The first lift-off lock with approval for TCC ceilings with birdsmouth joint design: the new REISSER KVB-connector
With the new KVB-connector, REISSER offers the first lift-off lock with approval for timber-concrete composite (TCC) ceilings with birdsmouth joint design. The new general type approval (aBG) of the REISSER KVB-connector is the first time that the load-bearing capacity of lift-off locks for such TCC constructions, which was previously not clearly regulated by the norms, has been clearly regulated for such a fastener.
The new REISSER KVB-connector with general type approval for TCC ceilings with birdsmouth joint design (aBG Z-9.1-916) is the first fastener that clearly regulates the load-bearing capacity for use as a lift-off lock. The bond between the timber component and the concrete slab is achieved via so-called birdsmouth joints in the timber component (birdsmouth joints: indentations across the width of the component). They are filled with concrete and thus connect both components to each other in a shear-resistant manner. TCC ceilings of this type only require screws for a force-fit connection so that the concrete slab does not lift away from the wood. The REISSER KVB-connectors, which are screwed vertically into the timber constructions, are used to absorb tensile forces between the concrete slab and the timber component in the area of the birdsmouth joints. Depending on the structural model, KVB-connectors can be used both inside and outside the birdsmouth joints. As a lift-off lock, they transfer axial tensile loads at right angles to the bond joint between the concrete and wood.
According to the currently valid norms (EC 5), a ceiling with a birdsmouth joint design cannot currently be clearly designed in accordance with the rules because, among other things, EC 5 does not provide any load-bearing capacity values for the head pull-through from the concrete. At the same time, however, the technical specification DIN CEN/TS 19103 (for the next generation of Eurocodes), which has not yet been introduced, requires a lift-off lock. At this point, the aBG Z-9.1-916 of the REISSER KVB-connector is now the only fastener that provides a remedy and planning security for the design and execution of TCC ceilings with birdsmouth joint design.
For example, the aBG contains load-bearing capacity tables to facilitate the design of the lift-off lock. This is because four failure criteria must be analysed, such as pull-out failure of the REISSER screws from the timber component, steel tension failure of the REISSER screws, conical concrete break-out and pull-out of the REISSER screws from the concrete. With the help of the load-bearing capacity values in the tables, the check can be carried out easily without having to recalculate each failure criterion individually.
Up to 6 times higher load-bearing capacity for a minimum number of fasteners
The embedment depths of the REISSER KVB-connector in wood and concrete guarantee up to 6 times higher load-bearing capacity compared to conventional pan head screws, which reduces the number of lift-off locks required to a minimum. Overall, this significantly
improves the cost-effectiveness of TCC ceilings with birdsmouth joint design. By reducing the number of fasteners alone, the REISSER KVB-connector contributes to the efficient use of fasteners and thus helps to save resources.
The screw head of the REISSER KVB-connector is set in concrete like a concrete anchor. In order to transfer the tensile force of the fastener exclusively via the head, the screw thread is separated from the concrete by a sleeve. This sleeve under the screw head or the washer of the connector also ensures an exact embedment depth in both the wood and the concrete and thus safety during installation. The KVB-connector is currently available with a diameter of 8 mm and in 20 mm increments in lengths from 200 mm to 300 mm. The REISSER KVB-connectors can be used flexibly in the aBG in terms of embedment depths in both wood and concrete. They can be adapted to the structural requirements of the respective TCC ceiling.
For example, the REISSER KVB-connector 8x200 (70/130) has almost twice the load-bearing capacity of conventional pan head screws compared to an 8x200 pan head screw. This in turn can halve the number of screws. The number of lift-off locks is further reduced if the TCC ceiling construction allows a greater embedment depth of the thread in the wood.
No additional tools are required when screwing in the REISSER KVB-connectors, which saves time during installation. A further time saving results from the smaller number of connectors to be installed. And last but not least, the issues of recycling management, dismantlability and the separation of components have already been taken into account in the development and then also in the aBG of the REISSER KVB-connector.
