Design of peripheral ties with the BT turnbuckle

Building with precast concrete elements is an established process. The design and execution of floors is often a virtually philosophical question. As frequently preferred solutions, they are often executed in complex in-situ concrete construction or with semi-precast elements, since with these construction methods it is easy to provide a load-transmitting diaphragm effect with load-transmitting peripheral ties.

For erection of floor elements – whether concrete-wood composite ceilings, weight-reduced hollow-core or solid-concrete floors – a peripheral tie is usually required. The common procedure is to execute the peripheral tie in the joints between floor element and wall and/or edge formwork. This is provided with reinforcement bars, in the required number and with the required lapping lengths, which are installed and cast with concrete. With higher tensile loads, the peripheral tie is executed with welded connections.

The peripheral tie, according to DIN EN 1992-1-1 and the National Annex, must have a tensile force of F_tie,per = l_i x q₁ or higher, or equal to 70 kN. As a rule, the peripheral tie must be placed within 1.2 m from the edge.

Better quality and less expenditure

However, subsequent installation of the reinforcement and casting slows the construction progress and – owing to the required curing times to establish the necessary concrete strength – may even lead to downtimes. To eliminate this obstacle, the BT turnbuckle system from B.T. innovation GmbH can be used. Here, the peripheral tie is integrated during precasting of the floor element. The principle can be seen on the drawing and consists of socket and/or double socket bars. In the case of double socket bars, these must be made to measure to the length of the structural member. Single socket bars are coupled with overlap into the reinforcement of the precast element. In the precast plant, additional blockouts for the BFT turnbuckles are provided.

The suitable turnbuckle size is selected depending on the resulting loads and in accordance with the usable floor height. The turnbuckles can be used beginning with floor thicknesses of 10 cm. The number and diameters of the peripheral tie reinforcing bars are dimensioned based on the resulting forces. For the tensile force of a peripheral tie of 70 kN, for example, three M12 BT turnbuckles or two M16 BT turnbuckles are required (Fig. 1). This sketch shows a peripheral tie capable of resisting a load of 130 kN; the loads are transmitted through socket bars and BT turnbuckles.

At the construction site, only the BT turnbuckles need to be installed in the blockouts and connected, with screws, to the socket bars (Fig. 2). Subsequently, the small grouting pockets are filled with concrete.

Compared to classic methods such as weld connections, the quality is notably increased and the time required for the work and coordination reduced. Tolerances are easily accommodated, and the danger of fire is diminished.

The described solution is currently being applied in the execution of a large office building. In this case, a conventionally welded peripheral tie is replaced by a BT turnbuckle solution and it is expected that this will result in notable cost reduction in all sub-areasof manufacture and erection, as well as in speeding up of erection. In addition, dependence on the weather is reduced and the cleanliness of the floor increased.