The Baumäckerhalle is located in tranquil Bihlafingen, a district of the town of Laupheim, and stands on a narrow plot of land wedged between the local kindergarten and school. The hall serves as a sports and communal venue. Since its opening, it has already become the heart of community life where schools run their sports classes, clubs organise their meetings and the municipality holds their celebrations and assemblies. The plot slopes steeply from north to south, a topographical challenge that Birk Heilmeyer and Frenzel Architekten, who won the competition for this public project in 2018, exploited in a most ingenious fashion.

Embedded in the landscape

The structure is 37.20 m long and 16.80 m wide. Its gable roof slopes gently by 20° and thus blends into the natural incline of the site, as one section is fully embedded into the ground, resulting in a moderate building height. The archetypal, compact building with its wooden façade fits perfectly into the rural surroundings. To the north, the hall appears as a low, flat, elongated structure. To the south, it features a floor-to-ceiling glass front, offering a view of the grass pitch.

Two entrances catering for different user groups

The building is designed as a filigree structure with a transverse grid of 5.55 m (reinforced concrete columns) and 2.775 m (BauBuche beams) respectively. It is essentially a timber construction, as only the elements in touch with the ground and the building sections exposed to the elements are made in reinforced concrete. In line with the sloped site, the building has two entrances. From the north, one enters the Baumäckerhalle on the upper floor with a unobstructed view into the interior of the hall. At this level, we find the changing rooms for sports activities. A single flight of stairs leads to the foyer on the lower ground floor, where the second entrance for pupils is located. The kitchen is placed at the centre of the foyer and caters for both indoor and outdoor events.

Filigree Polonceau trusses made of BauBuche

The centrepiece of the building is an extraordinary roof structure made of Polonceau trusses spanning the 28.65 m long hall. The design principle of this truss structure named after the French engineer Barthélemy Camille Polonceau (1813–1859) defines the rafters of a gable roof as underslung beams whose ends are connected by a horizontal tie rod. Posts, generally referred to as verticals, serve as compression members.

For the Polonceau trusses of the Baumäckerhalle, the designers chose BauBuche (GL 75). This material made it possible to use beams with particularly slim cross-sections to span the 16 m wide hall. All beam cross-sections are 16 cm wide and 12 cm high, with the exception of the approximately 8.50 m long square rafters that measure 16 cm x 16 cm. The underlays, verticals and tie rods also measure only 16 cm x 12 cm. The fact that tie rods with of such small dimensions could be used demonstrates the high load-bearing capacity and strength of BauBuche. For the installation of the beams, the members under compression were connected using multi-step joints in accordance with best timber construction practice. The tensioned members were connected using slotted plates and bar dowel fasteners. The result is a building where the filigree beam structure is one of the main features of the interior.

A key advantage of the chosen supporting construction is the fact that it forms a statically self-contained system that only needs to be mounted vertically. As a result, the Polonceau trusses are simply placed on top of the 2.65 m high BauBuche posts (w x h: 16 cm x 30 cm) at an axial distance of 2.775 m. These verticals are designed as hinged posts and the trusses are reversibly attached to them by means of shear pins and steel plates recessed into the posts.

Fully insulated, 32.50 cm thick timber panel elements consisting of 20 cm high purlins, 10 cm wood fibre boards on the upper side and 2.5 cm OSB boards on the lower side are laid on the BauBuche beams in longitudinal direction. The OSB planking on the underside is nailed to the BauBuche rafters of the Polonceau structure for shear-resistant connection. Each roof half forms a roof pane, and these planes are connected to the gable walls, providing transverse bracing for the building. Also here, the connections are shear-resistant. In longitudinal direction, the timber structure is reinforced by two diagonal compression struts – also made of BauBuche – in the manner of a traditional strut frame construction. Similar to a horizontally positioned lattice girder, these struts bridge the gap of 5.55 m between two beam ends at the bottom chord level and to the first node of the Polonceau truss. The horizontal compression elements required for this purpose along the eave edges are integrated into the timber panel level of the roof pane.

Recyclability as a guiding design principle

The planners paid particular attention to the sustainability of the construction. The Polonceau trusses were prefabricated as load-bearing elements and are reusable. Reusability is further ensured by the reversible connections at the supporting points on the BauBuche posts. Should the building be dismantled one day, the supporting elements can be individually removed and used elsewhere.

Logic of construction made visible

Inside the hall, the structural logic of the building is made visible through the combination of carefully chosen materials. The reinforced concrete chord marks the transition from solid construction to timber construction in the upper section of the hall.

Light-coloured wall and ceiling cladding made of three-layer softwood panels emphasises the supporting structure, while lightweight wood wool panels serving as acoustic elements are seamlessly integrated. In order to meet the structural strength requirements of a sports hall, a rebound wall with vertical slats made in white fir has been installed in the lower area. The light-coloured linoleum flooring blends harmoniously with the understated materials and features fetching colour accents by the floor markings.

Façade connecting exterior and interior

The closed exterior wall elements form a timber panel construction with façade cladding made of vertical, grey-glazed spruce slats. The geometry of the slats divides the two-storey structure and reveals the transition between the two construction methods. Expansive glazing along the long sides of the hall provides an unobstructed view of the attractive surroundings and uniform lighting throughout the interior. Rows of windows in the upper section ensure natural ventilation of the hall.

The load-bearing reinforced concrete columns in the south section are positioned outside the thermal envelope, allowing the glass façade at hall level to run flush behind the columns. The underfloor heating integrated into the sports floor ensures comfortable and even heat transfer.

Attractive meeting place

According to Ingo Bergmann, Lord Mayor of Laupheim, the Baumäckerhalle has already become a popular public venue in Bihlafingen where young and old like to meet. The hall is used for much more than just sports, as it serves as a meeting place for the adjacent school, for a newly founded club and for the entire community. The building is a place that fosters a sense of identity where small and large celebrations are held, enriching the social life of the people of Bihlafingen in a lasting way.

The Baumäckerhalle is a prime example of how innovative timber construction technology, respectful treatment of topography and landscape, and well-thought-out functionality can be combined to create a building that has significance for a community far beyond its sports hall function.

text by: Susanne Jacob-Freitag, Karlsruhe

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