The architects of Kaden + Lager based in the city came up with a construction that open, provides transparency and promotes interaction within the community. The timber hybrid construction was well received by the local community and has contributed to the general acceptance of the project within the city. The development consists of five 5-storey blocks and five 4-storey blocks. Altogether, this new estate in an established neighbourhood comprises 45 apartments and 2 maisonette units catering for 180 people.

Advantages of timber-concrete composite (TCC) floors

Timber-concrete composite floors combine the construction benefits and structural properties of two quality materials. While the timber structure deals with the tensile forces applied to the floor, the concrete bears the compressive forces. In addition, the hybrid system allows for greater spans without supports, while offering better impact sound insulation and fire safety than are achievable with timber floors. Although the total weight and installation height are smaller than with a concrete floor, the composite construction provides a greater load bearing capacity with less floor deflection. The two materials are force-fit by means special fastening elements, such as glued shear connectors. The timber-concrete composite system thus offers significant technical and commercial advantages.

New: timber-concrete composite floor with BauBuche

For the P1 project at Pistoriusplatz in Berlin, the engineers of Pirmin Jung Deutschland GmbH developed an innovative timber-concrete composite system where BauBuche replaces the traditional softwood in the tensile zone. Another first are the attractive ceilings with exposed vertical BauBuche veneer layers that give the rooms a unique atmosphere and reflect the timber character of the building. The uses of BauBuche, the TCC floors were laid indirectly onto the concrete support elements, and the downstand beam actually acts as an upstand beam. There was therefore no need for plastering or a suspended ceiling structure, which of course saved time and money. In addition, the elements could be laid in one go along the entire width of the building, so that the crane hire and installation time was considerable shorter than with a conventional construction. Each floor consists of 19 prefabricated 68 cm wide, 6 cm thick and 13 m long BauBuche elements. Expansion joints of 8 mm in width between the elements allow for the compensation of dimensional changes in the floor construction. A 12 cm thick layer of in-situ concrete was cast directly onto the timber construction. It provides for excellent fire and sound insulation and is connected to the timber floor by partially threaded wafer-head screws. The builders used a concrete mix that was fully set in many areas after only four days. The ducts for the electrical cables were therefore installed prior to casting. On top of the concrete, they installed 4 cm of impact sound insulation and a 7 cm screed with integrated underfloor heating covered by a 14 mm BauBuche parquet.

Slim floor structure for greater room heights

Compared with conventional softwood LVL or CLT elements, the chosen construction allows for much slimmer dimensions without compromising the floor’s load bearing capacity. In addition, this type of construction requires around 50% less timber that a conventional TCC structure. By using BauBuche beams, the height of the raw floors could be reduced to 18 cm, allowing for room heights of up to 3.80 m within the given building dimensions. At the centre upstand beams, the TCC slabs are not placed directly on the timber, but supported indirectly by the concrete structure, which in turn rests on the bottom flange of the integrated beams. The milled stud groves in the timber cross-sections were filled during the pouring of the in-situ concrete and further improve the bond between the timber and the concrete. The resulting concrete tappets transfer shear forces into the timber so that a significantly higher stiffness is achieved. The shear strength at these grooves and the REI60 fire safety requirements were the two main criteria for the dimensioning of the BauBuche beams. The downstand beams in the external walls are also in BauBuche, while upstand beams along the centre axes are made from welded profile steel, as they must carry a heavier load. From a structural strength point of view, the timber-concrete composite floors consist of panels that transfer the bracing loads into the stiffening external and internal walls. At each floor level, there are at least three such walls, namely two external walls of a lightweight timber panel construction, and a stiffening solid CLT internal wall.

Protection by hydrophobization

To protect the BauBuche in the TCC floors, the timber surface carrying the in-situ concrete was hydrophobized at the factory, using a water-soluble intermediate wood glaze based on pure fine-solid acrylate. This treatment makes the surface water-repellent without impairing diffusion. The cut ends were treated with a special end-grain coating consisting of the above hydrophobing varnish combined with a primer. To ensure that the BauBuche elements delivered with a wood moisture content of 7 % were properly protected against the elements, the installation was completed under a weather roof suspended from a crane. During heavy rain, construction even came to a complete halt. Tests carried out prior to construction showed that, when exposed to 85 % relative air humidity, the wood moisture content of BauBuche increased only in the edge zones, while the other zones remained unaffected, which indicates that the highly compact material delays and slows down the transfer of moisture towards the centre.

Contemporary design for urban living

Thanks to the double shell structure of the load bearing, stiffening and enclosing walls, and the slow burning rate of the solid timber construction elements, all apartment blocks are classified in fire safety class F60, which corresponds to building class IV. The burning rate of (softwood or beech) glulam with a characteristic density of ≥ 290 kg/m³ is around 0.7 mm/min, which means that people would have ample time to safely evacuate the building in the unlikely event of a fire. The excellent design of the apartment blocks is reflected in their overall dimensions, the quality of the materials and the top-standard finish. The architects succeeded in building homes that will that will stand the test of time. From an architectural point of view, this new estate in an established neighbourhood is thus built on solid foundations – and everything else that will make living here a valuable experience is bound to follow suit.

_{-Text by Marc Wilhelm Lennartz-}