaded punching shear area is the prerequisite for an equivalent load-bearing capacity of semi-precast Filigran floor slabs.
In addition to a rough precast component surface, the use of composite reinforcement increases the joint resistance. The Filigran punching shear reinforcement adopts this function as well. The verification by calculation of the composite joint is based on the principle of the Eurocode 2 design standard for reinforced concrete. This verification includes a maximum load-bearing capacity independent of the embedded reinforcement quantity. The Technical Report TR 508 specifies this upper limit with an increase factor for Filigran punching shear reinforcement. The maximum stress vRdi in the composite joint is dependent on the roughness factor n of the composite joint in accordance with Eurocode 2 and the concrete strength fcd. When Filigran punching shear reinforcement is used, the increase factor kmax,i is specified as kmax,i = 1.6 in the European Technical Assessment ETA-13/0521.
The increase factor for the maximum load-bearing capacity of the composite joint was verified in punching shear tests carried out on semi-precast floor slabs with horizontal composite joints. The high bond resistances are attributed to the stiff anchorage of the Filigran punching shear reinforcement on both sides of the composite joint as well as the fine distribution of the composite reinforcement within the punching shear area.
The increased maximum load-bearing capacity of the composite joint ensures the utilization of the maximum punching shear resistance in semi-precast floor slabs similar to cast-in-situ concrete slabs. When using the Filigran punching shear reinforcement a resistance of 2.1times of a slab without punching shear reinforcement applies as maximum load-bearing capacity for both cast-in-situ concrete slabs and Filigran floor slabs. Any verification required can easily be provided by the ProFi design program. This program is available for free download at www.filigran.de. The program also provides the verification of the composite joint beyond the punching shear area. Moreover, lower bond stresses occur in the joint.