Hager Forum in Obernai
For the Hager Forum in the French town of Obernai, timber construction company Holzbau Amann provided early advice to the architects Sauerbruch Hutton and structural engineers Werner Sobek (Fig.A–F). The original design was a 39 m ≈ 156 m timber grillage with a grid module of 3 m ≈ 3 m, which was to be supported on only a small number of steel columns. A timber roof with long edge cantilevers was envisaged to allow the roof loads to be transferred into the area of the concrete stairwells. The timber cross sections deepened towards the columns to clearly express the flow of forces. The plans showed individual elements being bolted together. The 3 m wide and up to 12 m long elements were manufactured out of two glued laminated timber beams with a glued-on, cross laminated timber panel. The shear forces and moments occurring in the transition between roof and columns were to be transmitted by concealed steel plates and continuously threaded bolts (Fig. B).
Having decided on prefabrication, the architects foresaw the decisions to be made about the methods of manufacture, connection techniques and installation tolerances and persuaded the client to involve us as the contractor to provide advice at an early stage. One of our tasks was to complete the structural engineering design of the erection phase and ensure the structure complied with the architectural requirements.
We identified several aspects that made construction more costly and complicated when considered from the point of view of glued timber elements and looked for alternatives.
Our first thought was a rafter-purlin alternative, but the shallowness of the beam required by the architect would have led to very high deflections because of the lack of grillage action. Instead, we developed a directional hybrid structure with steel main beams clad in glued laminated timber running transversely to the structure and timber secondary beams running longitudinally (Fig. C, E).
The connections of the partially haunched timber beams at the steel beams were stiff in bending so that the continuous beam effect in the longitudinal direction and the structural action of the grillage were maintained. The top layer of cross laminated timber boards was connected by nails at close centres to the secondary beams to ensure distribution of the vertical load. These measures made it possible to achieve the originally planned depth of beam. In order to take the loads due to climatic influences acting on the horizontal timber boards into account, cross-cut timber veneer, a measuring device for relative humidity and temperature and a system to monitor the moisture content of the wood were installed.
We worked in a mutually open and trusting relationship with Werner Sobek to develop this final alternative, which was adopted.
The advice from a specialist timber engineering firm paid off for the client: everything was very well prepared ahead of construction on site and the limited budget for time and cost was not exceeded. Our alternative was EUR 250,000 cheaper than the original solution.
Hermès Shop in the GUM store in Moscow
We received an enquiry for the manufacture of a feature staircase from timber at the end of 2014. The idea had been developed jointly by RDAI Architectes and consulting engineers Bollinger + Grohmann and consisted of 145 vertical, curved timbers with a cross section of 40 mm ≈ 60 mm, which together create a free-form surface (Fig. G–J). The structure of the feature staircase was made available to us in the form of a Rhino model along with the questions of practical feasibility and cost.
We had already developed curved timber sections working with the same multidisciplinary team for the “Hermès Rive Gauche” in Paris and therefore we were familiar with the expectations of the architect in terms of the finished quality. The particularly difficult challenge presented by these stair balustrades was that they were manufactured out of doubly curved sections which twisted at the same time.
Any incongruities in the model were eliminated in the detailed design working in conjunction with Bollinger + Grohmann and “design to production”. For example, eliminating any radii that were too tight or sharp kinks.
The architectural design envisaged that the individual verticals in the balustrade would stand up adjacent to one another without any form of connection. To position the sections such that the desired even gap pattern would be achieved, we suggested placing spacers between them. After this idea had been rejected by the architect, we developed what was the finally approved proposal to use the handrail and the top closure as spacers. The handrail was created out of doubly curved timber sections, while the top balustrade closure consisted of a singly curved flat steel profile. We made sample sections to confirm feasibility before the final order was placed (Fig. I).
Eckert Logistics Hall in Waldhut-Tiengen
Single-storey industrial buildings with steel roofs are usually cheaper than similar ones with timber roofs. The main reason for this is the smaller structural depth of the steel beams, the resulting lower overall building height and the frequently adopted two-pin frames with simple moment-stiff corners and low foundation costs. However, when fire protection requirements apply, the price advantage is reversed. According to industrial building construction regulations, load-bearing structures can be designed to have R30 fire resistance with fire compartments up to 3,000 m2, without the need to fit a fire detection system. Compared to steelwork, which requires an insulating coating, the R30 requirement imposes no significant additional costs on timber structures. On the other hand, with no requirement for fire resistance (R0) and no fire detection system, the permissible size of fire compartments is restricted to 1,800 m2.
We were requested directly by the client to prepare an alternative to a steel roof construction which had been planned for a logistics hall (Fig. K–M). The development plan for the area called for a minimum roof slope of 8°. The 37.5 m span between columns and the roof angle made it suitable for a timber truss solution. We decided on a system height of 3.5 m. To keep the system height to a minimum, we designed for single-span purlins – which meant a continuity factor could not be used in the design of the trusses. The connections were formed with multiple-shear slotted steel plates and thin dowels. Based on our input, the client commissioned the timber variant, which not only offered cost advantages but also created a pleasant room atmosphere.
The new, high-strength timber-based construction material Baubuche, a laminated veneer lumber made from beech, allows timber trusses to have still shallower system heights. We are currently designing a production hall roof with a span of 40 m and an overall truss height of 3 m.
Working together to achieve the objective
Close co-operative working between the architect, structural engineer and specialist firms, possibly in small networks, can pay dividends for all project stakeholders if they work together in a non-partisan manner and are open to new ideas. The feedback circle with the contractors allows architects and engineers to see how their designs may be optimally realised. This applies particularly to details, which can both fulfil the design ideals of the architect and provide practical and cost-secure solutions. Contractors should, of course, never forget to conclude an agreement for payment with the client for the advice they provide before they provide it.
