The Lokstadt area in Winterthur is an innovative urban renewal quarter on the site of the former locomotive and machinery manufacturer SLM. Since 1990, the site has been transformed step by step into a mixed-use district with residential, commercial and office buildings. The seven-storey ‘Elefant’ is located on Zürcherstrasse and is named after a locomotive model that was produced here in the past. The clear divisions along the longitudinal façade of the timber frame building, which adjoins a listed portico sections, reflect the industrial past of the site. The historic portico part of the building has been fully restored and upgraded whereby the existing steel beams and crane runways were preserved.

It now serves as the reception and consultation area of the regional headquarters of SWICA, a Swiss health insurance company. The total area used by SWICA is 11,000 m². Apart from general workplaces, there are meeting rooms, cafeterias, a canteen open to the public and a number of terrasses. There is also a dedicated area for project and creative work in the hall wing in front of the main building, while the ground floor houses a training and conference centre. The project was completed following the guidelines of the 2000 Watt Society and is certified according to the SIA Energy Efficiency Path as a Minergie building.

Supporting structure

Two concrete utility shafts serve as the stiffening cores of the office block designed with the help of BIM und Lean Construction Management. All other sections are built as a timber skeleton structure with load-bearing elements made in beech laminated veneer lumber, a product known as BauBuche. At the heart of the supporting structure are BauBuche posts. In beam span direction, they are positioned at distances of 5.40 m to 5.70 m to each other. In ceiling span direction, the gaps between the posts measure 7.41 m and 8.94 m respectively. In the annex, the span is 11.80 m, whereby the beams and the concrete above form a composite structure. The posts support the edge beams as well as the single-span beams that carry the timber-concrete composite floors. These single-span slabs between the girders serve as horizontal stiffening elements.

The design and the materials of the supporting structure were chosen with a view to achieving maximum flexibility as regards room division. The structure is thus a traditional post-and girder system with large beam spans. As the supporting structure has to withstand extremely high horizontal and vertical loads, special attention had to be paid to the design of the posts at ground floor level.

Given that the height of the storeys was limited by building regulations, the architects were looking for the lowest possible construction height of the beams and floors. Their requirements were met by BauBuche of strength class GL75. Thanks to the high load-bearing capacity of this currently most efficient industrial wood-based material, the height of the girders and thus the amount of wood used could be minimised. Additional load performance was achieved by installing composite floors made from 20 cm glulam topped by 12 cm concrete. For the posts at ground floor level, BauBuche proved to be the only timber material able to carry the huge load. While it would have been possible to install spruce or fir posts in the upper floors, the designers opted for BauBuche throughout the building in order to obtain a streamlined look.

For fire safety reasons, all cross-sections of the supporting structure elements – posts, beams and girders – are dimensioned for fire resistance class R 60, which means that they can withstand 60 minutes of direct exposure to flames. For the heating, air-conditioning and electric cable ducts at right angles to the supporting structure, recesses were produced in the girders.

BauBuche elements in detail

The BauBuche posts along the façade and in areas of the building where loads are relatively low have cross-sections of 31.5 cm x 32 cm. In high-load areas, the cross-sections of the posts measure 31.5 cm x 48 cm. The design includes a maximum tolerance of 5 mm between top chord and post along the façade.

The façade posts at the standardised storey levels carry BauBuche beams of 52 cm in height and 25.5 cm in width. BauBuche edge strips (6 cm x 12 cm) attached by screw press bonding support the glulam elements of the timber-concrete composite floors. Reinforcing bars (2 cm diameter, 1.50 m long) are installed at 1 m intervals and on both sides of the posts to prevent torsion under high load. The top plates (110 x 110 x 15 mm) of the bars are screwed to the posts and connect the timber elements to the concrete floor layers. As the edge beams below the attic level needed to withstand higher loads, they are slightly larger (up to 96 cm x 31.5 cm), and in some sections they are cambered by 15 mm.

At the centre of the building, the girders have standard dimensions of 65 cm x 31.5 cm. In areas with lower loads, they are 40 cm high and 31.5 cm wide. Notches measuring 20 cm x 6.5 cm in the upper edge areas serve as supports for the horizontal glulam elements. Here, the construction tolerance is 8 mm, catering for sealing with liquid mortar and 8 mm foam sealing tape. The mortar provides a friction-locked floor construction, which means that the timber-concrete composite floors could be considered continuous girder systems in the structural strength calculations.

In the hall section with spans of up to 11.80 m, girders of 31.50 cm in width and 84 cm in height with a camber of 25 mm are installed. These sections, in conjunction with the composite elements with topping concrete, could be considered in the structural calculations as T-beams. They are connected through steel shear cams with head bolts screwed to the beams.

At the nodes formed by the posts and the continuous girders, the latter are reinforced with round steel bars inserted into the girders. In combination with the load distribution plates at the base and the head of the posts respectively, vertical loads are transferred directly from post to post, without impacting on the wood fibres. Depending on the actual load, between one and three round steel profiles with a diameter of 80 mm per post connect the head and base plates (d = 30 mm) to the adjoining girder. Where timber-concrete composite floors bridge the span from one girder to the next, the length of the round steel bars was adjusted accordingly to extend to the top of the concrete cover, which means that the bars protruded by 12 cm above the girders before the floors were installed.

All channels and recesses for utility lines and pipes in the BauBuche elements were produced at the factory (see also text box). Installation on site took five months. During this period, a temporary roof was erected above the construction to protect the materials against the elements. Whenever elements and materials needed to be placed by crane in the building, sections of the roof were temporarily removed.

Conclusion

By using BauBuche, it was possible to design and realise a supporting structure that is not only economically viable and highly efficient, but also meets the requirements of a modern office block while ensuring maximum flexibility over the entire lifespan of the building. In this project, the exceptional properties of the high-strength hardwood material have once more come to the fore. Wide spans allowed for flexible floorplans, and all building components exposed to high loads could be produced and installed efficiently.

Text box:

In Germany, recesses and openings in BauBuche components are not yet covered in building regulations and are therefore not permissible. This also applies to glued threaded bars. In Switzerland, the structural engineer is responsible for determining whether recesses and openings in BauBuche components may be produced. If they can prove in accordance with the applicable Swiss standard that the required load-bearing capacity of the BauBuche components is maintained even with the planned recesses and openings, these may be executed, as was the case in the Elefant building.

text von: Susanne Jacob-Freitag, Karlsruhe

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Photos: beate bühler fotografie gmbh, SWICA/Michael Haug