Aiming High with Prefabricated Balconies

The example of the Grand Tower in Frankfurt shows that there is currently no quick and practical solution for the construction of balconies on high-rise facades. In this project, the balconies were prefabricated as in-situ concrete construction with the floor slabs and extensively scaffolded. The facade elements then had to be lifted between the existing balconies and laboriously installed on the concrete components. This method of construction was detrimental to the speedy construction process in the unfinished and expanded building and facade trades.

The aim of the research project is to develop an assembly-friendly and inexpensive solution for the realisation of balconies in high-rise construction. The balconies are to be assembled as prefabricated parts with the facade in one step.

A very light balcony is designed so that the connection point can be carried out economically: The supporting plate is planned in UHPC with a thickness of 5 cm, whereby the concrete is ready for surface and therefore requires no covering or coating. The balcony slab is reinforced with carbon mats to increase the structural stability and the ductility of the component. It is stored on rectangular webs that penetrate the facade at certain points. The penetration is sealed with appropriate facade profiles.

The challenge at the connection point of the prefabricated part is particularly complex: Given the high clamping torque — due to the cantilever of the balconies — it is important to constructional solve the penetration of the insulation level (sealing and insulation).

Two variants were developed for the connection, variant A is designed for floor slabs with a minimum thickness of 25 cm, while variant B is used for slabs up to 15 cm.

Variant A is an end connection on the edge of the ceiling. The steel built-in part attached to the prefabricates balcony is welded to the reinforcement in the area of the recessed ceiling. In order to achieve the load-bearing capacity in the final state, the support point is filled with grout.

Variant B is based on an extension of the precast balcony with a steel beam. This is placed on the top of the ceiling and anchored. For tolerance compensation, approx. 3 cm deep ceiling slits with subsequent grouting are provided.

The connection of the balconies forms selective thermal bridges due to the continuous steel components. For this reason and to ensure corrosion protection, stainless steel must be used in the penetration area. 3D temperature field calculations have shown that with appropriate insulation the node is detectable on the basis of construction physics. The component temperatures on the inside are sufficiently high to prevent the formation of dew or mold.

As part of this R&D project, it has been possible to develop a coherent design. Based on typical geometries of balconies and facade connections, all important topics have been dealt with and conceptually solved — from the load-bearing capacity to the insulation and sealing.

The focus of the work was the load-bearing capacity of the precast balcony and its connection to the shell. By using UHPC, a very slim and light balcony construction was realised, and a connection was developed that meets the requirements regarding load-bearing capacity and penetration of the facade.

A sophisticated system solution for the subsequent assembly of prefabricated balconies in combination with the facade can become a key technology for the construction of high-rise residential buildings and thus bring a clear competitive advantage. The previous studies show the basic feasibility. A construction ready for execution would have to be developed in individual cases, considering the prevailing geometry and other project-specific aspects.

The Henninger Tower in Frankfurt am Main would have been a real example in which prefabricated balconies could have been used.