Hybrid multi-storey structure without beams

A new building system 

The municipal administration of Kirchheim unter Teck (Baden-Württemberg) has implemented a climate protection plan with the goal of reducing its CO2 emissions by 37% within 20 years. A new municipal office building, largely made of timber, will help to achieve this goal. A beamless construction system was used for the first time in Germany, featuring large spacing between columns (× 7x5 m) and a load capacity of 1000 kN per column.  

A three-storey building in the industrial area of Bohnau is supposed to bring under one roof all the departments of the municipal authority, which until now have been distributed over several locations. This building is a perfect example of hybrid construction: the core and basement are made of concrete, while the upper floors are made of cross-laminated timber (CLT) with a column-floor structure. The non-load-bearing walls are made of lightweight timber frame. 

«In Germany, this new building is actually the first to be built with a column-to-floor system, without beams», said Uwe Ruckgaber of the Bankwitz architecture office at the site opening ceremony in July 2023. In addition to planning the timber construction, Bankwitz managed the design of this work. The reason why designers and clients have decided in favour of a construction technique that is still rarely used is its many advantages: this construction technology allows for large spans and a supporting grid of up to 7×7 m, resulting in flexibility in the arrangement of electrical and ventilation systems, flexibility in moving internal partitions, and thus in the creation of a large foyer on the ground floor and spacious meeting rooms. Other advantages of a beamless construction, which become evident as the size of the building increases, are the ease with which you can create fireproof sections over large areas and the reduction in storey height, and thus in façade area and associated costs. 

The structural project manager at Furche Geiger Zimmermann Tragwerksplaner learned about the potential of this construction technique during a timber seminar at Rothoblaas, introduced it into the design and convinced the Bankwitz architects. From the outset, the client had requested an innovative and sustainable timber implementation. 

From an architectural point of view, the Bankwitz design avoids cladding as much as possible: exposed concrete and CLT floors also contribute to resource-saving and transparent building aesthetics. 

Timber construction with concrete-like spans 

The idea of a CLT floor, with just a few support points, without beams and with a span as wide as possible, as is the case with reinforced concrete, was developed at the University of Innsbruck. With the cooperation of Rothoblaas, the SPIDER column connector was developed and presented to a wider audience for the first time at the Holzbau Forum in Garmisch. Several solutions were tested with regard to the connection of the CLT panels forming the floors. The TC FUSION system by Rothoblaas, which connects the panels using threaded screws and a reinforced concrete casting, was finally chosen. Rothoblaas' system was chosen because it boasts a European Technical Assessment (ETA) from the Austrian Institute for Building Technology (OIB). 

The floors of the new building in Kirchheim are supported by eight glulam fir columns (300×300x2800 mm), arranged in a 7×5 m grid. The connection of the columns to the floor was implemented in two versions: in the inner area of the storey, where higher loads occur, the connector in the reinforced SPIDER version was used, with six arms fixed with 48 VGS full thread screws. In the peripheral zone of the floor, it was possible to avoid this reinforcement and to use PILLAR connectors. In any case, 45° VGZ reinforcement screws were provided to check the rolling-shear strength of the floor in the areas of the point supports of the floors on the columns. 

The concrete joints of the floor disappear into the flooring, covered by a filler mixture that fulfils the soundproofing and fire insulation requirements and enables the reduction of vibrations. Above this, soundproofing rock wool sheets are installed and a quick-fix concrete screed with underfloor heating (flow temperature 38 °C) is installed. The CLT sheets above the ground floor and first floor are 240 mm thick, while the roof is 200 mm thick. The bottom layer of the seven-layer floor panels, which are approximately 14 m long, can be left exposed due to their quality. The CLT panels were produced by HolzBauWerk Schwarzwald. Threaded rods were screwed to the panels at a variable spacing of 25 cm for a depth of approximately 50 cm in the CLT panel and 23 cm in the concrete. The entire timber construction for this project was undertaken by the company Holzbau Layh. 

Practical implementation of the climate protection plan 

By implementing the timber envelope, the building exceeds the requirements of the KfW-55 standard by more than 25%. The flat roof is insulated with mineral wool and extensively covered with vegetation. It supports rainwater retention, acting as insulation in winter and as sun protection in summer, thus contributing to energy savings. The cooling capacity of the vegetation will also increase the efficiency of the photovoltaic system on the roof. The exterior walls comply with F 60-B fire protection requirements and are made of timber, while the interior walls comply with REI 60 fire protection requirements. A ventilation system with 80% heat recovery ensures the necessary air exchange in the building and reduces heat losses. A salt water heat pump system provides a cooling effect through the floor in summer. The heat pump is powered by a dedicated photovoltaic system. The investment costs for the construction project amount to approximately 5 million Euro according to the 2022 planning status. The construction of the heating system is financed with approximately 166,000 Euro by the Federal Office for Economics and Export Control (Bafa), while the timber construction is financed with approximately 200,000 Euro by the "Holz Innovativ Programm (HIP)" of the Baden-Württemberg Ministry of Agriculture, Rural Development and Consumer Protection (MLR). A subsidised credit line from the Kreditanstalt für Wiederaufbau (KfW) was also requested under the "Energieeffizientes Bauen" financing programme.