The multifunctional office building was designed on the basis of the cradle-to-cradle principle that is at the heart of the circular economy approach. The aim was to design building components in such a way that up to 90% of them could eventually be dismantled and re-used at the end of the useful life of the office block. With its ornamental façade, the 48 m long and 26 m wide building with a height of 22 m is one of the most eye-catching features of the MediaHafen district of Düsseldorf. The basement and ground floor are built in reinforced concrete, while the five storeys and the attic level are constructed from pre-fabricated timber elements. The striking diamond-shaped façade structure of the Cradle, which combines exposed design and structural elements protected behind glazing, signals clearly that the building is essentially a timber construction. The pre-fabricated façade elements resemble large lattice girders and provide the exterior vertical and horizontal load-bearing structure. The V-shaped and rhomboid sections are made in larch glulam while the horizontal elements that frame the V-shaped sections are made in spruce glulam. The elements weigh up to 6 tons and are 17.20 m long, 3.49 m high and 1.09 m deep.

Global innovation: adhesive-free plug-in connectors with cleats made in BauBuche

The individual façade elements are attached to each other by means of glue-free plug-in connectors designed specifically for this project. In combination with the high degree of prefabrication and standardisation of the timber elements, this type of connector facilitates future dismantling and reuse. Die plug-in connectors consist of a welded steel plate at the bottom and a cleat made from BauBuche at the top, serving as a stiffening element and transferring the load between the posts as well as the shear and parallel forces to the adjacent construction elements. In contrast to conventional methods where cleats are glued or screwed to plates, the BauBuche cleats are simply inserted into recesses in the steel plates. The decision to use BauBuche for this critical element was made based on the properties of the material, in particular its high load-bearing capacity and strength resulting from a 15 percent share of cross plies, and its exceptional resistance to warping. The plug-in connectors with BauBuche cleats are located at the top and bottom connecting points of the individual timber-frame façade elements.

Installation by means of steel-lifting structure

The CAD-designed façade elements were precision-milled on 5-axis CNC machines and fitted at the DERIX factory with steel inserts and fire protection cladding. At the construction site, they were then secured to each other through the plug-in connectors, without any need for adhesive or screws. This method is truly innovative, as the individual construction elements are kept firmly locked together by nothing more than their dead weight. This solution was primarily chosen to allow for easy disassembly and reuse of the building materials. In contrast, with glued and screwed connections, the various elements and materials can only be separated with great difficulty. By choosing the innovative plug-in connectors, the builders had to come up with a novel method of assembly on site to ensure that the elements do not become detached from each other during building construction. They therefore applied a steel-lifting method where the elements are pushed up from the bottom and then lowered into position for final assembly. With each installed façade girder, the overall construction gains stability due to the additional dead weight placed on the elements already in place.

Internal supporting structure made in BauBuche

Given the high load of five upper floors, the designers chose a high-strength hardwood for the internal supporting structure. The building grid in the upper floors features spans of between 6 m and 7.15 m. Square BauBuche posts of 3.50 m in height ( (w x d = 40 cm x 40 cm) and BauBuche girders measuring up to 6.80 m in length (w x d = 40 cm x 56 cm), both of strength class GL 75h are installed here, forming the timber skeleton.

The installation sequence at each floor was as follows: 14 internal BauBuche posts were positioned on the reinforced concrete floor around the utility blocks. Subsequently, the diamond-shaped façade elements were installed around this core structure. This took approximately one week per floor. This work was followed by the installation of 18 BauBuche girders and three steel beams connecting the two utility blocks and anchoring the timber supporting structure to the reinforced concrete parts of the building. In a last step, solid timber floors consisting of pre-fabricated glulam elements were installed on the beams and secure to the façade construction by means of fully threaded screws. At each floor, the procedure for the construction of the building shell took around four weeks, and the six upper storeys were completed in about six months.

Madaster documentation platform

The design of the Cradle is based on CAD models that were developed by multidisciplinary teams and optimised with the help of simulation programs. The resulting IFC (Industry Foundation Classes) model formed the basis for the integrated BIM (Building Information Modelling) process in which all construction parts and materials are included into what is known as a Building Material Passport. This passport is then used for an optimised and highly specific selection of the building materials. It lists individual component numbers and will also serve as a guide document for the future dismantling of the construction. In the passport, the building components are classified according to well-defined criteria such as dismantlability, CO₂ footprint and recyclability. The IFC model has been uploaded to the Madaster platform in order to calculate the residual raw material value and the recyclability of the installed building components and raw materials. The Madaster platform documents the entire life cycle of every construction element from its manufacture, installation and use to the demolition of the building. The Cradle is the first project in Germany whose data has been fully uploaded to this platform.

Circular economy and obligation to take back

A number of building products have actually been leased rather than bought from the manufacturer, which means that they can be returned after use. DERIX Group, the contracted timber construction firm, is the first company in the German-speaking region of Europe to commit to a circular economy concept that obliges it to take back used, standardised timber construction products supplied by it. At the end of the service life of the building, the timber elements will be re-used in other construction projects. The fully digitized 3D documentation enables future users to carry out their search for suitable components with pinpoint accuracy. The promoters behind Madaster want to establish their system based on the circular economy principle across the entire building industry, whereby the timber construction sector is currently playing a pioneering role. In order to keep return rates high, the designers chose glue-free joining and fastening techniques that are easy to assemble and disassemble. The high-performance plug-in connectors made from BauBuche that do away with adhesive are one of the key innovative components in this project.

Text by: Susanne Jacob-Freitag in cooperation with Marc Wilhelm Lennartz

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Photos: Derix Group, Derix Group/Ralph Richter, Ansgar van Treeck

Drawings: Derix Group